JPH01188653A - Austenitic steel having excellent high temperature characteristics and structural stability - Google Patents

Abstract

PURPOSE:To obtain the subject steel having excellent high temp. strength and high temp. toughness and having excellent structural stability at high temp. in spite of its low Ni contents at a low cost formed by incorporating each specific amounts of C, Si, Mn, Cr, Ni and N, one or two kinds of Mo and W and one or more kinds among Al, Mg and Ca. CONSTITUTION:The austenitic steel contg. specific amounts of 0.03-0.15% C, <=0.2% Si, <=10% Mn, 14-20% Cr, 4-20% Ni and 0.03-0.2% N, contg. one or two kinds of Mo and W and furthermore contg. one or more kinds among Al, Mg and Ca is obtd. Said austenitic steel has excellent structural stability even in the low Ni contents permitting its low cost and retains excellent high temp. characteristics for a long period of time. In this way, at the time of using the steel as the structural material for high temp. apparatus, etc., of boiler, chemical plant, etc., used in a high temp. environmemt, its durability and economical efficiency can sufficiently be improved.

Description

[Detailed Description of the Invention] [Industrial Application Field] This invention has excellent high-temperature strength and high-temperature toughness (hereinafter referred to as high-temperature properties), and has excellent structural stability at high temperatures despite low Ni content. It concerns an excellent and inexpensive austenitic steel.

[Conventional technology]

Conventionally, 18-8 series austenitic stainless steel has been mainly used in high-temperature equipment such as boilers and chemical plants used in high-temperature environments.

[Problem to be solved by the invention]

However, in recent years, as the high-temperature equipment described above has improved in performance and become more energy-saving, the conditions for its use have become even more severe, and the structural materials used in these equipment are required to have even more stable high-temperature properties over time. It has become so.

On the other hand, there is also a strict demand for cost reduction for these structural materials, and for this purpose it is essential to reduce the Ni content, but if the Ni content is reduced, the austenitic structure becomes unstable, especially Mo.

W-containing steel has a problem in that after long-term use, intermetallic compounds such as σ phase precipitate, resulting in deterioration of high-temperature properties.

[Failure to solve the problem]

Therefore, from the above-mentioned viewpoint, the present inventors have developed a method for reducing Ni.
As a result of conducting research to develop austenitic steel with excellent high-temperature properties and structural stability, we found that if the Si content is lowered, the Ni content can be reduced, especially in austenitic steel containing Mo and W components. However, we obtained the knowledge that the structure at high temperatures becomes stable and retains excellent high-temperature properties for an extremely long period of time, and that this tendency is further promoted by the inclusion of N components. This is reflected in the creep rupture strength and Charpy impact value shown for the two component systems in Figures 1 and 2, and the microstructure balance (N - N
) It is clear from the relationship diagram ■, and as shown in the figure, the low Sj material has poorer high temperature strength and structural stability, especially at high temperatures and long periods of time, and PHA
It has a lower Sj effect than expected with the COM method, and N
It can be seen that the i content can be significantly reduced.

This invention was made based on the above knowledge, and C:0.
03 to 0.15%, Si: 0.2% or less.

Mn: 10% or less, Cr: 14-20%.

Ni: 4-20%, N: 0.03-0.
2%.

Mo: OJ -4%, W: 0.5 -
7%.

Contains one or two of the following: All): 0.03% or less, Mg: 0.
001-0.01%.

Ca: 0.001 -0.01%.

B: o. oot~o. Old %, Zr: 0.
001 to 0.1%.

Nb: 0.01-1.5%.

It is characterized by being an inexpensive austenitic steel with excellent high-temperature properties and structural stability, having a composition containing one or more of these, with the remainder consisting of Fe and unavoidable impurities.

Next, the reason why the composition of the austenitic steel of the present invention is limited as described above will be explained.

(a) CC The C component is an effective component for ensuring a predetermined tensile strength and creep rupture strength, but the content is 0.03
If the content is less than 0.15%, the desired strength cannot be secured, while if the content exceeds 0.15%, the amount of undissolved carbide increases in the solution state, which adversely affects mechanical properties. The content should be set at 0.03~
It was set at 0.15%. In particular, the reinforcing effect is further improved by containing 0.05% or more.

(b) Since the 5i St acid component has a deoxidizing effect, the 5i St acid component has a deoxidizing effect of 0.
It is contained at a ratio of about 4 to 0.8%, but such a ratio not only deteriorates weldability but also causes brittle phases such as σ phase to precipitate during long-term use at high temperatures. This tendency becomes noticeable when the content exceeds 0.2%.Therefore, the structure is stabilized under high temperatures, and the high temperature properties do not deteriorate even after long-term use. In order to do so, the upper limit was set at 0.2%.

(c) Mn The Mn component has the effect of deoxidizing and improving processability, but if its content exceeds 10%, heat resistance properties will deteriorate, so its content should be reduced. It was set at 10% or less.

(d) Cr The Cr component has the effect of improving oxidation resistance and corrosion resistance, but if its content is less than 14%, the desired effect cannot be obtained, whereas if its content exceeds 20%. Since there is a possibility that the structure stability may be impaired, the content was set at 14 to 20%.

(e) Ni The Ni component has the effect of stabilizing the austenite structure, and its appropriate amount is Si, Cr, and Mn.

It is determined by the amount of Mo, W, and even Nb added, but if the content is less than 4%, it is difficult to stably maintain an austenitic structure, while if the content exceeds 20%, further improvement is not achieved. Since no effect was observed, the content was determined to be 4 to 20% in consideration of economic efficiency.

(f) N The N component has the effect of stabilizing the austenite structure and improving tensile strength and creep rupture strength like the C component, but if its content is less than 0.03%, the desired effect may not be achieved. On the other hand, if the content exceeds 0.2%, the amount of nitride precipitation will increase during long-term use at high temperatures, and mechanical properties will be impaired. was set at 0.03 to 0.2%.

(g) Mo and W These components have the effect of improving the high temperature strength of steel, but if their content is less than 0.3% Mo and 0.5% W, the desired high temperature strength improvement will not be achieved. On the other hand, if the content exceeds Mo: 4% and W near%, the high temperature strength improves but the workability of the steel deteriorates.
~4%.

W: 0.5 to 7%.

(h) Al, Mg, and Ca These components have a deoxidizing effect, and in particular, in the austenitic steel of this invention, the content of 81 as a deoxidizing agent is kept low, so deoxidizing by these components is Although it is essential, if the content exceeds 0.08% for A and 17, it will lead to deterioration of the structure stability due to promotion of precipitation of σ phase at high temperature and long time side, and for Mg and Ca,
If Mg: 0.01% and Ca: 0.01% are exceeded, the workability will deteriorate, so the respective upper limit values were set as follows: Al): 0.03%, Mg: 0.01%, and Ca. : determined as 0.01%. Also, Mg and Ca
has the effect of improving workability and creep rupture strength, but it is difficult to ensure the above effects if the content is less than Mg +0.001% and Ca: less than 0.0%. , its lower limit is Mg:O,001
%.

Ca: Set as 0.0 old%.

(i) B and Zr These components have the effect of improving high-temperature strength, especially creep rupture strength, so they may be included as necessary or added to the If the content is less than 0.001%, the desired improvement effect cannot be obtained; on the other hand, if the content is less than 0.01%, B: 0.01%.
and Zr: If it exceeds 0.1%, weldability deteriorates, so the content is reduced to B:
0.001~0.01%, Zr: 0.001~
It was set at 0.1%.

(j) Nb The Nb component has the effect of forming fine and uniformly dispersed carbides to further improve high-temperature strength, so it is included as necessary, but if the content is less than 0% The desired improvement effect cannot be obtained, and on the other hand, if the content exceeds 15%, the toughness will decrease.
%.

〔Example〕

Next, the austenitic steel of the present invention will be specifically explained using examples.

Using an ordinary vacuum melting method, prepare molten steel having the composition shown in Table 1, cast it into an ingot, then forge it under ordinary conditions, and cold-roll it into a plate material.
Austenitic steels 1 to 24 of the present invention, comparative austenitic steels 1 to 24, and conventional austenitic steels 1 to 3 were produced by solution treatment, respectively.

In addition, comparative austenitic steels 1 to 24 all contain Si
The Ni content is set to the normal level of about 0.5%, and the Ni content is adjusted so that it has the same microstructural balance (N - N ) as the austenitic steel of the present invention having the same number.

In addition, conventional austenitic steels 1 to 3 are each 18-
8 series stainless steel SUS 304H.

SUS 31BI (and equivalent to SUS 847H).

Next, for the purpose of evaluating the high temperature strength of the various austenitic steels obtained as a result, a creep rupture test was conducted at 700°C to determine the creep rupture strength for 300 hours and 3000 hours, and the structural stability was also evaluated. For this purpose, the Charpy impact values at 0°C of the materials heated at 700°C for 300 hours and 3000 hours were determined. These results are shown in Table 2.

From the results shown in Tables 1 and 2, it can be seen that the austenitic steels 1 to 24 of the present invention are the comparative austenitic steels 1 to 24 of the same number and the conventional austenitic steels 1 to 24, respectively.
3, it shows the same creep rupture strength at 300 hours and higher at 3000 hours, and the Charpy impact value is also the same at 300 hours, but at 3000 hours, the decrease in value is significantly smaller and more noticeable. It is clear that it has certain characteristics.

In addition, several types of austenitic steels were selected from the above-mentioned austenitic steels 1 to 24 of the present invention and comparison austenitic steels 1 to 24, and the amount of N in nitrides and Si in the steel after aging at 700°C for 3000 hours was determined. When we investigated the relationship between the amounts, the results shown in Figure 3 are shown, and the results show that reducing the amount of Si not only suppresses the precipitation of σ phase, etc., but also significantly reduces the amount of nitrides that precipitate even when heated for a long time. In high-temperature austenitic steel with a normal level of St content of about 0.5%, toughness decreases due to increased nitride content and solid solution N
It can be seen that, while a decrease in creep rupture strength is unavoidable as the amount decreases, the austenitic steel of the present invention does not suffer from a decrease in toughness or creep rupture strength due to the above-mentioned causes.

〔Effect of the invention〕

As mentioned above, the austenitic steel of the present invention has excellent structural stability even with a low Ni content, which enables cost reduction, and maintains excellent high-temperature properties over a long period of time, so it is particularly useful in high-temperature environments. When used as a structural material for high-temperature equipment such as boilers used in industrial plants and chemical plants, it brings about industrially useful effects such as sufficiently improving durability and economic efficiency.

[Brief explanation of the drawing]

Figures 1 and 2 show Charpy impact values and creep rupture strengths and PHACOM
Figure 3 is a diagram showing the relationship between the amount of Si in steel and the amount of N in nitride. Applicant: Sumitomo Metal Industries, Ltd.

Claims (4)

[Claims] (1) C: 0.03-0.15%, Si: 0.2% or less, Mn: 10% or less, Cr: 14-20%, Ni: 4-20%, N: 0.03-0. 2%, Mo: 0.3-4%, W: 0.5-7%, Contains one or two of the following, Al: 0.03% or less, Mg: 0.001 ~0.01%
, Ca: 0.001 to 0.01%, and a structure with high-temperature properties characterized by having a composition (weight %) containing one or more of the following, with the remainder consisting of Fe and unavoidable impurities. Austenitic steel with excellent stability. (2) C: 0.03-0.15%, Si: 0.2% or less, Mn: 10% or less, Cr: 14-20%, Ni: 4-20%, N: 0.03-0. 2%, Mo: 0.3 to 4%, W: 0.5 to 7%, Contains one or two of the following, AD: 0.03% or less, Mg: 0.001 ~0.01%
, Ca: 0.001-0.01%, Contains one or more of the following, furthermore, B: 0
．． 001-0.01%, Zr: 0.001-0.1%,
An austenitic steel having excellent high-temperature properties and structural stability, characterized by containing one or two of the above, with the remainder consisting of Fe and unavoidable impurities (weight percent). (3) C: 0.03-0.15%, Si: 0.2% or less, Mn: 10% or less, Cr: 14-20%, Ni: 4-20%, N: 0.03-0. 2%. Contains one or two of the following: Mo: 0.3-4%, W: 0.5-7%, Al: 0.03% or less, Mg: 0.001-0. 01%
, Ca: 0.001-0.01%, Contains one or more of the following, and further contains Nb:
An austenitic steel having excellent high-temperature properties and structural stability, characterized in that it contains 0.01 to 1.5% of the following, with the remainder consisting of Fe and unavoidable impurities (weight percent). (4) C: 0.03-0.15%, Si: 0.2% or less, Mn: 10% or less, Cr: 14-20%, Ni: 4-20%, N: 0.03-0. 2%, Mo: 0.3-4%, W: 0.5-7%, Contains one or two of the following, Al: 0.03% or less, Mg: 0.001 ~0.01%
. Contains one or more of the following: Ca: 0.001 to 0.01%, and further B: 0
．． 001-0.01%, Zr: 0.001-0.1%,
High-temperature properties and structure characterized by having a composition (weight %) containing one or two of the following, Nb: 0.01 to 1.5%, and the remainder consisting of Fe and unavoidable impurities. Austenitic steel with excellent stability.