GB2103131A - Magnetic stirring of molten metal in a mould, utilizing permanent magnets - Google Patents

Abstract

A rotatable permanent magnet member is provided around the mould. Said rotatable permanent magnet member is arranged such that a rotating magnetic field in a plane perpendicular to the longitudinal direction of said mould and a shifting magnetic field in a direction parallel to said longitudinal direction are produced upon the rotation of said permanent magnet member. In one form the magnet member comprises two helically twisted magnets (21) with interposed spacers (22). In another form the magnet member comprises two groups of magnets (21) arranged in each group in a staggered formation following a helical path, and interposed spacers (22). An electric motor rotates the magnet member about the axis of the continuous casting mould by way of gearing. <IMAGE>

Description

SPECIFICATION Method of magnetic stirring utilizing permanent magnets and apparatus therefor Background of the invention This invention relates to a method and apparatus for magnetically stirring a molten metal such as molten steel to be poured into a mould for casting. Particularly, this invention relates to a method and apparatus utilizing permanent magnets for magnetically stirring a molten metal to provide not only a sufficient degree of stirring of the metal, but also to remove a remarkably large proportion of inclusions from the molten metal. More particularly, this invention is suitable for the production of billets having a small width to thickness ratio, i.e. rectangular ratio, usually a ratio of not greater than 2.5:1.

As is well known in the art, central segregation and central porosity are sometimes found in a billet, bloom or slab manufactured through a continuous casting machine. The formation of these defects is caused by metallurgical factors and mechanical or structural problems of the casting machine. It is said, for example, that central porosity is caused by an insufficient supply of molten steel to the center portion due to bridging formed during the progress of solidification.

It is also known in the art that the so-called electro-magnetic stirring utilizing multiphase alternating current is very effective for solving these problems. Namely, according to said electromagnetic stirring, molten metal at the solidifying front is stirred due to the force caused by external magnetic fields and the flow of stirred molten metal, e.g. molten steel, to the front area of solidification is sufficient to prevent the formation of bridging. Thus, a sound billet having equiaxed crystals, with central segregation and central porosity markedly reduced, can be obtained. See "Iron and Steel International" February 1979, pp. 29-41, and "Fachberichte Huttenpraxis Metallweiterverarbeitung" 1 7 Jahrg, Heft 10/79, pp.820-831.

However, an electromagnetic stirring apparatus for the well known rotating (rotary) induction magnetic field type employs alternating rotary field-producing electromagnets which are fixed around the mould. And, with an increase in size of the mould it is necessary to increase the size of the electromagnets so as to secure a sufficient degree of stirring. Therefore, the equipment becomes large and expensive.

Sometimes when the continuous casting machine is of a large scale, it is impossible to provide a large enough stirring apparatus. Furthermore, there is a limit to the depth that an induced current can penetrate into a molten metal. The limitation depends on the frequency of alternating current used. For example, in case of an alternating current of a frequency of 50 Hz, the depth of penetration of induced current is approximately 1 00mm even when magnetic fields are applied from both the opposite sides of the billet. Therefore, when billets 1 00mm or more in width are manufactured, it is necessary to reduce the frequency to a several Hz or less through a frequency converter. This adds to the cost of the apparatus.Furthermore, regardless of whether a high frequency or low frequency is used, a great portion of the power is wasted in coils as well as in cores, so the efficiency is said to be only 0.051.5%. In case a commercial frequency of 50 Hz is used, the rotation of magnetic field is very high, e.g. 3000 rpm for bipole system. However, the rotation of molten metal is only 100--300 rpm. This means that there is much slipping within the molten metal. In addition, since the temperature of the molten metal being stirred varies depending on casting temperature, drawing speed and cooling conditions of billets and in turn the viscosity thereof varies, sometimes the effect of stirring is not reproducible even if a molten metal having the same alloy composition and the same stirring conditions are used.

In order to overcome these disadvantages of the prior art, one of the co-invertors of this invention has proposed to use permanent magnets in place of electromagnets utilizing a multiphase alternating current (See Japanese Patent Disclosure No. 55-149754). According to the method disclosed therein, permanent magnets are placed surrounding the mould and are moved around the mould along the peripheral edge thereof, so that a sufficient depth of penetration of induced current is obtained and substantially no slipping is found in the molten metal. Since the method above employs permanent magnets, there is no need to employ the large scale equipment necessary for producing alternating rotary magnetic fields. A small size apparatus may be used.

One of the internal defects of steel billets is the presence of inclusions, which are classified according to their form into two classes: spherical inclusions of the manganese-silicate type and alumina cluster inclusions. In general, when stirring by the rotating induction field method which utilizes electromagnets, the molten metal is stirred in a plane vertical to the drawing direction of the billet. Therefore, the steel melt, the specific gravity of which is large, is subjected to a centrifugal force directed to the edge portion of the billet. On the other hand, inclusions, the specific gravity of which is small, are concentrated~ at the center portion of the billet.Thus, the electromagnetic stirring has been thought to be effective for the purpose of cleaning the solidification area by improved removal of impurities or inclusions from just below the surface skin of the biliet. In addition, the inclusions which have been concentrated in the central portion of the billet are aggregated and bulky and tend to come up through the molten metal. Thus, the electromagnetic stirring has been thought to be effective for the treatment of molten steel.

However, in general, the rate at which aggregated inclusions rise is not higher than the drawing speed of the billet, and most inclusions remain in the drawn billet. A billet containing large inclusions causes troubles with respect to quality of the final product formed and since these inclusions are usually large ones, such a defect is fatal.

In the conventional induction field type method it has also been proposed to utilize a shifting field so as to provide a fluid flow of the molten steel in the plane parallel to the drawing direction of the billet. See, British Patent No. 1,326,728.

However, this concerns a magnetic field shifting upwardly, not a rotating magnetic field. According to the method disclosed therein, the shifting field creates a rising flow of molten metal in the solidification area to promote the rising of inclusions to the surface of the molten metal.

However, though the shifting field is effective for the rising of inclusions to the molten metal surface, the following defects are found. Namely, since electromagnets are employed even in this method, it is inevitable that the efficiency of the apparatus is very small and the apparatus has to be made large. In addition, in principle, this electromagnetic stirring process does not provide an effective agitating force to the marginal portions, particularly the opposite marginal portions in the width direction of the billet in a plane perpendicular to the drawing direction of the billet and is, therefore, not suitable for the production of a billet having a small rectangular ratio, i.e. not greater than 2.5:1.

Furthermore, according to the shifting induction field type method, the flow pattern of the molten metal is very complicated and not uniform. Due to the non-uniform flow pattern of molten metal, inclusions are sometimes entrained in the flow of molten metal and are found in a billet produced. In this respect, even in case of the method utilizing rotatable permanent magnets, since the fluid flow of a molten metal is still in a plane perpendicular to the drawing direction of the billet, the complete removal of inclusions from the core portion of the billet cannot be effected.

Object of the invention One of the objects of this invention is to provide a method of magnetically stirring a molten metal such as molten steel poured into the mould of a continuous casting machine, in which a rotating flow as well as forced convection flow are provided to the molten metal so that central segregation and central porosity may be markedly reduced and also inclusions may be completely removed.

Another object of this invention is to provide an apparatus for magnetically stirring a molten metal such as molten steel poured into the mould of a continuous casting machine. The apparatus of this invention is so simple in its structure that it may be manufactured at a low cost. The apparatus of this invention is also able to provide a sufficient force of agitation to the molten metal even when the magnetic field is rotated slowly, since the strength of the magnetic field in the center portion of the molten metal is very strong.

Summary of the invention The inventors of this invention have noted that the magnetic stirring method which utilizes rotatable permanent magnets provides substantially no or only a slight degree of slipping to the molten steel and that the center portion of the molten metal is uniformly stirred at a relatively low rate even when the mould is large in its cross-section. This is because the current penetrates deeply. Thus, after intensive study of flow pattern of a molten metal in a magnetic stirring process on the basis of the recognition above, the inventors have completed this invention.

Namely, this invention relates to a method and apparatus for magnetically stirring a molten metal, such as molten steel with improvement in removal of inclusions, which is characterized in that permanent magnets are provided around-the mould in a spiral form or in a step-wise form, and the permanent magnets move around the periphery of the mould to provide not only a rotating magnetic field but also a shifting magnetic field to the molten metal within the mould following the movement of the permanent magnets. The structure of the permanent magnet may be in any forms as long as it can provide a shifting field as well as a rotating field when it moves around the mould periphery. In general, the permanent magnets may be disposed in a spiral form or in a step-wise form surrounding the mould.

Brief description of the drawings Figure 1 is a cross-sectional view of an example of the apparatus of this invention; Figure 2 is a perspective view of a cylindrical permanent magnet employed in this invention; Figure 3 is a perspective view of another arrangement of permanent magnets employed in this invention; Figure 4 is a diagrammatical illustration showing the change in magnetic field upon the movement of the permanent magnet; Figure 5 is also a diagrammatical illustration of the flow of fluid molten steel according to this invention; Figure 6(a) is an illustrative macrostructure in section of the cast billet; and Figure 6(b) is an illustrative rnacrostructure in section of the cast billet without application of magnetic stirring according to this invention, in which the symbol "x" indicates equiaxed crystals, the small circle at the center indicates center porosity and the area surrounding the equiaxed crystal area indicates colùmnlar crystal area; Figure 7 is a graph showing central segregation of the cast billet; and Figure 8 is a graph showing central segregation of the cast billet of the comparative example.

Detailed description of the invention The magnetic stirring method of this invention comprises providing at least one permanent magnet along the outer wall of the mould, the permanent magnet being designed for changing the magnetic flux in a spiral form or a step-wise form in a direction vertical to the longitudinal direction of the mould upon the rotation thereof, rotating the permanent magnet around the periphery of the mould so that a rotating magnetic field caused by the magnetic flux traversing the mould and a shifting magnetic field caused by the rotating angle of said permanent magnet may be applied to the molten metal contained within the mould.

The permanent magnets which may be used for the purpose of this invention may be any one which can achieve such a purpose as in the above. A single sheet of permanent magnet disposed around the mould in a spiral form may be used. Alternatively, a cylindrical body of permanent magnet magnetized in a spiral form may be used. A series of permanent magnets disposed stepwise surrounding the mould may also be used.

This invention will be described in more detail in conjunction with the accompanying drawings in which an embodiment of a magnetic stirring apparatus of this invention is illustrated. The embodiment which will be described hereinafter is directed to that used a continuous casting machine. However, it is to be noted that this invention is not limited thereto.

Fig. 1 is a cross-sectional view of the apparatus of this invention incorporated in a continuous casting machine.

The illustrated apparatus 1 comprises a rotatable permanet magnet 2 surrounding the cooling zone of the mould 3 installed in a continuous casting machine 5. A conduit for a cooling fluid, i.e. water is provided surrounding the mould 3 and the cooling water is supplied through a cooling inlet 6 and is discharged out of the apparatus from the outlet 7. The rotatable permanent magnet 2 is provided surrounding the cooling zone defined by the mould between the inlet 6 and outlet 7. In the illustrated example, the permanent magnet 2 is cylindrical in form in which a spacer of a suitable non-magnetic material (see Figs 2 and 3) is disposed between magnets in a spiral form or in a step-wise form. A supporting member 8 provided with a bearing 9 to make the cylindrical magnet rotatable is provided outside the magnet.In order to protect the permanent magnet 2 it is desirable to fix a protecting ring (not shown) to the inner surface of the cylindrical magnet. As mentioned above, the supporting member 8 bearing the permanent magnet 2 is supported by the bearing 9 at the lower end and in turn the bearing 9 is placed on a base 10 for the mould 3. A mechanism for rotating the permanent magnet 2 comprises a motor 11 which actuates a driving gear 12 to rotate the cylindrical permanent magnet 2. In place of the driving gear 12 any of a variety of known rotary driving mechanisms may be used.

Motor 11 may be an electric motor or a hydraulic motor. The surface of the molten metal is indicated by the numeral reference 13 for further reference.

The permanent magnet may be a sheet of permanent magnet mounted on the inner surface of a cylindrical body made of magnetic material. Fig.

2 shows a perspective view of a rotatable permanent magnet member employed in this invention.

Permanent magnets 21 and non-magnetic members 22 are alternately disposed in a spiral form. As this cylindrical body is rotated around the mould, a unidirectional magnetic field is produced cross the body in a direction perpendicular to the drawing direction of the billet. The non-magnetic member may be a spacer made of a non-magnetic material. In addition, two or more sheets of magnets may be used, and they may be formed into a cylindrical magnet member in which a magnetic portion and a non-magnetic portion are alternately disposed in a spiral form.

Fig. 3 shows a perspective view of another embodiment of a cylindrical magnet member employed in this invention, in which permanent magnets 21 and non-magnetic member 22 are fabricated into a cylindrical body and are disposed in a step-wise form.

This invention will be described in more detail by referring to Fig. 4 which shows the principle on which this invention is based. In Fig. 4, the horizontal axis indicates the circumferential direction of the cylindrical magnet (0), or axial direction of the cylindrical magnet (z), and the vertical axis indicates the magnetic field at the point (z, j) which is close to the magnet surface.

As shown in Fig. 4, the magnetic field is moved in a circumferential direction and also in the axial direction, as the cylindrical magnet is rotated. As is well known in the art, the direction of the electromagnetic force induced in the conductor body (molten metal, in this case) coincides with the direction of the moving magnetic field, and then the molten metal within the mould is subjected to the rotating fluid flow (horizontal fluid flow) due to the rotating magnetic field and the conventional field flow (vertical fluid flow) due to the shifting magnetic field.

Fig. 5 shows a diagrammatical illustration of the fluid flows according to this invention.

Therefore, the inclusions at the solidification front are removed by the washing effect of the induced flow and the centrifugal force and raised to the meniscus level by the upward flow. The inclusions at the meniscus level are absorbed by a powder flux placed on the metal surface. As the cylindrical permanent magnet 31 provided around the mould 33 is moved around the mould in the direction indicated by an arrow 32, the molten steel 34 poured into the mould is rotated as shown by an arrow 35 in a plane perpendicular to the drawing direction of the cast billet. The molten steel is also subjected to the fluid flow shown by an arrow 36 in a plane parallel to the drawing direction of the cast billet.As hereinabove mentioned, aggregated inclusions concentrated in the central portion of the molten metal are, therefore, successfully removed by the fluid flow directed upwardly to the molten metal surface.

Therefore, according to the magnetic stirring of this invention, bridging may be prevented successfully by the rotating movement of the molten metal, and, further, central segregation and central porosity are also reduced markedly. In addition, the inclusions are forced to rise to the surface of the molten metal at a rate sufficiently higher than the drawing speed of the cast billet.

As is apparent from the foregoing to the person skilled in the art, the magnetic stirring apparatus of this invention may be modified to some extent according to the continuous casting machine on which it is to be installed. However, any modifications are covered by this invention as long as at least one permanent magnet is provided around the mould in a spiral or step-wise rorm and it is rotated around the mould.

This irm-vention will be further described in conjunction with a working example of this invention.

Example The magnetic stirring apparatus of this invention comprising a rotatable permanent magnet in a spiral form and a driving mechanism therefor as shown in Figs. 1 and 2 was installed on a continuous casting machine of the fourstrand type for manufacturing billets (100mmx80mm) having a rectangular ratio of 1.25.

D cylindrical body (360mm inner diameter, 440mm outer diameter) was magnetised in a spiral form as shown in Fig. 2. Between the spirally magnetised permanent magnet members a spacer made of a non-magnetic material (AISI 314 stainless steel) is provided. The thus magnetised cylindrical permanent magnet member was fixed to the inner surface of a protective sleeve. The outer diameter of the apparatus was 520mm and the height thereof was 300mm. The permanent magnet employed in this example was samarium-cobalt magnet with a maximum energy product of 20 MGOe and a magnetic flux density of 980 G at the central portion of the cylinder. The cylindrical magnet was rotated at a rate of 100--250rpm.

The magnetic stirring of this invention was applied to the two strands of the four strands. The other two strands were manufactured without apDlica,ii n of this invention. The drawing speed the billets was 1.5 m/min. The steel composit on was: C:0.433v0 Si:0.27% Mn:0.65% P:0.019% 5:0.010% sol. Al:0.0295% balance iron with incidental impurities Test pieces were cut from billets at the points corresponding to the total casting amounts of 10 tons and 30 tons, respectively. Macrostructure, segregation and the number of inclusions were examined on these test pieces.

A diagrammatic illustration of a cross-section of a billet produced with the application of this invention is shown in Fig. 6(a). As is clear from the illustration, almost all the structure is comprises of equiaxed crystals and the central segregation and central porosity were effectively eliminated. On the contrary, as shown in Fig. 6(b), the billet produced without application of magnetic stirring of this invention was comprised mostly of columnar crystals. There was only a small amount of equiaxed crystals. The data of central segregation with respect to sulfur (S) and carbon (C) were plotted in Graphs A and B in Fig.

7. The Graphs A show the data of the billet produced without application of this invention, and Graphs B show the data of the billets produced in accordance with this invention.

Graphs B show that central segregation was successfully eliminated by use of this invention. In addition, the number of inclusions was 1 3 per square meters for the comparative billet and 2 per square meters for the billet produced with this invention. This fact shows that this invention is also effective to remove inclusions.

The strength of the magnetic field produced in accordance with the conventional electromagnetic stirring is on the order of 400 G at the central portion of the molten metal.

However, according to this invention the strength of magnetic field in the working example mentionned above was 1 KG. According to this invention it is easy to provide a magnetic field with a strength of at least 1 KG. In addition, since the apparatus of this invention is very simple, it may be made compact and manufactured at a low cost.

Comparative example In this example, the previous example was repeated and a permanent magnet was provided around the mould and was rotated, but the rotatable magnet was not magnitized in a spiral form or step-wise. Therefore, there was no shifting magnetic field in this example. The molten steel was subjected only to the fluid flow in a plane perpendicular to the drawing direction of the billet.

The test data are summarizied in Fig. 8. The concentration of carbon and sulfur is still rather high at the central portion of the billet. This means that the removal of inclusions was not satisfactory without application of this invention.

As already mentioned hereinbefore, the magnetic stirring of this invention can be carried out with a simple apparatus in which permanent magnet is magnetized or disposed in a spiral form or step-wise and is rotated around the mould during casting. The molten metal poured into the mould is subjected to both a horizontal fluid flow and a vertical fluid flow, so that the inclusions concentrated at the central portion of the molten metal are entrained in the molten metal flowing upwardly and are successfully removed. Thus, according to this invention billets having improved quality, particularly clean billets can be successfully produced.

Claims (11)

Claims

1. A method of magnetically stirring a molten metal poured into a mould, which comprises providing at least one rotatable permanent magnet around the mould, said permanent magnet having been so disposed that a rotating magnetic field in a plane perpendicular to the longitudinal direction of said mould and a shifting magnetic field directed upwardly in a plane parallel to said longitudinal direction are produced upon the movement of said permanent magnet around said mould, and moving said permanent magnet around said mould to stir said molten metal in a plane perpendicular to said longitudinal direction as well as in a plane parallel to said longitudinal direction during the solidification of the molten metal having been poured into the mould.

2. A method as defined in Claim 1, in which said permanent magnet is disposed in a spiral form surrounding said mould.

3. A method as defined in Claim 1, in which said permanent magnet is disposed in a step-wise form surrounding said mould.

4. A method as defined in any of Claims 1 through 3, in which the combination of said rotating magnetic field with said shifting magnetic field provides a fluid flow directed upwardly along the solidification front of the molten metal having been poured into the mould.

5. An apparatus of magnetically stirring a molten metal poured into a mould, which comprises a rotatable permanent magnet member provided around said mould, said rotatable permanent magnet member having been so magnetized that a rotating magnetic field in a plane perpendicular to the longitudinal direction of said mould and a shifting magnetic field directed upwardly in plane parallel to said longitudinal direction are produced upon the movement of said permanent magnet member around said mould, and a driving means of moving said permanent magnet around said mould.

6. An apparatus as defined in Claim 5, in which said rotatable permanent magnet member is supported by a cylindrical sleeve and said cylindrical sleeve is rotated by said driving means.

7. An apparatus as defined in Claim 5 or 6, in which said rotatable permanent magnet member is comprised of at least one permanent magnet having been disposed in a spiral form surrounding the mould.

8. An apparatus as defined in Claim 5 or 6, in which said rotatable permanent magnet member is comprised of at least one permanent magnet having been disposed in a step-wise form surrounding the mould.

9. An apparatus as defined in Claim 6, in which said rotatable permanent magnet member is comprised of two or more pieces of permanent magnets mounted in a spiral form on the inner surface of said cylindrical sleeve.

1 0. An apparatus as defined in Claim 6, in which said rotatable permanent magnet member is comprised of two or more pieces of permanent magnets mounted in a step-wise form on the inner surface of said cylindrical sleeve.

11. An apparatus as defined in Claim 9 or 10, in which between said sheets of permanent magnets is provided at least one non-magnetic spacer.