JPS61143562A - Austenitic ni-cr stainless steel having superior elongation at creep rupture - Google Patents

Abstract

PURPOSE:To improve the long-time elongation at creep repture by adding pre scribed percentages of C, Si, Mn, P, Ni, Cr and Mo to Fe. CONSTITUTION:The titled stainless steel consists of, by weight, <=0.015% C, <=3% Si, <=3% Mn, 0.045-0.2% P, 7-22% Ni, 14-25% Cr and the balance Fe or further contains <=3% Mo. The steel ahs superior long-time elongation at creep rupture and is used as a heat resistant structural material in a creep region.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a Ni--Cr austenitic stainless steel having excellent creep rupture ductility.

(Prior Art) In recent years, with the rise in temperature of chemical equipment and the development of fast breeder reactors, cleave deformation of materials in high-temperature structures used in creep regions cannot be ignored. Such high-temperature structural materials need to have excellent creep rupture ductility in addition to conventional high-temperature tensile strength and Talino rupture strength.

Therefore, as materials for such high-temperature structures, for example, 17 of the Stainless Steel Handbook (published August 30, 1970) is recommended.
As shown in "2.5.7 Austenitic Stainless Steel" on page 3, austenitic stainless steels have been mainly used so far. However, for example, SUS 3, which is a typical austenitic stainless steel,
The creep rupture elongation of 04 steel tends to decrease as the rupture time increases; for example, the creep rupture elongation of 04 steel tends to decrease with increasing rupture time.
% will appear. This decrease in leap rupture ductility over time becomes a factor that limits the lifespan of high-temperature structures. In addition, creep rupture ductility has a correlation with cleave fatigue properties, and from this point of view as well, creep rupture ductility is an important property.

(Problem to be solved by the invention) As described above, the creep rupture ductility of conventional steel tends to decrease over a long period of time. This is thought to be due to the embrittlement of the -10,000 is also an extremely effective element in ensuring the creep strength of these steels. Research report Mol published in 1973 by the 123rd Committee of the Japan Society for the Promotion of Science Heat-Resistant Metal Materials Study Group, 14. A 1 (D 19
According to the page, it is stated that creep rupture ductility is improved by adding KP when the amount of C is 0.04- or more. However, it is said that such an effect of P is not observed when the amount of C is low in the broadcast text. On the other hand, the present inventors also published Tetsu to Hagane Vol. 1.67 in 1981,
On page 81147 of M13, P is up to 0.036%
It is clear that the addition of up to 10% had an effect of improving creep rupture ductility, but no improvement effect of creep rupture strength was observed. However, these conventional findings suggest that the former causes a decrease in creep rupture ductility on the long-term side;
Furthermore, since the latter method does not provide sufficient leap rupture strength, neither of these methods can be a sufficient measure to solve the above-mentioned problems with high-temperature structural materials. - (Means for solving the problem) Therefore, as a result of further studies, the inventors found that 5tJ
It has the strength of S304 or SUS316,
The purpose is to develop a steel with less deterioration in creep rupture ductility on the long-term side.In order to prevent grain boundary embrittlement due to carbide precipitation, K is reduced and cleave strengthening is performed using elements that do not cause grain boundary embrittlement. It was investigated.

As a result, unlike conventional knowledge, we have come to the completely new knowledge that adding a large amount of P is effective in terms of both long-term creep rupture ductility and creep rupture strength.

(Structure and operation of the invention) The present invention has been made based on the above knowledge, and its gist is that C50,015% by weight,
Si≦l OTo, Mn≦3.0%, P
More than O,045 to 0.20%, Ni 7. O~22.0
'1k, contains Cr 140-25.0To,
Or, it is a Ni-Cr austenitic stainless steel which further contains KMe≦3.0q/b and has excellent creep rupture ductility, with the remainder being F6 and unavoidable impurities.

The present invention will be explained in detail below.

First, in the component system of the present invention, C is an effective strengthening element as described above, but it is also an element that impairs ductility because it precipitates as carbide at grain boundaries. Therefore, the present inventors conducted the following experiment to examine the influence of C on creep rupture characteristics. That is, as the test steel, Sl
O, 5%, Mn 1.0%, P 0.08%,
Steel containing 1496 Ni and 18% Cr was melted with various C ranges, and this was hot rolled into a 12 m thick steel plate. A creep rupture test piece with a parallel part diameter of 6+W and a gauge distance of 30 m was prepared. Created and JIS Z 2272 K
A creep rupture test was conducted in accordance with the following.

The results are shown in FIG. In other words, Figure 1 shows the influence of the amount of C on creep rupture strength and creep rupture ductility.As seen in the figure, the strength increases and the fracture ductility decreases with C3, but the fracture ductility due to CK increases. It can be seen that the decrease becomes significant when it exceeds 0.015%. For these reasons, the amount of C was determined to be 0.015'A or less.

Both VcSi and Mn are necessary as dispersing agents, but since their presence in excess of more than 3% impairs hot workability, the content of both was set to 3qII or less.

On the other hand, P precipitates within the crystal grains as a phosphorus compound during creep, has a cleaving strengthening effect, and does not precipitate at the grain boundaries, so that grain boundary embrittlement does not occur.

Therefore, the present inventors conducted the following experiment to examine the influence of P on creep rupture properties.

That is, the sample steel contains 0.01% C, 0.5% Si, and Mn.
1.0% Ni, 14% Ni, 18% CrO steel was melted with various po ranges to create a steel plate with a thickness of 125 gm. After that, a creep rupture test piece with a parallel part diameter of 6 m and a gauge distance of 30 cm was prepared. was prepared, and a creep rupture test was conducted in accordance with JIS Z 2272. The results are shown in FIG. That is, the second
The figure shows the influence of weight on creep rupture strength and creep rupture elongation.
It can be seen that although the creep rupture strength increases with P content, the creep rupture ductility hardly changes when the P amount exceeds 0.045 inch. Therefore, the amount of P added is 0.0451 to ensure the creep rupture strength of conventional copper plates.
It's super necessary. However, if more than 0.20% of P is added, the hot workability and weldability are significantly impaired, so the upper limit was set at 0.20'16. In addition, from the point of view of ensuring sufficient leap rupture strength, the P content range is more preferably 0.06 to 0.20%.

Furthermore, KNi is necessary as an austenite-forming element, and the amount required to blunt the austenite structure based on component equilibrium with respect to the amounts of Cr and Si, which are ferrite-forming elements, is 7.
It ranges from 0chi to 22.096. Further, Cr is an element that improves oxidation resistance, and therefore K1-114.
Luka, who needs OTo or more, 25. If it exceeds 0%, embrittlement will occur due to long-term heating at high temperatures, so the upper limit is set at 25%.
It was set to 0%.

The above is the basic component system in the present invention, but in the present invention, it is effective to contain Mo within a predetermined range in order to further increase the strength. M.

3. is an element that has a solid solution strengthening effect and increases creep strength. (Adding more than 1 liter increases hot deformation resistance, making rolling or forging difficult. Therefore, the content was set to 3.Os or less.

The steel of the present invention having the above-mentioned composition is manufactured by various types of electric furnaces, etc., and then made into steel billets by ordinary ingot-forming, blooming rolling, or continuous casting, and then rolled or forged into steel products of various shapes. It is provided for use as a.

The effects of the present invention will be described in more detail below based on Examples.

(Example) Table 1 shows the chemical composition of the invention steel and comparative steel.

Table 2 shows the creep rupture properties at 550°C for the steels shown in Table 1. As is clear from these property investigation results, the steel of the present invention is superior to the comparison steel in terms of creep elongation at break and particularly in elongation at break on the long-term side.

(Effects of the Invention) As described above, the steel of the present invention is a material that has excellent creep rupture ductility over a long period of time, and is extremely effective industrially as a material for high-temperature structures used in the creep region. be.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the influence of the amount of C on the 10,000 hour creep rupture characteristics, and FIG. 2 is a diagram showing the influence of the amount of P on the 10,000 hour creep rupture characteristics. Figure 1 C (%)

Claims (2)

[Claims] (1) C≦0.015%, Si≦3.0%, M in weight%
n≦3.0%, P over 0.045% to 0.20%, Ni7
．． Ni-Cr austenitic stainless steel with excellent creep rupture ductility, containing 0 to 22.0% of Cr, 14.0 to 25.0% of Cr, and the remainder substantially consisting of Fe. (2) C≦0.015%, Si≦3.0%, M in weight%
n≦3.0%, P over 0.045 to 0.20%, Ni7.
Contains 0-22.0%, Cr14.0-25.0%,
Further, the Ni-Cr austenitic stainless steel contains Mo≦3.0%, and the remainder is substantially Fe, which has excellent creep rupture ductility.