Metal treatment – Process of modifying or maintaining internal physical... – Heating or cooling of solid metal
Reexamination Certificate
2001-12-13
2004-07-13
King, Roy (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Heating or cooling of solid metal
Reexamination Certificate
active
06761780
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention concerns a structural material for cryogenic temperature use and, more in particular, it relates to a non-magnetic structural material for cryogenic temperature use, required for constituting superconductive magnets. The steel sheet referred to this invention includes steel sheets and steel strips.
2. Description of the Related Art
In various techniques utilizing super conductivity such as nuclear fusion power generation, particle accelerators, and superconductive power storage, superconductive magnets are used in view of the requirement of supplying a large amount of current for generating strong magnetic fields. Since large electromagnetic forces are induced in the superconductive magnet and it is usually cooled to a cryogenic temperature at 2-4 K by liquid helium, structural materials supporting the superconductive magnet require high strength capable of withstanding the large electromagnetic forces under the cryogenic temperature. In addition, since it is a basic object to generate a strong magnetic field at a uniform and stable distribution and in a range as wide as possible, it is essential to minimize the effects of the structural materials on the magnetic fields. Accordingly, it is an essential condition for the materials that they are non-magnetic materials not causing interaction with the magnetic fields.
In view of the above, the structural materials used at the inside or the periphery of the superconductive magnet are required to have high mechanical characteristics and extremely low magnetic permeability at a cryogenic temperature and, further, it is also necessary to take a consideration on thermal deformation in order to firmly hold the superconductive magnet in a composite structure. Further, in the manufacture of the superconductive magnet, it is required for the structural materials that they are excellent in machinability such as punching or boring property or weldability and, further, also excellent in surface flatness or fitness required for laminating a plurality of sheets.
Existent materials considered as structural materials for supporting the superconductive magnet can include austenitic stainless steels, high Mn steels, aluminum alloys, titanium alloys and fiber-reinforced plastics. The mechanical strength, the magnetic permeability and the thermal expansion coefficient required for the structural materials for supporting the superconductive magnet vary depending on the designed intensity of the magnetic fields in the superconductive magnet to be manufactured or the aimed uniformity for the distribution of the magnetic fields and it is important for the selection of the materials that the strength is high, and the permeability and the thermal expansion coefficient are low at a cryogenic temperature.
The fiber-reinforced plastics are non-magnetic and easy to handle with being of low specific gravity and have lower thermal expansion coefficient compared with austenitic stainless steels but the strength per unit cross sectional area is lower. Further, while titanium alloys are low in the specific gravity and high in the strength and have high specific strength, they involve a problem that the toughness is low at a low temperature and is expensive.
Aluminum alloys are used in various applications at cryogenic temperatures since they are light in weight, and have high specific strength and extremely low permeability but they lack in the strength when the designed magnetic fields are applied as in large scale particle accelerators and also involve a problem in the weldability.
Since usual austenitic stainless steels are insufficient in the strength and the toughness at low temperatures, stainless steels of low carbon content with addition of nitrogen have been developed. However, since the stability in the austenitic phase is insufficient in such stainless steels, a portion of the austenitic phase is transformed into a ferromagnetic martensitic phase by deformation at a low temperature. Accordingly, this results in lowering of the toughness and involves a problem that the permeability can not be lowered sufficiently at a cryogenic temperature.
Subsequently, austenitic stainless steels with further increased Ni. content have been developed but they involve a problem of increased cost and high thermal expansion coefficient as the structural material for cryogenic temperature use.
In view of the problems described above, Japanese Patent Publications No. 11661/1984 and No. 18887/1993 propose relatively inexpensive high Mn non-magnetic steels and manufacturing methods thereof. However, the high Mn non-magnetic steels described in Japanese Patent Publication No. 11661/1984 have high permeability at a cryogenic temperature and involve problems as a large scale particle accelerator use. The technique disclosed in Japanese Patent Publication No. 18887/1993 involves problems of requiring long time aging treatment and lowering the productivity.
Further, in the superconductive magnet, a non-magnetic member referred to as a collar is required as fixing members for superconducting wires as conductor coils and the collar is formed by laminating a plurality of non-magnetic steel sheets. Then, the collar also requires an appropriate mechanical strength in order to withstand strong electromagnetic forces caused when it is cooled to a cryogenic temperature and a large amount of current is supplied as the superconductive magnet. However, when the mechanical strength of the non-magnetic steel sheet is excessively high or the residual stress therein is excessive, the working life of a punching die is shortened or warps are caused after punching the non-magnetic steel sheet into a predetermined shape of the collar.
In the superconductive magnet, the collar is often manufactured by precision punching such as fine blanking. With the view point as described above, the mechanical strength of the material used for the collar is determined while taking the strength and the distribution of the designed magnetic field into a consideration. Accordingly, it has been demanded for a method of manufacturing a non-magnetic steel sheet that can easily control the strength of the non-magnetic steel sheet as the material to a desired strength demanded in the design.
OBJECT OF THE INVENTION
An object of this invention is to effectively overcome the foregoing problems in the prior art and provide a method of manufacturing a high Mn non-magnetic steel sheet for cryogenic temperature use, capable of manufacturing, with industrial stability and high productivity, and a high Mn non-magnetic steel sheet which is suitable for use in large scale particle accelerators, and has a high yield point at a cryogenic temperature and low permeability at the cryogenic temperature.
SUMMARY OF THE INVENTION
In order to attain the foregoing subject, the present inventors have investigated characteristics required for supporting structural members used in superconductive magnets for use in large scale particle accelerators and have made an earnest study for the factors giving effects on the permeability and the yield stress at a cryogenic temperature of high Mn non-magnetic steel sheets. As a result, it has been found that the permeability of the high Mn non-magnetic steel at the cryogenic temperature can be lowered by further stabilizing the austenitic phase by increasing the content of Mn. Further, it has been found that the yield stress of the high Mn non-magnetic steel at the cryogenic temperature can be controlled easily to 900 MPa or more by applying temper rolling to a steel sheet after intermediate annealing.
This invention has been constituted based on the findings described above. That is, this invention provides a method of manufacturing a hot rolled high Mn non-magnetic steel sheet for cryogenic temperature use, which comprises:
heating a steel material containing, on the weight percent basis:
from 0.05 to 0.18% of C,
from 26.0 to 30.0% of Mn,
from 5.0 to 10.0% of Cr,
from 0.05 to 0.15% of N,
from 0.01 to 0.07% of Al,
from 0.01 to
Kobori Katsuhiro
Morito Nobuyuki
Nishiike Ujihiro
Nohara Kiyohiko
Yamashita Takako
JFE Steel Corporation
King Roy
Wilkins, III Harry D.
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