JPH1171655A - Stainless steel sheet excellent in cryogenic characteristic in weld zone for supporting intermetallic compound superconducting material and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a stainless steel sheet excellent in cryogenic characteristics in the weld zone by composing it of a steel of austenitc single phases contg. specified amounts of C, Si, Mn, Cr, Ni, N, Ca, Mg and Fe and regulating the maximum size of the oxides of Ca and Mg to be precipitated. SOLUTION: This steel sheet is composed of a stainless steel of austenitic single phases having a compsn. contg., by weight, <=0.05% C, <=2% Si, 0.1 to 16% Mn, 10 to 24% Cr, 8 to 25% Ni and 0.1 to 0.35% N, contg. one or more kinds of 0.0003 to 0.01% Ca and 0.0003 to 0.01% Mg and one or more kinds of <=4% Mo and 0.005 to 0.6% Nb, and the balance substantial Fe. Furthermore, the maximum size of the oxides of Ca and Mg to be precipitated is regulated to <=1 μm. In the process of subjecting the steel to melting, casting, hot rolling, cold rolling and heat treatment to form into a thin steel sheet, the content of dissolved oxygen in the stainless steel is regulated to 20 to 100 ppm, and Ca and Mg are added thereto.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a stainless steel sheet for supporting an intermetallic compound superconductive material having excellent cryogenic characteristics of a weld even after heat treatment for forming a superconductive material, and a method for producing the same.

[0002]

2. Description of the Related Art Intermetallic compound superconductors such as Nb ₃ Sn and Nb ₃ Al are very brittle and have poor strength and toughness. Therefore, they are produced in a state where the raw materials are held by a supporting structure or a coating reinforcing material. As it is, it is used after being cooled to a cryogenic temperature which is a superconducting temperature. Since austenitic stainless steel is used for these reinforcing materials because it is indispensable that they are nonmagnetic, it is inevitable that toughness at cryogenic temperatures is deteriorated by heat treatment for forming a superconducting material. In particular, it is necessary to form a pipe by welding in order to wrap the raw material of the superconducting material, but at present, the welded portion is severely deteriorated.

[0003] The superconducting phenomenon is being used in various applications because a strong electromagnetic force is obtained and there is no loss due to Joule heat of current. Accordingly, high-performance superconducting materials are under development. However, a superconducting wire that generates a high magnetic field naturally generates a large electromagnetic force. Therefore, in addition to being non-magnetic, the supporting structure and the coating reinforcing material have high strength and toughness in the cryogenic temperature range. Is required. For this reason, an austenitic stainless steel that has been conventionally solid-solution strengthened with N and has an austenite phase stabilized has been used.

[0004] Incidentally, the intermetallic compound superconducting material such as Nb ₃ Sn or Nb ₃ Al has a defect that the superconducting characteristics are remarkably deteriorated when plastic strain is applied. For this purpose, the intermetallic compound superconducting member is formed into a conductor member or a superconducting magnet in the form of a material using a coating reinforcing material and a supporting structure,
Thereafter, heat treatment for forming an intermetallic compound at 600 to 1000 ° C. for 10 hours or more (hereinafter, heat treatment for forming an intermetallic compound)
Manufactured. Naturally, the coating reinforcing material and the supporting structural material must be simultaneously subjected to the heat treatment for forming an intermetallic compound.

However, when heat treatment is performed in such a temperature range, it is recognized that even in a low-C austenitic stainless steel, a nitride of Cr is formed at a grain boundary and toughness at an extremely low temperature is deteriorated. Have been. On the other hand, Japanese Patent Publication No. 61-416 discloses a method of adding V to steel. However, not only is it necessary to add expensive elements, but if the N is increased to increase the strength, the effect is not always clear. Further, JP-A-62-2220
No. 48 discloses a method of suppressing the diffusion of impurity elements to grain boundaries by adding Mo, and a method of containing Nb to finely precipitate NbC and NbN to suppress the precipitation of Cr carbonitride. Have been. However, this method requires a stabilizing heat treatment, so that the production cost is high and the effect of the welded portion is lost. Also, JP-A-63-
JP-A-134627 discloses a method in which reheating is performed in a temperature range higher than the Nb ₃ Sn generation heat treatment temperature after solution treatment, but this method increases the cost because the process becomes complicated. Not only cannot it be prevented but also has the disadvantage of deteriorating the properties of the superconductor itself.

Further, the present inventors add a small amount of Ti to precipitate Ti-based precipitates in crystal grains in steel, and precipitate Cr carbonitrides in the grains using the precipitates as nuclei. To prevent the strength of grain boundaries from deteriorating (Japanese Patent Laid-Open No. 7-2383).
No. 20). Although this method is relatively effective at a relatively low cost, it has the drawback that the fine Ti precipitates precipitated in the welded area are re-dissolved and the effect is reduced. I couldn't.

On the other hand, since it has been recognized that Ni-based alloys have a small characteristic deterioration in an extremely low temperature range, application of high Ni alloys such as Incoloy has been attempted, but these alloys are expensive. Since a large amount of Ni is used, wide application in terms of cost has been difficult. As described above, there has been no low-cost thin steel sheet material capable of securing strength and toughness in a cryogenic temperature range and a corresponding method for supporting pipes of intermetallic compound superconductors. Accordingly, the present invention has been made in view of such circumstances, and suppresses the property deterioration due to heat treatment for generating a superconducting intermetallic compound, and provides a stainless steel sheet for supporting an intermetallic compound superconducting material having excellent cryogenic characteristics of a welded portion. And a method of manufacturing the same, and a method of manufacturing the same.

[0008]

The cause of the toughness deterioration of the austenitic stainless steel, which is the supporting structure material of the superconductor, due to the heat treatment for forming an intermetallic compound is that coarse Cr carbonitride precipitates at the grain boundaries during the heat treatment. Instead of suppressing the precipitation of Cr carbonitride, the technical idea of changing the precipitation site from grain boundary to intragranular, that is, the precipitation carbonitride deteriorates the toughness at cryogenic temperature is It was considered that the adverse effect was slight as long as the particles precipitated in the grains. Therefore, for this purpose, the present invention has been made based on a technical idea that a precipitation site of Cr carbonitride is introduced into grains by some method.

[0009] The inventors of the present invention based on this idea,
A detailed examination of the carbonitride precipitation sites revealed that, although a little, Cr carbonitride was also precipitated in grains other than the grain boundaries, and that fine Ti-based carbonitrides and oxides were found in the center. I found that there is. JP-A-7-23832
No. 0 discloses a method of positively utilizing Ti-based carbonitride as a nucleus, but has a disadvantage that Ti-based precipitates are re-dissolved by welding to reduce the effect. Therefore, in the heat treatment for forming an intermetallic compound, a precipitate that functions as a precipitation nucleus of Cr carbonitride and does not re-dissolve even by welding was further studied. As a result, they have found that fine high-melting oxides such as CaO and MgO are most suitable as the core for the above purpose. Oxide, Al ₂ O ₃ , SiO ₂ , CaO, MgO, etc. have a maximum diameter of 1 μm.
When a coarse substance exceeding m was contained, no effect as a nucleus was observed.

The present invention has been completed based on the above findings, and the gist thereof is as follows. (1) By weight%, C: 0.05% or less, Si: 2% or less, Mn: 0.1% or more and 16% or less, Cr: 10% or more and 24% or less, Ni: 8% or more and 25% or less, N: 0.1
% To 0.35%, Ca: 0.0003% to 0.01%, and Mg: 0.0003% to 0.01
% Or less, the balance consisting of Fe and unavoidable impurities, the austenitic single phase, and the maximum diameter of all oxides containing one or both of precipitated Ca and Mg is 1 μm or less. A stainless steel sheet for supporting an intermetallic compound superconducting material having excellent cryogenic characteristics at a welded portion. (2) By weight%, C: 0.05% or less, Si: 2% or less, Mn: 0.1% or more and 16% or less, Cr: 10% or more and 24% or less, Ni: 8% or more and 25% or less, N: 0.1%
Not less than 0.35%, Ca: 0.0003% to 0.01%, and Mg: 0.0003% to 0.01%
One or two of the following, and Mo: 4% or less and Nb:
All oxides containing one or two kinds of 0.005% or more and 0.6% or less, the balance being Fe and unavoidable impurities, an austenitic single phase, and containing one or both of precipitated Ca and Mg A stainless steel sheet for supporting an intermetallic compound superconducting material having excellent cryogenic characteristics at a welded portion, wherein the maximum diameter of the stainless steel is 1 μm or less. (3) In the step of forming a thin steel sheet by performing melting, casting, hot rolling, cold rolling and heat treatment, the dissolved oxygen content of the stainless steel is adjusted to 20 ppm or more and 100 ppm or less.
(1) or (2) for supporting an intermetallic compound superconducting material excellent in cryogenic characteristics of a welded part according to (1) or (2), wherein one or both of Mg and Mg are added to the above amount. Manufacturing method of stainless steel sheet.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The main point of the present invention is to add a trace amount of one or both of Ca and Mg to molten steel to disperse and precipitate oxides of these elements. Some of Ca and Mg become oxides in the state of added molten steel, but some precipitate as oxides simultaneously with solidification in the grains. Fine oxide particles typified by CaO and MgO, which are precipitated in the grains at the same time as the solidification, act as precipitation nuclei of Cr carbonitride during heat treatment for generating an intermetallic compound. As a result, the Cr carbonitride mainly precipitates in the grains and can suppress the deterioration of the toughness of the austenitic stainless steel.

The oxides of Ca and Mg finely precipitated in the grains do not disappear even when welding is performed. At present, it is not clear whether this mechanism does not form a solid solution by welding or re-precipitates after solid solution in steel, but it is present even after welding, and the Cr carbonitride The inventors have confirmed that they function as precipitation nuclei. Next, the reasons for limiting the components of the steel of the present invention will be described.

C is an element that stabilizes austenite and improves proof stress. However, it is necessary to keep C low because it tends to combine with Cr by aging to form carbides and cause deterioration in toughness after heat treatment for forming an intermetallic compound. , 0.05% or less. Si is an element necessary for deoxidation at the time of steel making, but is a ferrite stabilizing element. If it exceeds 2%, it becomes difficult to obtain a stable austenite structure.

Mn has an effect of increasing the solubility of N, and is an extremely effective element when a large amount of N is added. If it is less than 0.1%, the effect is small, so the lower limit is 0.1%, and if it exceeds 16%, it becomes difficult to melt, so the upper limit is 16%. Cr is a ferrite stabilizing element, but has an effect of increasing the solubility of N, and is an extremely effective element when adding a large amount of N. Since this effect is small when the content is less than 10%, the lower limit is set to 10%. If it exceeds 24%, the heat treatment for forming an intermetallic compound generates a σ phase, and the toughness is significantly deteriorated. Therefore, the upper limit is set to 24%.

Ni is an element necessary for stabilizing austenite and improving low-temperature toughness. In the steel of the present invention, 8% or more is required. However, when added in a large amount, the solubility of N becomes small, so that Cr nitride is easily precipitated by aging, and the toughness after the heat treatment for forming an intermetallic compound is deteriorated. Was set to 25%. N is an element necessary for stabilizing austenite and improving proof stress, and is required to be 0.1% or more in order to ensure proof stress at extremely low temperatures. However, it is easy to combine with Cr by aging to form a nitride, which causes deterioration of toughness after heat treatment for forming an intermetallic compound.
%.

In order for Ca to react with dissolved oxygen and to contribute as nuclei for forming Cr carbonitride, at least 0.001
The lower limit was set because 03% was required. On the other hand, it is considered that the addition of a large amount of Ca is not necessarily disadvantageous in principle, but in the stainless steel component system of the present invention, 0.01%
% Is extremely difficult to add, and a special scouring method needs to be used, which causes a large increase in cost. Therefore, the upper limit is set to 0.01%.

In order for Mg to also react with dissolved oxygen and contribute as a nucleus for forming Cr carbonitride, at least 0.1 mg of Mg is required.
Since 0003% is required, the lower limit was set. On the other hand, it is thought that the addition of a large amount of Mg is not necessarily disadvantageous in principle, but it is extremely difficult to add more than 0.01% in the component system of the steel of the present invention. Since it becomes necessary to use the iron oxide, which causes a large increase in cost, the upper limit is set to 0.01%.

Even if the chemical compositions described above match, if the precipitated Ca and Mg oxides have a coarse maximum diameter exceeding 1 μm, they do not function as precipitation nuclei for Cr carbonitride in the heat treatment for forming an intermetallic compound. From Ca and Mg
The maximum diameter of the oxide was limited to 1 μm or less. The size of the oxide may be measured by any method by which an oxide of about 1 μm can be sufficiently observed. For example, observation of several fields at about 5,000 times with a transmission electron microscope shows that the maximum diameter is 1 μm.
It may be sufficient to confirm that the number exceeding m does not exist. In stainless steel manufactured by the method disclosed in the present invention, no abnormal grain growth of the precipitated oxide is seen,
Since the average particle size of the oxide is much smaller than 1 μm,
This method was sufficient.

The amount of the element selectively added for improving the characteristics is limited for the following reasons. Mo is an element that improves the low-temperature strength by solid solution strengthening, but if it exceeds 4%, a large amount of Fe ₂ Mo precipitates at the crystal grain boundaries due to heat treatment for forming an intermetallic compound and lowers toughness, so the upper limit is 4%. And

Nb is simultaneously strengthened with NbC, Nb
N is an element that improves the low-temperature strength by strengthening the precipitation of N and NbCrN. However, if it is less than 0.005%, this effect is small, so the lower limit is set. If it is more than 0.6%, NbC, NbN, and NbCrN are coarsened and Fe ₂ Since Nb precipitates in large amounts and lowers toughness, the upper limit is set to 0.6%. In the production method of the present invention, the dissolved oxygen amount of molten steel when adding Ca or Mg is added in a state where a large amount remains,
Since it becomes a coarse oxide and does not function as a precipitation nucleus of Cr carbonitride in the heat treatment for forming an intermetallic compound, it itself deteriorates toughness in a cryogenic temperature range and deteriorates corrosion resistance. The upper limit of the oxygen concentration at the time of addition was 100 ppm. Conversely, if the amount of dissolved oxygen is too small, the amount of Ca and Mg oxides serving as precipitation nuclei of Cr carbonitride during intermetallic compound formation heat treatment is small, and the amount of Cr carbonitrides with grain boundaries as nuclei increases. Since the purpose of (1) was not achieved, the lower limit was set to 20 ppm.

[0021]

EXAMPLES Austenitic stainless steels having the chemical compositions shown in Table 1 were melted and cast into 20 kg slabs. In melting, Ca was added with a Ca-Si alloy, and Mg was added with a Mg-Ni alloy. The slab was hot-rolled into a thin steel sheet of 4.5 mm, then subjected to a solution treatment at 1100 ° C. for 10 minutes, further cold-rolled to a thickness of 2.5 mm after descaling by pickling, and finally to 1050 mm. A solution treatment was performed at -5 ° C for 5 minutes to produce a cold-rolled solution-treated steel sheet. Then, this thin steel plate was TIG-welded under an Ar seal,
A heat treatment corresponding to an intermetallic compound formation heat treatment of 200 h was applied. Table 1 also shows the dissolved oxygen content of the molten steel when Ca and / or Mg was added during the smelting of these steels. Table 2 shows the mechanical properties at 4K of these thin steel plate welds.

No. Nos. 1 to 6 are steels of the present invention, and show that the 0.2% proof stress and the Charpy impact value of the weld at 4K are high. As a result of examining the precipitates, it was confirmed that Cr carbonitrides with fine Ca or Mg oxide nuclei were precipitated in the crystal grains, and that precipitation of Cr carbonitrides at the grain boundaries was suppressed. Was done. No. In No. 7, since the amount of dissolved oxygen at the time of adding Ca was too large, fine CaO was recognized, but a large amount of coarse CaO was precipitated. Therefore, 0.2
% Proof stress was high, but the Charpy impact value was low.
No. 8 has too little dissolved oxygen when Ca is added,
Almost no CaO was found in the crystal grains, and Cr carbonitride was precipitated at the crystal grain boundaries. No. In No. 9, MgO was hardly found in the crystal grains due to the small amount of added Mg, and Cr carbonitride was precipitated at the crystal grain boundaries. No. In No. 10, since neither Ca nor Mg was added, most of the Cr carbonitrides were precipitated at the crystal grain boundaries. As a result, no. Steels 8 to 10 all had a high 0.2% proof stress, but exhibited low Charpy impact values.

[0023]

[Table 1]

[0024]

[Table 2]

[0025]

As described above, according to the present invention, a stainless steel for supporting an intermetallic compound superconducting material excellent in cryogenic characteristics of a welded part by suppressing deterioration of characteristics due to heat treatment for producing a superconducting intermetallic compound. It has become possible to provide a steel sheet and its manufacturing method. Therefore, it can be said that the present invention is a very valuable invention in using superconductivity in industry.

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Hideo Nakajima 1 801 Mukaiyama, Nakamachi, Naka-gun, Ibaraki Pref.

Claims (3)

[Claims] C .: 0.05% or less by weight, C: S
i: 2% or less, Mn: 0.1% or more and 16% or less, Cr:
10% or more and 24% or less, Ni: 8% or more and 25% or less, N
: 0.1% or more and 0.35% or less, and Ca: 0.0
003% or more and 0.01% or less and one or two kinds of Mg: 0.0003% or more and 0.01% or less, with the balance being Fe
And unavoidable impurities, a single phase of austenite,
A stainless steel sheet for supporting an intermetallic compound superconducting material excellent in cryogenic characteristics of a weld portion, wherein the maximum diameter of all oxides containing one or both of precipitated Ca and Mg is 1 μm or less. 2. In% by weight, C: 0.05% or less, Si:
2% or less, Mn: 0.1% or more and 16% or less, Cr: 10
% To 24%, Ni: 8% to 25%, N:
0.1% or more and 0.35% or less, and Ca: 0.000
One or two of 3% or more and 0.01% or less and Mg: 0.0003% or more and 0.01% or less, and Mo: 4%
And Nb: one or two of 0.005% or more and 0.6% or less, the balance being Fe and unavoidable impurities, an austenitic single phase, and containing one or both of precipitated Ca and Mg A stainless steel sheet for supporting an intermetallic compound superconducting material having excellent cryogenic characteristics of a weld, wherein the maximum diameter of all oxides is 1 μm or less. 3. In the step of performing melting, casting, hot rolling, cold rolling and heat treatment to form a thin steel sheet, after adjusting the dissolved oxygen content of the stainless steel to 20 ppm or more and 100 ppm or less, one or both of Ca and Mg are used. The method for producing a stainless steel sheet for supporting an intermetallic compound superconducting material having excellent cryogenic characteristics of a welded portion according to claim 1 or 2, wherein the amount of addition is adjusted to the above-mentioned amount.