FR3120635A1 - Process for manufacturing soft magnetic metal semi-finished product - Google Patents

Abstract

Process for manufacturing a soft magnetic metal semi-finished product Process for manufacturing a soft magnetic metal semi-finished product, endowed with a particularly good magnetization behavior, said process comprising the following process steps: manufacturing or providing of a metallic blank of a metallic product suitable for rolling, roughing the metallic blank in the rolling mill with a well-defined degree of deformation, down to an intermediate thickness, which degree of deformation is coordinated with a critical reduction rate or post-criticality to be observed during the subsequent rolling,heat treatment of the rough-milled blank, preferably annealing of said rough-milled blank,rolling said blank with a critical or postcritical reduction rate, down to a final thickness, and annealing carried out in the direct sequence in order to set a well-defined grain size, and completion of the semi-finished product. Figure to be published with abstract: Fig. 1

Description

Process for manufacturing soft magnetic metal semi-finished product

The invention relates to a process for the manufacture of a soft magnetic metal semi-finished product, endowed with a particularly good magnetization behavior.

Soft magnetic semi-finished products are known in the prior art as electrical foil.

By "soft magnetic materials", it is appropriate to understand materials which can be magnetized and demagnetized in a simple way, in particular under the influence of external electric fields, of conductors through which a current passes, and of magnetic fields, so that their use in electrical systems makes it possible to make optimal use of electrical energy.

The soft magnetic semi-finished product is transformed into finished products finding, for example, application in the construction of electrical machines. A semi-finished product with correspondingly suitable soft magnetic properties is selected and processed according to the application purpose and requirements.

If, for example, for the purpose of application, the magnetic flux is not firmly established in a specific direction, but if on the other hand it is necessary to establish good magnetic properties in all directions, a half - product with isotropic properties. The latter is also referred to as "electrical strip with non-oriented grains" and is implemented, for example, in generators, electric motors or relays.

Such non-grain-oriented materials are considered below.

These and other properties, such as permeability, remanence, flux density and saturation, as well as coercive field strength, describe the quality of soft magnetic properties of materials. These properties can and should be coordinated with the respective application purpose of the product.

A semi-finished product, preferably a hot rolled steel strip, is used as a starting material in the prior art to manufacture an electrical strip having such desired properties. In types of electric strip "with non-oriented grains, subjected to final annealing", after pickling the hot-rolled strip, cold rolling is carried out in several rolling passes up to the thickness desired final annealing, and to a subsequent final annealing, in which case the laminated structure is recrystallized, the carbon content is set to very low values and a formation of coarse grains is caused, thus allowing easy magnetization of the material. In direct continuity, the semi-finished product (cold rolled strip) is processed to obtain finished products, which finished products are, for example, punched from the cold rolled strip or debited by laser cutting. During this transformation, a deformation of the material must be absolutely avoided since a deformation generates, in the structure, mutations which would considerably degrade the aptitude of the material for magnetization and would result in an unsatisfactory finished product.

Another possibility, to manufacture an electrical strip having the desired properties, consists in that the hot-rolled steel strip is cold-rolled after pickling, subjected to a crystallization annealing, then cold-rolled with a low degree of deformation (critical deformation), so as to cause targeted grain growth and thereby facilitate the material's ability to magnetize. Finished products, for example parts coming from stamping-bending or parts coming from deep drawing, can then be produced from this semi-finished product, knowing that at the end of their completion, said finished products must be subjected to special annealing with a view, for example, to eliminating the mutations induced in the structure of the material during forming, and to provide a product endowed with optimal soft magnetic properties.

Such an electric strip is qualified as “electric strip with non-oriented grains, not subjected to final annealing”.

Subsequent special annealing of finished products is time-consuming and expensive, as each individual part must be dedicated to the annealing process.

The object of the invention is to provide a process which can be implemented economically, in which the soft magnetic and mechanical properties of a semi-finished product can be suitably adjusted, said properties being able in particular to be coordinated with a transformation putting shaping said semi-finished product, so as not to require any tedious heat treatment of finished products generated by forming said semi-finished product, and to provide a finished product endowed with good soft magnetic properties.

It is also appropriate to provide an enrichment of the multiple facets of the process, i.e. alternative solutions targeting the manufacture of semi-finished products endowed with particularly good soft magnetic properties.

• fabrication ou mise à disposition d’une ébauche métallique d’un produit métallique apte au laminage,
• dégrossissage de l’ébauche métallique au laminoir avec un degré de déformation bien défini, jusqu’à une épaisseur intermédiaire, lequel degré de déformation est coordonné avec un taux de réduction critique ou post-critique devant être respecté lors du laminage consécutif,
• traitement thermique de l’ébauche dégrossie au laminoir, de préférence recuit de ladite ébauche dégrossie au laminoir,
• laminage de ladite ébauche avec un taux de réduction critique ou post-critique, jusqu’à une épaisseur définitive, et recuit opéré dans l’enchaînement direct en vue de régler une grosseur de grains bien définie, et
• parachèvement du demi-produit.

To achieve this object, the invention proposes a method for manufacturing a soft magnetic metal semi-finished product endowed with a particularly good magnetization behavior, preferably a strip, a wire or a semi-finished product. - similar metal finish, said method comprising the following operating steps:

• manufacture or provision of a metal blank of a metal product suitable for rolling,
• roughing of the metal blank in the rolling mill with a well-defined degree of deformation, up to an intermediate thickness, which degree of deformation is coordinated with a critical or post-critical reduction rate which must be respected during subsequent rolling,
• heat treatment of the roughened blank in the rolling mill, preferably annealing of said roughened blank in the rolling mill,
• rolling said blank with a critical or post-critical reduction rate, down to a final thickness, and annealing operated in the direct sequence in order to set a well-defined grain size, and
• completion of the semi-finished product.

A metal blank, suitable for rolling, is produced or made available as a starting product. Said blank may consist, for example, of ferritic steel or another metallic material, and has isotropic properties.

First, the metal blank is roughened in the rolling mill, the preliminary rolling taking place down to an intermediate thickness. During said preliminary rolling, the degree of deformation is coordinated with a critical or post-critical reduction rate to be adjusted during subsequent rolling in such a way that, during said subsequent rolling targeting the final thickness, only a modest deformation entering the respectively critical or post-critical deformation range of the material, with a low rate of reduction. Thus, at the stage of said preliminary rolling targeting the intermediate thickness, the degree of deformation is dependent on the critical or post-critical reduction rate during rolling targeting the final thickness.

At the end of the preliminary rolling, the blank is subjected to a heat treatment, preferably to a recrystallization annealing.

In direct continuity, rolling is carried out during which the blank, having the critical or post-critical reduction rate, is rolled to its final desired thickness, then to an additional annealing in direct succession to said rolling. Rolling with the critical or post-critical reduction rate, and annealing directly following the latter, have the effect of causing grain growth in the metal structure, which allows easier and faster magnetization and demagnetization of the material.

During the forming of a material by rolling with a reduction ratio lower than the critical reduction ratio, there does not take place in the structure, at first, no grain growth or, at least, no noticeable growth. Grain growth is induced in the structure only when the critical reduction rate is reached, the maximum grain size being reached when forming with the critical reduction rate. If the reduction rate is greater than said critical reduction rate (post-critical reduction rate), the size of the grains decreases as the reduction rate increases. Said reduction rate is selected in such a way that a growth of the grains is caused in the structure, namely at least a critical reduction rate, or even post-critical, the size of the grains and, beyond, the magnetic properties which can then be regulated in a well-defined manner by the yardstick of the reduction rate. The grains generated in the structure are all the larger and the magnetization properties are all the better, that is to say that the material can be magnetized all the more easily and quickly, as the reduction rate is chosen weak. The mechanical properties of said material nevertheless deteriorate, at the same time, as the size of the grains increases.

The grain size and the magnetic and mechanical properties associated with it can, and therefore must, be suitably tuned to the critical or post-critical reduction rate, and adapted to the respective subsequent purpose of use of the material, so that a suitable semi-finished product can be adequately supplied for the intended application purpose.

The soft magnetic properties, induced in the material by the implementation of the process, allow a transformation of the semi-finished product also carried out by forming, for example by deep drawing or bending, into finished products endowed with particularly good soft magnetic properties, without said finished products necessarily having to be subjected to a particular subsequent annealing. Time-consuming and expensive post-processing of individual products is thus superfluous.

The method makes it possible to generate, and to make available, various semi-finished products endowed with particularly good soft magnetic properties, for example strips, wires or similar semi-finished metal products which, in direct continuity, are placed in form to obtain finished products.

It is preferably provided that the blank is a hot-rolled strip of a ferritic (or non-alloy) steel.

It is preferably provided that, during the preliminary rolling, the blank is roughened in the rolling mill with a degree of deformation of 30-80%.

A degree of deformation of this order of magnitude is required, for example, when forming hot-rolled steel strip, such that a deformation of the material, falling within the range of deformation respectively critical or post- critical, can be ensured during subsequent rolling with a critical or post-critical reduction rate.

A hot rolled steel strip, to be shaped according to the process, can have a thickness of up to 50 mm.

Insofar as a respectively different blank or material is used, the degree of deformation is adapted to the respective reduction rate, critical or post-critical, specific to the material.

It is then provided, preferably, that the annealing of the rough blank in the rolling mill takes place at a temperature of 550 ° C - 700 ° C, knowing that it is preferably provided that the annealing is carried out for up to 50 hours.

The structure is thus recrystallized at the end of the preliminary rolling.

It is preferably provided, moreover, that the critical or post-critical reduction rate is between 8 and 25%, preferably between 9 and 15%, knowing that it is preferably provided that the annealing, directly following the rolling with the rate of critical or post-critical reduction takes place at a temperature of up to 710°C and said annealing is carried out for a period of up to 80 hours.

Under the effect of the preliminary rolling with a reduction rate within the limits indicated, and of an annealing directly succeeding it, the grain sizes of a material of the non-alloy steel type can be regulated in a variable manner and with adaptation to the purpose of subsequent application of the semi-finished product. Depending on the choice of material, the critical, and thus also post-critical, reduction ratios lie within the mentioned range and several grain sizes can be set depending on the reduction ratio. In the case of steel (alloy or non-alloy), grain sizes with an ASTM index of 1 to 6 can be set, knowing that the grain size decreases as the degree increases. of deformation.

If another material is used, the specific critical or post-critical reduction rate must be determined and applied during rolling.

Provision is preferably made for a heat treatment carried out upstream, preferably annealing, to be carried out before the preliminary rolling of the blank, in which case it is preferably provided that said annealing carried out upstream takes place at a temperature between between 650 and 800°C and is preferably carried out over a period of up to 60 hours.

The heat treatment carried out upstream, preferably annealing, is used to prepare the metallic material of the blank for its shaping, and to bring the structure of said blank to an improved initial state.

In the presence of blanks in a material of the ferritic steel type, the heat treatment carried out upstream causes, for example, a modification of the carbon content. Under the effect of the heat treatment, the carbon, randomly distributed in the structure prior to said heat treatment, is deposited at the boundaries between the grains.

The heat treatment carried out upstream allows a further improvement in the soft magnetic properties of the semi-finished product, and it is possible to provide a semi-finished product of even better quality.

Provision is preferably made for the blank to be subjected to a finish rolling before finishing and after the final heat treatment, knowing that it is preferably provided that the finish rolling takes place with a degree of deformation situated between 0.1 and 2%.

In this way, it is possible to set the exact thickness of the material, as well as the flatness and quality of the surface.

An exemplary embodiment of the method according to the invention is illustrated in the drawings and is described in more detail below.

On the drawings:

is a schematic representation of the course of the process;

shows, in the form of a table, the magnetic and mechanical properties of an unalloyed steel, after implementation of said process, with different reduction rates.

There schematically illustrates the progress of a process for the manufacture of a soft magnetic metal semi-finished product, endowed with a particularly good magnetization behavior.

A semi-finished product, in the form of a cold-rolled steel strip, is produced using this process.

A rollable steel metal blank, i.e. a semi-finished product, preferably a hot-rolled unalloyed steel strip, is manufactured or made available as a starting product for this purpose, the material of said strip hot rolled with isotropic properties.

The hot-rolled strip is first roughened in the rolling mill, the preliminary rolling being carried out to an intermediate thickness. During said preliminary rolling, the degree of deformation is between 30 and 80% and is coordinated with a respectively critical or post-critical reduction rate, to be regulated during the subsequent rolling targeting a final thickness, so that, during said rolling targeting said final thickness, only a modest deformation still necessary entering the respectively critical or post-critical deformation range of the material, with a low rate of reduction. The degree of deformation, during the preliminary rolling targeting the intermediate thickness, is thus dependent on the critical or post-critical reduction rate during the rolling targeting the final thickness.

In the sequence, the blank roughed out in the rolling mill is subjected to a heat treatment, annealing preferably being carried out at a temperature between 550 and 700°C, for a duration of up to 50 hours.

Annealing is followed by rolling during which the blank, presenting the critical or post-critical reduction rate, is rolled to its final desired thickness; and in the direct succession of rolling, additional annealing at a temperature reaching up to 710°C for a period reaching up to 80 hours. Said rolling with the critical or post-critical reduction rate, and the annealing directly following it, have the effect of causing grain growth in the metallic structure, thus allowing easier and faster magnetization and demagnetization of the material.

The forming must be carried out at least with a critical reduction rate given that, during a forming of the material by rolling with a reduction rate lower than said critical reduction rate, no growth of the grains or, at the very least, no growth noticeable takes place in the structure. Obvious grain growth is caused, in the structure, only during a deformation during which the critical reduction rate is reached, the maximum grain size being set during forming with the critical reduction rate. The grain size is reduced again if the reduction rate is greater than said critical reduction rate (postcritical reduction rate), which grain size decreases as the postcritical reduction rate increases. -critical. At least one critical, or even post-critical, reduction rate is thereby set, knowing that the size of the grains and, beyond that, the magnetic properties can be set in a well-defined manner by the yardstick of the reduction rate. Said degree of reduction is between 11 and 25% in the example illustrated. The results obtained, concerning the magnetic and mechanical properties acquired by forming the material with such reduction rates, are represented in the table of the . The grains generated in the structure are all the larger and the magnetization properties are all the better, that is to say that the material can be magnetized all the more easily and quickly, as the reduction rate is chosen low. At the same time, however, the mechanical properties of said material deteriorate as the grain size increases.

The grain size, and the magnetic and mechanical properties associated with it, are therefore suitably regulated by the yardstick of the critical or post-critical reduction rate, and adapted to the respective subsequent purpose of use of the material, so well that a suitable semi-finished product can be adequately provided for the intended application purpose.

The soft magnetic properties induced in the material in this way allow a transformation of the semi-finished product, also taking place by forming, for example by deep drawing or bending, into finished products endowed with particularly good soft magnetic properties, without it is necessary to submit the said finished products to a particular subsequent annealing. This dispenses with tedious and expensive post-processing of individual products.

After rolling and subsequent annealing, the blank undergoes finish rolling with a degree of deformation of, for example, 0.7%. The exact thickness of the material, as well as the flatness and quality of the surface, are thus regulated.

The possibility is offered, moreover, of first devoting the blank to an annealing prior to the preliminary rolling, so as to prepare the material for the following operating steps and to bring the structure to an improved initial state. This annealing is preferably carried out at a temperature of between 650 and 800°C.

Annealing governs a modification of the carbon content in the structure. Prior to the heat treatment, the carbon is randomly distributed in the structure and is deposited, under the effect of the annealing, at the level of the boundaries between the grains. Because of this, the carbon does not negatively impede the magnetization of the material at a later stage and the soft magnetic properties of the semi-finished product can be additionally improved. It is possible to supply a semi-finished product with a particularly good quality.

The other operating steps are carried out after the heat treatment carried out upstream.

The method makes it possible to provide a semi-finished product endowed with particularly good soft magnetic properties, which can be adjusted so as to also allow subsequent forming of the material, for example into parts coming from deep drawing or drawing-bending, with particularly good soft magnetic properties, without the finished products necessarily having to be subjected, in direct continuity, to a particular annealing or comparable heat treatment.

The invention is not confined to the embodiment, but may undergo numerous modifications within the framework of the description.

All the individual and combined characteristics, disclosed in the description and/or on the drawings, are deemed to be of an essential character for the invention.

Claims (13)

Process for producing a soft magnetic metal semi-finished product with particularly good magnetization behavior, preferably a strip, a wire or a similar semi-finished metal product, said process comprising the following operating steps:

• manufacture or provision of a metal blank of a metal product suitable for rolling,
• roughing of the metal blank in the rolling mill with a well-defined degree of deformation, up to an intermediate thickness, which degree of deformation is coordinated with a critical or post-critical reduction rate which must be respected during subsequent rolling,
• heat treatment of the roughened blank in the rolling mill, preferably annealing of said roughened blank in the rolling mill,
• rolling said blank with a critical or post-critical reduction rate, down to a final thickness, and annealing operated in the direct sequence in order to set a well-defined grain size, and
• completion of the semi-finished product.

Process according to Claim 1, characterized in that the blank is a hot-rolled strip of a ferritic or unalloyed steel. Process according to Claim 1 or 2, characterized in that during the pre-rolling the blank is roughened in the rolling mill with a degree of deformation of 30-80%. Method according to one of Claims 1 to 3, characterized in that the annealing of the roughened blank in the rolling mill takes place at a temperature of 550°C - 700°C. Process according to Claim 4, characterized in that the annealing is carried out for up to 50 hours. Process according to one of Claims 1 to 5, characterized in that the critical reduction rate is between 8 and 25%, preferably between 9 and 15%. Process according to one of Claims 1 to 6, characterized in that the annealing, directly following the rolling with the critical reduction rate, takes place at a temperature reaching up to 710°C. Process according to Claim 7, characterized in that the annealing takes place over a period of up to 80 hours. Method according to one of Claims 1 to 6, characterized in that an upstream heat treatment, preferably an annealing, is carried out before the preliminary rolling of the blank. Process according to Claim 9, characterized in that the annealing carried out upstream takes place at a temperature of between 650 and 800°C. Process according to Claim 9 or 10, characterized in that the annealing carried out upstream is carried out over a period of up to 60 hours. Process according to one of Claims 1 to 11, characterized in that the blank is subjected to finish rolling before finishing and after the final heat treatment. Process according to Claim 12, characterized in that the finish rolling takes place with a degree of deformation situated between 0.1 and 2%.