JP2001240942A - Mn NONMAGNETIC SEAMLESS STEEL PIPE FOR CRYOGENIC USE - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high Mn nonmagnetic seamless steel pipe low in permeability under cryogenic conditions, having uniform low permeability in the circumferential direction and being suitable to a large beam pipe for a particle accelerator. SOLUTION: This steel pipe has a composition containing, by mass, 0.05 to 0.15% C, 26.0 to 30.0% Mn, 5.0 to 10.0% Cr and 0.05 to 0.15% N, or moreover containing 0.50 to 5.0% Ni and/or <=0.02% Ca, and the balance Fe with inevitable impurities.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a seamless steel pipe suitable as a structural material for cryogenic use, particularly to a strong magnetic field generator such as a superconducting magnet or a beam pipe necessary for constructing a particle accelerator. The invention relates to a nonmagnetic steel seamless steel pipe used at cryogenic temperatures required for construction.

[0002]

2. Description of the Related Art In various superconducting technologies such as nuclear fusion power generation, particle accelerators, and superconducting power storage, superconducting magnets are used because a large amount of current needs to flow to generate a strong magnetic field. A strong electromagnetic force is induced in such a superconducting magnet, and usually, liquid helium causes
Since the superconducting magnet is cooled to an extremely low temperature of 4K, a structural material supporting the superconducting magnet is required to have a mechanical strength capable of withstanding a strong electromagnetic force at an extremely low temperature. Moreover, since the basic purpose is to generate a uniform and stable strong magnetic field distribution as widely as possible, it is important to minimize the influence of the structural material on the magnetic field as much as possible. Therefore, a non-magnetic material that does not interact with a magnetic field is an essential condition.

[0003] From the above viewpoints, it is required that the structural material used inside or around the superconducting magnet has high mechanical properties at cryogenic temperatures and extremely low magnetic permeability. In particular,
Particle accelerator beam pipe is an essential component for particle accelerators in order to accelerate elementary particles in ultra-high vacuum.
It is important not to impair the uniform magnetic flux distribution constituted by the superconducting magnet. For this reason, the beam pipe is required to have an extremely low magnetic permeability so as not to interact with a magnetic field. Furthermore, since the beam pipe does not directly contact the superconducting coil, it does not require high mechanical strength to withstand strong electromagnetic force at cryogenic temperatures, but airtightness is required to maintain a high vacuum inside the beam pipe. Is done.

[0004] Materials which have been conventionally studied as structural materials used in and around the superconducting magnet include austenitic stainless steel, high Mn steel, aluminum alloys, titanium alloys, and fiber-reinforced plastics. The strength, magnetic permeability and coefficient of thermal expansion required for these structural materials vary depending on the design magnetic field strength of the superconducting magnet to be manufactured and the uniformity of the target magnetic field distribution. Low magnetic permeability and thermal expansion coefficient are important in selecting a material.

[0005] Fiber reinforced plastics are non-magnetic,
Low specific gravity, easy to handle, has low coefficient of thermal expansion compared to austenitic stainless steel, but low strength per unit cross-sectional area, airtightness, heat resistance is low,
There remains a problem as a large particle accelerator material for obtaining a high vacuum by degassing such as baking. Titanium alloys have a low specific gravity, a high strength and a high specific strength, but have the problem of low toughness at low temperatures and high cost.

Aluminum alloys are lightweight, have high specific strength, and are extremely low in magnetic permeability, and are therefore used in many applications at extremely low temperatures. However, when the design magnetic field is increased as in large particle accelerators, aluminum alloys are used. Insufficient strength and poor weldability. Further, when performing degassing such as baking, it is difficult to attain a high degree of vacuum, and there are many problems as a material for a large particle accelerator that achieves a high degree of vacuum by performing degassing such as baking.

[0007] General austenitic stainless steels are:
Due to insufficient strength and toughness at low temperatures, stainless steels with a low carbon content by adding nitrogen have been developed. However, in this stainless steel, since the stability of the austenite phase is insufficient, a part of the austenite phase is transformed into a ferromagnetic martensite phase by deformation at a low temperature. Therefore, there is a problem that the toughness is reduced and the magnetic permeability at an extremely low temperature is not sufficiently reduced.

Thereafter, an austenitic stainless steel having a further increased Ni content was developed. However, there were problems in that the structural material for cryogenic use had a high cost and a large thermal expansion coefficient. In response to such problems, Japanese Patent Publication No. 59-11
Japanese Patent Publication No. 661 and Japanese Patent Publication No. 5-18887 propose a relatively inexpensive high-Mn nonmagnetic steel and a method for producing the same.

However, the high-Mn nonmagnetic steel described in Japanese Patent Publication No. 59-11661 has high magnetic permeability at extremely low temperatures, and has a problem for use in large particle accelerators. In addition, 5-18
The technique described in Japanese Patent No. 887 has a problem that a long time aging treatment is required and productivity is reduced.

[0010]

Further, as a tube applied to a beam pipe for a particle accelerator, an inner surface is beautiful, excellent in airtightness, extremely low magnetic permeability at cryogenic temperature, and low uniform permeability in a circumferential direction. Small diameter tubes with magnetic susceptibility are preferred. Examples of the small-diameter pipe include a welded steel pipe in which a steel sheet is formed into a cylindrical shape and a joint is welded to form a pipe, and a seamless steel pipe in which a steel material is drilled and rolled to form a pipe. However, as long as conventional high-Mn non-magnetic steel is used, the magnetic permeability of the welded portion is too high compared to that of the base metal, and uniform properties in the circumferential direction cannot be obtained. Is difficult to obtain in a stable manner. Further, in the seamless steel pipe, as long as a conventional high-Mn nonmagnetic steel is used, the hot workability is poor and a problem remains in pipe forming such as drilling and hot rolling.

The present invention solves the above-mentioned problems of the prior art, and has a low magnetic permeability at cryogenic temperatures and a high Mn having a uniform low magnetic permeability in the circumferential direction, which is suitable for a beam pipe for a large particle accelerator. Aims to propose a non-magnetic steel seamless steel pipe.

[0012]

Means for Solving the Problems In order to achieve the above object, the present inventors first investigated the characteristics required for a beam pipe for a large particle accelerator, and at the same time, examined the characteristics of a high Mn nonmagnetic steel sheet at extremely low temperatures. The factors that affect the permeability were studied diligently. As a result, it has been found that the magnetic permeability of a high-Mn nonmagnetic steel sheet at an extremely low temperature can be lowered by further stabilizing the austenite phase by increasing the amount of Mn.

Further, the present inventors have made the steel material manufactured based on the above-mentioned knowledge hot-extrusion or hot-rolled by a Mannes mandrel method or a plug mill method.
Alternatively, by using a sizer, reducer, or the like, hot or cold, to make a seamless steel pipe of a predetermined size,
It has been found that the steel pipe has excellent airtightness, low permeability at cryogenic temperatures, low uniform permeability in the circumferential direction, and sufficient properties as a beam pipe for a particle accelerator.

The present invention has been made based on the above-mentioned findings.
It was completed after further consideration. That is,
In the present invention, C: 0.05 to 0.15%, Mn: 26.0 to 3% by mass%.
0.0%, Cr: 5.0-10.0%, N: 0.05-0.15%, or Ni: 0.50-5.0% and / or C
a: High Mn for cryogenic temperature, characterized by containing 0.02% or less and having a composition consisting of balance Fe and unavoidable impurities.
Non-magnetic steel seamless steel pipe.

In the present invention, C: 0.05 to 0.5% by mass.
15%, Mn: 26.0 to 30.0%, Cr: 5.0 to 10.0%, N: 0.05
It is a beam pipe for a particle accelerator composed of a seamless steel pipe containing about 0.15%, or further, 0.50 to 5.0% of Ni and / or 0.02% or less of Ca, and having a balance of Fe and unavoidable impurities.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First, the reasons for limiting the chemical components of the seamless steel pipe of the present invention will be described. Hereinafter, mass% in the composition is simply described as%. C: 0.05 to 0.15% C is an interstitial solid solution element and is effective for increasing the strength of steel by solid solution strengthening. To obtain the desired yield stress at cryogenic temperatures, a C content of 0.05% or more is required. On the other hand, if C exceeds 0.15%, the austenite phase becomes unstable, carbides are precipitated, and it becomes impossible to maintain low permeability at extremely low temperatures, and workability and weldability deteriorate. For this reason, C was limited to the range of 0.05 to 0.15%. The preferred range of C is 0.07 to 0.13%.

N: 0.05 to 0.15% N is an interstitial solid solution element like C, and is a useful additive element for stabilizing the austenite phase and increasing the low-temperature strength. Need. On the other hand, N
If it exceeds 0.15%, the rollability and weldability are impaired, and the magnetic permeability is increased by precipitation of nitrides and carbonitrides. For this reason, N was limited to the range of 0.05 to 0.15%.
The preferred range of N is 0.07 to 0.13%.

Mn: 26.0-30.0% Mn is an important element in the present invention and is useful for stabilizing the austenite phase and realizing extremely low magnetic permeability even at extremely low temperatures. To achieve this effect, Mn
Requires a content of 26.0% or more. On the other hand, if it exceeds 30.0%, the toughness, weldability and manufacturability will be reduced.
Mn was limited to the range of 26.0-30.0%.

Cr: 5.0-10.0% Cr not only contributes to an increase in mechanical strength by solid solution strengthening but also effectively acts to improve corrosion resistance. Such an effect
Although it is recognized at a content of 5.0% or more, if it exceeds 10.0%,
It hinders the stabilization of the austenite phase and causes an increase in magnetic permeability at low temperatures. For this reason, Cr is limited to the range of 5.0 to 10.0%. The environment in which the material targeted in the present invention is used is as follows.
Basically, it is in a very low temperature and high vacuum where the progress of the chemical reaction is extremely slow, is not inferior from the viewpoint of corrosiveness, and a sufficient corrosion resistance can be ensured with such a Cr content. Note that a preferable range of Cr is 6 to 8%.

Ni: 0.50 to 5.0% Ni contributes to stabilization of the austenite phase and improvement of toughness at cryogenic temperatures, and also improves corrosion resistance. In the present invention, it can be contained as needed. Such an effect is at least
Although it is recognized at a content of 0.50% or more, a large amount is not industrially preferable because Ni is expensive.
For this reason, Ni is preferably in the range of 0.50 to 5.0%. As a result, the steel material of the present invention can
Compared to Ni-based austenitic stainless steel, it has a great advantage not only in thermal expansion coefficient but also in price.

Ca: 0.02% or less Ca suppresses harm of S mixed as an inevitable impurity,
It can be added as needed for the purpose of improving hot workability. The preferable addition amount of Ca is in the range of 0.004 to 0.01%, and the contents of each element of Ca, S, and O are expressed in ppm by mass, and the following equation (1) 0.8 × Ca + 30> S + O (1) It is effective to satisfy the above to ensure hot workability. Ca / S ≧ 2 as a simpler criterion, preferably
Ca / S ≧ 3 can also be used.

The balance other than the above components is substantially Fe and unavoidable impurities. As inevitable impurities, S: 0.005% or less, P: 0.05% or less, O: 0.00
5% or less is acceptable from the viewpoint of industrial economy. Also,
Precipitates such as carbides, nitrides, carbonitrides, especially Fe ₃ C, Fe
It is desirable that the generation of ferromagnetic precipitates such as ₄ N and the inclusion of components such as Si that impair the stability of the austenite phase are small as long as the desired mechanical properties are satisfied.

In the method for producing a seamless steel pipe of high Mn nonmagnetic steel according to the present invention, first, molten steel having the above-mentioned chemical composition is melted by a commonly known melting method such as a converter or an electric furnace, and is continuously cast or formed. A lump-bulking method or other hot rolling method is applied to obtain a steel pipe material. It goes without saying that secondary refining, such as ladle refining of molten steel and vacuum degassing, may be performed as necessary. Then, without reheating these steel tube materials, or without reheating if the steel tube material has a predetermined temperature or more,
Pipes are formed by piercing and hot rolling such as a plug mill method and a mandrel mill method to obtain a seamless steel pipe.

[0024] The steel pipe material suitable for the present invention may contain a large amount of Mn, and Mn is easily oxidized at a high temperature. Therefore, if the heating temperature of the steel pipe material is excessively increased, only the burnout increases. No, it is not preferable because it leads to excessive generation of Mn fume. Therefore, the hot workability of the steel pipe material suitable for the present invention was evaluated by a high-temperature tensile test.
The composition of the steel material used is, by mass%, C: 0.12%,
Si: 0.05%, Mn: 27.9%, P: 0.029%, S: 0.002
%, Cr: 7.0%, N: 0.10%, Ni: 0.15%, Ca: 0.006
%. The result is shown in FIG. FIG. 1 shows that the cross-sectional shrinkage decreases when the temperature exceeds 1200 ° C., and signs of hot brittleness appear.

From the above, when the temperature exceeds 1200 ° C.,
There is a concern that ear cracks may occur. For this reason, it is preferable to set the upper limit of the rolling start temperature of hot rolling to 1200 ° C. Also,
If the rolling start temperature of the hot rolling is lower than 1050 ° C., the dissolution of carbides is insufficient, and a problem such as an increase in deformation resistance occurs. For this reason, the rolling start temperature of hot rolling is 10
The temperature is preferably in the range of 50 to 1200 ° C. The temperature is more preferably 1100 to 1180 ° C.

In the present invention, it is preferable to limit the hot rolling end temperature to 700 ° C. or higher. In addition, when the rolling end temperature of hot rolling exceeds 1000 ° C., there is a problem that crystal grains become coarse due to recrystallization. For this reason, the rolling end temperature of the hot rolling is preferably limited to the range of 700 to 1000 ° C. The temperature is more preferably from 800 to 950 ° C. from the viewpoint of preventing ear cracks.

The seamless steel pipe formed by hot rolling is as follows:
As it is or after reheating, it is further made into a steel pipe of desired dimensions by hot or cold working with a sizer, a reducer or the like. Hot working or cold working conditions do not need to be particularly limited as long as a seamless steel pipe having a desired size can be obtained. The annealing of the hot-rolled seamless steel pipe is preferably performed in order to homogenize the structure, and more preferably in a temperature range of 950 to 1200 ° C. If the annealing temperature is lower than 950 ° C, the cross-sectional shrinkage decreases,
If it exceeds 1200 ° C., embrittlement and scale formation become excessive.

The seamless steel pipe processed as described above is preferably subjected to a final annealing treatment. The final annealing treatment is performed mainly for the purpose of releasing internal strain by processing, recrystallization, and solid solution of precipitates. In particular, this is a process for completely dissolving carbides, nitrides, and carbonitrides in the austenite matrix phase to eliminate a precipitation phase that is disadvantageous for ensuring low magnetic permeability. The annealing temperature is preferably set to 1050 to 1200 ° C. If the annealing temperature is less than 1050 ° C., the solid solution of the precipitates is insufficient, while if it exceeds 1200 ° C., continuous annealing cannot be performed industrially stably. A preferred annealing temperature is 1050 to 1180 ° C. Further, the holding time of this annealing is desirably a time during which the steel pipe is held at the above-mentioned temperature for 10 to 120 seconds.

Further, according to the present invention, cooling is performed after the annealing temperature is maintained in the above range. The cooling is performed for the purpose of preventing the precipitation of carbides and carbonitrides, and the cooling means is not particularly limited as long as the cooling rate is 5 to 30 ° C./s. After annealing, it is preferable to perform a treatment such as pickling. The seamless steel pipe of the present invention is naturally excellent in airtightness because it is seamless, and naturally has no change in the magnetic permeability in the circumferential direction, and can maintain a low magnetic permeability of 1.0015 or less. The magnetic permeability at 4K is almost the same as that at room temperature, and there is little change in magnetic permeability due to temperature. This is considered to be due to the extremely stable austenite phase in the seamless steel pipe of the present invention.

As described above, the seamless steel pipe of the present invention is a steel pipe having airtightness and low uniform magnetic permeability in the circumferential direction, and is suitable as a beam pipe for a particle accelerator.

[0031]

EXAMPLE Molten steel having the chemical composition shown in Table 1 was melted in a converter.
It was made into a steel pipe material by the continuous casting method. These steel pipe materials were subjected to piercing by a piercer and hot rolling of elongation by a mandrel mill under the conditions shown in Table 2 to obtain a seamless steel pipe (110 mmφ x wall thickness).
2.3 mm). Then, these steel pipes were subjected to intermediate annealing (atmosphere: in the air) shown in Table 2 and then subjected to a 50 mmφ steel pipe (thickness 2.0) with a hot stretch reducer under the conditions shown in Table 2.
mm) as a final annealing treatment, annealing was performed at 1100 ° C. × 60 s (atmosphere: in the air), and after the annealing, the product was cooled to a normal temperature at a cooling rate of 20 ° C./s, and then pickled to obtain a product tube.

[0032]

[Table 1]

[0033]

[Table 2]

Test pieces were collected from these seamless steel pipes,
A tensile test was performed at room temperature and 4K, and a permeability test was performed at room temperature and 4K using a vibration sample type magnetometer. In addition, a measurement test of the average thermal expansion coefficient between room temperature and liquid nitrogen temperature was performed on these seamless steel pipes. Further, an airtightness test of the steel pipe was performed to evaluate the airtightness of the steel pipe.

A He leak test was performed as an airtightness test of the steel pipe. The airtightness was evaluated as ○ when complete, and x when slight leakage was observed. Table 3 shows the results.

[0036]

[Table 3]

In each of the examples of the present invention, at a very low temperature, 0.1
The 2% proof stress and tensile strength also show high values and have excellent mechanical properties at cryogenic temperatures. In addition, all of the examples of the present invention show low magnetic permeability at 4K as well as at room temperature, have a low coefficient of thermal expansion between room temperature and liquid nitrogen temperature, and have excellent characteristics at extremely low temperatures. Furthermore, the steel pipe of the example of the present invention has a low average thermal expansion coefficient between room temperature and liquid nitrogen temperature of the steel pipe, furthermore, has excellent airtightness of the steel pipe and can maintain a high degree of vacuum. Can be sufficiently applied as a beam pipe for a particle accelerator.

On the other hand, Comparative Examples outside the range of the present invention have a large average thermal expansion coefficient between room temperature and liquid nitrogen temperature,
In addition, it has a high magnetic permeability at extremely low temperatures and cannot be used as a beam pipe for a particle accelerator.

[0039]

According to the present invention, a high-Mn nonmagnetic steel seamless steel pipe having a low magnetic permeability at a very low temperature and a low average thermal expansion coefficient and capable of being applied to a beam pipe for a large particle accelerator. Can be provided industrially at a low cost, and an industrially significant effect is achieved.

[Brief description of the drawings]

FIG. 1 is a graph showing a relationship between a sectional shrinkage rate and a heating temperature in a high-temperature tensile test.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H05H 13/04 H05H 13/04 C (72) Inventor Takaaki Toyooka 1-1-1 Kawasaki-cho, Handa-shi, Aichi Kawasaki 2G085 BA16 EA01 EA04 in Chita Works, Steel Works

Claims (3)

[Claims] 1. A composition containing, by mass%, C: 0.05 to 0.15%, Mn: 26.0 to 30.0%, Cr: 5.0 to 10.0%, and N: 0.05 to 0.15%, the balance being Fe and unavoidable impurities. A high-Mn nonmagnetic steel seamless steel pipe for cryogenic use, characterized by having: 2. In addition to the above composition, N
2. The composition according to claim 1, wherein i: 0.50 to 5.0%.
2. A high Mn nonmagnetic steel seamless steel pipe for cryogenic use according to item 1. 3. A beam pipe for a particle accelerator, comprising the seamless steel pipe according to claim 1 or 2.