Austenitic Stainless Alloy Material

The present disclosure relates to an alloy material, and more particularly relates to an austenitic stainless alloy material.

Austenitic stainless alloy materials are used as a raw material for boilers such as coal-fired power boilers, biomass boilers, and HRSG (Heat Recovery Steam Generators). Raw materials that are used in these boilers are required to have excellent creep strength in high-temperature environments.

Austenitic stainless alloy materials in which the creep strength is increased are proposed in International Application Publication No. WO2009/044796 (Patent Literature 1) and Japanese Patent Application Publication No. 2004-250783 (Patent Literature 2).

Patent Literature 1 discloses an austenitic stainless alloy material that consists of, in mass %, C: 0.04 to 0.18%, Si: 1.5% or less, Mn: 2.0% or less, Ni: 6 to 30%, Cr: 15 to 30%, N: 0.03 to 0.35%, and sol. Al: 0.03% or less, and also contains one or more types among Nb: 1.0% or less, V: 0.5% or less, and Ti: 0.5% or less, with the balance being Fe and impurities. In addition, in this alloy material, P1 (=S+{(P+Sn)/2}+{(As+Zn+Pb+Sb)/5}) is 0.06 or less, and P2 (=Nb+2 (V+Ti)) is 0.2 to 1.7-10×P1. In the alloy material disclosed in Patent Literature 1, by making P2 that is an index of Nb, V, and Ti 0.2 or more, precipitates are formed during use in a high-temperature environment and the creep strength is increased.

Patent Literature 2 discloses an austenitic stainless alloy material that consists of, in mass %, C: 0.03 to 0.12%, Si: 0.2 to 2%, Mn: 0.1 to 3%, P: 0.03% or less, S: 0.01% or less, Ni: more than 18% to less than 25%, Cr: more than 22% to less than 30%, Co: 0.04 to 0.8%, Ti: 0.002% or more and less than 0.01%, Nb: 0.1 to 1%, V: 0.01 to 1%, B: more than 0.0005% to 0.2% or less, sol. Al: 0.0005% or more to less than 0.03%, N: 0.1 to 0.35%, and O (oxygen): 0.001 to 0.008%, with the balance being Fe and impurities. In the alloy material disclosed in Patent Literature 2, by containing Ti, Nb, and V, precipitates are formed during use in a high-temperature environment and the creep strength is increased.

In this connection, when austenitic stainless alloy materials for boiler use are applied for use in a boiler, the austenitic stainless alloy materials are welded or subjected to bending. Austenitic stainless alloy materials that are applied for use in boilers are used for long periods of time in a high temperature range of 500 to 750° C. At such time, relaxation of residual stress occurs at a weld zone of the austenitic stainless alloy material or at a portion subjected to bending. Due to the relaxation of residual stress, precipitates form within grains and the interior of the grains hardens. Consequently, creep strain may sometimes accumulate at grain boundaries and a crack may occur at the grain boundaries. A crack of this kind is called a “stress relaxation crack”.

An austenitic stainless alloy material for boiler use is required to have not only excellent creep strength, but also excellent stress relaxation cracking resistance. In the aforementioned Patent Literatures, although creep strength is discussed, there is no discussion regarding stress relaxation cracking resistance.

An objective of the present disclosure is to provide an austenitic stainless alloy material that has excellent creep strength and excellent stress relaxation cracking resistance.

An austenitic stainless alloy material according to the present disclosure consists of, in mass %,

The austenitic stainless alloy material of the present disclosure has excellent creep strength and excellent stress relaxation cracking resistance.

The present inventors conducted studies regarding an austenitic stainless alloy material which can achieve both excellent creep strength and excellent stress relaxation cracking resistance. First, the present inventors attempted to achieve both excellent creep strength and excellent stress relaxation cracking resistance from the viewpoint of the chemical composition. As a result, the present inventors considered that if the chemical composition consists of, in mass %, C: 0.03 to 0.12%, Si: 0.05 to 2.00%, Mn: 0.05 to 3.00%, P: 0.03% or less, S: 0.010% or less, Ni: 18.0 to less than 25.0%, Cr: 22.0 to less than 30.0%, Co: 0.04 to 0.80%, Ti: 0.002 to 0.010%, Nb: 0.1 to 1.0%, V: 0.01 to 1.00%, Al: 0.001 to less than 0.030%, N: 0.10 to 0.35%, Mo: 0 to 1.00%, W: 0 to 1.00%, B: 0 to 0.010%, and Ca: 0 to 0.0100%, with the balance being Fe and impurities, there is a possibility that both excellent creep strength and excellent stress relaxation cracking resistance can be achieved.

Therefore, in addition, the present inventors conducted studies from the viewpoint of the microstructure with regard to achieving both excellent creep strength and excellent stress relaxation cracking resistance in an austenitic stainless alloy material that satisfies the chemical composition described above.

Usually, in an austenitic stainless alloy material for boiler use, during use in a high-temperature environment, fine precipitates such as Ti precipitates, Nb precipitates, and V precipitates are formed and the creep strength is increased by precipitation strengthening by these precipitates. Therefore, in the austenitic stainless alloy material disclosed in Patent Literature 1 or Patent Literature 2, a heat treatment (solution treatment) is performed in the final stage of the production process. By this means, precipitates in the austenitic stainless alloy material are melted as much as possible, and Ti, Nb, and V are placed in a dissolved state. This is because, when the austenitic stainless alloy material is being used in a high-temperature environment, fine precipitates are formed by these dissolved elements and the creep strength is thereby increased.

However, the present inventors considered that rather than reducing precipitates in the austenitic stainless alloy material and placing Ti, Nb, and V in a dissolved state as has been done in the past, by intentionally causing fine precipitates to be present in advance in the austenitic stainless alloy material, it would be possible to increase not only the creep strength but also the stress relaxation cracking resistance.

The grains of an austenitic stainless alloy material can be kept fine by the pinning effect of fine precipitates that are present in advance in the austenitic stainless alloy material. In this case, the grain boundary area in the alloy material increases. By such increase in the grain boundary area, the stress relaxation cracking resistance can be increased.

On the other hand, in a case where precipitates are present in advance in an austenitic stainless alloy material, it is difficult for new fine precipitates to form during use in a high-temperature environment. In addition, it is also conceivable that during use in a high-temperature environment, the precipitates that are already present will coarsen. In such case, there is a possibility that sufficient creep strength will not be obtained. However, as a result of studies conducted by the present inventors, it has been revealed that if precipitates having an equivalent circular diameter of 0.5 to 2.0 μm are present in an amount equivalent to a number density of 5000 pieces/mmor more in an austenitic stainless alloy material having the above chemical composition before being applied for used in a boiler, sufficient creep strength is obtained even during use in a high-temperature environment.

Based on the above findings, the present inventors have discovered that instead of increasing the creep strength in a high-temperature environment by reducing precipitates as much as possible in an alloy material as in the case of the conventional austenitic stainless alloy materials such as are disclosed in Patent Literature 1 and Patent Literature 2, excellent creep strength and excellent stress relaxation cracking resistance can both be achieved by intentionally causing fine precipitates to be present in an amount equivalent to a number density of 5000 pieces/mmor more in the austenitic stainless alloy material, and thus completed the austenitic stainless alloy material of the present embodiment.

The austenitic stainless alloy material of the present embodiment, which has been completed based on the technical idea described above, is as follows.

[1]

An austenitic stainless alloy material consisting of, in mass %,

[2]

The austenitic stainless alloy material according to [1], containing one kind of element or more selected from a group consisting of:

Hereunder, the austenitic stainless alloy material of the present embodiment is described in detail.

The austenitic stainless alloy material of the present embodiment satisfies the following Feature 1 and Feature 2.

The chemical composition consists of, in mass %, C: 0.03 to 0.12%, Si: 0.05 to 2.00%, Mn: 0.05 to 3.00%, P: 0.03% or less, S: 0.010% or less, Ni: 18.0 to less than 25.0%, Cr: 22.0 to less than 30.0%, Co: 0.04 to 0.80%, Ti: 0.002 to 0.010%, Nb: 0.1 to 1.0%, V: 0.01 to 1.00%, Al: 0.001 to less than 0.030%, N: 0.10 to 0.35%, Mo: 0 to 1.00%, W: 0 to 1.00%, B: 0 to 0.010%, and Ca: 0 to 0.0100%, with the balance being Fe and impurities.

The number density of precipitates having an equivalent circular diameter of 0.5 to 2.0 μm is 5000 pieces/mmor more.

Hereunder, Feature 1 and Feature 2 are described.

The chemical composition of the austenitic stainless alloy material of the present embodiment contains the following elements.

Carbon (C) increases the creep strength of the alloy material in a high-temperature environment. If the content of C is less than 0.03%, the aforementioned advantageous effect will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment.

On the other hand, if the content of C is more than 0.12%, even if the contents of other elements are within the range of the present embodiment, MC-type Cr carbides will form at grain boundaries. In such case, Cr-depleted zones will form at the grain boundaries. Consequently, the stress relaxation cracking resistance of the alloy material will decrease.

Therefore, the content of C is 0.03 to 0.12%.

A preferable lower limit of the content of C is more than 0.03%, more preferably is 0.04%, and further preferably is 0.05%.

A preferable upper limit of the content of C is 0.11%, more preferably is 0.10%, and further preferably is 0.09%.

Silicon (Si) deoxidizes the alloy in the steelmaking process. Si also increases the oxidation resistance of the alloy material in a high-temperature environment. If the content of Si is less than 0.05%, the aforementioned advantageous effects will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment.

On the other hand, if the content of Si is more than 2.00%, the weld hot cracking resistance will decrease even if the contents of other elements are within the range of the present embodiment.

Therefore, the content of Si is 0.05 to 2.00%.

A preferable lower limit of the content of Si is 0.10%, more preferably is 0.15%, further preferably is 0.18%, and further preferably is 0.20%.

A preferable upper limit of the content of Si is 1.80%, more preferably is 1.60%, further preferably is 1.40%, further preferably is 1.30%, and further preferably is 1.25%.

Manganese (Mn) deoxidizes a weld zone of the alloy material during welding. Mn also stabilizes austenite. If the content of Mn is less than 0.05%, the aforementioned advantageous effects will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment.

On the other hand, if the content of Mn is more than 3.00%, even if the contents of other elements are within the range of the present embodiment, sigma phase (σ phase) will easily form during use in a high-temperature environment. The σ phase will reduce the toughness and creep ductility of the alloy material in a high-temperature environment.

Therefore, the content of Mn is 0.05 to 3.00%.

A preferable lower limit of the content of Mn is 0.10%, more preferably is 0.15%, further preferably is 0.20%, further preferably is 0.30%, further preferably is 0.40%, and further preferably is 0.45%.

A preferable upper limit of the content of Mn is less than 3.00%, more preferably is 2.99%, further preferably is 2.95%, further preferably is 2.90%, further preferably is 2.80%, further preferably is 2.60%, further preferably is 2.40%, further preferably is 2.35%, further preferably is 2.20%, and further preferably is 2.00%.

Phosphorus (P) is unavoidably contained. In other words, the content of P is more than 0%.

P segregates to grain boundaries of the alloy material. If the content of P is more than 0.03%, even if the contents of other elements are within the range of the present embodiment, the aforementioned segregation will occur and the stress relaxation cracking resistance will decrease.

Therefore, the content of P is 0.03% or less.

The content of P is preferably as low as possible. However, excessively reducing the content of P will raise the production cost of the alloy material. Therefore, when normal industrial manufacturing is taken into consideration, a preferable lower limit of the content of P is 0.01%.

A preferable upper limit of the content of P is 0.02%.

Sulfur(S) is unavoidably contained. In other words, the content of S is more than 0%.