Metal treatment – Stock – Magnetic
Reexamination Certificate
2000-03-13
2002-07-02
Sheehan, John (Department: 1742)
Metal treatment
Stock
Magnetic
C148S302000
Reexamination Certificate
active
06413327
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a nitride-type, rare earth magnet material made of an R—T—M(—B)—N alloy and an isotropic, bonded rare earth magnet formed from such a nitride-type, rare earth magnet material, particularly to a nitride-type, rare earth magnet material comprising Sm and La as R and an isotropic, bonded rare earth magnet having good magnetizability.
BACKGROUND OF THE INVENTION
Bonded rare earth magnets comprising Nd—Fe—B magnet powder have conventionally been used widely, though their applications at high temperatures are restricted because they have as low Curie temperatures as about 300° C. and high temperature coefficients of coercivity iHc.
Sm
2
Fe
17
N
x
compounds formed by making Sm
2
Fe
17
compounds absorb nitrogen have recently been finding industrial applications as magnet powder for bonded magnets, because they show higher Curie temperatures (470° C.) and anisotropic magnetic field (260 kOe) than those of Nd
2
Fe
14
B compounds. However, Sm
2
Fe
17
N
x
compounds fail to show usefully high iHc unless they are pulverized to as small a particle size as a few &mgr;m, corresponding to the size of a single magnetic domain. Also, Sm
2
Fe
17
N
x
compounds in a state of fine powder having a few &mgr;m size are easily oxidized in the air at room temperature, resulting in drastic deterioration of their magnetic properties. In addition, Sm
2
Fe
17
N
x
compounds in a state of fine powder having a few &mgr;m cannot be filled in the bonded magnets at high density, failing to achieve usefully high maximum energy products (BH)
max
.
To solve the above problems in connection with fine pulverization, Japanese Patent Laid-Open No. 4-260302 describes that nitrided magnet powder having a composition comprising 5-15 atomic % of Sm, 0-10 atomic % of M which is at least one element selected from the group consisting of Zr, Hf, Nb, Ta, W, Mo, Ti, V, Cr, Ga, Al, Sb, Pb and Si, and 0.5-25 atomic % of N, the balance being substantially Fe or Fe and Co (Fe content is 20 atomic % or more) is obtained by heat-treating the Sm
2
Fe
17
compounds in a hydrogen atmosphere and then under reduced pressure and further nitriding it, and that when M is contained, the resultant magnet powder has an average crystal grain size of 1 &mgr;m or less and an average particle size of 20 &mgr;m or more, showing magnetic anisotropy. The inventors' research has revealed, however, that the nitrided magnet powder produced under the conditions of Japanese Patent Laid-Open No. 4-260302 is magnetically isotropic, having an average crystal grain size of more than 1 &mgr;m. The reason therefor is considered that a hydrogen absorption temperature in Examples of Japanese Patent laid-Open No. 4-260302 is as low as 650° C., lower than a hydrogenation/decomposition temperature.
As a result of the inventors' investigation, it has been found that nitrided magnet powder having an average particle size of 10 &mgr;m or more and an average crystal grain size of 1 &mgr;m or less can be produced, when thin ribbons obtained from a mother alloy melt for a nitride-type, rare earth magnet material by rapid quenching at as high a peripheral speed of a quenching roll as, for instance, 45 m/sec. or more are heat-treated under the conditions of Japanese Patent Laid-Open No. 4-260302 and then nitrided. However, because thin mother alloy ribbons rapidly-quenched under the above conditions are extremely as thin as less than 50 &mgr;m, magnet powders obtained by finally nitriding them have ragged shapes, reflecting the shapes of the thin ribbons. As a result, such magnet powders cannot be compression-molded well. Accordingly, such nitrided magnet powder cannot be formed into isotropic, bonded magnets having as high a density as more than 6.1 g/cm
3
, making less likely the expectation that (BH)
max
is improved by increasing the filling density of the nitrided magnet powder.
Magnetizability is extremely important for isotropic, bonded rare earth magnets, and a magnetic field intensity for magnetization is preferably 25 kOe or less at room temperature in practical applications. However, conventional R—T—M—N-type, isotropic, bonded rare earth magnets are not well magnetized under the above conditions.
OBJECT OF THE INVENTION
Accordingly, an object of the present invention is to provide a nitride-type, rare earth magnet material of an R—T—M(—B)—N alloy, particularly a (Sm, La)—T—M(—B)—N alloy, wherein R is at least one rare earth element including Y, as a rare earth element Sm must be present, T is Fe alone or a combination of Fe and Co and/or Ni, and M is at least one element selected from the group consisting of Al, Ti, V, Cr, Mn, Cu, Ga, Zr, Nb, Mo, Hf, Ta, W and Zn, the nitride-type, rare earth magnet material containing an extremely small amount of &agr;-Fe, if any, and being substantially composed of a fine, hard magnetic phase of an R
2
T
17
-type structure.
Another object of the present invention is to provide an isotropic, bonded rare earth magnet containing such a nitride-type, rare earth magnet material and having good magnetizability.
DISCLOSURE OF THE INVENTION
With respect to nitride-type, rare earth magnet material powder and an isotropic, bonded rare earth magnet containing such magnet powder, research has been carried out to achieve the following objectives:
(1) To provide nitride-type, rare earth magnet material particles substantially composed of a hard magnetic phase of R
2
T
17
-type structure, &agr;-Fe being preferably 5% or less, more preferably 2% or less, particularly 0% by average area ratio;
(2) To provide a small decrease in magnetic properties by temperature elevation (good heat resistance);
(3) To provide high (BH)
max
;
(4) To easily form isotropic, bonded rare earth magnets under practical molding pressure;
(5) To provide bonded rare earth magnets with magnetizability sufficient for practical applications; and
(6) To provide bonded rare earth magnets having a density of more than 6.1 g/cm
3
.
As a result, it has been found that nitride-type, rare earth magnet materials satisfying the above requirements (1)-(6) can be produced by preparing by a melting method a mother alloy having a composition corresponding to the basic composition of an R—T—M(—B)—N-type, nitrided rare earth magnet alloy, wherein R is at least one rare earth element including Y, Sm being indispensable, T is Fe alone or Fe and Co and/or Ni, and M is at least one element selected from the group consisting of Al, Ti, V, Cr, Mn, Cu, Ga, Zr, Nb, Mo, Hf, Ta, W and Zn, and subjecting the resultant mother alloy to a homogenizing heat treatment at 1010-1280° C. for 1-40 hours in an inert gas atmosphere containing no nitrogen, if necessary, and then to a hydrogenation/decomposition reaction treatment, a dehydrogenation/recombination reaction treatment and a nitriding treatment in this order.
It has particularly been found that nitride-type, rare earth magnet materials satisfying the above requirements (1)-(6) can be produced by rapidly cooling a mother alloy melt having a composition corresponding to the basic composition of an R—T—M—B—N nitride-type, magnet alloy, wherein R is at least one rare earth element including Y, as a rare earth element Sm must be present, T is Fe alone or a combination of Fe and Co and/or Ni, and M is at least one element selected from the group consisting of Al, Ti, V, Cr, Mn, Cu, Ga, Zr, Nb, Mo, Hf, Ta, W and Zn, wherein Ti must be present, at a peripheral speed of a quenching roll that is preferably 0.05-15 m/second, more preferably 0.08-10 m/second, particularly preferably 0.1-8 m/second, and then subjecting the resultant quenched alloy to a hydrogenation/decomposition reaction treatment and a dehydrogenation/recombination reaction treatment described below, and then to a nitriding treatment. It has further been found that a combination of Sm and La is advantageously selected as the R element to improve the magnetizability. The present invention has been completed based on these findings.
Thus, the nitride-type, rare earth magnet material according to the present invention has a bas
Okajima Hiroshi
Shindo Mikio
Tobise Masahiro
Hitachi Metals Ltd.
Sheehan John
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