EP0849371A1 - Non-oriented magnetical steel sheet - Google Patents

Abstract

A sheet steel contains less than 0.005 wt.% carbon, less than 0.1 wt.% phosphorous, 0.2 wt.% silicon, 0.1 wt.% manganese and additional quantities of Si, Mn and aluminium defined by equations 80Mn+90Si ≥ 85 (I) and 5.38 ≤ 6Mn+13Al+12Si ≤ 16 (II). Maximum levels of Si and Mn are 0.6 wt.% and 1.3 wt.% respectively. The steel is formed into a sheet, e.g. by cold-rolling, then annealed at 800-900 degrees C, remaining below the ferrite-austenite transformation temperature to produce a structure with non-oriented grains.

Description

The present invention relates to the field of steel sheets to electrical use, used in particular for the construction of transformers, motors and alternators.

It relates specifically to magnetic grain sheets not oriented cold rolled delivered in the finished state called "Fully Process" sheets.

Sheets for electrical use are traditionally divided into two families.

Cold rolled magnetic sheets delivered in semi-finished state so-called "Semi Process" sheets, used in low to medium devices power (motors of small and medium household appliances, transformers, automobile alternators for example) constitute the first family.

These sheets contain by weight a carbon content included between 0.03 and 0.08%, a manganese content of between 0.2 and 1.0%, a phosphorus content of between 0.01 and 0.25%, and optionally but not necessarily aluminum and / or silicon, the content of aluminum being less than 1.0% and the silicon content being less than 0.2%.

These sheets are manufactured by hot and cold rolling, then recrystallization annealing and passage through a skinning cage (skin pass) to harden the metal by a deformation of 4 to 10%, sometimes more.

It is in this form that the sheets are sent to the customer, but at this stage, their magnetic properties are poor because they have been degraded during work hardening.

This work hardening is however necessary so that the metal can easily be cut by the customer.

This must, after cutting and shaping the sheet, perform a heat treatment aimed firstly at decarburizing the steel up to a carbon content of less than 0.005%, and on the other hand increase its grain size (operation facilitated by the work hardening which has been skin pass).

Cold rolled magnetic sheets delivered in the so-called finished state "Fully Process" sheets constitute the second family.

These sheets have the distinction of being usable by the customer directly in the state of delivery of the producer, without processing thermal insulation is necessary to obtain the magnetic properties targeted.

These "Fully Process" sheets are divided into two categories.

Oriented grain "Fully Process" sheets are the first category.

They have the particularity to contain silicon contents high (of the order of 3%), and having a very texture orientation marked with type (110-001), direction 100 being oriented in the direction of rolling, which corresponds, in the use of sheet metal, to the direction in which the magnetic field is applied.

The silicon increases the resistivity of the sheet and thus limits the eddy current losses.

Sheets of this type are applied in particular to manufacturing of power transformers.

Non-oriented grain "Fully Process" sheets constitute the second category.

They contain silicon in proportions that can be lower than for the previous ones (from 0.2 to 3.0%), and also phosphorus.

This last element plays the same role as silicon in the limitation of eddy current losses, and its partial substitution for silicon does not increase the fragility of the metal too much.

Sheets of this type do not have a particular texture and are applied in particular to the manufacture of rotating machines, engines or alternators for example.

• on élabore, coule et lamine sous forme de tôle un acier ayant la composition ci-dessus,
• puis on fait subir à la tôle ainsi élaborée un traitement de recuit, par exemple un recuit en continu, sous atmosphère neutre à température élevée (dans la gamme 800 à 900°C) et de manière à rester en dessous du point de transformation ferrite-austénite (α → γ) de la nuance d'acier traitée.

This second family of sheets called "fully process" sheets generally contain by weight a carbon content of less than 0.005%, a manganese content of between 0.1 and 0.7%, a silicon content of more than 0.2%, a phosphorus content of between 0.1 and 0.2%, and are produced as follows:

• a steel having the above composition is produced, cast and rolled in the form of sheet metal,
• then the sheet thus prepared is subjected to an annealing treatment, for example a continuous annealing, under a neutral atmosphere at high temperature (in the range 800 to 900 ° C) and so as to remain below the ferrite transformation point- austenite (α → γ) of the treated steel grade.

As will be understood, the process for manufacturing such a "Fully Process" sheet consists of obtaining a carbon content of less than 0.005%, then subjecting it to annealing to give it the magnetic properties required.

To obtain a carbon content of less than 0.005%, one can either carry out a decarburization treatment at the steelworks by using a apparatus for degassing liquid steel under vacuum of the RH or DH type, either carry out decarburizing annealing under cold atmosphere after cold rolling containing a mixture of oxidizing gases.

Each of these two families of sheets for electrical use have certain advantages and disadvantages.

We know that the magnetic characteristics of a steel sheet are obtained on the one hand by certain alloying elements, and on the other hand by the grain size bias of the metal structure.

Indeed, certain alloying elements, such as manganese, phosphorus, silicon and aluminum increase the resistivity of the metal, and thus gives it good magnetic properties.

Similarly, the larger the grain size (in a certain upper limit), the better the characteristics magnetic of the metal.

In the case of "Semi Process" sheets, obtaining magnetic characteristics is guaranteed by the annealing operation of recrystallization and grain enlargement carried out after cutting sheet metal.

In addition to the critical work hardening effect, the transition to the skin pass enables favorable mechanical characteristics to be obtained cutting the sheet.

So the chemical composition of the metal, in terms of elements alloy is relatively secondary from the characteristic point of view mechanical and cuttability, as these are ensured by the operation of skin pass hardening.

So a "Semi Process" sheet does not pose any problems particular cutting, but it requires an additional step of treatment (decarburization annealing, recrystallization and magnification of grains) and perform this step on cut pieces, which results lower productivity than if this step were carried out by the producer during the sheet metal manufacturing cycle.

In the case of "Fully Process" sheets, it is impossible optimize the mechanical properties of the metal after annealing grain enlargement, otherwise its characteristics will deteriorate magnetic.

The chemical composition in terms of alloying elements is so in this case of particular importance because it conditions strongly obtaining the mechanical characteristics necessary for good cuttability of the sheet.

These "Fully Process" sheets can be cut more difficult than "Semi Process" sheets because the ratio between the limit of elasticity and the breaking load Re / Rm is between 0.65 and 0.70, and the elongation A% is high (greater than 30).

The object of the invention is to provide steel sheets for use electrical, for low to medium power electrical appliances, with excellent cuttability, and produced using a short die and economical, of the "Fully Process" type.

To this end, the subject of the invention is a steel sheet for electrical use, of the type comprising less than 0.005% by weight of carbon, obtained in two stages, a first stage consisting in manufacturing a steel sheet, then a second step consisting in subjecting said sheet to an annealing treatment at a temperature taken in the range 800 ° C to 900 ° C and so as to remain below the ferrite-austenite transformation point (α → γ), characterized in that that the sheet includes in weight a carbon content of less than 0.005%, a phosphorus content of less than 0.1%, a silicon content of 0.2%, a manganese content of 0.1%, and a or several elements chosen from silicon, manganese and aluminum, with contents such as: 80 Mn + 90 If ≥ 85 and 5.38 ≤ 6 Mn + 13 Al + 12 Si ≤ 16
and the total silicon content is less than 0.6%
and the total manganese content is less than 1.3%, the remainder being inevitable iron and residual impurities.

• la teneur en poids en phosphore est comprise entre 0,01 % et 0,09 %;
• le rapport entre la teneur en silicium total et la teneur en manganèse total est inférieur à 0,5 ;
• la tôle comprend en poids une teneur en carbone inférieure ou égale à 0,004 %, une teneur en silicium comprise entre 0,25 et 0,35 %, une teneur en manganèse comprise entre 0,8 et 1,0 %, une teneur en phosphore comprise entre 0,05 et 0,09 %, une teneur en aluminium comprise entre 0,08 et 0,12 %, le reste étant du fer et des impuretés résiduelles inévitables.

According to other features of the invention,

• the content by weight of phosphorus is between 0.01% and 0.09%;
• the ratio between the total silicon content and the total manganese content is less than 0.5;
• the sheet comprises by weight a carbon content less than or equal to 0.004%, a silicon content of between 0.25 and 0.35%, a manganese content of between 0.8 and 1.0%, a phosphorus content between 0.05 and 0.09%, an aluminum content between 0.08 and 0.12%, the rest being iron and unavoidable residual impurities.

The characteristics and advantages of the invention will become apparent the following description which is given only by way of example.

The first important characteristic of the invention resides in limiting the phosphorus content of the steel to 0.1% by weight.

The inventors have found that the presence of phosphorus in steel, with a content between 0.10 and 0.20% by weight, is harmful for the cuttability of the sheet made from such steel, and this regardless of mechanical characteristics.

When cutting a sheet blank with a device comprising a punch and a matrix with sharp edges, this cutting takes place in two phases: a first phase of shearing the metal, and a second phase of metal tearing. The cutting edge therefore has two zones: a first sheared zone located on the punch side, and a second ruptured area located on the matrix side.

In the first cutting phase, the metal is therefore sheared, then cracks in the metal start at the cutting edge of the punch and cutting edge of the die, which meet in the phase end of the cut, causing the metal to break.

In a "Fully Process" sheet with a phosphorus content greater than 0.10% by weight, this tends to segregate into layers, generally located between a quarter and three quarter of the thickness of said sheet metal.

These phosphorus segregation layers are layers having a hardness greater than the rest of the sheet, and this phenomenon is detrimental to the homogeneity of the cutting.

The cracks that start in the metal after the first cutting phase of a blank taken from such a sheet are deflected when they encounter a phosphorus segregation layer and take a horizontal orientation, which affects the quality of the broken area of the cutting. Indeed, this broken area of the cut is all the clearer that the orientation of the cracks is vertical.

The cut thus presents a non-homogeneous broken zone with the appearance of burrs.

In addition, the horizontal orientation of the cracks often results the detachment of certain metal particles, causing wear abrasive cutting tools.

It therefore appeared important to limit the phosphorus content in steel, and the inventors have found that this phenomenon is no longer significant when this phosphorus content is less than 0.10% by weight.

Preferably, the phosphorus content is between 0.01% and 0.09%.

This limitation of the phosphorus content, favorable to the cuttability of the sheet, however has two harmful effects.

On the one hand from a mechanical characteristics point of view, the phosphorus is one of the most effective alloying elements to give the sufficient hardness, and on the other hand, the decrease in the content of phosphorus leads to a degradation of the magnetic characteristics, in particularly in terms of specific total losses.

It is therefore necessary to replace some of the phosphorus by one or more other elements which increase the characteristics sheet metal mechanics as well as its magnetic characteristics.

The inventors have found that these elements can be advantageously chosen from manganese, silicon and aluminum.

The second important characteristic of the invention is therefore that the sheet comprises by weight, in addition to a carbon content of less than 0.005% and a phosphorus content of less than 0.1%, a silicon content of 0.2%, a manganese content equal to 0.1%, and one or more elements chosen from silicon, manganese and aluminum, with contents such as: 80 Mn + 90 If ≥ 85 and 5.38 ≤ 6 Mn + 13 Al + 12 Si ≤ 16
and the total silicon content is less than 0.6%
and the total manganese content is less than 1.3%, the remainder being inevitable iron and residual impurities.

The total silicon content can also be limited to 0.5% to stay in the field of so-called non-alloy steels. Indeed, if the content total silicon is greater than 0.5%, the standards in force classify such steel in the alloy steel category.

Minimum silicon content of 0.2% is required to limit the eddy current losses.

Limitation of the total manganese content is necessary in order to guarantee that the annealing carried out in the second stage of the steel production is carried out correctly, i.e. in the field ferritic while allowing sufficient grain enlargement. Indeed, if the total manganese content is more than 1.3%, the temperature of the ferrite-austenite transformation point (α → γ) is lowered and given that annealing in the ferritic field is essential, the temperature of this annealing will no longer be sufficient to obtain a structure with grains large enough to give this steel the magnetic characteristics required. Manganese is said to be a γ-gene element.

So, preferably, we will choose to replace phosphorus by an element called α-gene, such as for example silicon, or several elements at least one of which is α-gene.

It is also important to respect the condition according to which 80 Mn + 90 If ≥ 85, because if it is not respected, the mechanical characteristics of the sheet obtained will be insufficient.

Finally, it is important to respect the condition that 5.38 ≤ 6 Mn + 13 Al + 12 Si ≤ 16, in order to guarantee the characteristics sheet metal, in terms of specific total losses and permeability.

To obtain a carbon content of less than 0.005%, one can either carry out a decarburization treatment at the steelworks by using a apparatus for degassing liquid steel under vacuum of the RH or DH type, either carry out decarburizing annealing under cold atmosphere after cold rolling containing a mixture of oxidizing gases, which can be combined with annealing recrystallization and grain enlargement (second step).

The table below indicates the composition of several exemplary embodiments of the invention, the contents being expressed in percent by weight. Steel carbon phosphorus silicon manganese aluminum AT 0.002 0.09 0.4 0.7 0.001 B 0.002 0.05 0.3 1.2 0.001 VS 0.004 0.05 0.5 0.6 0.2 D 0.004 0.01 0.01 1.2 0.3 E 0.004 0.03 0.2 1.0 0.3

According to a preferred embodiment of the invention, the steel has a additional feature of limiting the ratio between the content total silicon and the total manganese content to 0.5.

Indeed, if the ratio between the total silicon content and the content in total manganese is greater than 0.5, the steel may pose some problems in terms of weldability, in particular in welding by sparking, which is detrimental, on the one hand to the implementation of the sheet made of this steel, and on the other hand for the implementation of the process of continuous production of a sheet metal strip made of this steel.

Thus, the preferred embodiment of the invention consists in make a steel sheet which includes by weight a carbon content less than or equal to 0.004%, a silicon content of between 0.25 and 0.35%, a manganese content between 0.8 and 1.0%, a content phosphorus between 0.05 and 0.09%, an aluminum content between 0.08 and 0.12%, the rest being iron and impurities inevitable residuals.

Such a sheet has the advantage of having good magnetic characteristics and to be properly cut and weldable especially by sparking.

Tests were carried out using a sheet according to the invention in order to show the improvement on the cuttability compared to a sheet of the state of the art.

The exact composition of the steels taken into account in these tests is mentioned in the table below, the contents being expressed in weight percent. Steel carbon phosphorus silicon manganese aluminum F 0.003 0.159 0.486 0.404 0.132 G 0.002 0.180 0.325 0.420 0.001 H 0.005 0.047 0.326 0.885 0.094

Steels F and G correspond to steel conforming to the prior art and steel H corresponds to an embodiment of the invention.

These three steels were produced in the same way, that is to say in two stages, a first stage consisting in manufacturing the steel under sheet metal thickness equal to 1 mm, then a second step consisting of subjecting said sheet to an annealing treatment at a temperature taken in the range 800 ° C to 900 ° C and so as to remain below the point of ferrite-austenite transformation.

This annealing step was identical for these three steels and has specifically constituted by continuous annealing at a temperature equal to 870 ° C for 2 minutes.

The three sheets F, G and H were compared in terms of mechanical characteristics, magnetic characteristics and in terms cuttability.

The magnetic characteristics were measured on a cell at single strip with samples taken in the rolling direction of the sheets.

Cutability was compared during cutting tests according to two criteria, the energy dissipated by friction during cutting on the one hand, and the cutting force at the end of rupture on the other. This last criterion allows in particular to estimate the impact of segregation. The cutting was carried out using a carbide punch of diameter equal to 12 mm, the clearance between the punch and the die being equal to 7%.

The following table shows the average result of these tests. Steel Re (MPa) Rm (MPa) AT% Losses at 1.5 T (W / kg) Induction at 5000A / m (Tesla) Wear criteria (J / mm) Force at end of failure (N) F 300 420 34 11.8 1,724 13900 2420 G 302 422 30.8 12.0 1,722 13420 2746 H 300 415 24.5 12.2 1,722 8490 840

From the point of view of mechanical characteristics, we see that the steel H of the invention has a yield strength Re and a load at fracture Rm almost identical to that of steels F and G of the technique anterior. On the other hand, the elongation A% is considerably reduced, less than 30, which is favorable to the cuttability of the sheet of the invention.

From the point of view of magnetic characteristics, we see that these are almost identical in the three cases.

Finally we see that the energy dissipated by friction as well as the force at the end of rupture is considerably reduced in the case of the steel H of the invention, which clearly shows the better cuttability of this H steel according to the invention compared to F and G steels of the technique anterior.

Claims (4)

Steel sheet for electrical use, of the type comprising less than 0.005% by weight of carbon, obtained in two stages, a first stage consisting in manufacturing a steel from sheet metal, then a second stage consisting in subjecting said sheet to a treatment annealing at a temperature taken in the range 800 ° C to 900 ° C and so as to remain below the ferrite-austenite transformation point (α → γ), characterized in that it comprises by weight a lower carbon content at 0.005%, a phosphorus content of less than 0.1%, a silicon content of 0.2%, a manganese content of 0.1%, and one or more elements chosen from silicon, manganese and aluminum, with contents such as: 80 Mn + 90 If ≥ 85 and 5.38 ≤ 6 Mn + 13 Al + 12 Si ≤ 16
and the total silicon content is less than 0.6%
and the total manganese content is less than 1.3%, the remainder being inevitable iron and residual impurities. Steel sheet according to claim 1, characterized in that the content by weight of phosphorus is between 0.01% and 0.09%. Steel sheet according to claim 1 or 2, characterized in that the ratio between the total silicon content and the content of total manganese is less than 0.5. Steel sheet according to claims 1, 2 or 3, characterized in that it comprises by weight a carbon content less than or equal to 0.004%, a silicon content of between 0.25 and 0.35%, a content of manganese between 0.8 and 1.0%, a phosphorus content between between 0.05 and 0.09%, an aluminum content of between 0.08 and 0.12%, the remainder being iron and unavoidable residual impurities.