JPS644577B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to austenitic stainless steel for cryogenic structures, particularly for use at cryogenic temperatures ranging from liquid helium temperature (4〓) to LNG temperature (111〓).
This relates to a stable austenitic stainless steel with excellent strength and toughness. (Prior art) The demand for materials used at cryogenic temperatures is increasing, as is the demand for materials used at extremely low temperatures, such as LNG tanks, piping, containers for rockets that use liquid hydrogen as fuel, and structural materials for superconducting magnets that must be used at liquid helium temperatures. Coupled with conversion, demand is increasing year by year, and demand for nuclear fusion devices, linear motor cars, superconducting generators, etc. is expected to increase dramatically in the near future. The required properties of materials used at extremely low temperatures are:
First, from a safety standpoint, it must not cause brittle fracture at the operating temperature, and secondly, it must have high strength, especially high yield strength, and when used as a material for magnets such as superconductors, it must be non-magnetic. Since austenitic stainless steel maintains its ductility even down to extremely low temperatures, it has the potential to be used as a low-temperature material and has been used for several purposes. However, austenitic stainless steel has the disadvantage of low yield strength at low temperatures.
As a structural material, it cannot be said to be sufficient from the viewpoint of strength. It has long been well known that the most effective means for improving this low yield strength is the addition of N, and it has been put to practical use as nitrogen-containing austenitic stainless steel. The degree of increase in yield strength is
The larger the amount of N, the larger the amount, and the lower the temperature, the larger the amount, but it is said that the addition of N has the disadvantage of deteriorating the toughness at low temperatures, so those with an N amount of at most 0.20% or less are suitable for cryogenic use, such as SUS304LN,
It has only been put into practical use under names such as SUS316LN. However, with this level of N addition, it is not possible to obtain the high yield strength required at cryogenic temperatures, so other steel types, such as high manganese austenitic steel, have recently come into the spotlight as promising candidates for cryogenic materials. It has become like that. Therefore, it has a high yield strength of 1000Mpa (megapascal) or more at 4〓 and high toughness with an energy absorption value of 100J (joule) or more in the V-notch shear py test.
Moreover, there is a strong desire to develop stable austenitic stainless steels that are completely non-magnetic. Here, Figure 1 shows C: 0.02%, Si 0.8%,
The amount of N in austenitic stainless steel with the components of Mn: 0.5%, Cr: 25%, and Ni: 13%.
This shows the relationship with 0.2% yield strength. As is clear from the figure, in order to obtain a yield strength of 1000 MPa or more in 4〓, it is necessary to add at least 0.20% or more of N. By further increasing N, the low-temperature yield strength further increases, but there is a limit to the solubility of N, and in austenitic stainless steel, when the Cr content is 20%,
The solid solubility limit of N is 0.2%, and at 25% it is about 0.3%. Therefore, in order to obtain a high nitrogen stainless steel having a yield strength of 1000 MPa or more at 4〓, the Cr content must be 20% or more. It is a well-known fact that by adding a large amount of N in this way, the proof stress required for cryogenic structural materials can be ensured.
Addition of N causes a sharp decrease in toughness at low temperatures and makes the material brittle, so it has been considered difficult to put it into practical use. (Objective of the Invention) An object of the present invention is to provide a stable austenitic stainless steel for cryogenic structures that has high yield strength and high toughness at cryogenic temperatures and is non-magnetic. (Structure and operation of the invention) The present inventors have discovered that the decrease in toughness at low temperatures is not due to N itself, but is due to the presence of a second phase such as inclusion precipitates, δ ferrite, and martensite. Ta. In other words, when N exists in steel in a solid solution state, there is no deterioration in low temperature toughness;
The low-temperature toughness of N-added stainless steel is said to deteriorate due to (1) when N combines with other elements and precipitates, (2) when the amount of nonmetallic inclusions is large, and (3) when austenite is reduced. It was found that this was because the material was not completely stable and had one or more of the following three conditions in which δ ferrite or martensite was formed, and was mistakenly assumed to be due to solid solution N. Therefore, if N-added austenitic stainless steel has a composition that does not cause the above-mentioned (1) to (3), a material with excellent strength and yield strength can be obtained. First, in order to prevent condition (1) from occurring, it is necessary to adjust the other components and the amount of N so that N is within the solid solubility limit, and to carefully perform heat treatment. In order to control the amount of non-metallic inclusions, it is necessary to control the amount of non-metallic inclusions.Concerning condition (3), not only does δ-ferrite not exist in the first place, but it also requires fairly severe processing at extremely low operating temperatures. In addition, complete austenitic stability is required so that deformation-induced martensitic transformation does not occur. Such a material is also completely non-magnetic, making it an extremely advantageous material for applications such as superconducting magnets. The present invention was obtained based on the above findings, and its gist is that C:
0.05% or less, N: 0.20 to 0.50%, Si: 1.0% or less,
Mn: 4.0% or less, Cr: 20-35%, Ni: 8-25%
The austenitic stainless steel for cryogenic structures is characterized in that the remainder is substantially Fe, and the amount of nonmetallic inclusions is 0.1% or less in cleanliness. The present invention will be explained in detail below. First, although C is an austenite stabilizing element, it easily combines with Cr to form carbides and causes toughness deterioration, so it should be kept low at 0.05%.
The following was made. Next, N should be used at least to ensure proof strength at low temperatures.
0.20% is necessary. The larger the amount of N, the higher the yield strength, but it is difficult to contain N in a solid solution state of 0.50%, and even if N exists in the form of precipitates, it is of little use in increasing low-temperature yield strength, and instead increases the toughness. Therefore, the upper limit of N was set at 0.50%. Si is a necessary element for deoxidation during steel manufacturing, and is a ferrite stabilizing element, with a concentration of 1.0%
If it exceeds 1.0%, it becomes difficult to obtain a stable austenite structure, so it was set at 1.0% or less. Mn has the effect of increasing the solubility of N,
It is an extremely effective element when adding a large amount of N, but in steel with Cr of 20% or more, it is a ferrite stabilizing element, and when it is added in excess of 4.0%, δ
Since ferrite is easily generated and the low-temperature toughness is rapidly deteriorated, the upper limit of the content was set at 4.0%. Cr has a large relationship with the amount of solid solution of N; when the amount of Cr is 20%, the amount of solid solution of N is about 0.20%, and as Cr increases, the amount of solid solution of N also increases. however,
Cr is a ferrite stabilizing element, and in order to maintain stable austenite, the amount of Ni must be increased in proportion to the amount of Cr.
If the amount of Cr increases too much, there is a risk of exhibiting ferromagnetism at extremely low temperatures, so the amount of Cr added is limited to 35%. Therefore, the Cr content of the steel of the present invention is reduced to 20 to 35%.
It was determined that Ni is a necessary element for austenite stabilization and is determined by the balance with Cr, but since N is also an austenite stabilizing element, it does not need to be used in as large a quantity as in general stable austenitic stainless steels that do not contain N. . According to the test results of the present inventors, in order to obtain stable austenite even at low temperatures, the steel of the present invention requires 8% or more Ni, and when Ni exceeds 25%,
Due to the risk of becoming ferromagnetic, the amount of Ni should be 8 to 8.
It was set at 25%. Other elements cause the formation of inclusions and precipitates, so it is desirable to keep them as low as possible. With the above ingredients, it is possible to obtain a material that has high yield strength at cryogenic temperatures and a stable austenitic structure, but this alone has problems with toughness, and it is difficult to obtain excellent toughness values even at cryogenic temperatures. If so, it is necessary to control the amount of nonmetallic inclusions and precipitates. Figure 2 shows C: 0.02%, N: 0.35%, Si: 0.8
%, Mn: 0.5%, Cr: 25%, Ni: 13%, the relationship between the amount of nonmetallic inclusions and the V-notch shear py impact absorption energy value at 77〓 is shown. As is clear from the figure,
The amount of nonmetallic inclusions has a strong correlation with the impact absorption energy value, and in order to obtain a sufficient toughness value even at extremely low temperatures, the cleanliness of nonmetallic inclusions should be reduced to 0.1%.
Must be kept to the following (according to JIS G055 Microscopic Test Method for Nonmetallic Inclusions in Steel): In other words, when the cleanliness exceeds 0.1%, in 4〓,
The disadvantage is that the impact absorption energy value does not reach 100J. Therefore, the cleanliness of non-metallic inclusions is
It is limited to 0.1% or less. (Example) Next, Examples showing the effects of the steel of the present invention shown in Table 1 will be described in more detail. In the table, A, B, and C are the steels of the present invention, which have excellent 0.2% proof stress and shock absorption energy values at both 4〓 and 77〓 temperatures, and are nonmagnetic with a magnetic permeability μ of 1.02 or less. . Steel D is a stable austenitic steel, but it does not contain N and has a very low yield strength. E
Since the amount of N in the steel is less than the range of the present invention, the yield strength is low, and at the same time, the austenite stability is insufficient, and the magnetic permeability is 1.5 or more, so it cannot be said to be a non-magnetic steel. Steel F and steel G have a lower Cr content than the range of the present invention, lower yield strength, and also exhibit magnetism. In particular, F steel has poor cleanliness and has a very low impact absorption energy value.
H steel is within the range of the present invention in terms of chemical composition range, and has satisfactory proof strength and non-magnetic properties, but its impact absorption energy value is inferior due to its poor cleanliness. I steel has Cr, Ni, and N contents within the range of the present invention and has good cleanliness, but due to excessive Mn content, δ
Ferrite is formed, and therefore the impact absorption energy value and magnetic permeability do not have sufficient values. In steel J, since the amount of Ni is less than the range of the present invention, the austenite is not sufficiently stabilized and the magnetic permeability is high.Also, despite the good cleanliness, the amount of Mn is excessive, so the impact energy is not sufficient.

【table】

[Table] (Effects of the Invention) As is clear from the above description, according to the present invention, it is possible to provide a non-magnetic austenitic stainless steel for cryogenic structures that has excellent yield strength and toughness at cryogenic temperatures. The industrial benefits are extremely large.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing the effect of the amount of N on the yield strength of austenitic stainless steel, and Fig. 2 is a diagram showing the relationship between the V-notch shear py impact absorption energy value and the amount of nonmetallic inclusions in stable austenitic stainless steel. .

Claims (1)

[Claims] 1% by weight: C: 0.05% or less, N: 0.20-0.50%,
Si: 1.0% or less, Mn: 4.0% or less, Cr: 20-35%,
Austenitic stainless steel for cryogenic structures, containing 8 to 25% Ni, the remainder being substantially Fe, and having a cleanliness level of 0.1% or less of nonmetallic inclusions.