JPH0225545A - Ni-cr austenitic stainless steel having excellent high temperature long time stability - Google Patents

Abstract

PURPOSE:To hold the excellent creep rupture characteristics of the title steel even after the use at a high temp. for a long time by specifying the content of Ni and Co and their relationship in an Ni-Cr austenitic stainless steel. CONSTITUTION:A stainless steel having the compsn. constituted of, by weight, <=0.060% C, <=3.0% Si, <=3.0% Mn, 0.02 to 0.08% P and <=0.15% N, in which the amounts of Ni and Cr are regulated to the range of A-B-C-D in the Figure showing the relationship between the Ni equivalent and the Cr equivalent, and the balance substantial Fe. If required, either or both of <=3.0% Mo and <=5.0% W are furthermore incorporated thereto. By this compsn., excellent creep rupture strength can be obtd. even after the use at a high temp. for a long time even by saving the Ni amt.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a Ni--Cr austenitic stainless steel having excellent long-term stability at high temperatures.

(Prior Art) Recently, with the rise in temperature of chemical equipment and the development of fast breeder reactors, creep deformation of the material No. 1 cannot be ignored in high-temperature structures used in creep regions.

Such high-temperature structural materials need to be stable even when used at high temperatures for long periods of time.

Therefore, such high-temperature structures B, 4th material, and C are, for example, as shown in Stainless Steel Handbook (Published 8B30 [-], 1973), page 173 r2.5.1 Austenitic Stainless Steel J. Until now, mainly austenite-based stainless steels have been used.

However, for example, 5US304 steel or 5O531, which is a typical auste→-ite stainless steel,
In steel No. 6, carbides and intermetallic compounds precipitate during high-temperature use, and material changes such as deterioration of creep rupture strength or creep rupture ductility are unavoidable. Material deterioration due to use at such high temperatures is a factor that limits the lifespan of high-temperature structures.

(Problems to be Solved by the Invention) As described above, conventional steels exhibit low creep rupture strength and creep rupture ductility during long-term use at high temperatures. It is related to the precipitation and coarsening of carbides at grain boundaries and within grains during use at high temperatures. That is, it is known that carbides precipitated at grain boundaries cause grain boundary embrittlement, which causes a decrease in ductility or a deterioration in creep rupture strength. It is also well known that the σ and χ phases that precipitate during high-temperature use cause embrittlement and deteriorate creep rupture properties. A new cause of embrittlement was discovered through detailed microstructural observation. However, as shown in Figure 2, the G phase precipitates at the grain boundaries and a ferrite phase is formed in the vicinity, but this ferrite phase has significantly lower resistance to creep deformation than the austenite phase, so this region As a result of concentrated deformation, early fracture occurs and fracture ductility and fracture strength decrease.

The formation of ferrite phase, which is the direct cause of the deterioration of creep rupture properties, is caused by Ni, a strong austenite-forming element.
is a major constituent element of the G phase. Zunawara, 1: Due to phase precipitation, Austi J near the G phase...
As a result of the decrease in the prior concentration of ferrite-forming elements, ferrite is produced. Therefore, we have obtained completely new knowledge that suppressing the precipitation of this ferrite phase is important for preventing the deterioration of creep rupture properties.

(Means and effects for solving the problems) The present invention has been made based on the above findings, and its gist is that the percentage of heavy rings (20,060% or less, Sl 3
．． 0% or less, Mn 3.0% or less,) Agriculture 0.02-0.
08%, N 0.15% or less, and Ni and Cr
The amount of is within the region ^-B-C-n shown in FIG. 1, or in addition -o 3.0% or less, W5. Ni-Cr containing either or both of the following OX, with the remainder consisting of Pe and unavoidable impurities, with excellent high-temperature, long-term room stability.
Located in austenitic stainless steel.

The present invention will be explained in detail below.

First, in the component system of the present invention, although C is an effective strengthening element, it is also an element that impairs high-temperature mechanical properties such as creep rupture properties after long-term use at high temperatures because it precipitates as carbides at grain boundaries. . From this point of view, Cl is 0.
However, if particularly high creep rupture ductility is required, it is desirable to set it to 0.030% or less.

Next, both Si and Mn are deoxidized +A',! : L
-r is necessary, but if it exists in excess of 3.0%, the hot workability will be impaired, so both amounts were set at 3.0% or less.

P precipitates within crystal grains as a phosphide during high-temperature storage and has a strengthening effect, and also has the effect of strengthening grain boundaries, so it is an effective element, especially from the viewpoint of creep rupture properties. Since the effect occurs from 0.02%, the lower limit was set at 0.02%. However, excessive addition significantly impairs weldability and hot workability, so the upper limit is set at 0.0.
It was set at 8%.

Ni is an essential austenite-forming element, but it is also a main constituent element of the G phase that precipitates during long-term use at high temperatures. Therefore, as the G phase precipitates, the degree of N t tI, which is an austenite forming element in the vicinity of the G phase, decreases, resulting in the formation of ferrite.

Therefore, the present inventors conducted the following experiment in order to investigate the influence of Nil on the precipitation for ferrite. That is, as the test steel, co, ois%, Si 1.2%, Mn
1.4%, P 0.035%, Cr18.5%, N
After melting 0.07% steel with various Ni content and hot rolling it into a steel plate of IV, size 12,
Solution treatment was performed at 50°C. This steel plate was heated to 550℃ for 5
After aging for 000 hours, a creep rupture test piece was prepared with a parallel part diameter of 6 ms and a distance between I1 points of 30 fins, and
A creep rupture test was conducted in accordance with 2272. The results are shown in FIG. In other words, Figure 3 shows the influence of Ni1L on creep rupture strength and creep rupture ductility.As seen in the figure, creep rupture strength and creep rupture ductility improve as Ni increases;
A tendency to saturate at 1t 0.5% or more is observed. Third
The figure also shows the state of precipitation of the ferrite phase, but for ferrite, C
It can be seen that the above creep rupture strength and creep rupture ductility correspond to the disappearance of the ferrite phase.

This effect of a single amount of Cr, which is a ferrite-forming element,
Since it depends on the amount of Cr, the limits of Ni and l were determined by changing the amount of Cr and conducting a similar investigation. The boundary value of the component thus obtained is the line segment BC shown in FIG. The line segment ^Old is a boundary to prevent transformation to martensite, the line segment CD is a limit to prevent the precipitation of the gray σ phase, and the line 〇 indicates the normally considered high-temperature usage conditions. This is the boundary line where no ferrite phase is generated.
In addition, since Ni is a highly vortex element, it was determined to avoid adding more than necessary. In addition, in FIG. 1, C is excluded from Ni and 16.6xC is reduced from Cr to take into account that C!, tCrrsC4, and ζ precipitate during high-temperature use.

Cr also improves oxidation resistance - Sa・1! This element requires 15.0% or more, but 25.0% or more is required.
If it exceeds 25.0%, embrittlement will occur due to long-term heating at high temperatures, so the upper limit was set at 25.0%.

N, together with C, is a strengthening element for austenitic stainless steel. Since N has a higher solubility than C, it remains stable in a solid solution state during high temperature maintenance and can exist at °C. I wanted to”
If ζ and N are used within their solubility ranges, a stable strengthening effect can be expected even during long-term use at high temperatures, and grain boundary embrittlement due to nitrides will not occur. From the viewpoint of structural materials, the upper limit of Nl was set to 0.15%. The reason for not setting a limit is that the strength can be increased to 1.0% depending on the application. This is to control If, but the level 0.
01% is strictly speaking the lower limit.

The above is the basic component system in the present invention, but in the present invention, it is effective to include NO or W within a predetermined range in order to further increase the strength. No is an element that has a solid solution strengthening effect and is an element that increases re-creep rupture strength, but if it is added in an amount exceeding 3.0% or less, it increases hot deformation resistance and makes rolling or forging difficult. Therefore, the content of ffff1 was set to 3.0% or less. Td 4
) It is a solid solution strengthening element similar to Mo, but if the addition exceeds 5.0%, it increases the hot deformation resistance and makes rolling or forging difficult, so the content is 5.0%. % or less.

The steel of the present invention having the above-mentioned composition is manufactured using various electric furnaces, etc., then made into steel ingots or slabs by ordinary ingot making or continuous casting, and then rolled or forged into steel of various shapes (as A). It is intended for use.

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

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

Table 2 shows the Cree time aging properties of the steels in Table 1 after aging at 550° C. for 5000 hours. As is clear from these property investigation results, the steel of the present invention has superior creep rupture strength and creep rupture ductility after long-term use at high temperatures, compared to comparative steels.

(Effects of the Invention) As described above, the steel of the present invention is a material that saves Ni as much as possible and has high-temperature properties such as creep rupture properties that are superior even after long-term use at high temperatures compared to conventional Ni-stirred steels. It is industrially extremely effective as a high temperature structural material used in the creep region.

4. fIn't' Explanation of the Drawings FIG. 1 is a diagram showing the claims of Ni and Crff1 of the present invention. Further, FIG. 2 is a photograph of the metal structure of the 550"C long-term creep rupture 1] obtained by a transmission electron microscope, and FIG. 3 is a diagram showing the influence of NiQl on the creep rupture characteristics of a material aged at 550"C for 5,000 hours.

Patent applicant: Nippon Smelting Co., Ltd.

Claims (2)

[Claims] (1) C0.060% or less, Si 3.0% or less, Mn 3.0% or less, P0.02-0.08%, N0.
15% or less, and the amounts of Ni and Cr are within the range A-B-C-D shown in FIG. Austenitic stainless steel. (2) C0.060% or less, Si 3.0% or less, Mn 3.0% or less, P0.02-0.08%, N0.
15% or less, and the amounts of Ni and Cr are within the region A-B-C-D shown in FIG. 1, and further contains Mo3.0% or less, W5.0% or less, or both. death,
The remainder is a Ni-Cr austenitic stainless steel that is essentially Fe and has excellent long-term stability at high temperatures.