Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
1999-07-22
2001-07-03
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C148S120000, C148S121000, C148S308000, C148S309000, C148S310000, C335S296000, C428S611000, C428S638000, C428S686000, C428S900000, C428S928000
Reexamination Certificate
active
06255005
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a composite magnetic member combining a ferromagnetic portion and a non-magnetic portion in a single material, which member can be used in industrial products utilizing a magnetic circuit, such as a motor.
Industrial products requiring a magnetic circuit, such as the rotor of a motor and a magnetic scale etc., conventionally have a structure in which a non-magnetic portion is provided in a part of a ferromagnetic body (generally, a soft magnetic material). Techniques such as the brazing and laser welding of a ferromagnetic part and a non-magnetic part have been employed to provide a non-magnetic portion in a part of the ferromagnetic part. In contrast to these techniques of bonding dissimilar materials, the present inventors propose the use of a single material as the material for a composite magnetic member which is formed by providing a ferromagnetic portion and a non-magnetic portion by cold working or heat treatment. When such composite magnetic members made of a single material are used, it is possible to obtain parts superior to those obtained by bonding a ferromagnetic portion and a non-magnetic portion regarding the respects of ensuring airtightness, ensuring reliability, such as prevention of breakage by vibrations, etc., and reducing the cost thereof.
In JP-A-9-157802 based on the proposal by the present inventors, for example, a martensitic stainless steel containing 0.5 to 4.0% Ni is disclosed as a composite magnetic member suitable for an oil controlling device of an automobile. This proposal is such that in a martensitic stainless steel composed of ferrite and carbides in an annealed condition, by adding Ni of an appropriate amount in a Fe—Cr—C base alloy in which such a ferromagnetic characteristic as to be not less than 200 in maximum magnetic permeability, a non-magnetic portion having magnetic permeability not more than 2 is obtained and is stabilized in the martensitic stainless steel through the steps of heating the portion and then cooling it, and that the Ms point (at which the austenite begin to be changed to martensite) can be lowered to a temperature not more than −30° C.
Also, JP-A-9-228004 based on another proposal by the present applicant discloses that, by adding more than 2% but not more than 7% Mn and 0.01 to 0.05% N to a C—Cr—Fe-base alloy containing 10 to 16% Cr and 0.35 to 0.75% C and having ferromagnetic properties with a maximum magnetic permeability of not less than 200, there is obtained a composite magnetic material used in magnetic scales, etc., in which material a retained austenite with a magnetic permeability of not more than 2 is obtained and is stabilized by cooling after heating, and it becomes possible to lower the Ms point to not more than −10° C. These proposals are excellent in the respect that a ferromagnetic portion with a maximum magnetic permeability of not less than 200 and a stable non-magnetic portion with a magnetic permeability of not more than 2 and a low Ms point can be obtained in a single material.
The composite magnetic members disclosed in the above JP-A-9-157802 and JP-A-9-228004 are based on the proposal that a non-magnetic portion stable down to low temperatures can be formed in a part of a ferromagnetic body by adding an appropriate amount of Ni and Mn, which are austenite-forming elements, to a martensitic stainless steel from which ferromagnetic properties can be obtained, and by performing partial solution treatment, and it can be said that these composite magnetic members are excellent in the respect that a single material can combine a ferromagnetic portion with a maximum magnetic permeability (&mgr;m) of not less than 200 and a stable non-magnetic portion with a magnetic permeability (&mgr;) of not more than 2.
According to examinations by the present inventors, some of the composite magnetic members used as a magnetic circuit are required to have better soft magnetic properties (hereinafter referred to as soft magnetism) than those of conventional members, i.e., high maximum magnetic permeability and low coercive force, for example, as in the rotor of a motor. In contrast to this, in the above two proposals there were limits to the soft magnetism obtained in the ferromagnetic portion.
SUMMARY OF THE INVENTION
An object of the present invention is to obtain, by solving the above problems, a composite magnetic member combining a ferromagnetic portion and a non-magnetic portion in a single material, which ferromagnetic portion has better soft magnetism than conventional members and which non-magnetic portion has stable properties comparable to those of conventional members, a method of producing the ferromagnetic portion of this composite magnetic member, and a method of forming the non-magnetic portion.
According to the researches of the inventors, the microstructure of the ferromagnetic portion of the conventional composite magnetic member made of an Fe—Cr—C-base alloy steel is composed of ferrite matrix and carbides precipitated in this ferrite matrix. However, in order to obtain high maximum magnetic permeability, which is one of indices indicative of excellent soft magnetism, it is necessary to decrease precipitates in the member as little as possible and to thereby produce such a condition as domain walls are readily moved. When there are many carbides whose grain size is not less than 0.1 &mgr;m, in particular, there was a limit to the maximum magnetic permeability obtained in the ferromagnetic portion due to the carbides acting as obstacles to the movement of the domain walls.
Furthermore, in order to obtain low coercive force, which is another index indicative of excellent soft magnetism, it is effective to increase the size of crystal grains of the matrix.
However, when many carbides are present, the growth of the ferrite grains that form the matrix is suppressed and, therefore, the size of ferrite grains become very fine. This becomes the cause of impeding decrease in coercive force obtained in the ferromagnetic portion.
As a method of enhancing the soft magnetism in the ferromagnetic portion of the composite magnetic member, the present inventors discovered the addition of Al that had not been positively added as a ferrite-forming element. The composite magnetic member previously proposed by the present inventors in JP-A-9-157802 contains at least one kind selected from the group consisting of Si, Mn and Al as deoxidizers in an amount of not more than 2.0% in total.
In this proposal, the present inventors expected only the effect of the removal of the oxygen in molten steel by these elements of Si, Mn, Al, etc. as deoxidizers and considered that it is better if these elements do not remain in the member. According to their further examination, however, the present inventors found out that in a composite magnetic member made of an Fe—Cr—C-base alloy steel, the soft magnetism of the ferromagnetic portion is remarkably improved by positively adding Al to the alloy steel, which is used as a stock for producing the composite magnetic member, in amounts of 0.1 to 5.0%
Subsequently, the present inventors made an detailed research regarding the effect of the amount of Al added in the microstructure of the ferromagnetic portion. As a result, they found out that in the ferromagnetic portion having a microstructure mainly composed of ferrite and carbides irrespectively of the addition or non-addition of Al, when Al is added, the number of carbides per unit area decreases together with increase in the size of individual carbides and that the grain size of ferrite grains increases.
Next, the present inventors investigated the relationship between microstructure and soft magnetism. As a result, they found out that in the ferromagnetic portion mainly composed of ferrite and carbides, magnetic properties with a maximum magnetic permeability (&mgr;m) of not less than 400 can be realized by providing such a state as the number of carbides with a grain size of not less than 0.1 &mgr;m is not more than 50 in an area of 100 &mgr;m
2
and as
Inui Tsutomu
Yamada Hideya
Yokoyama Shin-ichiro
Hitachi Metals Ltd.
Jones Deborah
Koehler Robert R.
Sughrue Mion Zinn Macpeak & Seas, PLLC
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