JPH0243817B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a non-magnetic steel with high strength and high corrosion resistance for cryogenic applications. In recent years, there have been remarkable advances in technologies for utilizing various fuel gases such as liquefied natural gas and hydrogen, and in technologies for utilizing cryogenic temperatures such as superconductivity. The implementation of these technologies requires cryogenic structural materials. Traditionally, JIS standard materials have been used as structural materials for cryogenic temperatures.
Austenitic stainless steels such as SUS304 or 316 and aluminum alloys are used.
Recently, high manganese nonmagnetic steel has also been attracting attention as a structural material for low temperatures. However, conventional austenitic stainless steels and aluminum alloys, such as the above-mentioned SUS304 and 316, have a drawback of low strength, especially low 0.2% proof stress. Furthermore, the austenite in conventional austenitic stainless steels is generally unstable, and there is a high possibility that martensitic transformation will occur during use and the stainless steel will become ferromagnetic, which is inconvenient for equipment used in high magnetic field environments such as superconducting electromagnets. Furthermore, since martensitic transformation is accompanied by volumetric expansion, it may cause excessive deformation of the structure, which is undesirable. On the other hand, 0.2%
A high manganese non-magnetic steel with high yield strength and extremely stable austenite at extremely low temperatures was published in 1986-53428.
However, corrosion resistance and rust resistance are low, and rust may be generated during use, which may cause problems in equipment used in high magnetic fields such as superconducting electromagnets. The purpose of the present invention is to reduce the 0.2
This gas has the advantages of both, and has an extremely low liquefaction temperature, compensating for the disadvantages of low % yield strength and unstable austenite, and also compensating for the disadvantages of high manganese nonmagnetic steel, such as low corrosion resistance and rust resistance. To provide structural high-strength, high-corrosion-resistant nonmagnetic steel for use in storage, refining, transportation, and utilization equipment. Therefore, the structure of the present invention has C:
0.08% or less, Si: 2.0% or less, Mn: 3.0 to 18.0%,
Ni: 5.0~15.0%, Cr: 8.0~14.0%, Mo: 2.0~
7.0%, N: 0.30% or less, balance Fe and unavoidable impurities, austenite is extremely stable at cryogenic temperatures up to 4゜, and is a high-strength, highly corrosion-resistant nonmagnetic steel with excellent corrosion resistance, strength, toughness, and ductility. , and the above composition as the basic component, with Nb: 1.0
% or less, V: 1.0% or less, Ti: 1.0% or less, W: 1.0
% or less, Ta: Contains one or more of 1.0% or less, and if two or more are contained, the sum of them is
It is a high-strength, high-corrosion-resistant non-magnetic steel for cryogenic use with a content not exceeding 1.0%. Next, the reason why the components are limited as described above in the present invention will be explained. C: 0.08% or less; C is a strong austenite-forming element and at the same time has a significant strengthening effect, but in the steel of the present invention, C is 0.08% or less.
If the content exceeds 0.08%, grain boundary carbides will be formed due to the effects of welding heat and the toughness will decrease.
The following was made. Si: 2.0% or less; Si is a necessary element for melting and refining as a deoxidizing element, but if the content exceeds 2.0% in the steel of the present invention, it promotes the generation of delta ferrite and sigma phase and impairs toughness. The following was made. Mn: 3.0-18.0%; Mn is an austenite-forming element and at the same time stabilizes austenite. It also significantly increases the solubility of N, which is one of the important strengthening elements in the steel of the present invention. However, in the steel of the present invention, if the content exceeds 18.0%, it will lead to the precipitation of a brittle phase, which is not preferable.
On the other hand, if the content is less than 3.0%, the above effects will be extremely small. Therefore, the range was set to 3.0 to 18.0%. Ni: 5.0-15.0%; Ni is important as an austenite-forming element and at the same time as an austenite-stabilizing element.
Ni is expensive, and even if it is contained in an amount of 15% or more in the steel of the present invention, its effect is small. Further, the effect is small even at a content of less than 5.0%. Therefore, the range was set to 5.0 to 15.0%. Cr: 8.0-14.0%; Cr is an important element that improves corrosion resistance and rust resistance. The corrosiveness of the environment in which the steel of the present invention is used is relatively mild, and excessive Cr content causes the generation of delta ferrite, so it is not preferable to contain more than 14%, while a content of less than 8% will prevent corrosion. ，
Rust resistance becomes insufficient. Therefore, the range is 8.0 ~
It was set at 14.0%. Mo: 2.0 to 7.0% Mo is an important strengthening element along with N in the steel of the present invention. At the same time, it significantly stabilizes austenite. However, if the content exceeds 7.0%, a brittle phase will appear and the toughness will be impaired. On the other hand, if the content is less than 2.0%, the above effect is extremely small. Therefore, the range was set to 2.0 to 7.0%. N: 0.30% or less; Like C, it is a strong austenite-forming element, and at the same time has a significant strengthening effect. Moreover, compared to C, grain boundary precipitation has less harmful effect on toughness. However, excessive content deteriorates low-temperature toughness, so the upper limit was set at 0.30% in the steel of the present invention. Nb, V, Ti, W, Ta: 1.0% or less; All have strong affinity with C and N, disperse and precipitate carbonitrides in steel, and increase strength. However, excessive content deteriorates low-temperature toughness, so the upper limit was set at 1.0%, and even in the case of combined addition, the sum of the contents was kept below 1.0%. Next, the effects of the present invention will be explained by comparing examples of the present invention with conventional austenitic stainless steel. Table 1 shows examples of the present invention and conventional austenitic stainless steel SUS304 and 316 steel (comparative steel).
Comparison of chemical composition with Also, in Table 2,
The mechanical properties and magnetic permeability of each steel shown in the table are compared at the liquid nitrogen temperature (-196°C) and in part at the liquid helium temperature (-269°C).

【table】

【table】

[Table] Magnetic permeability is measured on the fracture surface of a tensile test piece.
It gives an indication of the stability of austenite against martensitic transformation. As is clear from Table 2, each of the steels of the present invention has a significantly higher 0.2% yield strength than the comparative steels. Moreover, although the steel of the present invention has such an extremely high 0.2% proof stress, its elongation and reduction of area are at approximately the same level as the comparative steel. In addition, the Charpy absorbed energy is at a slightly lower temperature than that of the comparative steel, and shows a sufficient value considering the intended use. Furthermore, the magnetic permeability at the fracture surface of a tensile test specimen is at a lower level in the inventive steel than in the comparative steel, and this indicates that the austenite in the inventive steel is more stable than that in the comparative steel, and is not susceptible to stress or strain during use. It can be seen that the possibility of becoming ferromagnetic is small. Furthermore, since the steel of the present invention has an austenitic structure containing a considerable amount of Ni and Cr as a solid solution, it also has excellent corrosion resistance and rust resistance. Table 3 shows the results of the corrosion test of the steel of the present invention in seawater at 80°C.
Mn-7%Cr steel, 18%Mn-5%Cr steel) and 9
%Ni steel. As can be seen, the corrosion resistance of the steel of the present invention is almost the same as, or slightly better than, that of the austenitic stainless steel SUS304. As described above, the present invention has made it possible to provide a nonmagnetic steel in which austenite is extremely stable at extremely low temperatures and has excellent corrosion resistance, rust resistance, strength, particularly 0.2% proof stress, toughness, and ductility.

【table】

Claims (1)

[Claims] 1. C: 0.08% or less, Si: 2.0% or less, on a weight basis
Mn: 3.0~18.0%, Ni: 5.0~15.0%, Cr: 8.0~
14.0%, Mo: 2.0-7.0%, N: 0.30% or less, balance
A high-strength, highly corrosion-resistant nonmagnetic steel for cryogenic use, consisting of Fe and unavoidable impurities, with extremely stable austenite at cryogenic temperatures of up to 4 〓, and characterized by excellent corrosion resistance, strength, toughness, and ductility. 2 C: 0.08% or less, Si: 2.0% or less, on a weight basis
Mn: 3.0~18.0%, Ni: 5.0~15.0%, Gr: 8.0~
Contains 14.0%, Mo: 2.0 to 7.0%, N: 0.30% or less, further Nb: 1.0% or less, V: 1.0% or less,
Contains one or more of Ti: 1.0% or less, W: 1.0% or less, Ta: 1.0% or less, and if two or more are contained, the sum of these is 1.0% or less, and the remainder is
A high-strength, highly corrosion-resistant nonmagnetic steel for cryogenic use, consisting of Fe and unavoidable impurities, with extremely stable austenite at cryogenic temperatures of up to 4 〓, and characterized by excellent corrosion resistance, strength, toughness, and ductility.