EP0441674B1 - Aluminising method for oriented magnetic steel plates and oriented magnetic steel sheets so obtained - Google Patents

Abstract

The process for aluminising a sheet of magnetic steel with oriented grains which has been subjected to at least one cold rolling, at least one intermediate annealing under hydrogen atmosphere, followed by a decarbonising annealing, the grains of the sheet being then in the primary recrystallisation state, is characterised in that a deposition of aluminium is carried out on the said sheet between two stages of a final annealing, the first stage of which produces a partial or total secondary recrystallisation of the grains of the sheet and the second stage of which allows the aluminium to diffuse and the impurities to be removed. <IMAGE>

Description

The subject of the present invention is a process for the aluminization of magnetic grain oriented steel sheets.

Oriented grain magnetic steel sheets, for example GOSS textured, are used in the construction of electrical machines, in particular transformers where their magnetic characteristics play an important role.

It is particularly advantageous to reduce the losses in the magnetic sheets used in electrical machines, both by hysteresis and by eddy currents, by increasing the resistivity of the metal by adding alloying elements such as aluminum and taking into account the thickness of said sheets, which is related to the skin thickness, which itself depends on the working frequencies envisaged.

The oriented texture must imperatively be retained after this addition of aluminum.

We know in FR-A-1 525 034, a process which consists in covering the sheet on each of its faces with a layer of aluminum, silicon, or germanium, to diffuse this element by heat treatment in the matrix, and then to carry out on the metal thus obtained a heat treatment in a megetic field to improve the configuration of the Weiss domains and obtain the desired structure.

In this process, the diffusion takes place at a temperature between 750 ° C to 1050 ° C, more particularly at 900 ° C for three hours under a hydrogen atmosphere. After this static treatment, a hot leveling treatment is essential. This last heat treatment is carried out in an oven when passing at a temperature of the order of 900 ° C., for a few minutes, followed by cooling in a magnetic field, when the temperature is between 600 ° C. and 300 ° C.

Such a method has the major drawback of requiring a particular heat treatment at high temperature so as to diffuse the aluminum. The aluminization stage is a stage preceding the development of the texture of the grains of the sheets.

We also know in FR-A-2 067 409, a process for manufacturing oriented grain sheets with high magnetic properties in the rolling direction, from a steel strip capable of acquiring the texture of GOSS, but not having it yet, and this thanks to a suitable composition of the steel and to an appropriate cold rolling followed by a decarburization annealing to less than 0.005% of carbon. An aluminum deposit is made on said strip, followed by a hot diffusion of this aluminum throughout the mass of the strip to be treated, in solid phase, this diffusion of aluminum taking place before recrystallization is carried out hot. secondary in GOSS texture.

The decarburized cold rolled strip is wound with an aluminum strip. This winding is followed by two successive heat treatments, at two different temperatures, which may or may not take place in the same oven, with or without return to room temperature between the two treatments. The first treatment, between 600 and 800 ° C, carries out the diffusion of the aluminum in the strip to be treated, and the second treatment, between 950 and 1250 ° C, realizes the secondary recrystallization in texture of GOSS.

In the two methods described, the aluminum diffusion step is carried out before recrystallization in GOSS texture.

The method according to the present invention relates to the manufacture of aluminized magnetic steel sheets with oriented grains, comprising an aluminizing operation carried out during the recrystallization process of the grains of the magnetic sheet, which, compared to the aforementioned prior techniques, eliminates the specific heat treatment for diffusion of aluminum in the manufacture of sheets while maintaining the grain-oriented texture.

For this purpose, the process for the aluminization of a magnetic steel sheet with oriented grains having undergone at least one cold rolling, at least one annealing under a hydrogen atmosphere, followed by a decarburization annealing, the grains of the sheet then being in the primary recrystallization state, is characterized in that an aluminum deposit is made on said sheet between two stages of a final annealing, the first step of which performs partial or total secondary recrystallization of the grains of sheet metal, the second stage of which allows the diffusion of aluminum and the elimination of impurities.

• une montée en température à environ 40°C par heure sous atmosphère de gaz neutre,
• un premier palier, dont la température est comprise entre 800°C et 1050°C et la durée entre une demi-heure et cinq heures, sous atmosphère de gaz neutre,
• un refroidissement naturel sous gaz neutre,

   et en ce que la deuxième étape du recuit final comprend :

• une montée de température rapide à environ 450°C par heure jusqu'à la température du premier palier, puis une montée en température d'environ 40°C par heure,
• un second palier dont la température est comprise entre 1080°C et 1200°C et la durée entre une demi-heure et cinq heures sous atmosphère d'hydrogène.

The method is further characterized in that the first step of the final annealing comprises:

• a temperature rise to around 40 ° C per hour under an atmosphere of neutral gas,
• a first stage, the temperature of which is between 800 ° C and 1050 ° C and the duration between half an hour and five hours, under an atmosphere of neutral gas,
• natural cooling under neutral gas,

and in that the second stage of the final annealing comprises:

• a rapid rise in temperature to approximately 450 ° C. per hour up to the temperature of the first level, then a rise in temperature to approximately 40 ° C. per hour,
• a second stage whose temperature is between 1080 ° C and 1200 ° C and the duration between half an hour and five hours under a hydrogen atmosphere.

• la température du premier palier est de 890°C,
• la durée du premier palier est de quatre heures,
• la température du second palier est de 1180°C,
• la durée du second palier est de quatre heures.

Preferably:

• the temperature of the first level is 890 ° C.,
• the duration of the first stage is four hours,
• the temperature of the second stage is 1180 ° C.,
• the duration of the second stage is four hours.

• le dépôt d'aluminium est effectué à l'aide d'au moins un feuillard d'aluminium mis en contact avec la tôle,
• le dépôt d'aluminium est effectué par évaporation sous vide,
• le dépôt d'aluminium est effectué par trempage dans un bain d'aluminium fondu,
• après traitement, la teneur en aluminium dans la tôle est comprise entre 1 et 5%.

According to other characteristics of the invention:

• aluminum is deposited using at least one aluminum strip brought into contact with the sheet,
• the aluminum is deposited by vacuum evaporation,
• the aluminum is deposited by dipping in a bath of molten aluminum,
• after treatment, the aluminum content in the sheet is between 1 and 5%.

The invention also relates to a magnetic steel sheet obtained by this process.

Such an aluminization process is now described with reference to comparative tests which, with reference to the accompanying drawings, will highlight the characteristics of the magnetic sheet according to the invention.

FIG. 1 represents an example of a temperature profile of a final annealing.

FIG. 2 represents three hysteresis cycles corresponding to three comparative aluminization tests.

FIG. 1 shows an example of the temperature profile of a final annealing comprising a temperature rise of approximately 40 ° C. per hour. The sheet to be treated is then placed under an atmosphere of neutral gas which can for example be nitrogen, and maintained at a first level P₁ which can vary from thirty minutes to five hours at a temperature T₁ of between 800 ° C and 1050 ° C . This level allows the germination of crystals having for example the GOSS orientation. After the passage of the bearing P pal, the sheet is naturally cooled to a temperature allowing deposition of aluminum in different forms such as evaporation, soaking in a molten bath, contact with aluminum strips.

Then, the aluminized sheet is maintained at a second level P₂, under a hydrogen atmosphere, for a time comprised between thirty minutes and five hours at a temperature T₂ comprised between 1080 ° C and 1200 ° C.

The temperature increases at a constant speed of 450 ° C per hour up to the temperature T₁ of the bearing P₁, then at 40 ° C per hour to reach the temperature T₂ of the bearing P₂, the cooling then taking place. naturally under hydrogen.

• 1. traitement thermique de recuit final en deux étapes sans aluminisation,
• 2. aluminisation après le recuit en deux étapes suivi d'une opération de diffusion de l'aluminium,
• 3. aluminisation avant le recuit final en deux étapes,
• 4. aluminisation après la première étape du recuit final.

For comparison of the samples taken on a Fe-Si sheet 3% in coil were subjected to the following different treatment tests:

• 1. two-stage final annealing heat treatment without aluminization,
• 2. aluminization after annealing in two stages followed by an aluminum diffusion operation,
• 3. aluminization before final annealing in two stages,
• 4. aluminization after the first stage of final annealing.

• a) un premier laminage à froid pour atteindre une épaisseur intermédiaire à 0,49 mm,
• b) un recuit intermédiaire au passage à 950°C sous atmosphère d'hydrogène, suivi d'un second laminage à froid à l'épaisseur finale de 0,23 mm,
• c) une décarburation au passage à 820°C sous une atmosphère d'hydrogène et d'azote,
• d) une enduction d'un revêtement de magnésie (MgO) ou d'alumine (Al₂O₃),
• e) et un recuit final en deux étapes, comprenant :
  • . un premier recuit de quatre heures à 890°C correspondant à la température T₁ du palier P₁, sous une atmosphère d'azote, avec une montée en température lente à 40°C par heure, le recuit étant suivi d'un refroidissement naturel,
  • . un second recuit de quatre heures à 1100°C correspondant à la température T₂ du palier P₂, sous une atmosphère d'hydrogène avec montée rapide jusqu'à la température T₁, puis une montée lente à 40°C par heure, jusqu'à la température T₂ du palier P₂, le recuit étant suivi d'un refroidissement naturel.

The Fe-Si 3% alloy sheet in coil is produced from a hot-rolled sheet two millimeters thick subjected to:

• a) a first cold rolling to reach an intermediate thickness of 0.49 mm,
• b) intermediate annealing on passing to 950 ° C. under a hydrogen atmosphere, followed by a second cold rolling to the final thickness of 0.23 mm,
• c) decarburization when passing at 820 ° C. under an atmosphere of hydrogen and nitrogen,
• d) coating with a coating of magnesia (MgO) or alumina (Al₂O₃),
• e) and a final annealing in two stages, comprising:
  • . a first four-hour annealing at 890 ° C corresponding to the temperature T₁ of the bearing P₁, under a nitrogen atmosphere, with a slow rise in temperature to 40 ° C per hour, the annealing being followed by natural cooling,
  • . a second four-hour annealing at 1100 ° C corresponding to the temperature T₂ of the bearing P₂, under an atmosphere of hydrogen with rapid rise to the temperature T₁, then a slow rise to 40 ° C per hour, until the temperature T₂ of the bearing P₂, the annealing being followed by natural cooling.

It is necessary to cover the sheets superficially with an anti-stick coating which can be easily removed. The texture of the sheets is not modified by the use of an anti-stick coating such as magnesia or alumina.

The magnetic characteristics, magnetic losses and induction B 800 (under an excitation field of 800 A / m) for a sheet having undergone the heat treatment 1, without aluminization, are indicated in table I:

The saturation magnetization Bs = 2.01 Tesla. $$\text{The report}\frac{\text{B800}}{\text{Bs}}\text{=}\frac{\text{1.86}}{\text{2.01}}\text{= 0.925}$$

The aluminum used for the aluminization step is a strip whose weight composition of the residual elements is as follows: Iron: 0.20%; Silicon: 0.20 to 0.30%; Titanium: 0.015%

The strip is, moreover, in the annealed state.

For example, aluminum foil 9 microns thick is used for the 0.23 mm thick sheets. After treatment, the average aluminum content is then close to 1.3% and the density of the samples is close to 7.52.

The density of the aluminized sheets deviates significantly from the density of the Fe-Si 3% base alloy. A fair comparison of physical properties requires expressing the losses per unit of volume, for example in mW / cm³.

The aluminizing treatment 2 is carried out by placing a strip on both sides of the magnetic sheet after the latter has undergone the two stages of the final annealing.

• une montée en température de 40°C par heure,
• un palier de quatre heures à 1050°C sous gaz neutre.
• un refroidissement naturel sous gaz neutre.

It is therefore necessary to carry out on this sheet-metal strip assembly an additional heat treatment intended to diffuse the aluminum. The broadcasting operation includes the following stages:

• a temperature rise of 40 ° C per hour,
• a four hour stop at 1050 ° C below neutral gas.
• natural cooling under neutral gas.

The magnetic characteristics are indicated in table II below:

Ce rapport diffère très peu de celui obtenu avant aluminisation (Tableau 1). La texture n'est donc pas modifiée.The saturation magnetization is then Bs = 1.93 Tesla. $$\text{The report}\frac{\text{B800}}{\text{Bs}}\text{=}\frac{\text{1.81}}{\text{1.93}}\text{= 0.937}$$

This ratio differs very little from that obtained before aluminization (Table 1). The texture is therefore not modified.

The aluminizing treatment 3 is carried out by placing the aluminum strip on the two faces of the sheet before the two stages of the final annealing, as recommended in FR-A-2 067 409.

The magnetic characteristics obtained are grouped in Table III below.

The saturation magnetization is Bs = 1.93 $$\text{The report}\frac{\text{B800}}{\text{Bs}}\text{= 0.827}$$

The aluminization before the two stages of the final annealing plays a very unfavorable role on the secondary recrystallization mechanisms. In macrographic observation, the grains appear irregular, of small size, as they are after the primary growth stage. The magnetic loss characteristics confirm that the alumization process before the two stages of final annealing gives less good results than in the final annealing test without aluminum.

The aluminizing treatment 4 according to the invention is carried out by placing the aluminum strip on the two faces of the sheet between the two stages of the final annealing, that is to say after a secondary recrystallization of the grains of the sheet .

The magnetic characteristics are grouped in Table IV below.

The saturation magnetization is Bs = 1.93 $$\text{The report}\frac{\text{B800}}{\text{Bs}}\text{= 0.935}$$

The magnetic characteristics highlighted during treatment 3 and grouped in Table III confirm that the aluminization process, before final annealing, does not improve the loss characteristics of the sheet. This is why it was carried out, by cutting the final secondary recrystallization annealing in two stages, with the addition of aluminum at the intermediate stage, that is to say after the secondary recrystallization mechanisms have developed at least partially.

The germination of the texture takes place during the first stage of the final annealing during which the recrystallization mechanisms have been able to intervene.

This study shows that the aluminization carried out during the final annealing gives results which are substantially identical to those obtained with an aluminization carried out after the final annealing.

The deposition of aluminum can be carried out either by vacuum evaporation or by dipping in a bath of molten aluminum.

After treatment, the aluminum content in the sheet is between 1 and 5%, and equal to 1.3% in the example.

The variations in the hysteresis cycle shown in FIG. 2 for an induction B = 1.5 Tesla as a function of the aluminization shows the advantage of such a process.

• la courbe 1 représente le cycle de l'alliage Fe-Si avant aluminisation et après l'opération de recuit final,
• la courbe 2 représente le cycle d'hystérésis de l'alliage Fe-Si-Al pour un traitement l'aluminisation après le recuit en deux étapes suivi d'une opération de diffusion de l'aluminium,
• la courbe 3 représente le cycle d'hystérésis de l'alliage Fe-Si-Al après aluminisation, l'aluminisation étant effectuée après la première étape du recuit final.

In this figure 2, the curves represent hysteresis cycles of the Fe-Si alloys (treatment 1) and Fe-Si-Al: (treatments 2 and 4).

• curve 1 represents the cycle of the Fe-Si alloy before aluminization and after the final annealing operation,
• curve 2 represents the hysteresis cycle of the Fe-Si-Al alloy for an aluminization treatment after annealing in two stages followed by an aluminum diffusion operation,
• curve 3 represents the hysteresis cycle of the Fe-Si-Al alloy after aluminization, the aluminization being carried out after the first stage of the final annealing.

It is noted that the differences between the hysteresis cycles for the two aluminization processes are small and that the variation between the two cycles with aluminization and the Fe-Si cycle without aluminum is significant.

A 0.23 mm thick sheet used in devices working in particular at 400 Hz is the seat of large induced currents which generate additional losses and limit the response time in, for example, control circuits using power components semiconductor. In these conditions, it is very advantageous to work with materials of smaller thickness.

In another example, a magnetic steel sheet with grains oriented by the method according to the invention corresponding to treatment 4 has been produced, the sheet having a thickness of approximately 0.15 mm.

The characteristics of the sheet thus produced are grouped in Table V below.

Note that the 0.15mm magnetic sheet obtained by the process has a substantial decrease in the magnetic loss characteristics compared to the loss characteristics of 0.23 mm thick sheets obtained by the same process.

The method according to the invention allows aluminization of the oriented grain sheets during their preparation using the second stage of the final annealing to ensure the diffusion of the aluminum and the removal of sulfur and other impurities.

In addition, this process allows a significant saving, in particular of energy, by eliminating a heat treatment step at high temperature, in comparison with the process shown by curve 2.

The heat treatment temperatures and the durations are liable to change as a function of the thicknesses and of the initial compositions of the sheets to be aluminized, the example which has just been given relates to magnetic sheets with oriented grains commonly used.

Likewise, the deposition of aluminum is carried out by means of strips, but identical results are obtained by deposition under vacuum or dipping in a bath of molten aluminum.

Claims (12)

1. Process of aluminization of a sheet of magnetic steel with oriented grains which has undergone at least one cold rolling, at least one annealing under a hydrogen atmosphere, followed by a decarbonization annealing, the grains of the sheet then being in the state of primary recrystallization, characterized in that aluminium is deposited on the said sheet between two stages of a final annealing, the first stage of which brings about partial or total secondary recrystallization of the grains of the sheet and the second stage of which permits diffusion of the aluminium and elimination of impurities.
2. Process according to claim 1, characterized in that the first stage of the final annealing comprises:

   - an increase in temperature of about 40°C per hour under an atmosphere of neutral gas,

   - a holding step P₁, at which the temperature T₁ is between 80°C and 1050°C and which lasts between half an hour and five hours, under an atmosphere of neutral gas,

   - a natural cooling under neutral gas, (sic)
3. Process according to claim 2, characterized in that the temperature T₁ of the holding step P₁ is 890°C.
4. Process according to claim 2, characterized in that the duration of the holding step P₁ is four hours.
5. Process according to claim 1, characterized in that the second stage of the final annealing comprises:

   - a rapid increase in temperature of about 450°C per hour up to the temperature T₁ of the first stage, then an increase in temperature of about 40°C per hour,

   - a holding step P₂, at which the temperature T₂ is between 1080°C and 1200°C and which lasts between half an hour and five hours under a hydrogen atmosphere,

   - a natural cooling under hydrogen.
6. Process according to claim 5, characterized in that the temperature T₂ of the holding step P₂ is 1180°C.
7. Process according to claim 5, characterized in that the duration of the holding step P₂ is four hours.
8. Process according to any of the preceding claims, characterized in that the aluminium deposition is carried out by means of at least one aluminium foil placed in contact with the sheet.
9. Process according to one of claims 1 to 7, characterized in that the aluminium deposition is carried out by vacuum evaporation.
10. Process according to one of claims 1 to 7, characterized in that the aluminium deposition is carried out by immersion in a bath of molten aluminium.
11. Process according to one of the preceding claims, characterized in that, after treatment, the aluminium content in the sheet is between 1 and 5%.
12. Sheet of magnetic steel with oriented grains, characterized in that it is obtained by the process according to claims 1 to 11.

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