JPH01149944A - Austenitic steel with excellent high temperature properties and structural stability - Google Patents

Abstract

PURPOSE:To inexpensively manufacture an austenitic stainless steel which has superior strength at high temp. and toughness at high temp. even if Ni content is reduced by incorporating specific small amounts of N and reducing Si content in an austenitic stainless steel. CONSTITUTION:A Cr-Ni austenitic stainless steel has a composition containing, by weight, 0.03-0.15% C, <0.2% Si, <10% Mn, 14-20% Cr, 4-20% Ni, 0.03-0.2% N, and one or >=2 kinds among <0.03% Al, 0.001-0.01% Mg, and 0.001-0.01% Ca or further containing 0.001-0.01% B and/or 0.001-0.1% Zr or 0.01-1.5% Nb independently or in combination. By incorporating small amounts of N and reducing Si content, the austenitic stainless steel excellent in high-temp. characteristics even if the content of expensive Ni is reduced can be obtained.

Description

[Detailed Description of the Invention] [Field of Industrial Application] This invention has excellent high-temperature strength and high-temperature toughness (hereinafter referred to as high-temperature properties), and excellent structural stability at high temperatures despite a low N1 content. This relates to low-cost austenitic steel.

[Conventional technology]

Conventionally, l8-8 series austenitic stainless steel has been mainly used in high-temperature equipment such as boilers and chemical plants used in high-temperature environments.

[Problem that the invention seeks to solve]

However, in recent years, as the high-temperature equipment mentioned 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 this 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 N1 content, but if the N1 content is low, the austenite structure becomes unstable, and after long-term use, σ There is a problem in that intermetallic compounds such as phases precipitate and the high temperature characteristics deteriorate.

[Means for solving problems]

Therefore, from the above-mentioned viewpoint, the present inventors have developed a low Ni
As a result of conducting research to develop an austenitic steel with excellent high-temperature properties and structural stability, we found that if the Si content is lowered, even if the N1 content is reduced, We have obtained the knowledge that the structure under high temperature is stable and maintains excellent high temperature properties for an extremely long period of time, and this is shown for several component systems in Figures 1 and 2. It is also clear from the relationship between the creep rupture strength and Charpy impact value and the microstructure balance (N - N') obtained by the PHACOM method that, as shown in the figure, the low-Si material is better at high temperatures, especially at long periods of time. It can be seen that it has excellent strength and structural stability, has a lower Si content than expected by the PHACOM method, and can significantly reduce the Ni content.

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%.

N1: 4-20%, N: o. oa
~0.2%.

and further contains Al1:0. H% or less, Mg: 0.001~
0.01%.

Ca: 0.001 to 0.01%.

Contains one or more of the following, and further contains, if necessary, B: 0.001 to 0.01%, Zr: 0.
OOL~0.1%.

Nb: 0. Ol~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. Since it becomes effective, its 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) 5i The Si component has a deoxidizing effect, so in normal steel, 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; on the other hand, if the content is less than 20%, If the content exceeds this, the structure stability may be impaired, so the content was set at 14 to 20%.

(e) Ni The N1 component has the effect of stabilizing the austenite structure, and its appropriate amount is Si, Cr, and Mn.

Furthermore, it is determined by the content of Nb, etc., but if the content is less than 4%, it is difficult to stably maintain an austenitic structure, while if the content exceeds 20%, no further improvement effect will appear. Therefore, in consideration of economic efficiency, the content was determined to be 4 to 20%.

(f') N The N component has the effect of stabilizing the austenite structure and improving the 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. The amount was determined to be 0.03-0.2%.

(g) l! , Mg, and Ca These components have a deoxidizing effect, and especially in the austenitic steel of this invention, the content of 81 as a deoxidizing agent is kept low, so deoxidizing with these components is essential. For A and Q, if the content exceeds 0.03%, the structure stability will deteriorate due to promotion of precipitation of elongated phase at high temperature and long time side, and for Mg and Ca, Mg: 0.01%.
and Ca: If it exceeds 0.01%, the workability will deteriorate, so the respective upper limit values were set as A, Q: 0.01%.
03%, Mg: 0.01%, and Ca:O,01
%. Furthermore, Mg and Ca have the effect of improving workability and creep rupture strength, but when the content is less than 0.001% for Mg and 0.001% for Ca.
Since it is difficult to ensure the above-mentioned effect at less than 0.001% of Mg, the lower limit of Mg: 0.001%, Ca
: determined as 0.001%.

(h) B and Zr These components have the effect of improving high-temperature strength, especially creep rupture strength, so they are included as necessary especially when higher high-temperature strength is required. 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 0.001%, respectively, B:0. Old% and Zr: If it exceeds 0.1%, weldability deteriorates, so the content is reduced to B: o.
, oot ~o, ot%, Zr: 0.001~0
．． It was set at 1%.

(1) 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 its content is less than 0.601%, it will not reach the desired level. No improvement effect was obtained, while the content was 1.5
If the content exceeds 0.01% to 1.5%, the toughness decreases, so the content was set at 0.01% to 1.5%.

〔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 26 of the present invention, comparative austenitic steels 1 to 26, and conventional austenitic steels 1 to 3 were produced by solution treatment, respectively.

In addition, comparison austenites n41 to n26 are all S
1 content at a normal level of about 0.5%, and after adjusting the Ni content, it was made to have the same microstructural balance (N - N' value) as the austenitic steel of the present invention with the same number. .

In addition, conventional austenitic steels 1 to 3 are each 18-
8 series stainless steel SUS 804H.

It corresponds to SUS 321H and 5US374H.

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, the austenitic steels 1 to 2G of the present invention are the comparative austenitic steels 1 to 26 of the same number and the conventional austenitic steels 1 to 3, respectively.
In comparison, the creep rupture strength was the same at 300 hours and higher at 3000 hours, and the Charpy impact value was also the same at 300 hours, but at 3000 hours, the decrease in the value was 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 26 of the present invention and comparative austenitic steels 1 to 26, and the amount of N in nitrides and 51ffi in the steel after aging at 700°C for 3000 hours were determined. The results shown in Figure 3 are shown when the relationship between the two is investigated, and the results show that by reducing the amount of Si, not only the precipitation of σ phase etc. is suppressed, but also the amount of nitrides that precipitate even when heated for a long time is significantly reduced.
In general N-containing high-temperature austenitic steels, a decrease in toughness due to an increase in the amount of nitrides and a decrease in creep rupture strength due to a decrease in solid solute Nff1 are unavoidable, whereas in the austenitic steel of the present invention, the toughness decreases due to the above reasons. It can be seen that there is no decrease in creep rupture strength or decrease in creep rupture strength.

〔Effect of the invention〕

As mentioned above, the austenitic steel of the present invention has excellent structural stability even with a low Nl content, which enables cost reduction, and maintains excellent high-temperature properties over a long period of time, so it is particularly suitable for use 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 Nfl in nitride.

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%, Al: 0.03% or less, Mg: 0.001-0.01%
, Ca: 0.001 to 0.01%, and a composition (weight %) characterized by containing one or more of the following, with the remainder consisting of Fe and unavoidable impurities (weight %). 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%, 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 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%, 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%, Al: 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%,
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.