JPH0830247B2 - Austenitic steel with excellent high temperature strength - Google Patents

Description

TECHNICAL FIELD The present invention relates to an austenitic steel which has excellent high temperature strength and therefore exhibits excellent performance when used as a structural material for a high temperature device.

[Conventional technology]

Conventionally, austenitic steel mainly composed of 18-8 type austenitic stainless steel has been widely used as a structural material for equipment such as boilers and chemical plants generally used in a high temperature environment.

[Problems to be solved by the invention]

On the other hand, in recent years, the performance and size of boilers and chemical plants have been remarkably increased, and along with this, the operating conditions of these devices have become even more severe, and there is a situation where operations at higher temperatures are performed. The structural material of the device is required to have excellent high-temperature strength. However, since the above-mentioned 18-8 austenitic stainless steel is not the main component of the austenitic steel, it does not have sufficient high-temperature strength. I cannot respond to satisfaction.

[Means for solving problems]

Therefore, the inventors of the present invention have conducted research to develop an austenitic steel excellent in high temperature strength from the above viewpoints, and as a result, (a) in the N-containing austenitic steel, depending on the contents of Cu, B and Mg, High temperature strength should be further improved.

(B) Reducing the Si and Al contents is effective for improving the high temperature strength of the austenitic steel of (a) above. Particularly, Si promotes the precipitation of nitrides, resulting in strength reduction and toughness deterioration. It is necessary to keep the content below 0.3%.

(C) When Nb as a carbonitride dispersion strengthening element or Mo and / or W as a solid solution strengthening element is contained in the austenitic steel of the above (b) alone or in combination, the high temperature strength is further improved. thing.

The research results shown in (a) to (c) above were obtained.

The present invention was made based on the above research results, and in weight ratio (hereinafter,% indicates weight ratio) C: 0.15% or less, Si: 0.3% or less, Mn: 10% or less, Cr : 14-27%, Ni: 6-30%, Cu: 2-6%, Al: 0.003-0.030%, Ag: 0.001-0.015%, B: 0.001-0.010%, N: 0.05-0.35% In addition, if necessary, it contains at least one of Mo: 0.3-3.0%, W: 0.5-5.0%, Nb: 0.05-1.5%, and Fe and inevitable impurities: the composition of the remainder. It is characterized by an austenitic steel having excellent high temperature strength.

Next, in the austenitic steel of the present invention, the reason why the content ratio of each component is limited as described above will be described.

(A) CC C is an element effective for ensuring the tensile strength and creep rupture strength required for austenitic steel,
If the content exceeds 0.15%, the amount of undissolved carbide in the solution state increases and the mechanical properties are adversely affected. Therefore, the C content is set to 0.15% or less. Even if the C content is very small, a certain effect can be obtained, but 0.01% or more is preferably contained.

(B) Si Si is an element effective as a deoxidizer, and is contained in ordinary austenitic steel in an amount of 0.4 to 0.8%, but if the content is large, the weldability deteriorates, and high temperature strength and ductility / toughness Will also be reduced. That is, when Si is contained, the amount of σ phase generated during heating for a long time increases with the increase of Si, and the amount of nitride also increases, resulting in serious deterioration of mechanical properties. Since this tendency becomes remarkable when the Si content exceeds 0.3%, the Si content is set to 0.3% or less, but it is desirable to suppress it to 0.2% or less if possible.

(C) Mn Mn is an element that is also effective as a deoxidizer and is also effective for improving workability, but if it is contained in excess of 10%, the heat resistance of austenitic steel will deteriorate. , Mn content was determined to be 10% or less.

(D) Cr Cr is an element necessary from the viewpoint of improving oxidation resistance and corrosion resistance, and in order to exert its sufficient effect, it is necessary to secure a content of 14% or more. By the way, from the viewpoint of corrosion resistance, the larger the Cr content, the more desirable, but if the content exceeds 27%, the workability is deteriorated and the structure may become unstable. Therefore, the Cr content is 14 to 27. Defined as%.

(E) Ni Ni is an essential component for ensuring a stable austenite structure, and its proper amount is determined by the content of Cr, Mo, W, Nb, etc., but its content is less than 6%. It is difficult to secure a stable austenite structure, while containing more than 30% is economically disadvantageous.
The content was set to 6 to 30%.

(F) Cu Cu has an effect of improving high temperature strength, but if the content is less than 2%, the desired effect cannot be obtained in the above operation.
On the other hand, if the content of Cu exceeds 6%, the ductility decreases and the workability deteriorates. Therefore, the Cu content was set to 2 to 6%.

(G) Al Al is an element required as a deoxidizing agent and must be contained in an amount of 0.003% or more. However, if it is contained in excess of 0.030%, σ phase precipitation at high temperature and long time use is promoted and mechanical This will lead to deterioration of properties. Therefore, the Al content is 0.003 ~
Although it was set to 0.030%, it is desirable to adjust it to 0.003 to 0.020% if possible.

(H) Mg Mg is an element necessary as a deoxidizing agent and as a workability improving component, but is absent in the austenitic steel of the present invention when the amounts of Si and Al as deoxidizing elements are limited to a low level. It is a component that cannot be obtained, and also contributes to the improvement of high temperature strength. And, when the Mg content is less than 0.001%, it is not possible to exert its sufficient effect, while when the content exceeds 0.015%, the workability becomes deteriorated again, so the Mg content is 0.001 to It was set at 0.015%.

(I) B B has the effect of improving the high temperature strength by carbonitride dispersion strengthening and grain boundary strengthening, but its content is 0.001%.
If less than 0.01, a sufficient effect cannot be obtained for the above-mentioned action, while on the other hand, 0.01
If the content exceeds 0%, the weldability deteriorates, so the B content was set to 0.001 to 0.010%.

(J) N N is an element effective for improving the tensile strength and creep rupture strength like C, but if its content is less than 0.05%, a sufficient effect cannot be obtained, while N is C In comparison, the amount of solid solution is large, but if the content exceeds 0.35%, the amount of nitride that precipitates during long-term use at high temperature increases, leading to deterioration of mechanical properties. Therefore, the N content is 0.05 to 0.35%. I decided.

(K) Mo and W Since these elements have the effect of improving the high temperature strength, one or two kinds are contained if necessary, but the Mo content is
If it is less than 0.3% or the W content is less than 0.5%, the desired effect cannot be obtained. On the other hand, from the viewpoint of improving the high temperature strength, the higher the content of these elements, the more preferable. However, when the Mo content exceeds 3.0% or the W content exceeds 5.0%, the effect tends to be saturated, and the workability is improved. However, the Mo content is 0.3 to 3.0% and the W content is 0.5%.
〜5.0% respectively.

(L) Nb Nb is an element effective for improving high temperature strength by carbonitride fine dispersion strengthening, but if its content is less than 0.05%, the above effect is not sufficiently exerted, while it exceeds 1.5%. If it is contained as a solution, the amount of undissolved nitride in the solution state increases and the mechanical properties deteriorate, so the Nb content is 0.05 to 1.5.
Defined as%.

Next, the austenitic steel of the present invention will be specifically described with reference to examples.

[Examples] First, the present invention steels 1-35, conventional steels and comparative steels 1-12 having the component compositions shown in Tables 1 to 4 were melted by vacuum melting,
After forging and cold rolling, solution treatment was performed. In addition,
In Table 3, the conventional steel is SUS316H steel having the highest high-temperature strength among the 18-8 type austenitic stainless steels, and in Comparative Steels 1 to 12 in Table 4, at least one component is within the scope of the present invention. It is an austenitic steel with a deviated composition.

Subsequently, the present invention steels 1 to 35, conventional steels and comparative steels 1 to 12
For the purpose of evaluating the high temperature strength, a creep rupture test at 750 ° C was performed to obtain the 1000 hour creep rupture strength, and further, in order to evaluate the structural stability
After heating for a long time at 1000C for 1000 hours, a Charpy impact test was performed at 0C.

 The results of these tests are shown in Tables 5 and 6.

〔The invention's effect〕

From the results shown in Table 5 and Table 6, the invention steel 1
Nos. 35 to 35 show significantly higher creep rupture strength than the SUS316H steel (conventional steel), which has the highest high-temperature strength among the 18-8 type austenitic stainless steels.
It is understood that Nos. 35 to 35 have good impact properties, and Comparative Steels 1 to 12 having a composition outside the scope of the present invention have unfavorable creep rupture strength.

As described above, the austenitic steel of the present invention has excellent high-temperature strength and also has good microstructure stability, and therefore, when used as a structural material of a high-temperature device such as a boiler or a chemical plant, it is more severe. Even in high temperature operation, it exhibits excellent performance over a long period of time.

Claims (4)

[Claims] 1. By weight ratio, C: 0.15% or less, Si: 0.3% or less, Mn: 10% or less, Cr: 14 to 27%, Ni: 6 to 30%, Cu: 2 to 6%, Al : 0.003 to 0.030%, Mg: 0.001 to 0.015%, B: 0.001 to 0.010%, N: 0.05 to 0.35%, Fe and inevitable impurities: Fe and unavoidable impurities: Excellent in high temperature strength Austenitic steel. 2. By weight ratio, C: 0.15% or less, Si: 0.3% or less, Mn: 10% or less, Cr: 14 to 27%, Ni: 6 to 30%, Cu: 2 to 6%, Al : 0.003 to 0.030%, Mg: 0.001 to 0.015%, B: 0.001 to 0.010%, N: 0.05 to 0.35%, and Mo: 0.3 to 3.0%, W: 0.5 to 5.0% An austenitic steel excellent in high-temperature strength, characterized in that it contains at least one kind and has a composition of Fe and inevitable impurities: the rest. 3. By weight ratio, C: 0.15% or less, Si: 0.3% or less, Mn: 10% or less, Cr: 14 to 27%, Ni: 6 to 30%, Cu: 2 to 6%, Al : 0.003 to 0.030%, Mg: 0.001 to 0.015%, B: 0.001 to 0.010%, N: 0.05 to 0.35%, Nb: 0.05 to 1.5%, Fe and unavoidable impurities: Characteristic composition having the rest Austenitic steel with excellent high temperature strength. 4. By weight ratio, C: 0.15% or less, Si: 0.3% or less, Mn: 10% or less, Cr: 14 to 27%, Ni: 6 to 30%, Cu: 2 to 6%, Al : 0.003-0.030%, Mg: 0.001-0.015%, B: 0.001-0.010%, N: 0.05-0.35%, Nb: 0.05-1.5%, and Mo: 0.3-3.0%, W: 0.5 An austenitic steel excellent in high-temperature strength, characterized in that it contains at least one of 5.0 to 5.0% and has a composition of Fe and inevitable impurities: the rest.