JPS5930786B2 - Austenitic stainless steel for LNG piping - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an austenitic stainless steel for liquefied natural gas (LNG) piping, and particularly proposes a stainless steel with excellent fatigue strength and toughness at low temperatures.

As efforts are being made to prevent air pollution and diversify energy sources, LNG is attracting attention as a clean energy source, and its import volume is increasing every year.

There are many plans to increase imports in the future, and construction of tankers to transport LNG and storage tanks to receive it continues. In addition, in connection with this, LNG is 1162
Due to the low temperature of ℃, a large amount of cold energy can be obtained for actual gasification and use, so various refrigeration plants are being developed. Incidentally, since piping accounts for a large proportion of these fields, the selection of piping materials and piping construction methods are extremely important from the standpoint of equipment functionality and safety. The special requirements for this piping material are: (1) It must not become brittle even when kept at low temperatures and maintain high toughness (2) It must be able to resist thermal expansion and contraction caused by external mechanical vibrations and temperature fluctuations. (3) Excellent machining bending and weldability. Conventionally, 9% Ni steel, 36% Ni-Fe upper alloy, At alloy, austenitic stainless steel, etc. have been used for the purpose of the present invention, but among them, austenitic stainless steel is difficult to obtain as a material of stable quality. It is most commonly used due to its advantages such as being easy to use, relatively easy to work with, and relatively easy to weld.

Stainless steels for piping that have been used in Ibara include SUS304, 304L, 316, 316L, and some 3105. However, the following problems have been pointed out to austenitic stainless steel as well.

(1) Various measures have been taken in piping construction to absorb the thermal strain caused by the loading and unloading of LNG.
This is not necessarily sufficient as a measure to prevent thermal fatigue failure. Therefore, in order to increase the reliability of piping materials, it is necessary to select the appropriate material, but there has been little consideration from the material aspect.
(With the exception of expensive materials such as 21310S, several percent of δ ferrite remains in the deposited metal of the welded part, but in that case, the higher the N content, including the N mixed in during welding, the lower the low-temperature toughness. Regarding (1), there is a demand for the development of a material that has less price increase and has better thermal fatigue resistance than the materials currently used, in order to increase reliability in terms of materials.

In addition, regarding (2), it has conventionally been considered that it is unavoidable to some extent from the viewpoint of preventing hot cracking during welding work.
In the application of the present invention, there are few flange joints and most of the joints are welded joints, so it is necessary to improve the reliability of the welded part from the material standpoint. The development of materials was desired. In this case, welding materials of the same type of steel as the pipes are generally used to weld the pipes together, but in many cases the required properties cannot be met by simply selecting the welding materials, so it is necessary to improve the material of the pipes themselves. It has become. In other words, it has been desired to develop an austenitic stainless steel for LNG piping that is relatively inexpensive and has the above-mentioned customization requirements. Therefore, the present inventors have conducted various experiments and continued research in response to the above-mentioned development requirements.

As for thermal fatigue, we examined a custom-made low cycle fatigue at -162℃ and found that the fracture life has a strong relationship with the stability of the austenite phase and low-temperature strength, and the more stable the austenite phase and the higher the strength, the longer the fracture life will be. It became clear that it was the end of its lifespan. In this case, it has been found that adding an appropriate amount of N is effective in increasing austenite stability and increasing low-temperature strength. In addition, by reducing the amount of delta ferrite that forms in the weld metal to about 1.0% or less, the negative effect of N on the low-temperature toughness of the weld is significantly improved, and by further lowering the impurity elements P and S, austenite It was found that the welding hot cracking that tends to occur in stainless steels can also be improved. After integrating these findings and conducting performance tests on the actually designed steel, we found that LN
It has become clear that this steel has excellent properties as a steel for G piping. That is, in the present invention, C<0.03%, Si≦1
．． 0%, Mn≦5%, P≦0.03%, S≦0.005
%, Ni: 8-15%, Cr: 16-20%, N: 0
．． Exceeding 0.05% and 0.20% or less, consisting of other elements unavoidably mixed in during manufacturing and the remainder Fe, in which case, the A value of the austenite stability index = Ni% + 0.7 x Cr% +
0.6XMn%+0.5XSi%+30X(C%10N%
) is 27 or more, and B of the index that defines the structure of the welded part
Value = Cr% + 1.5XSi% - Ni% - 0.5XMn%
The gist of the present invention is an austenitic stainless steel for LNG piping, which is characterized in that its composition is adjusted so that -30X (C% and N%) is 6 or less.

Hereinafter, the reason why the constituent components, austenite stability, and structure of the welded part of the steel of the present invention are limited as described above will be explained.

C: Being an austenite-forming element, it is effective in increasing austenite stability, but when welded, it precipitates as carbides and causes intergranular corrosion and deterioration of low-temperature toughness.
In order to prevent these problems, the content was set at 0.03% or less.

Si: It is necessary to add an appropriate amount as a deoxidizing agent and also from the viewpoint of welding workability.

The amount is limited to 1.0% or less, which is included in ordinary austenitic stainless steel. Mn: Mn is an element that stabilizes the austenite phase and is effective in preventing hot cracking during welding, so it can be actively used as a substitute element for Ni.

If added in large amounts, productivity will be significantly impaired, so the upper limit should be set at 5.
It was set to 0%. P, S: Elements that adversely affect low-temperature toughness and fatigue strength, and are directly responsible for hot welding cracks, so they must be kept as low as possible.

The range in which these characteristics can be obtained is 0,0 for P.
3% or less, and S was specified to be 0.005% or less.
Ni: This is the most important element for stabilizing the austenite phase and improving fatigue properties at low temperatures, especially low cycle fatigue properties.

It is also necessary to maintain high low-temperature toughness. For this purpose, 8% or more is required. However, from the point of view of economy, the upper limit was set at 15%. Cr: Most important for maintaining corrosion resistance, 16% or more is required for use in the present invention.

However, if it is too large, it will be necessary to add a large amount of Ni in terms of component balance, and because it is not necessary to use that much Ni, it is limited to 16 to 20%. N: Effective in increasing the stability of austenite and improving low temperature tensile strength and low cycle fatigue customization.

To obtain these results, an additive surface of 0.05% or more is required. Adding a large amount significantly reduces manufacturability, especially hot workability, and also reduces low-temperature toughness. The appropriate range of N is defined as more than 0.05% and less than 0.20%. A value: For low cycle fatigue special orders which are important in the application of the present invention, it is necessary to make the austenite phase more stable than SOS3O4.

The A value is used as an indicator of stability, and we have investigated the relationship between this value and low cycle fatigue life and found that adjusting the ingredients so that the A value is 27 or higher can significantly extend fatigue life. Ta. Therefore, it was defined as 27 or above. B value: In a steel to which N is added, such as the steel of the present invention, if more than about 1% delta ferrite phase is generated in the deposited metal during welding, low temperature tough pouring will be significantly reduced.

To improve this, it is necessary to specify the B value to 6 or less and control the components. The austenitic stainless steel of the present invention described above can be easily manufactured on a normal stainless steel manufacturing line.

Moreover, there is no particular problem in making pipes. Note that the steel of the present invention can also be used as a welding material. Next, specific examples of the present invention will be described.

Table 1 shows the steels of the present invention (a) to (h) produced by the usual method.
) and comparative steels (1 to n) are shown, and Table 2 shows the mechanical properties of those cold-rolled steel sheets. Comparative steel j had a high N content of 0.25%, so edge breakage occurred frequently during hot rolling, and the hot working envelope was poor. As is clear from the mechanical properties in Table 2, the steel of the present invention has high strength and ductility even at -162°C, which is not as good as conventional steel.

Table 3 shows the results of a low cycle fatigue test conducted under strain control at -162°C and the amount of martensitic phase at the fractured part. From the table, it can be seen that all specimens with an A value of 27 or more have a long fatigue life. Furthermore, since both have high austenite stability, the amount of martensite phase produced is small. Comparison steels K and N also have long fatigue lives, but K has the drawback that the welded part has a significantly low ligament as described later, and N has a high Ni content, making it expensive and prone to weld hot cracking. . Table 4 shows the base material and TIG at -162℃.
Charpy impact value and delta ferrite amount of deposited metal in weld zone are shown.

From the table, it can be seen that all the steels of the present invention having a B value of 6 or less have high low-temperature toughness in both the base metal and the weld metal.
The comparative steels also have considerably high toughness, except for k. The comparative steel K has a 2.7% teltar ferrite content and 0.12% N, so its low-temperature toughness is extremely low (G). In addition, comparative steel l (SUS3O4), m(
SOS3Ol) has a high B value and contains a few percent delta ferrite phase, but has a relatively high toughness due to the low N content. Table 5 shows the results of a weld hot cracking test conducted using the arc strike test method using TIG welding.

Claims (1)

[Claims] 1 C: 0.03% or less, Si: 1.0% by weight.
0% or less, Mn: 5% or less, P: 0.03% or less, S:
0.005% or less, Ni: 8-15%, Cr: 16-2
0%, N: more than 0.05% and less than 0.20%, consisting of other elements unavoidably mixed during manufacturing and the remainder Fe, in which case, A value of austenite stability index = Ni% +
0.7×Cr%+0.6×Mn%+0.5×Si%+3
0 x (C% + N%) is 27 or more, and the B value of the index that defines the structure of the weld = Cr% + 1.5 x Si% - Ni% -
An austenitic stainless steel for LNG piping, the composition of which is adjusted so that 0.5×Mn%−30×(C%+N%) is 6 or less.