NL192522C - Process for the manufacture of a Fe-Cr-Co type sintered magnet. - Google Patents

Description

1 192522

Process for the manufacture of a Fe-Cr-Co-type sintered magnet

The invention relates to a method for manufacturing a sintered magnet of the Fe-Cr-Co type, consisting of 20-35%, Cr, 3-15% Co and the rest of Fe, with a maximum of 5 energy product ( BH) of at least 5.0 MGOe, whereby a powder mixture is compressed to form a pellet, which is sintered in a vacuum or non-oxidizing atmosphere.

Such a method is known from the article of Chin et al., J. Appl. Phys. 52 (3), 1981, biz. 2536-2541. This article destroys maximum energy products of 4.4 MGOe and 5.2 MGOe of sintered compositions of 5% Co, 31% Cr and Fe, and of 12% Co, 25% Cr and Fe, respectively. A lubricant is also added to these compositions. Furthermore, no values are mentioned in the article for the ratio of the density of the material to the theoretical density.

The object of the invention is to provide a method of manufacturing a Fe-Cr-Co-type sintered magnet, with a density of 97% or more of theoretical density and a maximum energy product (BH) of at least 5.0 MGOe.

In order to achieve this aim, the method according to the preamble according to the invention is characterized in that the powder mixture is formed by a Fe-Cr powder and or Fe-Cr-Co powder mixed with a carbonyl Fe- powder and, if necessary, a Co powder, of which Fe-Cr powder, Fe-Cr-Co powder and carbonyl-Fe powder have an average particle size of 0.074 mm or less.

Many methods exist for manufacturing an object from a magnetic material on an industrial scale, for example by rolling, casting and sintering. However, it is impossible to use ALNICO (trade name) and ferrite-type magnetic alloys rollers, but manufacturing the Fe-Cr-Co-type magnetic alloy articles by rolling has advantages because the magnetic alloy of this type is crushable. Therefore, with respect to a Fe-Cr-Co type magnetic alloy, many alloy compositions and a number of different laws for their manufacture have been proposed. However, for mass production, the rolling of a magnetic alloy is usually not desirable, while further rolling is expensive, because a series of complicated steps, i.e., forging, rolling and tempering, is necessary. The use of the rolling is therefore generally limited to the manufacture of special shapes, for example products in the form of plate or wire. In addition, it has been considered necessary to use a number of different additives to prevent oxidation and nitride formation of chromium during melting and heat treatment in the alloy preparation process.

Casting can generally be applied to material that is hard and brittle, such as the ALNICO type alloy. Since the Fe-Cr-Co type material is crushable, it is sometimes difficult to remove castings from a runner, while casting errors such as sand containment and failed run are unattainable. In addition, the operating return is not satisfactory. Furthermore, casting is not attractive from an economic point of view since, to prevent oxidation and nitride formation of chromium during melting, a number of different additives must be added.

Such problems associated with the rolling or casting method do not exist with sintering. Sintering is suitable for mass production of Fe-Cr-Co type magnets on an industrial scale. Sintering has disadvantages in that the density and magnetic properties of sintered products are not satisfactory. Japanese Patent Laid-Open Nos. 54-33205 and 53-43006 show that the sintering density is improved by the addition of B, Si or C. However, in these cases the resulting magnetic properties are not satisfactory: (BH) ^ is no greater than 5.0 MGO. In addition, the cobaft content is more than 20%, making the product expensive.

The invention relates to a method for manufacturing an article obtained from a sintered magnetic alloy of the Fe-Cr-Co type, wherein this alloy mainly consists of 20-35% Cr, 3-15% Co and for the rest mainly from Fe, which method consists of the following steps: mixing at least one Fe-Cr powder or a Fe-Cr-Co powder with a carbonyl iron powder and, if a 'make-up' cobalt powder is necessary, with a Co-powder, whereby a powder mixture is obtained, wherein the average particle size of the Fe-Cr powder and Fe-Cr-Co powder is 0.074 mm or smaller, and these powders have undergone surface activation; compressing the resulting powder mixture into a pressed product; sintering the resulting pressed product in an atmosphere in which the total amount of oxygen and nitrogen is limited to carbon black more than 3 ppm; cooling the sintered body from 800 ° C to 500 ° C at a cooling rate of 5 ° C / min or greater, 55 without effecting a mixed crystal formation treatment (treatment in which a solid solution is formed) and performing a heat treatment and magnetization of the resulting sintered alloy.

192522 2

According to the invention, the chromium content is limited to 20-35%, since the required magnetic properties, such as residual flux density and resistivity, cannot be obtained when the content is less than 20% or greater than 35%. Furthermore, the cobalt content is limited to 3-15%, since the required magnetic properties including residual flux density and resistivity cannot be obtained when the content is less than 3%. On the other hand, when the content is greater than 15%, it is difficult to apply a mixed crystal formation treatment, which is necessary under the usual conditions to provide the required degree of magnetic properties. Moreover, since cobalt is expensive, the cost of the product increases with an increasing cobalt content, making this method unsuitable for practice. The rest of this alloy consists of Fe (barring 10 unavoidable impurities).

According to the invention, therefore, additives previously considered essential are intentionally omitted from the alloy composition. Therefore, one of the important features of the invention is that to obtain a magnetic alloy having a density of 97% or more of theoretical density and a (BH) van of 5.0 MGO or more, it is not necessary adding an element other than the above-mentioned elements. According to the invention, the magnetic alloy has a residual flux density (Br) of 13000 G or more, a coefficient of force (Hc) of 580 Oe or more, a theoretical density.

In the atmosphere used in the sintering according to the invention, the oxygen and nitrogen content together is limited to a maximum of 3 ppm. This atmosphere corresponds to a reduced pressure of 0.1333 Pa or less or to an inert atmosphere with a dew point of -70 ° C or less. Thus, this atmosphere can easily be created on an industrial scale.

Of the metal powder particles used in accordance with the invention, at least the Fe-Cr powder particles, the Fe-Cr-Co powder particles and the carbonyl Fe powder particles should have a diameter of 0.074 mm or less. Preferably, the particle size of all metal powder particles used according to the invention is 0.074 mm or less. It is undesirable to use extremely fine metal particles, because the additional steps required to make the particles smaller increase the manufacturing cost, and moreover the finer the particles, the more easily they are oxidized. However, producing powder particles with a particle size of 0.074 mm or smaller is not that difficult. In particular, it is easy to obtain powder particles of the Fe-Cr or Fe-Cr-Co type with a diameter of 0.074 mm or smaller, because the alloy of the Fe-Cr or of the Fe-Cr-Co type has a brittle sigma phase, which is formed in a wide range of 40-60% Cr.

One of the commercially available metal powders is a water spray obtained powder. Although the surface of each of the individual particles is usually oxidized, this type of metal powder can also be used according to the invention, insofar as the particles have an activated surface which can be obtained by the powder in a reducing atmosphere, for example a hydrogen atmosphere , to temper.

According to the invention, the addition of a Co powder is necessary when a ferrochromium powder is used. However, when a sigma powder, which is a Fe-Cr-Co powder, is used, a 40 Co powder is not necessarily required. The Co powder can only be applied when a "makeup" cobalt powder is required to provide a predetermined alloy composition.

The reasons why the process according to the invention does not require the use of element components other than chromium, cobalt and iron can be given on the basis of the following points in combination: 45 (1) According to the invention, a pressed starting mixture is sintered in an atmosphere where the total amount of oxygen and nitrogen is limited to a maximum of 3 ppm, preventing oxidation and nitride formation.

In the prior art processes, on the other hand, it is necessary to use some additives to prevent oxidation and nitride formation of chromium during melting and heat treatment, which are necessarily associated with the conventional rolling or casting method. Therefore, as long as either casting or rolling is used, the oxidation and nitride formation of chromium is unavoidable unless some additives are used.

As will be seen from the foregoing, metallurgical powder techniques are employed in the invention in this regard, which do not require a melting step or, in some cases, mixed crystal voiding treatment. The 55 alloy according to the invention therefore has no risk of being subjected to oxidation or nitride formation; and the invention does not require the use of additives which are absolutely necessary in the prior art process.

3 192522 (2) According to the invention, a finely divided powder of the Fe-Cr type or of the Fe-Cr-Co type is used as the starting powder, most of the particles having a diameter of 0.074 mm or less. It is known that it is difficult to obtain a high density Fe-Cr-Co type sintered magnetic alloy without the addition of auxiliary elements. A powder with a particle size of 0.147 mm has been used as the starting powder. However, it has been found that it would be easy to obtain a higher density article if a starting powder, especially Fe-Cr or Fe-Cr-Co powder having a particle size of 0.074 mm or smaller, is used. Therefore, the use of additives is not required in the process of the invention. In this regard, it should be remembered that the use of a starting powder with a particle size of 0.074 mm or smaller is necessary, but not sufficient to achieve a density of 97% or more of the theoretical density.

(3) Water spraying is one of the most widely used methods for mass production of metal powder on an industrial scale. However, since the surface of the powder particles obtained by the water-spraying method is oxidized, addition of C or B is essential to improve the sintering density while the addition of these additives adversely affects the resulting magnetic properties.

This means that an object of high density and improved magnetic properties can be obtained if a starting powder whose surface is activated is used. This also means that even when the metal powder obtained by the water-spraying method is used as the starting powder, these improved effects can be obtained as long as the surface of the powder is activated.

Therefore, a preferred embodiment of the method according to the invention is characterized in that powders with an activated surface are used.

(4) In addition to the carbonyl powder, many other types of Fe powders exist, for example atomized Fe powder, electrolytically obtained Fe powder or reduction powder Fe. However, it is difficult to obtain a high density article when these powders are used alone or in combination. On the other hand, it has been believed that sintering the carbonyl powder at a high temperature of 1400 ° C is necessary to achieve a high degree of density and that it is not possible to obtain a high density without using additives. Moreover, since the carbonyl powder contains finely divided particles and is also highly activated, it is expected that a relatively high density can be achieved.

Another preferred embodiment of the method according to the invention is therefore characterized in that as starting powders a sigma powder and a carbonyl Fe powder are used.

A further preferred embodiment is characterized in that in the starting material the carbonyl Fe powder is partially replaced by Fe powder with an activated surface and a particle size of 0.074 or less.

According to the inventors, however, it is impossible to achieve a density of 97% or more of the theoretical density solely by mixing the carbonyl powder with an Fe-Cr or Fe-Cr-Co powder, the particle size of which is 0.147 mm. In this regard, it has been found that the use of Fe-Cr or Fe-Cr-Co metal powder particles with a diameter of 0.074 mm or smaller in combination with the 40 carbonyl powder enables a high density magnetic alloy with improved magnetic properties to prepare. An additive is not required.

(5) As stated previously, in one embodiment of the invention, it is not necessary to use a mixed crystal formation treatment which was believed to be essential. In this embodiment, therefore, there is no chance of adversely affecting the powder used by the ambient atmosphere in heat treatment because the mixed crystal formation treatment is eliminated and the other heat treatment including tempering using a magnetic field or aging is usually carried out at a lower temperature. temperature than the temperature at which the mixed crystal formation treatment is carried out. In addition, since in the process of the invention the total amount of oxygen and nitrogen in the sintering atmosphere is limited to no more than 3 ppm, there is practically no adverse effect caused by the ambient atmosphere and it is not necessary to add additives in the alloy mixture of the invention to to avoid oxidation and nitride formation of alloying elements.

Therefore, since according to the invention no additives are used which are usually required to avoid oxidation and nitride formation of powders during sintering and those of 55 sintered alloys in the heat treatment processes, the magnetic alloy of the invention does not have the disadvantages that are caused by the additives, such as B, C, Ti, Si, Mn, Cu or Sn. In this regard, it has been found that such additives are of no use in providing a sintered 192522 4 Fe-Cr-Co type magnetic alloy. According to the invention, these additives are therefore intentionally excluded from the alloy composition of the invention.

Figures 1 and 2 form a series of graphs showing the adverse effects caused by the inclusion of said additives.

The invention is described in more detail in connection with the examples below.

Example I

A sigma powder containing 48% Cr, 18% Co and the remainder Fe was mechanically pulverized to a particle size of 0.074 mm or smaller. The resulting powder was mixed with a carbonyl-Fe powder with an average particle size of 5 microns and a Co powder with a particle size of 0.038 mm or smaller to obtain a powdery mixture containing 25% Cr, 9.5% Co and the residue contained Fe, which powder was then compressed at a pressure of 490,332.5 kPa into a rod with a diameter of 13 mm and a length of 10 mm. The resulting crushed starting product was sintered under a reduced pressure of 0.1333 Pa for 2 hours at a temperature of 1280-1450 ° C. The resulting sintered rod was subjected to a mixed crystallization bead at a temperature of 1250 ° C for 30 minutes and then to an isothermal heat treatment in a magnetic field of 3000 Oe at a temperature of 640¾ 1 hour. Thereafter, the thus-magnetized rod was cooled to 500 ° C at a cooling rate of 3 ° C / hr from 620 ° C and held at that temperature to age. The magnetic properties of the magnetic alloy thus obtained are summarized in Table A.

TABLE A

25 No. Sintering Ratio to Magnetic Coercitive - Magnetic temperature theoretical flux density, force Jc (Oe) energy product (° C) density (%) Br (G) ((BH) ^ (MGO) 1 1280 98.1 13500 620 5.6 30 2 1330 99.0 13600 620 5.8 3 1380 99.6 13800 630 6.2 4 1450 99.8 14100 610 6.8

Example II

(comparative example).

In the method of this example, a sigma powder and a Co powder as described in Example I were mixed with a sputtered Fe powder having a particle size of 0.147 mm or less to obtain a powdered mixture containing 25% Cr and 9.5% Co and the remainder was Fe. The resulting powdered mixture was sintered and heat treated in the same manner as described in Example I. The magnetic properties of the magnetic alloy thus obtained are summarized in Table B.

TABLE B

45 -

No. Sintering Ratio to Magnetic Coercitive- Magnetic temperature theoretical flux density, force Hc (Oe) energy product, (eC) density (%) Br (G) ((BH) ** »(MGO) 50 5 * 1280 94.3 12600 580 4 .2 6 * 1330 95.5 12 900 590 4.6 * = For comparison.

55 As is apparent from the data set forth in Tables A and B, the Fe-Cr-Co type magnetic alloy obtained in accordance with the present invention exhibits improved magnetic properties. In particular, this is evident from Table A, which relates to cases where a sigma powder having a particle size of 0.074 to 192522 mm or smaller and a carbonyl-Fe powder were used as starting powders and a magnetic alloy of the Fe-Cr- Co-type with a density of 98% or more of the theoretical density and a magnetic energy product (BH) of 5.5 or more was obtained over a wide range of sintering temperatures.

5

Example lil

To the powdery mixture mentioned in Example II was added TiH2 powder with a particle size of 0.041 mm or less or a powdered Fe-Si alloy (76.7% Si, the remainder Fe) with a particle size of 0.061 mm or less or a powdered Fe-Mn alloy (77.3% Mn, the remainder Fe) with a particle size of 0.061 mm or smaller or an electrolytically obtained copper powder with a particle size of 0.061 mm or smaller or an Sn powder with a particle size of 0.074 mm or smaller added in varying amounts of 0.5 wt%, 1.0 wt% and 2.0 wt%. Each of the resulting mixtures was sintered and heat-treated in the same manner as described in Example I. The results are summarized in Figure 1, plotting the data of the resulting magnetic properties and density of each of the magnetic alloys against the amount of any of these additives used. The process conditions are summarized in Table C.

TABLE C

20 Sintering Solid solution Magnetization Tempering Remark 1300 ° C x 4h none 640 ° Cx1h 620 ° Cx1h 0 cooled at 12.5eC / h to

25 5750 ° C, and 575 ° C

x 3h "” 645 ° C x 1 h "-Δ- 1350 ° C x 2h 1250 ° C x 20 '640 ° C x 1h cooled to - · - 500 ° C with 20eC / h 30" "645 ° Cx1h" - A-

Figure 1 shows that the magnetic properties are adversely affected by the incorporation of additives. The best properties were obtained when no additive was used.

35

Example IV

To the powder mixture described in Example II, in an amount of 0.05-0.3%, a powder of an Fe-B alloy (20% B, the remainder Fe) with a particle size of 0.061 mm or smaller was added. The resulting mixture was then sintered and heat-treated in the same manner as described in Example I.

The results are summarized in Figure 2, in which the amount of B added is indicated with respect to the density and magnetic properties of the alloy thus obtained. According to the results shown in Figure 2, the magnetic properties deteriorate with increasing amount of B added, although the addition of B in an amount of 0.05-0.1% seems to slightly increase the density of the alloy. The best grazing results were obtained when B was not used.

Example V

In the same manner as described in Examples I and II, several starting powders, indicated in Table 50, were mixed into powder mixtures containing 25% Cr and 9.5% Co, the balance of which was Fe. Each of the powder mixtures thus obtained was compressed into a pressed starting product which was then sintered at a temperature of 1330 ° C in a hydrogen atmosphere containing oxygen in an amount of less than 3 ppm. The resulting sintered body was then subjected to aging in the same manner as described in Example 1. The magnetic properties of the final products are summarized in Table D.

192522 6

TABLE D

No. Output powder Ratio to Magnetic Coercive Force Magnetic theoretical flux density, Br Hc (Oe) energy product 5 density (%) (G) ((BH) ^ (MGO) 7 sigma powder + 98.8 13700 640 6.2 carbonyl-Fe powder 10 8 * ferro-chrome - 95.7 13000 540 4.5 powder *

Fe powder +

Co-powder 9 ferro-chromium- 98.3 13800 580 6.0 15 powder * carbonyl-Fe- ___ powder * - ------------------------ ------------

Co-powder 20 * For comparison.

As is apparent from the data relating to the Fe-Cr-Co type sintered magnetic alloy of the present invention (Nos. 7 and 9) disclosed therein, an improved high density magnetic alloy and improved magnetic properties were obtained according to the invention, wherein the sintering was conducted in a non-oxidizing atmosphere.

25

Example VI

A sigma powder (48% Cr, 18% Co and the remainder Fe) or a ferro-chromium powder (62% Cr and the remainder Fe), which was mechanically pulverized to a particle size of 0.074 mm or smaller, was combined with several kinds of powders, as indicated in table E and with a Co powder, whereby a 30 powder mixture! 25% Cr, 9.5% Co and the balance Fe were obtained. The resulting mixture was then compressed into a pressed starting product in the same manner as described in Example I. The pressed starting product was sintered for 2 hours at a temperature of 1330 ° C under a reduced pressure of 0.1333 Pa. The resulting sintered body was subjected to the same heat treatment as described in Example 1. The density of the magnetic alloy thus obtained with respect to its theoretical density is shown in Table E.

TABLE E

No. Starting powder Ratio to the theoretical 40 density 10 * ferro-chrome powder + atomized Fe powder (particle- 95.1 size 0.147mm or smaller) + Co-powder 11 ferro-chrome powder + carbonyl-Fe powder + 98.8 45 Co- powder 12 * sigma powder + atomized Fe powder (particle size 93.9 0.147mm or smaller) * Co powder 13 sigma powder + atomized Fe powder + carbonyl Fe- 97.1 powder + Co polymer 50 14 sigma powder * carbonyl Fe powder * Co powder 98.4 15 * sigma powder + reduction Fe powder (particle size 96.2 0.048mm or smaller) + Co powder 16 * sigma powder * electroytic Fe powder (particle 94.2 size 0.147mm or smaller) * Co powder 55 - * For comparison.

As shown in Table E, the sintered magnetic alloy of the present invention (Nos. 11,13 and 14)

Claims (4)

7 192522 a large ratio with respect to the theoretical density, namely of 97% or more. EXAMPLE VII The procedure described in Example I was repeated, except that the sintering was carried out at a temperature of 1350 ° C for 2 hours and the cooling rate was varied from 800 ° C to 500 ° C during the cooling in the table. F indicated manner. The resulting alloys were subjected to heat treatment without the use of mixed crystal formation treatment (treatment, where a solid solution is filled). The magnetic properties of the magnetic alloys thus obtained are summarized in Table F. TABLE F No. Cooling rate Ratio to Magnetic Coercive - Magnetic 15 upon cooling theoretical flux density, force, Hc (Oe) energy product, from 800 ° C to density (%) Br (G) ((BH) ^ (MQO) —------- ----------------- §00? C ----------------------------- ---------------------------------------------- ---- --------------------------------------. (° C / minute) 20 17 100 98, 3 13700 615 5.9 18 40 98.0 13700 610 6.2 19 20 97.7 13700 605 6.1 20 5 97.3 13 500 595 5.7 * 21 2 98.0 13 300 555 5.05 25 22 1 97.9 13000 525 4.8 * For comparison, Table F shows that satisfactory magnetic properties can be obtained by cooling from 800 ° C to 500 ° C with a cooling rate of 5 ° C / minute or higher. that it is possible to omit the solid solution treatment which was considered essential in the prior art A method for manufacturing a sintered magnet of the Fe-Cr-Co type, consisting of 20-35% Cr, 3-15% Co and the rest of Fe, with a maximum energy product (BH) max of at least 5.0 MGOe, in which a powder mixture is compressed to form a pellet, which is sintered in vacuum or a non-oxidizing atmosphere, characterized in that the powder mixture is formed by an Fe-Cr powder and / or Fe- Mix Cr-Co powder with a carbonyl Fe powder and, if necessary, a Co powder, of which Fe-Cr powder, Fe-Cr-Co powder and carbonyl-Fe powder have an average particle size of 0.074 mm or less amounts. Method according to claim 1, characterized in that powders with an activated surface are used. The method according to claim 2, that the starting powders are a sigma powder and a carbonyl-Fe powder. Method according to claim 2 or 3, characterized in that in the starting mixture the carbonyl-Fe powder is partially replaced by Fe powder with an activated surface and a particle size of 0.074 mm or less. Hereby 2 sheets drawing