JPS645104B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to austenitic stainless steel for cryogenic structures, and in particular has excellent yield strength and toughness for use at cryogenic temperatures ranging from liquid helium temperature (4°K) to LNG temperature (111°K). The invention relates to stable austenitic stainless steels. (Prior art) The demand for materials used at cryogenic temperatures is increasing due to the demand for materials used at extremely low temperatures, such as LNG tanks, piping, containers for rockets that use liquid hydrogen as fuel, and holding 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:
Firstly, from a safety point of view, it must not cause brittle fracture at the operating temperature, and secondly, it must have high strength, especially high yield strength, and furthermore, when used as a material for superconducting magnets,
One example is that it is 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 increase in strength is N
The larger the amount, the larger the size, and the lower the temperature, the larger the size, but it is said that the addition of N has the disadvantage of deteriorating the toughness at low temperatures, so those with a N content of at most 0.20% or less are suitable for cryogenic use, such as SUS 304LN, SUS
It has only been put into practical use under common names such as 316LN. However, with this amount of N added, 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 potential candidates for cryogenic materials. I'm getting used to it. Therefore, 77°K
High yield strength of over 700Mpa (megapascals) and energy absorption value in V-notch sial py test
There is a strong desire to develop a stable austenitic stainless steel that has a high toughness of 120 J or more and is completely non-magnetic. Here, Figure 1 shows C: 0.02%, Si: 0.8%,
N content and 0.2% in austenitic stainless steel with components of Mn: 5%, Cr: 22%, and Ni 13%
This shows the relationship with yield strength. As is clear from the figure, in order to obtain a yield strength of 700 MPa or more at 77°K, it is necessary to add at least 0.20% of N. By further increasing N, the low-temperature yield strength further increases, but there is a limit to the solubility of N. In austenitic stainless steel, when the amount of Cr is 20%, the solid solubility limit of N is 0.2%, 25 % is about 0.3%. Therefore
In order to obtain a high nitrogen stainless steel with a yield strength of 700 MPa or more at 77°K, 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 strength required for cryogenic structural materials can be ensured. It has been considered difficult to put it into practical use because of the (Object of the Invention) The object of the present invention is to provide a stable austenitic stainless steel that has excellent yield strength and toughness at extremely low temperatures and is completely 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 second phases such as inclusions, precipitates, δ ferrite, and martensite. I found it. In other words, when N exists in steel in a solid solution state, there is no deterioration of low temperature toughness and N
The reason why low-temperature toughness is said to deteriorate in stainless steels with additives is that (1) when N combines with other elements and precipitates,
(2) When there are many nonmetallic inclusions; (3) When austenite is not completely stable and δ-ferrite or martensite is formed; It turned out that this was due to the misunderstanding that it was caused by 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 this, it is necessary to control the amount of non-metallic inclusions.
Regarding condition (3), not only does δ-ferrite not exist in the first place, but also complete austenitic stability is achieved, such that deformation-induced martensitic transformation does not occur even if extremely severe processing is applied at extremely low operating temperatures. is required. Such a material is also completely non-magnetic, making it an extremely advantageous material for applications such as superconducting magnets. Furthermore, the present inventors succeeded in further increasing the solubility of N by adding Cr and Mn in combination. Figure 2 shows C: 0.02%, Si: 0.8%, Ni: 13
This figure shows the influence of Cr and Mn on the solubility of N in austenitic stainless steel with a composition of It was found that both Cr and Mn contribute to increasing the solubility of N, and that the solubility of nitrogen can be expressed by the following empirical formula: N = 0.021 (Cr + 0.9Mn) - 0.204 (1). Therefore, in order to increase the solubility of N to 0.20% or more, Cr
+0.9 All you have to do is design the alloy so that Mn is 19.2% or more, preferably 20% or more, and compared to the case where you have to control the solubility of N with Cr alone as mentioned above, the Mn and Since the solubility of N can be controlled by both Cr and N, the range of component selection can be widened.
It is also possible to add The present invention was obtained based on the above findings, and its gist is that C:
0.05% or less, N: 0.20% to 0.70%, Si: 1.0% or less,
Cr: 13% to 25%, Ni: 5% to 25%, and
A cryogenic structure characterized by containing Mn: more than 4% and not more than 25%, and Cr+0.9Mn≧20%, the remainder being substantially Fe, and the amount of nonmetallic inclusions being 0.1% or less in cleanliness. For austenitic stainless steel. 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, and should be kept at 0.05% or less. Next, N is at least 0.20 to ensure proof strength at low temperatures.
% is required. The larger the amount of N, the higher the yield strength, but even if the amounts of Mn and Cr are increased, it is difficult to contain N in a solid solution state of more than 0.70%, and even if N exists in the form of precipitates, it will not increase the low temperature yield strength. is of little use,
The upper limit of N should be set at 0.70, as it would actually deteriorate the toughness.
%. Si is a necessary element for deoxidation during steel manufacturing, but it is a ferrite stable element, and if it exceeds 1%, it becomes difficult to obtain a stable austenite structure, so it was set at 1% or less. Cr is an element that increases the solid solubility of N together with Mn, and is also an element that imparts corrosion resistance to stainless steel, so a minimum content of 13% is required.
However, Cr is a ferrite stabilizing element, and in order to maintain stable austenite, the amount of Ni or Mn must be increased in proportion to the amount of Cr, but if the amount of Ni is too large, it becomes ferromagnetic at extremely low temperatures. If the amount of Cr increases, Mn changes from an austenite-stabilizing element to a ferrite-stabilizing element, so taking these effects into consideration, the upper limit for the Cr amount is 25%. Therefore, the Cr content of the steel of the present invention was determined to be 13% to 25%. Ni is an element necessary for stabilizing austenite and is determined by the balance with the amount of Cr, but since N and Mn (when the amount of Cr is small) are also austenite stabilizing elements, general austenite that does not contain N and Mn It does not require as much as stainless steel. According to the test results of the present inventors, in order to obtain stable austenite even at low temperatures, the steel composition of the present invention requires 5% or more Ni.
If it exceeds 25%, there is a risk of ferromagnetism at extremely low temperatures, so the Ni content was set to 5% to 25%. Mn is an element that increases the solubility of N together with Cr,
In order for N to exist in a solid solution state of 0.20% or more, it is necessary to determine the amount of Mn so that Cr+0.9Mn≧20% as described above. Also, there is a limit to the solid solubility of N, which is 0.70 as mentioned in the reason for the limitation of N.
% is the upper limit for the steel of the present invention. Therefore, by substituting N=0.70% into equation (1) above, Cr
+0.9Mn≒43, and in case of Cr20%, Mn is almost 25
%. When the amount of Cr exceeds 20%, Mn acts as a ferrite stabilizing element, so adding too much Mn should be avoided from the perspective of austenite stability. Further, when Cr is 20% or less, even if Mn is added in excess of 25%, the amount of solid solution of nitrogen does not exceed 0.70%. Therefore, the upper limit of Mn is set to 25
%. On the other hand, Mn acts to prevent the formation of deformation-induced martensite that occurs when deformation is applied at low temperatures and to stabilize austenite. However, when the amount of Mn is 4% or less, this stabilizing effect is not effective, and martensite is likely to be formed, causing toughness deterioration and magnetic permeability increase.
The lower limit of Mn was set at more than 4%. 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 has 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 3 shows C: 0.02%, N: 0.35%, Si: 0.8%, Mn: 5.2
%, Cr: 21.0%, Ni: 11.5%, the relationship between the amount of non-metallic inclusions and the V-notch shear py impact absorption energy value at 77°K. 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 must be reduced by 0.1%. Below (JIS
Based on the microscopic test method for non-metallic inclusions in G055 steel)
must be kept to a minimum. In other words, the cleanliness
If it exceeds 0.1%, there will be an inconvenience that the impact absorption energy value at 77°K will not reach 120J. Therefore, the cleanliness of nonmetallic inclusions is limited to 0.1% or less. (Example) Next, the effects of the present invention will be described in more detail with reference to Examples. Table 1 shows the temperature of Cr-Ni-Mn austenitic steel at 77°K.
In the same table, A,
B and C are steels of the present invention, and have a yield strength of 0.2% at 77°K.
Excellent impact energy value and magnetic permeability μ
Shows non-magnetism of 0.02 or less. D steel, E steel, F
Steel has a content of Cr, Mn, and Ni within the range of the present invention, but has a small amount of N and has a very low yield strength. Moreover, the stability of austenite is insufficient and the magnetic permeability μ is large, making it a non-magnetic steel. I can't say that. Especially F steel is Cr
The amount of +0.9Mn is less than the range of the present invention, and the yield strength is not sufficient. Since G steel has C exceeding the range of the present invention, there are many precipitates and the impact energy value is low. Since H steel has Ni lower than the range of the present invention,
The austenite is not stabilized and the magnetic permeability μ is 1.5 or more, and the impact energy value is low because the cleanliness is poor. Since the Cr content of steel I exceeds the range of the steel of the present invention, the austenite is not stabilized and the permeability μ is large, making it non-magnetic. Steel J is within the range of the present invention in terms of chemical composition. However, the impact energy value is very poor due to poor cleanliness.

[Table] (Effects of the Invention) As is clear from the above description, the present invention provides a non-magnetic stable austenitic stainless steel with excellent yield strength and toughness at extremely low temperatures.
The industrial advantages are extremely obvious.

[Brief explanation of the drawing]

Figure 1 shows the effect of the amount of N on the yield strength of austenitic stainless steel, Figure 2 shows the effect of Cr and Mn on the solubility of nitrogen in austenitic stainless steel, and Figure 3 shows the addition of Mn. FIG. 3 is a diagram showing the relationship between the V-notched pea shock absorption energy value and nonmetallic inclusions in stable austenitic stainless steel.

Claims (1)

[Claims] 1 Weight%: C: 0.05% or less, N: 0.20% to 0.70
%, Si: 1% or less, Cr: 13% to 25%, Ni: 5%
~25%, and Mn: more than 4% and less than 25%, and Cr
+0.9Mn≧20%, the remainder is substantially Fe, and the amount of nonmetallic inclusions is at a cleanliness level.
Austenitic stainless steel for cryogenic structures characterized by 0.1% or less.