GB1558526A

GB1558526A - Process for electromagnetically stirring molten metal during continuous casting operations

Info

Publication number: GB1558526A
Application number: GB2936977A
Authority: GB
Original assignee: Institut de Recherches de la Siderurgie Francaise IRSID
Current assignee: Institut de Recherches de la Siderurgie Francaise IRSID
Priority date: 1976-07-13
Filing date: 1977-07-13
Publication date: 1980-01-03
Also published as: NL182941B; CA1078043A; IT1077321B; FR2358223A1; ES460692A1; FR2358223B3; LU77743A1; SE7707979L; BE856670A; SE433184B; NL182941C; DE2731239C2; JPS6011585B2; NL7707821A; JPS5328034A; DE2731239A1

Description

(54) PROCESS FOR ELECTROMAGNETICALLY STIRRING MOLTEN METAL DURING CONTINUOUS CASTING OPERATIONS (71) We, INSITTUT DE RECHERCHES DE LA SIDERURGIE FRANCAISE, of 185, rue President Roosevelt, 78104 Saint Germain-en Laye, France, a body corporate organised and existing under the laws of France, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to a process for stirring electromagnetically by a travelling unidirectional field molten metals and especially steel in a vertically-disposed continuous casting mould.

Generally, when continuously casting steel, the non-metallic inclusions inevitably present in the liquid bath, and especially the largest which are the most troublesome, tend to be concentrated immediately below the surface of the product. For obvious reasons, regarding the surface quality of rolled products, especially when the latter are intended for the production of sheets for deep drawing, there is therefore every inducement to begin with to flame the first millimetres of skin of the products as cast. For wide-section products, such as slabs such a flaming commonly leads to a loss of steel near to 4% in weight, and to costs that cannot be neglected in oxygen and labour.

It is known at the present time that the quantity and the distribution of the inclusions are conditioned by the nature of the flowing of the liquid metal in the mould and it has been demonstratable that it has been possible to avoid the flaming by correctly controlling the convection movements which develop in the liquid steel in the interior of the mould.

To this end, it is known, and the Applicant's operations have contributed considerably to this (French patents Nos. 2187465, 2324397 and 2352430 and United Kingdom patents Nos.

1466008 and 1507444, that it is possible to create in the metal bath convection movements effectively cleaning the solidification face by means of a travelling unidirectional magnetic field, travelling along the walls of the mould, and preferably in an upwards direction, so that a rapid settling of the inclusions at the free surface of the metal is encouraged.

Generally, the magnetic field is created by a polyphase inductor, similar to a linear motor stator, tubular in construction and surrounding the cast product at the mould. The results obtained are positive in the sense that they effectively show a transfer of the accumulations of subcutaneous inclusions towards the axis of the products. However it is then a question of a relatively recent technique which has not up to the present time reached full maturity and which still poses a certain number of problems, especially from the electromagnetic point of view. One of the major difficulties encountered lies in not being able to proportion the action of the inductor judiciously so as to obtain quickly and reliably a desired industrial result, without needlessly long and expensive experimentation.

The precise object of the present invention is to provide the means for easily overcoming such a difficulty.

To this end, the subject of the invention is a process for electromagnetically stirring molten metals, in a vertically-disposed continuous casting mould, by means of a travelling unidirectional magnetic field, travelling along the inner walls of the mould, in which the propagation of said magnetic field is effected in the upwards direction from the lower part of the mould up to at least the level of the free surface of the liquid metal in the mould and in which the magnetic field is regulated as a function of the displacement d of the accumulations of non-metallic inclusions from the cast surface towards the axis of the cast products, so that F, the force per unit volume, acting in the metal bath meets the relation: F = 35d2 +260d where F is expressed in Newtons /m3 and d in mm.

In accordance with an equivalent definition of the present invention, the effective value Beff of the magnetic field is regulated consistent with the relation: Beff = (1 (35d2 +260d))1/2 where Beff is expressed in Tesla; d depicts, as before, the desired displacement of the accumulations of inclusions from the surface of the products inwards, to a desired extent, expressed in mm; Y denotes in Ohms - ,m v denotes the rate of propagation of the travelling unidirectional magnetic field expressed in m/s.

As will be understood the object of the present invention is to improve the cleanliness as regards inclusions of continuously cast products. To this end, it enables the casting operator to regulate the effect on the inductor in such a way that the strong concentration of inclusions, which, if there is no electromagnetic stirring are localized immediately below the surface of the products, will be transferred inwardly, towards the axis of the latter to a desired and predetermined extent.

Of course, the extent of this transfer cannot exceed the thickness of skin solidified at the trailing or lower end of the air-gap of the inductor, and which is peculiar to the proportioning of the equipment in use. Beyond that, indeed, the inductor has practically no longer any control action on the convection movements within the metal bath.

Industrial trials carried out by the Applicant have shown, on the one hand, that the upwards - travelling, electromagnetic stirring, that is to say going upwards along the mould, up to a level at least corresponding with that of the free surface of the cast metal, causes not only a decrease in the total quantity of inclusions, but also, and mainly, a displacement of the zone, where the content of inclusions is high, towards the axis of the product, and, on the other hand, that this displacement increases with the power of the inductor action.

The explanation advanced by the Applicant is the following: the molten metal introduced into the mould penetrates with a certain momentum (mv) into the liquid metal contained in the mould and because of this causes the formation of a descending axial flow within the metal. On the other hand the magnetic field travelling upwards confers to the liquid metal in the peripheral zone (in contact with the solidification face) an upwards movement in the direction opposite to that of the introduction of the metal.

Through the combined action of these two happenings, a constant circulatory flow is thus established in the mould, the liquid metal ascending at the periphery and descending at the centre. The inclusions are therefore brought back towards the face surface of the cast metal where some of them naturally settle and the remainder are again carried along axially within the liquid metal as far as a distance, below the inductor, where, as the electromagnetic effects are no longer noticeable, they are trapped by the solidification face.

The applicant continued their researches with the object of confirming the existence of a relation between the power of action of the inductor and the displacement of the nonmetallic inclusions from the surface towards the axis of the cast products, and has succeeded in representing this relation by an analytical expression, which can be easily worked and is of the form: F = 35d2 + 260d where F, expressed in N/m3, depicts the force per unit volume which has to be created in the metal bath by the electromagnetic inductor in order to push the accumulations of inclusions back to the interior of the cast product over a distance d, expressed in mm, and calculated from the surface of the product.

As a matter of fact the occurrence of the force per unit volume in this relation can present some disadvantages in the sense that it does not have a concrete physical reality to which direct access can be had by experimentation. In order to determine the force density, it is in fact necessary to calculate it from measurements carried out on outer electrical or magnetic parameters of the inductor. The result is that, in practice, the casting operator can modulate the effect of the inductor by influencing directly, not only the force density F, but other parameters of which the most readily adjustable is doubtless the magnetic field as it suffices therefor simply to modify the strength of the electric current in the inductor.

It is known in the technical field under consideration, that the force per unit volume F (or more simply the electromagnetic force) can be determined according to the expression F = yv B2 eff where zy denotes the electrical conductivity of the metal bath (in -', m-1), Beff denotes the effective value of the magnetic field (in Tesla) at the zone of contact between the mould and the cast product, and v denotes the speed of propagation of the sliding field (in m/s) The speed v may be expressed by the relation: v=2 r N where r denotes the pole-pitch of the inductor (in m) and N denotes the frequency of the electrical excitation current (in Hz) circulating in the inductor.

The electromagnetic force F can then be set down in the following analytical form: F = 2wyr NB2eff From the foregoing it appears possible to modify the force F, for example to increase it, by taking action on the frequency N and/or on the magnetic field Beff. However, the Applicant has already ascertained in United Kingdom patent no. 1525546 that because of the presence of an ingot mould of electrically conductive material, the magnetic field passing through the mould is not insensitive to the frequency of the excitation current. Hence, for a given mould, there is an optimum current frequency beyond which the electromagnetic force decreases.

One preferred manner of carrying out the invention consists therefore in keeping the current frequency, i.e. the speed v, at a constant value and in adjusting the magnetic field Beff in compliance with the following relation: = = (1(35d2 +260d))Y2 It will be advantageous to fix the current frequency at its optimum value, as is taught in French patent no. 2301325, but it goes without saying that that is not indispensable to the carrying out of the invention.

There will now be described one embodiment by way of illustration and without in any way intending to restrict the scope of the invention.

A continuous casting mould for square steel billets of 120 mm side is equipped with a tubular electromagnetic inductor made up by a stack of six coils (preferably but not necessarily identical with one another), of which that located at the upper end is at a level corresponding with the mean level of the free surface of the case metal.

These six coils are connected together in pairs in series - opposition, and connected with a three-phase supply in such manner as to generate, in the mould cavity, a magnetic flux wave which ascends along the walls of the mould. In the specific case being described, the mould wall defining the passage for the cast product is of copper with chrome - zirconium for precipitation hardening and its thickness is 8 mm.

Of course it is therefore a matter of a feature in no way necessary to the carrying out of the invention but the choice of which the Applicant recommends all the same as it enables the use of walls of small thickness, and so more permeable to the magnetic field without for all that being prejudicial to their good mechanical behaviour. The rate of extraction of the billets is permanently close to 2m per minute and, taking into account the cooling conditions, the thickness of the solidified skin at the end of the air-gap of the inductor is about 12 mm. The useful length of the inductor is 0.48m, a value corresponding to twice its pole-pitch. The inductor was proportioned in order to provide per phase a maximum current strength of 350Aeff at a phase voltage of 55v. The casting operator wants, for example, to push the peak of inclusions about 8 mm from the surface towards the axis of the billets. In compliance with the invention, the inductor must then establish in the metal bath an electromagnetic force F per unit volume of 4320 N/m3. The frequency of the excitation current was fixed at 10 Hz, which in the specific case described corresponds to the optimum value. Taking into account the electrical conductivity of molten steel (close to 6.25 x 105 - 1, m~l) the operator will therefore have to adjust the effective value of the magnetic field to 0.038 Tesla, at 380G.

At the time of the tests, a peak of inclusions located 8 mm from the surface towards the axis of the billets was obtained by a magnetic field of 420G. The aforesaid relations therefore give values approximate to 10% which is quite suitable.

The process in accordance with the invention can be applied to any continuously cast metal product whatever may be its composition and form and size.

Accumulations of inclusions can be pushed back at will from the surface towards the axis of the cast products up to depths predetermined and chosen by the operator as a function of the metallurgical treatments such as rolling subsequently to be undergone, so as to obtain with certainty a good surface quality of these products without having recourse to preliminary treatments such as flaming or others.

WHAT WE CLAIM IS: 1. A process for electromagnetically stirring molten metals, in a vertically-disposed continuous casting mould, by means of a travelling unidirectional magnetic field, travelling along the inner walls of the mould, in which the propagation of said magnetic field is effected in the upwards direction from the lower part of the mould up to at least the level of the free surface of the liquid metal in the mould, and in which the magnetic field is regulated as a function of the displacement d of the accumulations of non-metallic inclusions from the cast surface towards the axis of the cast products, so that F, the force per unit volume, acting in the metal bath meets the relation: F = 35d2 + 260d where F is expressed in Newtons /m3 and d in millimetres.

2. A process according to Claim 1, in which the effective value Beff of the magnetic field is controlled in conformity with the relationship Beff = F / where Beff is expressed in Tesla, v, denoting the electric conductivity of the metal,bath, is expressed in Ohms ~ . m-1 and v, denoting the rate of propagation of the magnetic field, is expressed in m/s.

3. A process for electromagnetically stirring molten metals within a continuous casting mould by means of a travelling unidirectional magnetic field travelling along the inner walls of the mould, in which the propagation of the magnetic field is effected in the upwards direction and in that the magnetic field is regulated as a function of the displacement d of the accumulations of non-metallic inclusions from the cast surface towards the axis of the cast products in such a manner that the effective value Beff of the magnetic field expressed in Tesla satisfies the relation:

where d is

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. of an ingot mould of electrically conductive material, the magnetic field passing through the mould is not insensitive to the frequency of the excitation current. Hence, for a given mould, there is an optimum current frequency beyond which the electromagnetic force decreases. One preferred manner of carrying out the invention consists therefore in keeping the current frequency, i.e. the speed v, at a constant value and in adjusting the magnetic field Beff in compliance with the following relation: = = (1(35d2 +260d))Y2 It will be advantageous to fix the current frequency at its optimum value, as is taught in French patent no. 2301325, but it goes without saying that that is not indispensable to the carrying out of the invention. There will now be described one embodiment by way of illustration and without in any way intending to restrict the scope of the invention. A continuous casting mould for square steel billets of 120 mm side is equipped with a tubular electromagnetic inductor made up by a stack of six coils (preferably but not necessarily identical with one another), of which that located at the upper end is at a level corresponding with the mean level of the free surface of the case metal. These six coils are connected together in pairs in series - opposition, and connected with a three-phase supply in such manner as to generate, in the mould cavity, a magnetic flux wave which ascends along the walls of the mould. In the specific case being described, the mould wall defining the passage for the cast product is of copper with chrome - zirconium for precipitation hardening and its thickness is 8 mm. Of course it is therefore a matter of a feature in no way necessary to the carrying out of the invention but the choice of which the Applicant recommends all the same as it enables the use of walls of small thickness, and so more permeable to the magnetic field without for all that being prejudicial to their good mechanical behaviour. The rate of extraction of the billets is permanently close to 2m per minute and, taking into account the cooling conditions, the thickness of the solidified skin at the end of the air-gap of the inductor is about 12 mm. The useful length of the inductor is 0.48m, a value corresponding to twice its pole-pitch. The inductor was proportioned in order to provide per phase a maximum current strength of 350Aeff at a phase voltage of 55v. The casting operator wants, for example, to push the peak of inclusions about 8 mm from the surface towards the axis of the billets. In compliance with the invention, the inductor must then establish in the metal bath an electromagnetic force F per unit volume of 4320 N/m3. The frequency of the excitation current was fixed at 10 Hz, which in the specific case described corresponds to the optimum value. Taking into account the electrical conductivity of molten steel (close to 6.25 x 105 - 1, m~l) the operator will therefore have to adjust the effective value of the magnetic field to 0.038 Tesla, at 380G. At the time of the tests, a peak of inclusions located 8 mm from the surface towards the axis of the billets was obtained by a magnetic field of 420G. The aforesaid relations therefore give values approximate to 10% which is quite suitable. The process in accordance with the invention can be applied to any continuously cast metal product whatever may be its composition and form and size. Accumulations of inclusions can be pushed back at will from the surface towards the axis of the cast products up to depths predetermined and chosen by the operator as a function of the metallurgical treatments such as rolling subsequently to be undergone, so as to obtain with certainty a good surface quality of these products without having recourse to preliminary treatments such as flaming or others. WHAT WE CLAIM IS:

1. A process for electromagnetically stirring molten metals, in a vertically-disposed continuous casting mould, by means of a travelling unidirectional magnetic field, travelling along the inner walls of the mould, in which the propagation of said magnetic field is effected in the upwards direction from the lower part of the mould up to at least the level of the free surface of the liquid metal in the mould, and in which the magnetic field is regulated as a function of the displacement d of the accumulations of non-metallic inclusions from the cast surface towards the axis of the cast products, so that F, the force per unit volume, acting in the metal bath meets the relation: F = 35d2 + 260d where F is expressed in Newtons /m3 and d in millimetres.

where d is

expressed in mm, , expressed in #-1 .m-1 , represents the electrical conductivity of the metal bath and v, expressed in m/s, denotes the rate of propagation of the magnetic field.

4. A process for electromagnetically stirring molten metals, in a continuous casting mould, substantially as hereinbefore described.