JPH0693382A - Ni-Cr austenitic stainless steel with excellent creep rupture ductility - Google Patents

Abstract

(57) [Summary] [Purpose] To improve the creep rupture ductility of austenitic stainless steels used for the purpose of extending the life of various high temperature equipment. [Structure] The decrease in creep rupture ductility during high temperature use is due to grain boundary embrittlement due to the precipitation of carbides at the grain boundaries. Therefore, the first requirement is to reduce the amount of C in steel to improve the creep rupture ductility, but at the current industrial level, the solubility of C at the operating temperature of high-temperature equipment (several ppm) or less is achieved. Is extremely difficult. Therefore, it has been found that by introducing δ-ferrite, which is a preferential precipitation site of carbide, C is absorbed in the δ-ferrite part during high temperature use, and as a result, carbide precipitation at grain boundaries can be suppressed. The present invention is a Ni-C in which the strengthening element is changed from precipitation type C to solid solution type N.
Provided is a steel material having improved creep rupture characteristics by allowing a suitable amount of δ ferrite to be present in r-austenitic stainless steel.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Ni-Cr austenitic stainless steel having excellent creep rupture ductility.

[0002]

2. Description of the Related Art In high temperature structures used in the creep region of various high temperature devices such as fast breeder reactors, the creep characteristics of materials are important. As such a high temperature structural material, it is necessary that the material is stable even when used at high temperature for a long time. Therefore, as a material for such a high temperature structure, for example, the Stainless Steel Handbook (Showa 48)
As described in “2.5.7 Austenitic Stainless Steel” on page 173, issued on Aug. 30, 2013, austenitic stainless steel has been mainly used so far. However, for example, SUS304 steel or SUS316 which is a typical austenitic stainless steel.
In steel, carbide precipitates at grain boundaries during high temperature use,
Changes in materials such as deterioration of creep rupture ductility and creep rupture strength are unavoidable. Deterioration of the material due to use at such a high temperature is a factor that limits the life of the high temperature structure.

[0003]

As described above, the conventional steel is
Creep rupture ductility and creep rupture strength tend to decrease during long-term use at high temperatures. An object of the present invention is to solve such conventional problems, that is, to provide an austenitic stainless steel exhibiting excellent creep rupture ductility and creep rupture strength even when used at high temperature for a long time.

[0004]

The cause of deterioration of the above-mentioned creep rupture ductility and strength is, for example, C existing in steel.
Is associated with precipitation and coarsening as carbides at grain boundaries and in grains during use at high temperature. That is, it is known that carbides precipitated at grain boundaries cause grain boundary embrittlement, which leads to a decrease in ductility or deterioration of creep rupture strengthening. Therefore, the amount of C is reduced to control the precipitation of carbides during high temperature use. However, there is a limit to the reduction of C in stainless steel, and although the amount of C has been greatly reduced by the development of vacuum refining technology such as VOD, for example, the C at 600 ° C., which is said to be several ppm.
It is difficult to reduce the solubility to below. It should be noted that although it is possible to make the carbon content extremely low by means such as EB melting, it is not practically applicable because it requires a significantly high cost.

The present inventors have found that the C content is 0.03% or less and low C content.
By observing the microstructure of the creep rupture test material of the Ni-Cr austenitic steel of the system, as shown in FIG. 1, when a small amount of δ ferrite is present, the precipitation amount of carbides at the grain boundaries is small. I found it. That is, in a sample in which a material containing 1.6% of δ ferrite was subjected to a creep rupture test at 550 ° C. and ruptured in 6520 hours, carbide was mainly precipitated in δ ferrite, and precipitation of carbide at grain boundaries was slight. (See FIG. 1A). On the other hand, a similar test was conducted on a material containing no δ ferrite, and a considerable amount of carbide was precipitated at the grain boundaries in the sample fractured at 6981 hours (see FIG. 1 (b)). It is considered that this is because carbide is preferentially precipitated in δ ferrite having a high concentration of Cr or Mo which is a carbide forming element, and as a result, carbide precipitation at grain boundaries is reduced. Therefore, it has become possible to obtain a completely new finding that the precipitation of carbides at grain boundaries during high temperature use can be controlled by leaving an appropriate amount of δ ferrite in the steel.

The present invention has been made on the basis of the above findings, and the gist of the present invention is C in% by weight.
0.030% or less, Si 3.0% or less, Mn 3.0% or less, P0.01 to 0.08%, Al 0.06% or less, N
0.15% or less, Ni 6 to 16%, Cr 15 to 22% are contained, and δ ferrite is 0.2 to 5.0% in volume%.
Content, or in addition to each of the above elements, M by weight%
A Ni-Cr austenitic stainless steel having an excellent creep rupture ductility, containing either or both of 3.0% or less and W5.0% or less, and the balance consisting of Fe and unavoidable impurities. The present invention will be described in detail below.

[0007]

First, in the component system of the present invention, C is also an element that impairs mechanical properties at high temperature such as creep rupture properties after long-time use at high temperature because it precipitates as carbides at grain boundaries during high-temperature use. From such a viewpoint, the amount of C was set to 0.030 or less. Next, both Si and Mn are necessary as deoxidizing agents, but if present in excess of 3.0%, hot workability is impaired, so both were made 3.0% or less. P is an element that is particularly effective from the viewpoint of creep rupture properties, because it precipitates as a phosphite in the crystal grains during high temperature retention and has a strengthening action, and also has the action of strengthening the grain boundaries. Since the effect occurs from 0.01%, the lower limit was made 0.01%. However, excessive addition significantly impairs weldability and hot workability, so the upper limit was made 0.08%.

Ni is an essential element as an austenite-forming element. Therefore, 6% or more is required, but 16% is required.
The addition of more than 16% makes it impossible to introduce δ-ferrite in relation to the range of Cr content described later, so the upper limit was made 16%.
Cr is an element that improves the oxidation resistance, and for that purpose 15% or more is required, but if it exceeds 22%, embrittlement occurs due to heating at high temperature for a long time, so the upper limit was made 22%. Al is an element used for deoxidation, but 0.06%
If it exceeds, the precipitation of AlN is caused during heating at a high temperature for a long time, and the ductility is reduced. N, together with C, is a strengthening element for austenitic stainless steel. Since N has a higher solubility than C,
It can exist stably as a solid solution while it is kept at high temperature. Therefore, if N is used within the range of solubility, a stable strengthening effect can be expected even during long-term use at high temperature, and grain boundary embrittlement due to nitride does not occur. From the viewpoint of a structural material, the upper limit of the amount of N is set to 0.15%. The reason for not providing the lower limit is to control the strength by the amount of N according to the application, but the level of 0.01% in ordinary industrial-scale melting is the lower limit if strongly stated.

As described above, δ-ferrite serves as a preferential precipitation site of carbides, and as a result, is a structural factor that controls the precipitation of carbides at grain boundaries. FIG. 2 shows the creep rupture properties of Ni--Cr austenitic stainless steel containing 0.01% C and 0.07% N.
This shows the effect of the amount of ferrite. That is, δ
Creep rupture properties (elongation) as the amount of ferrite increases
It can be seen that the improvement of elongation is saturated at about 5%. On the other hand, the creep rupture strength slightly decreases with an increase in the amount of δ ferrite, but this decreasing tendency becomes remarkable when it exceeds 5%. Thus, δ-ferrite is effective in improving creep rupture ductility by controlling the grain boundary precipitation of carbides, but 0.2% or more is required to obtain this effect, so the lower limit is set to 0.2%. did. On the other hand, the effect of improving the creep rupture ductility by δ-ferrite is saturated at about 5%, and the presence of more than this causes a large decrease in creep rupture strength because of the small creep resistance of δ-ferrite, so 5% was made the upper limit. The amount of δ ferrite is N
It is controlled by adjusting chemical components such as i and Cr and heating and rolling conditions.

The above is the basic component system and the metallographic conditions in the present invention. In the present invention, it is effective to contain Mo and / or W in a predetermined range in order to further increase the strength. Mo is an element having a solid solution strengthening action and an element that enhances creep rupture strength.
If added in excess of 3.0%, the hot deformation resistance increases and rolling or forging becomes difficult. Therefore, the content is set to 3.0% or less. W is also a solid solution strengthening element similar to Mo, but if added in excess of 5.0%, hot deformation resistance increases and rolling or forging becomes difficult, so the content was made 5.0% or less. .

The steel of the present invention having the above-mentioned composition of components is
After steel is manufactured in various electric furnaces, etc., it is used as a steel ingot of various shapes by ordinary ingot making or continuous casting to make a steel ingot or a billet, and then rolling or forging. The effects of the present invention will be described more specifically below based on examples.

[0012]

EXAMPLES Table 1 shows the chemical composition of the steel of the present invention and the comparative steel.
From these samples, diameter of parallel part: 6 mm, distance between voting points: 30
mm tensile test pieces and creep rupture test pieces were taken and
Tested at 50 ° C. Table 2 shows the results of these tests, namely the tensile properties at 550 ° C. and the creep rupture ductility at 550 ° C.-1000 hours and 5000 hours. As is clear from the results of these characteristic investigations, the steel of the present invention is superior to the comparative steel in creep rupture ductility after long-term use at high temperature. On the other hand, in the comparative steel,
For example, Alloy No. 9 has a large amount of δ ferrite and thus has a high amount of N, but has a low creep rupture strength, and Alloy No. 10 has a large amount of C, and thus has a low creep rupture ductility.
Alloy No. 11 has low creep rupture ductility because it does not contain δ ferrite.

[0013]

[Table 1]

[0014]

[Table 2]

[0015]

As described above, the steel of the present invention is a material having high temperature characteristics such as creep rupture ductility even after being used for a long time at high temperature due to the introduction of δ ferrite. It is extremely effective industrially as a material.

[Brief description of drawings]

1A and 1B are metallographic diagrams showing the influence of δ-ferrite on the precipitation of carbides at grain boundaries during high temperature creep.

FIG. 2 is a diagram showing the influence of the amount of δ ferrite on creep rupture properties.

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[Procedure amendment]

[Submission date] August 31, 1993

[Procedure Amendment 1]

[Document name to be corrected] Drawing

[Name of item to be corrected] Figure 1

[Correction method] Change

[Correction content]

[Figure 1]

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Hajime Komatsu Hajime Komatsu 1618 Ida, Nakahara-ku, Kawasaki, Kanagawa Nippon Steel Corporation Advanced Technology Research Laboratories (72) Masanori Tashita Wadazaki, Hyogo-ku, Kobe, Hyogo Prefecture 1-1-1 Machi Mitsubishi Heavy Industries, Ltd. Kobe Shipyard (72) Inventor Takashi Nishida 2-1-1 Niihama, Niihama, Arai-cho, Takasago-shi, Hyogo Prefecture Mitsubishi Heavy Industries Ltd. Takasago Laboratory (72) Inventor Keizo Okada Kobe Hyogo Prefecture 1-1 1-1 Wadasaki-cho, Hyogo-ku, Yokohama Mitsubishi Heavy Industries, Ltd. Kobe Shipyard

Claims (2)

[Claims] 1. By weight%, C: 0.030% or less, Si: 3.0% or less, Mn: 3.0% or less, P: 0.01 to 0.08%, Al: 0.06% Hereinafter, N: 0.15% or less, Ni: 6 to 16%, Cr: 15 to 22% are contained, the balance is substantially Fe, and δ ferrite is 0.2 to 5.0% by volume. % Ni-Cr austenitic stainless steel with excellent creep rupture ductility. 2. By weight%, C: 0.030% or less, Si: 3.0% or less, Mn: 3.0% or less, P: 0.01 to 0.08%, Al: 0.06% Hereinafter, N: 0.15% or less, Ni: 6 to 16%, Cr: 15 to 22% are contained, and further, Mo: 3.0% or less, W: 5.0% or less, or both are contained. However, Ni-Cr austenitic stainless steel having excellent creep rupture ductility, the balance being substantially Fe, and containing 0.2 to 5.0% by volume of δ ferrite.