JP7131318B2 - austenitic stainless steel - Google Patents

Description

TECHNICAL FIELD This disclosure relates to stainless steels, and more particularly to austenitic stainless steels.

Conventionally, in equipment such as thermal power boilers and chemical plants used in a high-temperature carburizing environment, austenitic stainless steel with increased Cr content and Ni content, or Ni-based stainless steel with increased Cr content is used as heat-resistant steel. alloy is used. These heat-resistant steels are austenitic stainless steels or Ni-based alloys containing about 20-40% by mass of Cr and about 20-70% by mass of Ni.

Piping for facilities such as thermal power boilers and chemical plants is manufactured into a tube shape by hot working after the austenitic stainless steel or Ni-based alloy described above is melted. After being manufactured into a tube shape, the piping of facilities such as thermal power boilers and chemical plants is used in a high temperature environment of 1000° C. or higher. Therefore, the heat-resistant steel described above is required to have excellent creep strength at high temperatures of 1000° C. or higher (hereinafter also referred to as high-temperature creep strength) for the purpose of extending the service life.

By the way, recently, cheap shale gas is being produced due to the so-called shale revolution. When shale gas is used as a raw material gas in facilities such as chemical plants, the carbon (C) derived from the raw material gas reduces the amount of metal pipes used in facilities such as chemical plants (for example, reaction Carburization, which is a corrosion phenomenon of pipes), is likely to occur. Therefore, steels used in facilities such as chemical plants are required to have excellent carburization resistance.

Examples of austenitic stainless steels that can be used in high-temperature carburizing environments include International Publication No. 2017/119415 (Patent Document 1), Japanese Patent Application Publication No. 2018-3064 (Patent Document 2), and International Publication No. 2010/113830 (Patent Document 2). 3).

The austenitic heat-resistant alloy disclosed in Patent Document 1 has, in mass %, C: 0.03 to less than 0.25%, Si: 0.01 to 2.0%, Mn: 2.0% or less, Cr: 10 to less than 30%, Ni: more than 25 to 45%, Al: more than 2.5 to less than 4.5%, Nb: 0.2 to 3.5%, N: 0.025% or less, Ti: 0 to Less than 0.2%, W: 0-6%, Mo: 0-4%, Zr: 0-0.1%, B: 0-0.01%, Cu: 0-5%, Rare earth element: 0- Contains 0.1%, Ca: 0 to 0.05%, and Mg: 0 to 0.05%, the balance is Fe and impurities, and P and S in the impurities are each P: 0.04 % or less, and S: 0.01% or less, and the total volume fraction of precipitates having an equivalent circle diameter of 6 μm or more in the structure is 5% or less.

The austenitic stainless steel disclosed in Patent Document 2 has, in mass %, C: 0.25 to 0.7%, Si: 0.01 to 2.0%, Mn: 2.0% or less, P: 0 .04% or less, S: 0.01% or less, Cr: 10 to 19%, Ni: 20 to 40%, Al: more than 2.5 to less than 4.5%, Nb: 0.01 to 3.5% , N: 0.03% or less, Ti: 0 to less than 0.2%, Ca: 0 to 0.05%, and Mg: 0 to 0.05%, the balance being Fe and impurities It has a composition and satisfies formula (1). _0.4≤ (CCr'/ _CAl ')/( _CCr /CAl) _≤0.8 (1). Here, the Cr concentration (% by mass) in the range from the surface of the austenitic stainless steel to a depth of 2 μm is substituted for C _Cr ' in equation (1). The Al concentration (% by mass) in the range from the surface of the austenitic stainless steel to a depth of 2 μm is substituted for C _Al '. Also, the Cr concentration (% by mass) of the austenitic stainless steel base material is substituted for C _Cr . The Al concentration (% by mass) of the austenitic stainless steel base material is substituted for C _Al .

The cast product disclosed in Patent Document 3 has C: 0.05 to 0.7%, Si: more than 0% and 2.5% or less, and Mn: more than 0% and 3.0% by mass. % or less, Cr: 15 to 50%, Ni: 18 to 70%, Al: 2 to 4%, rare earth elements: 0.005 to 0.4%, and W: 0.5 to 10% and / or Mo A barrier layer is formed on the surface of the cast body that is in contact with a high-temperature atmosphere, and the barrier layer is An Al ₂ O ₃ layer having a thickness of 0.5 μm or more, wherein 80 area % or more of the outermost surface of the barrier layer is Al ₂ O ₃ , and an alloy is present at the interface between the Al ₂ O ₃ layer and the casting. It is characterized by dispersing Cr-based particles having a Cr concentration higher than that of the matrix.

WO2017/119415 JP 2018-3064 A WO2010/113830

The techniques disclosed in Patent Documents 1 to 3 above provide austenitic stainless steels that can be used in high-temperature carburizing environments.

In particular, according to the techniques disclosed in Patent Document 1 and Patent Document 2, austenitic stainless steel having excellent carburization resistance and high high-temperature creep strength can be obtained. However, it would be desirable if the carburization resistance and high-temperature creep strength of austenitic stainless steel could be improved by a method different from the techniques of Patent Documents 1 and 2.

The technique of Patent Document 3 evaluates the ductility in a room temperature environment, but does not evaluate the high temperature creep strength in a high temperature environment.

An object of the present disclosure is to provide an austenitic stainless steel with excellent carburization resistance and high high temperature creep strength.

The austenitic stainless steel of the present disclosure has, in mass%, C: 0.250 to 0.700%, Si: 0.01 to 2.00%, Mn: 2.00% or less, P: 0.040% or less , S: 0.010% or less, Cr: 10.00 to less than 25.00%, Ni: 30.00 to 60.00%, Al: more than 2.50% to 3.50%, Nb: 0.20 ~ 3.50%, further Zr: 0.0001 ~ 0.1000%, Hf: 0.0001 ~ 0.1000%, Sn: 0.0001 ~ 0.1000% and As: 0.0001 ~ One or more selected from the group consisting of 0.1000%, Ti: 0 to less than 0.20%, Mo: 0 to 0.10%, W: 0 to 0.20%, B: 0 to 0.1000%, V: 0-0.500%, Cu: 0-5.0%, Ca: 0-0.0500%, Mg: 0-0.0500%, REM: 0-0.1000%, It contains N: 0 to 0.030%, the balance being Fe and impurities, and has a chemical composition that satisfies formula (1).
0.06≦1.96×Zr+Hf+1.50×Sn+2.38×As≦0.55 (1)
Here, the content (% by mass) of the corresponding element is substituted for the symbol of the element in formula (1).

The austenitic stainless steel of the present disclosure has excellent carburization resistance and high high temperature creep strength.

The present inventors have investigated and studied the carburization resistance and high-temperature creep strength of austenitic stainless steel in a high-temperature carburizing environment, and obtained the following findings. A high-temperature carburizing environment refers to an environment of 1000° C. or higher in a hydrocarbon gas atmosphere.

If the austenitic stainless steel or Ni-based alloy contains Cr, a protective film of Cr ₂ O ₃ is formed on the surface of the steel, increasing carburization resistance. However, Cr ₂ O ₃ is thermodynamically unstable. Therefore, in the present invention, Al is added to form an Al ₂ O ₃ coating on the steel surface. Al ₂ O ₃ acts as a protective coating. Al ₂ O ₃ is thermodynamically more stable than Cr ₂ O ₃ in high temperature carburizing environments. That is, Al ₂ O ₃ can improve the carburization resistance of the austenitic stainless steel even in an environment of 1000° C. or higher.

On the other hand, if the austenitic stainless steel or Ni-based alloy contains excessive Al, coarse γ'-Ni ₃ Al precipitates. Coarse γ'-Ni ₃ Al significantly reduces the high-temperature creep strength of austenitic stainless steels or Ni-based alloys. Therefore, it is necessary to limit the Al content to a certain amount or less.

Therefore, the present inventors investigated a method of promoting the formation of the Al ₂ O ₃ coating while suppressing the Al content to a certain amount or less. As a result, it was found that the proper addition of Zr, Hf, Sn and As promotes the formation of protective uniform Al ₂ O ₃ . As a result, the carburization resistance of the austenitic stainless steel or Ni-based alloy can be improved without excessively increasing the Al content.

It is not clear why Zr, Hf _, Sn and As promote uniform formation of the _Al2O3 coating. Al combines with C, N, and O, which inhibit Al ₂ O ₃ formation, to form carbonitrides and oxides, thereby promoting the external oxidation of Al, thereby improving the uniformity of the Al ₂ O ₃ coating. Presumed to promote formation.

On the other hand, when Zr, Hf, Sn and As are contained in excess, coarse precipitates are formed in the microstructure of the austenitic stainless steel or Ni-based alloy. Coarse precipitates significantly reduce the high-temperature creep strength of austenitic stainless steels or Ni-based alloys. Therefore, in order to achieve both excellent carburization resistance and high high temperature creep strength, it is necessary to appropriately adjust the contents of Zr, Hf, Sn and As. Specifically, when the contents of Zr, Hf, Sn and As satisfy the formula (1), an austenitic stainless steel or Ni-based alloy having both excellent carburization resistance and high high-temperature creep strength can be obtained. .
0.06≦1.96×Zr+Hf+1.50×Sn+2.38×As≦0.55 (1)
Here, the content (% by mass) of the corresponding element is substituted for the symbol of the element in formula (1).

The austenitic stainless steel of the present disclosure, which was completed based on the above findings, has C: 0.250 to 0.700%, Si: 0.01 to 2.00%, and Mn: 2.00% by mass. Below, P: 0.040% or less, S: 0.010% or less, Cr: 10.00 to less than 25.00%, Ni: 30.00 to 60.00%, Al: more than 2.50% to 3 50%, Nb: 0.20-3.50%, Zr: 0.0001-0.1000%, Hf: 0.0001-0.1000%, Sn: 0.0001-0. 1000% and As: one or more selected from the group consisting of 0.0001 to 0.1000%, Ti: 0 to less than 0.20%, Mo: 0 to 0.10%, W: 0 to 0.20%, B: 0-0.1000%, V: 0-0.500%, Cu: 0-5.0%, Ca: 0-0.0500%, Mg: 0-0.0500%, It contains REM: 0 to 0.1000%, N: 0 to 0.030%, and the balance consists of Fe and impurities, and has a chemical composition that satisfies formula (1).
0.06≦1.96×Zr+Hf+1.50×Sn+2.38×As≦0.55 (1)
Here, the content (% by mass) of the corresponding element is substituted for the symbol of the element in formula (1).

The austenitic stainless steel of the present disclosure satisfies formula (1). Therefore, it has excellent carburization resistance and high high-temperature creep strength.

The chemical composition of the austenitic stainless steel described above is, in mass%, Ti: 0.005 to less than 0.20%, Mo: 0.005 to 0.10%, W: 0.005 to 0.20%, B : 0.0001 to 0.1000%, V: 0.001 to 0.500%, and Cu: 0.005 to 5.0% good too.

The chemical composition of the austenitic stainless steel described above is, in mass%, Ca: 0.0001 to 0.0500%, Mg: 0.0001 to 0.0500%, and REM: 0.0005 to 0.1000%. You may contain 1 type(s) or 2 or more types selected from the group which becomes.

The chemical composition of the austenitic stainless steel described above may contain N: 0.0005 to 0.030% in mass %.

The austenitic stainless steel according to this embodiment will be described in detail below. "%" for elements means % by weight unless otherwise specified.

[Chemical composition]
The chemical composition of the austenitic stainless steel according to this embodiment contains the following elements.

C: 0.250-0.700%
Carbon (C) primarily combines with Cr to form Cr carbides in the steel, enhancing high temperature creep strength during use in high temperature carburizing environments. If the C content is too low, this effect cannot be obtained. On the other hand, if the C content is too high, a large number of coarse eutectic carbides are formed in the solidified structure of the steel after casting, reducing the toughness of the steel. Therefore, the C content is 0.250-0.700%. A preferred lower limit for the C content is 0.280%, more preferably 0.300%. A preferable upper limit of the C content is 0.650%, more preferably 0.600%.

Si: 0.01-2.00%
Silicon (Si) deoxidizes steel. If other elements can sufficiently deoxidize, the Si content may be as low as possible. On the other hand, if the Si content is too high, the hot workability deteriorates. Therefore, the Si content is 0.01-2.00%. A preferred lower limit for the Si content is 0.02%, more preferably 0.03%. A preferred upper limit for the Si content is 1.00%.

Mn: 2.00% or less Manganese (Mn) is inevitably contained. Mn combines with S contained in steel to form MnS and enhances the hot workability of steel. However, if the Mn content is too high, the steel will be too hard, resulting in poor hot workability and weldability. Therefore, the Mn content is 2.00% or less. When stably obtaining the above effect, the lower limit of the Mn content is preferably 0.05%, more preferably 0.10%. A preferable upper limit of the Mn content is 1.50%, more preferably 1.30%.

P: 0.040% or less Phosphorus (P) is an impurity. P deteriorates the weldability and hot workability of steel. Therefore, the P content is 0.040% or less. A preferable upper limit of the P content is 0.030%. The lower the P content is, the better. However, excessively reducing the P content increases the cost. Therefore, the lower limit of the P content is, for example, 0.0005%.

S: 0.010% or less Sulfur (S) is an impurity. S reduces the weldability and hot workability of steel. Therefore, the S content is 0.010% or less. A preferable upper limit of the S content is 0.008%. It is preferable that the S content is as low as possible. However, excessively reducing the S content increases the cost. Therefore, the lower limit of the S content is, for example, 0.001%.

Cr: 10.00 to less than 25.00% Chromium (Cr) promotes the formation of an Al ₂ O ₃ film during the heat treatment process and in a high-temperature carburizing environment. Cr also combines with C in the steel to form Cr carbides in the steel, increasing the high temperature creep strength. If the Cr content is too low, these effects cannot be obtained. On the other hand, if the Cr content is too high, in a high-temperature carburizing environment, Cr combines with C from the atmospheric gas (hydrocarbon gas) to form Cr carbides on the steel surface. When Cr carbide is formed on the steel surface, Cr on the steel surface is locally depleted. In this case, a so-called TEE effect (Third Element Effect) is not obtained, and a uniform Al ₂ O ₃ film is not formed. Furthermore, if the Cr content is too high, Cr carbides on the steel surface physically impede the formation of a uniform Al ₂ O ₃ coating. Therefore, the Cr content is between 10.00 and less than 25.00%. A preferable lower limit of the Cr content is 11.00%, more preferably 12.00%. A preferable upper limit of the Cr content is 24.00%, more preferably 23.00%.

Ni: 30.00% to 60.00%
Nickel (Ni) stabilizes austenite and increases high temperature creep strength. Ni further enhances the carburization resistance of steel. If the Ni content is too low, these effects cannot be obtained. On the other hand, if the Ni content is too high, not only will these effects saturate, but raw material costs will increase. If the Ni content is too high, intermetallic compounds containing Al, such as γ'-Ni ₃ Al, precipitate during the manufacturing process, and the hot workability is significantly reduced. Therefore, the Ni content is 30.00% to 60.00%. A preferable lower limit of the Ni content is 31.00%, more preferably 32.00%. A preferable upper limit of the Ni content is 55.00%, more preferably 50.00%.

Al: more than 2.50% to 3.50%
Aluminum (Al) forms an Al ₂ O ₃ film on the steel surface during the heat treatment process and in a high-temperature carburizing environment, increasing the carburizing resistance of the steel. Especially in the high-temperature carburizing environment assumed in the present invention, the Al ₂ O ₃ coating is thermodynamically more stable than the conventionally used Cr ₂ O ₃ coating. If the Al content is too low, these effects cannot be obtained. On the other hand, if the Al content is too high, intermetallic compounds containing Al, such as γ'-Ni ₃ Al, are coarsely precipitated during the manufacturing process. Coarse intermetallic compounds significantly reduce the high temperature creep strength of steel. Therefore, the Al content is more than 2.50% to 3.50%. A preferable lower limit of the Al content is 2.55%, more preferably 2.60%. A preferable upper limit of the Al content is 3.45%, more preferably 3.40%. In the austenitic stainless steel according to the present invention, the Al content means the total amount of Al contained in the steel material.

Nb: 0.20-3.50%
Niobium (Nb) forms intermetallic compounds (Laves phase and Ni ₃ Nb phase) that serve as precipitation strengthening phases, strengthens grain boundaries and grain interiors, and increases the creep strength of steel. On the other hand, if the Nb content is too high, an excessive amount of intermetallic compounds will be formed and the toughness of the steel will be lowered. If the Nb content is too high, the toughness after long-term aging also decreases. Therefore, the Nb content is 0.20-3.50%. A preferred lower limit for the Nb content is 0.25%, more preferably 0.30%. A preferable upper limit of the Nb content is less than 3.20%, more preferably 3.00%.

Zr: 0.0001 to 0.1000%, Hf: 0.0001 to 0.1000%, Sn: 0.0001 to 0.1000% and As: selected from the group consisting of 0.0001 to 0.1000% One or more kinds The chemical composition of the austenitic stainless steel of the present embodiment is one or two selected from the group consisting of zirconium (Zr), hafnium (Hf), tin (Sn) and arsenic (As). contains more than The respective contents of Zr, Hf, Sn and As when contained are as follows. However, the contents of Zr, Hf, Sn and As satisfy formula (1) described later.

Zr: 0.0001 to 0.1000%
Zirconium (Zr) combines with C, N, and O, which inhibit the formation of Al ₂ O ₃ , to form carbonitrides and oxides during the heat treatment process and in a high-temperature carburizing environment. As a result, it promotes the formation of the Al ₂ O ₃ film. On the other hand, if the Zr content is too high, the volume fraction of short nitrides and oxides in the steel becomes excessively high, deteriorating hot workability. Therefore, the Zr content is 0.0001-0.1000%. A preferred lower limit for the Zr content is 0.0010%, more preferably 0.0020%. A preferred upper limit for the Zr content is 0.0800%, more preferably 0.0500%.

Hf: 0.0001-0.1000%
Hafnium (Hf) combines with C, N, and O, which inhibit Al ₂ O ₃ formation, to form carbonitrides and oxides during the heat treatment process and in a high-temperature carburizing environment. As a result, it promotes the formation of the Al ₂ O ₃ film. On the other hand, if the Hf content is too high, the volume fraction of short nitrides and oxides in the steel becomes excessively high, deteriorating hot workability. Therefore, the Hf content is 0.0001-0.1000%. A preferable lower limit of the Hf content is 0.0010%, more preferably 0.0020%. A preferred upper limit of the Hf content is 0.0800%, more preferably 0.0500%.

Sn: 0.0001-0.1000%
Tin (Sn) combines with C, N, and O, which inhibit the formation of Al ₂ O ₃ , to form carbonitrides and oxides during the heat treatment process and in a high-temperature carburizing environment. As a result, it promotes the formation of the Al ₂ O ₃ film. On the other hand, if the Sn content is too high, the volume fraction of short nitrides and oxides in the steel becomes excessively high, deteriorating hot workability. Therefore, the Sn content is 0.0001-0.1000%. A preferred lower limit for the Sn content is 0.0010%, more preferably 0.0020%. A preferable upper limit of the Sn content is 0.0500%, more preferably 0.0100%.

As: 0.0001-0.1000%
Arsenic (As) combines with C, N, and O, which inhibit the formation of Al ₂ O ₃ , to form carbonitrides and oxides during the heat treatment process and in a high-temperature carburizing environment. As a result, it promotes the formation of the Al ₂ O ₃ film. On the other hand, if the As content is too high, the volume fraction of short nitrides and oxides in the steel becomes excessively high, deteriorating hot workability. Therefore, the As content is 0.0001-0.1000%. A preferable lower limit of the As content is 0.0010%, more preferably 0.0020%. A preferable upper limit of the As content is 0.0500%, more preferably 0.0100%.

One or more selected from the group consisting of Zr, Hf, Sn and As may be contained. Therefore, if one of Zr, Hf, Sn and As is contained, other elements out of Zr, Hf, Sn and As need not be contained.

The rest of the chemical composition of the austenitic stainless steel of this embodiment consists of Fe and impurities. Here, impurities are those that are mixed from ore, scrap, or the manufacturing environment as raw materials when austenitic stainless steel is industrially manufactured, and are allowed within a range that does not adversely affect the present invention. means to be

[Regarding arbitrary elements]
The chemical composition of the austenitic stainless steel described above may further contain one or more selected from the group consisting of Ti, Mo, W, B, V and Cu in place of part of Fe. . All of these elements are optional elements, and all increase the creep strength of steel.

Ti: 0 to less than 0.20% Titanium (Ti) is an optional element and may not be contained. When included, Ti forms intermetallic compounds (Laves phase and Ni ₃ Ti phase) that become precipitation-strengthening phases and increases creep strength by precipitation strengthening. On the other hand, if the Ti content is too high, an excessive amount of intermetallic compounds is produced, resulting in deterioration of hot ductility and hot workability. If the Ti content is too high, the toughness after long-term aging is further reduced. Therefore, the Ti content is between 0 and less than 0.20%. A preferred lower limit for the Ti content is 0.005%, more preferably 0.010%. A preferable upper limit of the Ti content is 0.15%, more preferably 0.10%.

Mo: 0-0.10%
Molybdenum (Mo) is an optional element and may not be contained. When contained, Mo forms a solid solution in the mother phase, austenite. The dissolved Mo increases the creep strength by solid solution strengthening. On the other hand, if the Mo content is too high, the hot workability deteriorates. Therefore, the Mo content is 0-0.10%. A preferable lower limit of the Mo content is 0.005%, more preferably 0.010%. A preferred upper limit for the Mo content is 0.08%, more preferably 0.05%.

W: 0-0.20%
Tungsten (W) is an optional element and may not be contained. When contained, W forms a solid solution in austenite, which is the mother phase. Solid-soluted W increases the creep strength by solid-solution strengthening. On the other hand, if the W content is too high, the hot workability deteriorates. Therefore, the W content is 0-0.20%. A preferable lower limit of the W content is 0.005%, more preferably 0.010%. A preferable upper limit of the W content is 0.15%, more preferably 0.10%.

B: 0 to 0.1000%
Boron (B) is an optional element and may not be contained. When included, B segregates at grain boundaries and promotes precipitation of intermetallic compounds at grain boundaries. This increases the creep strength of the steel. On the other hand, if the B content is too high, the weldability and hot workability of the steel deteriorate. Therefore, the B content is 0-0.1000%. A preferable lower limit of the B content is 0.0001%, more preferably 0.0005%. A preferable upper limit of the B content is 0.0800%, more preferably 0.0600%.

V: 0-0.500%
Vanadium (V) is an optional element and may not be contained. When included, V forms intermetallic compounds similar to Ti and enhances the creep strength of steel. On the other hand, if the V content is too high, the deposition rate of intermetallic compounds in the steel becomes excessively high, degrading the hot workability. Therefore, the V content is 0-0.500%. A preferable lower limit of the V content is 0.001%, more preferably 0.002%. A preferable upper limit of the V content is 0.300%, more preferably 0.100%.

Cu: 0-5.0%
Copper (Cu) is an optional element and may not be contained. When included, Cu stabilizes austenite. Cu also increases the strength and creep strength of steel through precipitation strengthening. On the other hand, if the Cu content is too high, the ductility and hot workability of the steel are reduced. Therefore, the Cu content is 0-5.0%. A preferred lower limit for the Cu content is 0.005%, more preferably 0.010%. A preferable upper limit of the Cu content is 3.0%, more preferably 1.0%.

The chemical composition of the austenitic stainless steel described above may further contain one or more selected from the group consisting of Ca, Mg and REM in place of part of Fe. All of these elements are optional elements and improve the hot workability of steel.

Ca: 0-0.0500%
Calcium (Ca) is an optional element and may not be contained. When included, Ca fixes S as a sulfide and enhances hot workability. On the other hand, if the Ca content is too high, toughness and ductility will decrease. Therefore, hot workability is deteriorated. If the Ca content is too high, the detergency is further reduced. Therefore, the Ca content is 0-0.0500%. A preferable lower limit of Ca is 0.0001%, more preferably 0.0005%. A preferable upper limit of the Ca content is 0.0300%, more preferably 0.0100%.

Mg: 0-0.0500%
Magnesium (Mg) is an optional element and may not be contained. When included, Mg fixes S as sulfides and enhances the hot workability of the steel. On the other hand, if the Mg content is too high, toughness and ductility will decrease. Therefore, hot workability is deteriorated. If the Mg content is too high, the detergency is further reduced. Therefore, the Mg content is 0-0.0500%. A preferred lower limit for Mg is 0.0001%, more preferably 0.0005%. A preferable upper limit of the Mg content is 0.0300%, more preferably 0.0100%.

REM: 0-0.1000%
A rare earth element (REM) is an optional element and may not be contained. When included, REM fixes S as a sulfide and enhances hot workability. REMs also form oxides to enhance corrosion resistance, creep strength, and creep ductility. However, if the REM content is too high, the amount of inclusions such as oxides increases, degrading hot workability and weldability and increasing manufacturing costs. Therefore, the REM content is 0-0.1000%. A preferred lower limit for the REM content is 0.0005%, more preferably 0.0010%. A preferred upper limit for the REM content is 0.0900%, more preferably 0.0800%.

As used herein, REM is a generic term for a total of 17 elements including Sc, Y and lanthanoids. When the REM contained in the austenitic stainless steel is one of these elements, the REM content means the content of that element. When two or more REMs are contained in the austenitic stainless steel, the REM content means the total content of those elements. REM is generally contained in misch metal. Therefore, for example, it may be added in the form of misch metal so that the REM content is within the above range.

The chemical composition of the austenitic stainless steel described above may further contain N instead of part of Fe. N is an optional element and stabilizes austenite.

N: 0-0.030%
Nitrogen (N) is an optional element and may not be contained. When included, N stabilizes austenite. On the other hand, if the N content is too high, coarse nitrides and/or carbonitrides remain undissolved even after the heat treatment. Coarse nitrides and/or carbonitrides reduce the toughness of steel. Therefore, the N content is 0-0.030%. A preferred lower limit for N is 0.0005%. A preferred upper limit for N is 0.010%.

The austenitic stainless steel of this embodiment satisfies the chemical composition described above and the following formula (1). Only then can an austenitic stainless steel having excellent carburization resistance and high high-temperature creep strength be obtained.

[Regarding formula (1)]
The austenitic stainless steel of this embodiment satisfies formula (1).
0.06≦1.96×Zr+Hf+1.50×Sn+2.38×As≦0.55 (1)
Here, the content (% by mass) of the corresponding element is substituted for the symbol of the element in formula (1).

In order to improve the carburization resistance of austenitic stainless steel, it is important to form Al ₂ O ₃ on the steel surface as a protective film. Increasing the Al concentration in the austenitic stainless steel is the most effective way to promote the formation of Al ₂ O ₃ . However, as described above, when more than 3.50% of Al is added, the high-temperature creep strength is remarkably lowered. In the present disclosure, one or more selected from the group consisting of Zr, Hf, Sn and As are added to the austenitic stainless steel. Thereby, even if Al is 3.5% or less, Al ₂ O ₃ can be uniformly formed on the steel surface.

Define FN1=1.96*Zr+Hf+1.50*Sn+2.38*As. If the value of FN1 is less than 0.06, the effects of Zr, Hf, Sn and As cannot be obtained. On the other hand, excessive addition of these elements not only saturates the effect of promoting the formation of the Al ₂ O ₃ film, but also forms coarse precipitates in the microstructure of the austenitic stainless steel. Coarse precipitates significantly reduce the high-temperature creep strength of austenitic stainless steel. Specifically, if the value of FN1 exceeds 0.55, the high-temperature creep strength decreases. Therefore, the value of FN1 should be 0.06 to 0.55 in order to obtain the effect of promoting the formation of Al ₂ O ₃ while maintaining good high-temperature creep strength.

The lower limit of the value of FN1 is preferably 0.08, more preferably 0.09, still more preferably 0.10. The upper limit of the value of FN1 is preferably 0.50, more preferably 0.48, even more preferably 0.46, still more preferably 0.44.

The shape of the austenitic stainless steel according to this embodiment is not particularly limited. Austenitic stainless steels are, for example, steel pipes. Austenitic stainless steel tubes are used as reactor tubes for chemical plants. The austenitic stainless steel may be plate material, bar material, wire material, or the like.

[Production method]
Hereinafter, a method for manufacturing a steel pipe will be described as an example of the method for manufacturing austenitic stainless steel according to the present embodiment.

[Preparation process]
A molten steel having the above chemical composition is produced. A well-known degassing treatment is performed on the molten steel as necessary. Molten steel is used to manufacture raw materials by casting. The raw material may be an ingot by ingot casting, or may be a slab, bloom, billet, or other slab by continuous casting. Shot processing and/or cutting processing may be performed on the material after the pickling treatment.

[Hot forging process]
A cylinder material may be manufactured by performing hot forging on the manufactured material. By performing hot forging, the internal structure of the molten steel produced in the preparation process can be changed from a solidified structure to a homogeneous regular grain structure. Although the temperature of hot forging is not particularly limited, it is, for example, 900 to 1300°C.

[Hot working process]
A raw steel tube may be manufactured by performing hot working on the raw material manufactured in the preparatory step or the hot forged raw material (cylindrical raw material). For example, a through hole is formed in the center of the cylindrical material by machining. Hot extrusion is performed on the columnar material in which the through holes are formed to manufacture a steel tube. The processing temperature for hot extrusion is not particularly limited, but is, for example, 900 to 1300°C. A cylindrical raw material may be pierced and rolled (Mannesmann method or the like) to produce a steel tube.

[Cold working process]
An intermediate material may be produced by cold working the steel tube after hot working. Cold working is, for example, cold drawing or the like. If strain is applied to the steel surface in the cold working process, Al can easily move to the steel surface. The working rate of cold working is not particularly limited, but is, for example, 10 to 90%.

[Heat treatment process]
The material manufactured in the preparatory step, the steel tube after hot working, or the intermediate material after cold working may be subjected to heat treatment in an air atmosphere as a solution treatment. Heat treatment in an air atmosphere recrystallizes the crystal grains in the structure to obtain a uniform regular grain structure.

Although the heat treatment conditions are not particularly limited, for example, the heat treatment temperature is 900 to 1300° C. and the heat treatment time is 1.0 to 60.0 minutes.

Scale formed on the steel surface may be removed from the intermediate material after the heat treatment. Scale removal may be performed by, for example, pickling, shot processing and/or cutting. Shot processing and/or blast processing may be performed after the pickling treatment.

The pickling treatment can remove the scale formed on the steel surface. Although pickling conditions are not particularly limited, for example, a mixed acid solution of nitric acid and hydrochloric acid is used for pickling, and the pickling time is, for example, 30 to 60 minutes.

The scale formed on the steel surface can be removed by shot processing and/or cutting. Strain can be further imparted to the steel surface by performing shot processing and/or cutting. Although the material and shape of shot grains in shot processing and the processing conditions for shot processing and/or cutting are not specified, the material, shape, and processing conditions sufficient to remove scale from the steel surface or impart strain to the steel surface. and

The austenitic stainless steel of this embodiment is manufactured by the manufacturing method described above. In addition, the manufacturing method of a steel pipe was demonstrated above. However, the same manufacturing method (preparation process, hot forging process, hot working process, cold working process, heat treatment process) may be used to produce a plate material, a bar material, a wire material, and the like. The austenitic stainless steel of this embodiment is particularly preferably applied to steel pipes. Therefore, preferably, the austenitic stainless steel of this embodiment is an austenitic stainless steel pipe.

[Production method]
Molten steel having the chemical composition shown in Table 1 was produced using a vacuum melting furnace. In Table 1, REM is the sum of detected amounts of La, Ce and Nd.

A cylindrical ingot was produced using the molten steel. The ingot was subjected to solution treatment to produce an intermediate material.

[Carburizing test]
A test piece of 8 mm×20 mm×30 mm was produced from the obtained intermediate material by machining. The entire surface of the test piece was polished using #600 emery wet abrasive paper. After polishing finish, ultrasonic degreasing was performed in acetone. 67 vol. % H ₂ -30 vol. % CH ₄ -3 vol. It was held at 1100°C for 96 hours in an atmosphere of % CO ₂ . After carburizing, the surface of the test piece was dry hand-polished with #600 abrasive paper to remove surface scales and the like. Four layers of chips for analysis were collected at a pitch of 0.5 mm from the surface of the test piece after descaling. The C concentration of the obtained analysis chips was measured by a high-frequency combustion infrared absorption method. From the measurement results, the concentration of C originally contained in the steel was subtracted to obtain the amount of infiltrated C. The average amount of C that penetrated into the four layers was taken as the average amount of C that penetrated (% by mass). The results are shown in Table 2, "Average Penetrating C Amount (% by Mass)".

[Creep rupture test]
A creep test piece was produced from the intermediate material obtained above. A creep test piece was taken parallel to the length of the cylinder from the center of the intermediate material. The creep test piece was a round bar test piece, the diameter of the parallel portion was 6 mm, and the distance between marks was 30 mm. Using the obtained creep test piece, a creep rupture test was carried out. The creep rupture test was performed in an air atmosphere at 1000° C. under a tensile load of 15 MPa. Those with a rupture time of 1.0×10 ³ hours or more were evaluated as acceptable (○). Samples with a rupture time of less than 1.0×10 ³ hours were rejected (×). The results are shown in Table 2, "High Temperature Creep Strength".

[Evaluation results]
With reference to Tables 1 and 2, steel grades A to J and steel grades V to DD have appropriate element contents, and FN1 has a chemical composition that satisfies formula (1). As a result, the penetration C was 0.20% or less, indicating excellent carburization resistance. Further, steel grades A to J and steel grades V to DD had a rupture time of 1.0×10 ³ hours or more in a creep rupture test at 1000° C., indicating high high-temperature creep strength.

On the other hand, in steel type K, the C content was too low. As a result, the rupture time in the creep rupture test at 1000° C. was less than 1.0×10 ³ hours, indicating low high-temperature creep strength.

In steel grade L, the Ni content was too low. As a result, the penetrating C content was 0.35%, indicating low carburization resistance. Steel type L also had a rupture time of less than 1.0×10 ³ hours in a creep rupture test at 1000° C., indicating low high-temperature creep strength.

In steel grade M, the Al content was too low. As a result, the penetrating C content was 0.52%, indicating low carburization resistance.

In steel type N, the Al content was too high. As a result, the rupture time in the creep rupture test at 1000° C. was less than 1.0×10 ³ hours, indicating low high-temperature creep strength.

In steel grade O, the Zr content was too high and the value of FN1 was too high. As a result, the rupture time in the creep rupture test at 1000° C. was less than 1.0×10 ³ hours, indicating low high-temperature creep strength.

In steel type P, the Hf content was too high and the value of FN1 was too high. As a result, the rupture time in the creep rupture test at 1000° C. was less than 1.0×10 ³ hours, indicating low high-temperature creep strength.

In steel grade Q, the Sn content was too high and the value of FN1 was too high. As a result, the rupture time in the creep rupture test at 1000° C. was less than 1.0×10 ³ hours, indicating low high-temperature creep strength.

In steel type R, the As content was too high and the value of FN1 was too high. As a result, the rupture time in the creep rupture test at 1000° C. was less than 1.0×10 ³ hours, indicating low high-temperature creep strength.

In steel type S, the content of each element was appropriate, but the value of FN1 was too low. As a result, the penetrating C content was 0.36%, indicating low carburization resistance.

In steel type T, the content of each element was appropriate, but the value of FN1 was too high. As a result, the rupture time in the creep rupture test at 1000° C. was less than 1.0×10 ³ hours, indicating low high-temperature creep strength.

In steel type U, the contents of all of Zr, Hf, Sn and As were below the detection limit values, so the value of FN1 was too low. As a result, the penetrating C amount was 0.46%, indicating low carburization resistance.

The embodiments of the present invention have been described above. However, the above-described embodiments are merely examples for implementing the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately modifying the above-described embodiment without departing from the spirit of the present invention.

Claims (4)

in % by mass,
C: 0.250 to 0.700%,
Si: 0.01 to 2.00%,
Mn: 2.00% or less,
P: 0.040% or less,
S: 0.010% or less,
Cr: 10.00 to less than 25.00%,
Ni: 30.00 to 60.00%,
Al: more than 2.50% to 3.50%,
Nb: 0.20 to 3.50%,
Zr: 0.0001 to 0.1000%, Hf: 0.0001 to 0.1000%, Sn: 0.0001 to 0.1000% and As: selected from the group consisting of 0.0001 to 0.1000% 1 or 2 or more,
Ti: 0 to less than 0.20%,
Mo: 0-0.10%,
W: 0 to 0.20%,
B: 0 to 0.1000%,
V: 0 to 0.500%,
Cu: 0-5.0%,
Ca: 0 to 0.0500%,
Mg: 0-0.0500%,
REM: 0 to 0.1000%,
N: 0 to 0.030%, and
An austenitic stainless steel, the balance of which is Fe and impurities, and which has a chemical composition that satisfies formula (1).
0.06≦1.96×Zr+Hf+1.50×Sn+2.38×As≦0.55 (1)
Here, the content (% by mass) of the corresponding element is substituted for the symbol of the element in formula (1). The austenitic stainless steel according to claim 1,
The chemical composition, in mass %,
Ti: less than 0.005 to 0.20%,
Mo: 0.005-0.10%,
W: 0.005 to 0.20%,
B: 0.0001 to 0.1000%,
V: 0.001 to 0.500%, and
Cu: Austenitic stainless steel containing one or more selected from the group consisting of 0.005 to 5.0%. The austenitic stainless steel according to any one of claims 1 and 2,
The chemical composition, in mass %,
Ca: 0.0001 to 0.0500%,
Mg: 0.0001 to 0.0500%, and
REM: Austenitic stainless steel containing one or more selected from the group consisting of 0.0005 to 0.1000%. The austenitic stainless steel according to any one of claims 1 to 3,
The chemical composition, in mass %,
Austenitic stainless steel containing N: 0.0005 to 0.030%.