JP6627662B2 - Austenitic stainless steel - Google Patents

Description

  The present invention relates to stainless steel, and more particularly, to austenitic stainless steel.

  Conventionally, equipment such as boilers and chemical plants used in high-temperature environments use austenitic stainless steel with an increased Cr content and Ni content or a Ni-based alloy with an increased Cr content as heat-resistant steel. Have been. These heat-resistant steels are austenitic stainless steels or Ni-based alloys containing about 20 to 30% by mass of Cr and about 20 to 70% by mass of Ni.

  By the way, recently, the so-called shale revolution has produced inexpensive shale gas. When shale gas is used as a raw material gas in a facility such as a chemical plant, the C derived from the raw material gas causes a metal pipe (for example, a reaction tube) used in a facility such as a chemical plant to be compared with a conventional raw material such as naphtha. Caulking easily due to carburization, which is a corrosion phenomenon, and precipitation of carbon on the surface of a metal tube. For this reason, steels used for equipment such as chemical plants are required to have excellent carburization resistance and coking resistance.

  A stainless steel having improved carburization resistance and coking resistance is proposed in, for example, Japanese Patent Application Laid-Open No. 2005-48284 (Patent Document 1).

The stainless steel disclosed in Patent Literature 1 is, by mass%, C: 0.01 to 0.6%, Si: 0.1 to 5%, Mn: 0.1 to 10%, and P: 0.08%. Hereinafter, S: 0.05% or less, Cr: 20 to 55%, Ni: 10 to 70%, N: 0.001 to 0.25%, O (oxygen): 0.02% or less, the balance being Fe and It consists of a base material having a chemical composition consisting of inevitable impurities. This stainless steel has a Cr-deficient layer in the surface layer portion, the Cr-depleted layer has a Cr concentration of 10% or more, less than the Cr concentration of the base material, and the thickness of the Cr-depleted layer is within 20 μm. Patent Document 1 describes that by forming a protective film mainly composed of a Cr ₂ O ₃ film, carburization resistance and coking resistance are enhanced.

However, in the stainless steel disclosed in Patent Document 1, the main component of the protective film is a Cr ₂ O ₃ film. Therefore, especially in a high-temperature environment, the function of preventing intrusion of oxygen and carbon from the external atmosphere is not sufficient. As a result, internal oxidation and carburization may occur in the material.

Therefore, WO 2010/113830 (Patent Document 2), WO 2004/067788 (Patent Document 3), and JP-A-10-140296 (Patent Document 4) replace the Cr ₂ O ₃ film. A technique relating to a protective film is disclosed. Specifically, in these documents, a protective film mainly composed of Al ₂ O ₃ that is thermodynamically stable is formed on the surface of heat-resistant steel as a protective film instead of the Cr ₂ O ₃ film.

The cast product disclosed in Patent Literature 2 is, by mass%, C: 0.05 to 0.7%, Si: more than 0% to 2.5% or less, and Mn: more than 0% to 3.0%. Hereinafter, Cr: 15 to 50%, Ni: 18 to 70%, Al: 2 to 4%, rare earth element: 0.005 to 0.4%, and W: 0.5 to 10% and / or Mo: 0.1 to 5%, with a cast of heat-resistant alloy from the balance Fe and inevitable impurities. A barrier layer is formed on the surface of the casting in contact with the high-temperature atmosphere. The barrier layer is an Al ₂ O ₃ layer having a thickness of 0.5 μm or more, and 80% by area or more of the outermost surface of the barrier layer. Al ₂ O ₃ , wherein Cr-based particles having a higher Cr concentration than the base of the alloy are dispersed at the interface between the Al ₂ O ₃ layer and the casting. Patent Literature 2 describes that by adding Al, a protective film mainly composed of an Al ₂ O ₃ film is formed to enhance the carburization resistance.

The nickel-chromium casting alloy disclosed in U.S. Pat. No. 6,059,086 is disclosed in U.S. Pat. % Iron, 1.5 wt% to 7 wt% aluminum, niobium up to 2.5 wt%, titanium up to 1.5 wt%, zirconium 0.01 wt% to 0.4 wt%, nitrogen up to 0.06 wt%, Up to 12 wt% cobalt, up to 5 wt% molybdenum, up to 6 wt% tungsten, from 0.019 wt% to 0.089 wt% yttrium, balance nickel. Patent Literature 3 describes that by adding REM in addition to Al, it is possible to obtain a nickel-chromium casting alloy with improved peel resistance of Al ₂ O ₃ as a protective film.

The austenitic stainless steel disclosed in Patent Document 4 is, by weight%, C: 0.15% or less, Si: 0.9% or less, Mn: 0.2 to 2%, P: 0.04% or less, S: 0.005% or less, and a total of S (%) and O (%) of 0.015% or less, Cr: 12 to 30%, Ni: 10 to 35%, Al: 1.5 to 5.5 %, B: 0.001 to 0.01%, N: 0.025% or less, Ca: 0 to 0.008%, Cu: 0 to 2%, 1 of Ti, Nb, Zr, V and Hf 0 to 2% in total, 0 to 3% in total of one or more of W, Mo, Co and Re, and 0 to 0.05% in total in one or more of rare earth elements , The remainder consists of Fe and inevitable impurities. Patent Document 4 describes that by adding Al, a protective film mainly composed of an Al ₂ O ₃ film is formed, and the oxidation resistance is increased.

JP 2005-48284 A International Publication No. 2010/113830 International Publication No. WO 2004/067788 JP-A-10-140296

However, in Patent Document 2, the heat-resistant alloy contains up to 50% Cr. Therefore, in a high-temperature environment such as a hydrocarbon gas atmosphere, Cr may form as carbide on the steel surface. In this case, Al ₂ O _{3 as} a protective film is not formed uniformly. Therefore, carburization and coking may occur.

  In Patent Document 3, since the Ni content is high, the raw material cost is significantly increased.

  Patent Document 4 does not consider carburization resistance and coking resistance. Therefore, carburization resistance and coking resistance may be low.

  An object of the present invention is to provide an austenitic stainless steel having excellent carburization resistance and coking resistance even in a high-temperature environment such as a hydrocarbon gas atmosphere.

The austenitic stainless steel according to the present embodiment is, by mass%, C: 0.25 to 0.7%, Si: 0.01 to 2.0%, Mn: 2.0% or less, P: 0.04%. Hereinafter, 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: It contains 0.03% or less, Ti: 0 to less than 0.2%, Ca: 0 to 0.05%, and Mg: 0 to 0.05%, and the balance has a chemical composition of Fe and impurities. . The austenitic stainless steel according to the present embodiment satisfies Expression (1).
0.4 ≦ (C _Cr ′ / C _Al ′) / (C _Cr / C _Al ) ≦ 0.8 (1)
Here, 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 the formula (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 ′. 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 austenitic stainless steel according to the present embodiment has excellent carburization resistance and coking resistance even in a high-temperature carburizing environment such as a hydrocarbon gas atmosphere.

  The present inventors have investigated and studied the carburization resistance and coking resistance of austenitic stainless steel in a high-temperature carburizing environment (hereinafter, also simply referred to as a high-temperature environment), and obtained the following knowledge. The high-temperature carburizing environment refers to a high-temperature environment of 1000 ° C. or higher in a hydrocarbon gas atmosphere.

(A) If Cr is contained in the austenitic stainless steel or Ni-based alloy, Cr ₂ O _{3 as} a protective film is formed on the steel surface, and the corrosion resistance is enhanced. However, as described above, Cr ₂ O ₃ is thermodynamically unstable. Therefore, in the present invention, an Al ₂ O ₃ film is formed on the steel surface as a protective film. Al ₂ O ₃ is more thermodynamically stable than Cr ₂ O ₃ in a high temperature environment.

(B) When Cr is excessively contained in the Al-containing austenitic stainless steel or Ni-based alloy, it combines with C derived from atmospheric gas in a high-temperature environment to form Cr carbide on the steel surface. Cr carbides physically hinder uniform formation of an Al ₂ O ₃ coating on the steel surface. As a result, the carburization resistance and the coking resistance of the steel decrease.

On the other hand, Cr promotes uniform formation of the Al ₂ O ₃ film. Hereinafter, this effect is referred to as Cr Third Element Effect (hereinafter, referred to as TEE effect). The mechanism of the TEE effect is as follows. At the very beginning of the oxidation, Cr is preferentially oxidized first on the steel surface to form Cr ₂ O ₃ . For this reason, the oxygen partial pressure on the steel surface locally decreases. Thereby, Al is formed as a uniform Al ₂ O ₃ film near the surface without internal oxidation. After that, oxygen used as Cr ₂ O ₃ is taken into Al ₂ O ₃ , and finally, a protective film of only Al ₂ O ₃ is formed. Therefore, in order to form a uniform protective Al ₂ O ₃ film, it is necessary to contain a certain amount of Cr or more.

Therefore, in order to suppress the generation of Cr carbide and promote Cr ₂ O ₃ , in the present invention, the Cr content is set to 10 to 19%.

(C) In the austenitic stainless steel, if the ratio between the Cr concentration and the Al concentration in the range from the surface to a depth of 2 μm is appropriately made smaller than the ratio between the Cr concentration and the Al concentration of the base material, that is, the austenitic stainless steel When the stainless steel satisfies the formula (1), the carburization resistance and the coking resistance in a high temperature environment are improved.
0.4 ≦ (C _Cr ′ / C _Al ′) / (C _Cr / C _Al ) ≦ 0.8 (1)
Here, 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 the formula (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 ′. 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 .

When the austenitic stainless steel satisfies the formula (1), the TEE effect by Cr is sufficiently obtained on the steel surface, and the formation of the Al ₂ O ₃ film is promoted. When the austenitic stainless steel satisfies the formula (1), the Cr concentration is sufficiently low, and the formation of Cr carbide is suppressed. Therefore, the Al ₂ O ₃ film is formed uniformly. As a result, carburization resistance and coking resistance are improved.

The austenitic stainless steel according to the present embodiment completed on the basis of the above findings has, in mass%, C: 0.25 to 0.7%, Si: 0.01 to 2.0%, and Mn: 2.0%. Hereinafter, 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 ~ 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 a chemical composition of impurities. The austenitic stainless steel according to the present embodiment satisfies Expression (1).
0.4 ≦ (C _Cr ′ / C _Al ′) / (C _Cr / C _Al ) ≦ 0.8 (1)
Here, 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 the formula (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 ′. 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 chemical composition may contain 0.005 to less than 0.2% Ti.

  The chemical composition may contain at least one selected from the group consisting of Ca: 0.0005 to 0.05% and Mg: 0.0005 to 0.05%.

  Hereinafter, the austenitic stainless steel of the present embodiment will be described in detail. “%” With respect to an element means “% by mass” unless otherwise specified.

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

C: 0.25 to 0.7%
Carbon (C) mainly combines with Cr to form carbides and enhances high temperature creep strength. 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, and the toughness of the steel is reduced. Therefore, the C content is 0.25 to 0.7%. A preferred lower limit of the C content is 0.27%, more preferably 0.3%. A preferred upper limit of the C content is 0.6%, and more preferably 0.5%.

Si: 0.01 to 2.0%
Silicon (Si) deoxidizes steel. When deoxidation can be sufficiently performed with another element, the content of Si may be as small as possible. On the other hand, if the Si content is too high, the hot workability decreases. Therefore, the Si content is 0.01 to 2.0%. A preferred lower limit of the Si content is 0.02%, and more preferably 0.03%. A preferred upper limit of the Si content is 1.0%.

Mn: 2.0% or less Manganese (Mn) is inevitably contained. Mn combines with S contained in steel to form MnS and enhances hot workability of the steel. However, if the Mn content is too high, the steel becomes too hard, and the hot workability and the weldability decrease. Therefore, the Mn content is 2.0% or less. A preferred lower limit of the Mn content is 0.1%, more preferably 0.2%. The preferred upper limit of the Mn content is 1.2%.

P: 0.04% or less Phosphorus (P) is an impurity. P reduces the weldability and hot workability of steel. Therefore, the P content is 0.04% or less. A preferable upper limit of the P content is 0.03%. The P content is preferably as low as possible.

S: 0.01% or less Sulfur (S) is an impurity. S reduces the weldability and hot workability of steel. Therefore, the S content is 0.01% or less. A preferable upper limit of the S content is 0.008%. The S content is preferably as low as possible.

Cr: 10 to 19%
Chromium (Cr) forms a protective Cr ₂ O ₃ film on the surface of the steel in a high temperature environment. The Cr ₂ O ₃ coating enhances the corrosion resistance (oxidation resistance, steam oxidation resistance, etc.) of the steel. Cr further combines with C in the steel to form Cr carbides and enhances high-temperature strength. In the austenitic stainless steel of the present invention, Cr further promotes the formation of an Al ₂ O ₃ film by the above-described TEE effect. 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 hydrocarbon gas atmosphere, Cr combines with C derived from the atmosphere gas to form Cr carbide on the steel surface. When Cr carbide is formed, Cr is locally depleted. For this reason, the TEE effect is reduced, and a uniform Al ₂ O ₃ film is not formed. If the Cr content is too high, the Cr carbides also physically inhibit the formation of the Al ₂ O ₃ film. Therefore, the Cr content is 10 to 19%. A preferred lower limit of the Cr content is 11%, more preferably 12%. A preferred upper limit of the Cr content is 18.5%, and more preferably 18%.

Ni: 20-40%
Nickel (Ni) stabilizes austenite and increases high-temperature strength. Ni further enhances the corrosion resistance of the steel. If the Ni content is too low, these effects cannot be obtained. On the other hand, if the Ni content is too high, these effects are not only saturated, but also the raw material cost is increased. Therefore, the Ni content is 20 to 40%. A preferred lower limit of the Ni content is 22%, and more preferably 25%. The preferred upper limit of the Ni content is 39%, and more preferably 38%.

Al: 2.5 Super to 4.5% less than aluminum (Al) forms a Al ₂ O ₃ film on the steel surface in a high temperature environment, increase the corrosion resistance of steel. Particularly, in the high-temperature environment assumed in the present invention, the Al ₂ O ₃ film is thermodynamically stable as compared with the conventionally used Cr ₂ O ₃ film. If the Al content is too low, these effects cannot be obtained. On the other hand, if the Al content is too high, the structural stability is reduced, and the high-temperature strength is significantly reduced. Therefore, the Al content is more than 2.5 to less than 4.5%. A preferred lower limit of the Al content is 2.55%, and more preferably 2.6%. The preferred upper limit of the Al content is 4.2%, and more preferably 4.0%. 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: 0.01 to 3.5%
Niobium (Nb) forms intermetallic compounds (Laves phase and Ni ₃ Nb phase) to be a precipitation strengthening phase, precipitates and strengthens the crystal grain boundaries and inside the crystal grains, and increases the creep strength of steel. On the other hand, if the Nb content is too high, an intermetallic compound is excessively generated, and the toughness of the steel decreases. If the Nb content is too high, the toughness after long-term aging also decreases. Therefore, the Nb content is 0.01 to 3.5%. A preferred lower limit of the Nb content is 0.05%, and more preferably 0.1%. A preferred upper limit of the Nb content is less than 3.2%, and more preferably 3.0%.

N: 0.03% or less Nitrogen (N) stabilizes austenite and is inevitably contained. However, if the N content is too high, coarse carbonitrides that remain undissolved and remain after the solution treatment are generated. Coarse carbonitrides reduce the toughness of the steel. Therefore, the N content is 0.03% or less. A preferable upper limit of the N content is 0.01%.

  The balance of the chemical composition of the austenitic stainless steel of the present embodiment consists of Fe and impurities. Here, impurities are those that are mixed in from the ore, scrap, or production environment as a raw material when industrially producing austenitic stainless steel, and are allowed within a range that does not adversely affect the present invention. Means what is done.

[About optional elements]
The chemical composition of the austenitic stainless steel described above may further contain Ti instead of part of Fe.

Ti: 0 to less than 0.2% Titanium (Ti) is an optional element and may not be contained. When contained, Ti forms intermetallic compounds (Laves phase and Ni ₃ Ti phase) to be a precipitation strengthening phase, and increases creep strength by precipitation strengthening. However, if the Ti content is too high, an excessive amount of the intermetallic compound is generated, and the high-temperature ductility and hot workability are reduced. If the Ti content is too high, the toughness after long-term aging is further reduced. Therefore, the Ti content is 0 to less than 0.2%. A preferred lower limit of the Ti content is 0.005%, and more preferably 0.01%. The preferable upper limit of the Ti content is 0.15%, and more preferably 0.1%.

  The chemical composition of the austenitic stainless steel described above may further include one or more selected from the group consisting of Ca and Mg instead of part of Fe. These elements are all optional elements and enhance the hot workability of steel.

Ca: 0 to 0.05%
Calcium (Ca) is an optional element and may not be contained. When contained, Ca fixes S as a sulfide and enhances hot workability. On the other hand, if the Ca content is too high, toughness, ductility, and cleanliness deteriorate. Therefore, the Ca content is 0 to 0.05%. A preferred lower limit of Ca is 0.0005%. A preferred upper limit of the Ca content is 0.01%.

Mg: 0 to 0.05%
Magnesium (Mg) is an optional element and need not be contained. When contained, Mg fixes S as a sulfide and enhances the hot workability of the steel. On the other hand, if the Mg content is too high, toughness, ductility, and cleanliness deteriorate. Therefore, the Mg content is 0 to 0.05%. The preferred lower limit of Mg is 0.0005%. A preferable upper limit of the Mg content is 0.01%.

[About Equation (1)]
The austenitic stainless steel of the present embodiment further satisfies Expression (1).
0.4 ≦ (C _Cr ′ / C _Al ′) / (C _Cr / C _Al ) ≦ 0.8 (1)
Here, the Cr concentration (% by mass) in the surface layer of the austenitic stainless steel is substituted for C _Cr ′ in equation (1). The Al concentration (% by mass) in the surface layer of the austenitic stainless steel is substituted for C _Al ′. 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 .

  In the present specification, the surface layer of austenitic stainless steel means a range from the surface of austenitic stainless steel to a depth of 2 μm. The depth of 2 μm from the surface means a depth of 2 μm from the surface of the base material. Further, the Cr concentration (% by mass) of the base material and the Al concentration (% by mass) of the base material mean the average Cr concentration (% by mass) and the Al concentration (% by mass) of the base material excluding the surface layer, respectively.

As shown in Expression (1), in the austenitic stainless steel of the present embodiment, the ratio between the Cr concentration and the Al concentration in the surface layer is appropriately smaller than the ratio between the Cr concentration and the Al concentration in the steel base material. In this case, as described above, the formation of the Al ₂ O ₃ film is promoted. As a result, carburization resistance and coking resistance are increased in a high-temperature environment.

F1 = (C _Cr ′ / C _Al ′) / (C _Cr / C _Al ) is defined. F1 is an index of Cr behavior.

If F1 exceeds 0.8, the ratio between the Cr concentration and the Al concentration in the surface layer is too large than the ratio between the Cr concentration and the Al concentration in the base material. That is, the Cr concentration C _Cr ′ of the surface layer is too high. In this case, Cr carbide is formed on the steel surface, and the formation of a uniform Al ₂ O ₃ film is physically inhibited.

If F1 is less than 0.4, the ratio between the Cr concentration and the Al concentration in the surface layer is too small than the ratio between the Cr concentration and the Al concentration in the base material. That is, the Cr concentration C _Cr ′ of the surface layer is too small. In this case, the TEE effect of Cr cannot be obtained. Therefore, a uniform Al ₂ O ₃ film is not formed on the steel surface.

  Therefore, F1 is 0.4 to 0.8. A preferred lower limit of F1 is 0.42, and more preferably 0.44. A preferred upper limit of F1 is 0.79, and more preferably 0.78.

The Cr concentration C _Cr ′ and the Al concentration C _Al ′ of the above-mentioned surface layer are obtained by the following method. Austenitic stainless steel is cut perpendicular to the surface. Five arbitrary points (measurement points) are selected in a range from the surface of the cut austenitic stainless steel to a depth of 2 μm, and the Cr concentration and the Al concentration are measured by EDX (energy dispersive X-ray spectroscopy). The values obtained by averaging the measured values are defined as C _Cr ′ and C _Al ′ (%).

Ladle values are used for the Cr concentration C _Cr and the Al concentration C _Al of the base material described above. The ladle value is an analysis value of a steel material obtained in a steelmaking process. The ladle value is, for example, a result of analysis of a sample taken with a tundish in a steel manufacturing process. The analysis method is, for example, spark discharge spectroscopy. The peak value is determined for each sample, and the average thereof is defined as the Cr concentration C _Cr and the Al concentration C _{Al of the} base material. The ladle value shall be equivalent to the check analysis value.

  The shape of the austenitic stainless steel according to the present embodiment is not particularly limited. The austenitic stainless steel is, for example, a steel pipe. Austenitic stainless steel tubes are used as reaction tubes for chemical plants. The austenitic stainless steel may be a plate, a bar, a wire, or the like.

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

[Preparation process]
A molten steel having the above chemical composition is manufactured. A well-known degassing process is performed on the molten steel as needed. The material is manufactured by casting using molten steel. The raw material may be an ingot obtained by an ingot-making method, or a slab, a bloom, a billet or the like obtained by a continuous casting method. Further, a tube-shaped casting may be manufactured by a centrifugal casting method.

[Hot forging process]
Hot forging may be performed on the manufactured material. By performing hot forging, the internal structure of the molten steel produced in the preparation step can be changed from a solidified structure to a uniform sized structure.

[Hot / cold working process]
Hot working and / or cold working may be performed on the material manufactured in the preparation step or the hot forged material to manufacture a steel tube as an intermediate material. The hot working is, for example, hot extrusion or hot rolling. The cold working is, for example, cold drawing or cold rolling. Through the above steps, an intermediate material is manufactured.

[Heat treatment process]
A heat treatment is performed on the manufactured material or intermediate material in an air atmosphere. The heat treatment in the air atmosphere forms a scale mainly composed of Cr ₂ O ₃ on the steel surface. Thereby, Cr on the steel surface is deficient, and an austenitic stainless steel satisfying the expression (1) can be obtained.

  The heat treatment temperature is 900 to 1300 ° C., and the heat treatment time is 3.0 minutes to 30.0 hours.

If the heat treatment temperature is less than 900 ° C. or the heat treatment time is less than 3.0 minutes, the scale mainly composed of Cr ₂ O ₃ is not sufficiently formed on the steel surface, and the Cr concentration C _Cr ′ in the steel surface layer increases. And does not satisfy equation (1). For this reason, Cr carbide is formed on the steel surface, and a uniform Al ₂ O ₃ film is not sufficiently formed. As a result, carburization resistance and coking resistance are reduced. If the heat treatment temperature is less than 900 ° C. and / or the heat treatment time is less than 3.0 minutes, the crystal grains become too fine, and the high-temperature strength decreases.

On the other hand, if the heat treatment temperature exceeds 1300 ° C. or if the heat treatment time exceeds 30.0 hours, the scale mainly composed of Cr ₂ O ₃ is excessively formed on the steel surface, so that the Cr in the steel surface layer is excessively increased. Lack. In this case, the Cr concentration C _Cr ′ in the steel surface layer is too low, and does not satisfy the expression (1). For this reason, the TEE effect of Cr is lowered, and a uniform Al ₂ O ₃ film is not sufficiently formed. As a result, carburization resistance and coking resistance are reduced. If the heat treatment temperature exceeds 1300 ° C. and / or the heat treatment time exceeds 30.0 hours, the crystal grains are further coarsened, the diffusion of Al grain boundaries is inhibited, and the Al ₂ O ₃ film is not sufficiently formed. The crystal grain size is preferably 80 μm or less.

When the heat treatment temperature is 900 to 1300 ° C. and the heat treatment time is 3.0 minutes to 30.0 hours, a scale mainly composed of Cr ₂ O ₃ is sufficiently formed on the steel surface, and the chemical composition satisfying the formula (1) is obtained. Is obtained. As a result, carburization resistance in a high temperature environment is increased.

  The intermediate material after the heat treatment may be subjected to an acid washing treatment for the purpose of removing scale formed on the surface. For pickling, for example, a mixed acid solution of nitric acid and hydrochloric acid is used. The pickling time is, for example, 30 minutes to 60 minutes.

  Further, a blast treatment using a shot material may be performed on the intermediate material after the pickling treatment. For example, blast processing is performed on the inner surface of the steel pipe. In this case, a processed layer is formed on the surface, and corrosion resistance (eg, oxidation resistance) is increased.

  By the above manufacturing method, the austenitic stainless steel of the present embodiment is manufactured. In addition, the manufacturing method of the steel pipe was demonstrated above. However, a plate, a bar, a wire, etc. may be manufactured by the same manufacturing method (preparation step, hot forging step, hot / cold working step, heat treatment step).

[Production method]
Molten steel having the chemical composition shown in Table 1 was produced using a vacuum melting furnace.

[Measurement of Cr concentration C _Cr and Al concentration C _{Al of base} metal]
The molten steel that had been tapped into a ladle was collected, and the Cr concentration and the Al concentration of the base metal were measured by spark discharge spectroscopy. The peak values were determined for each sample, and the average thereof was defined as the Cr concentration C _Cr and the Al concentration C _{Al of the} base material.

  Using the molten steel, a cylindrical ingot (30 kg) having an outer diameter of 120 mm was manufactured. Hot forging was performed on the ingot to produce a rectangular material. Hot rolling and cold rolling were performed on the rectangular material to produce an intermediate material having a thickness of 15 mm. From the obtained intermediate material, a plate material of 8 mm × 20 mm × 30 mm was manufactured by machining, each of two steel types. The heat treatment was performed on the plate material at the temperature and time shown in Table 2. After the heat treatment, the plate was cooled with water to produce a test steel.

[Measurement of austenite grain size]
Specimens for microscope observation were prepared from the center of the cross section perpendicular to the rolling direction of the steel material of each test number. Using a surface (referred to as an observation surface) corresponding to the above-described cross section among the surfaces of the test pieces, a microscopic test method specified in ASTM E112 was performed to measure the crystal grain size. Specifically, after the observation surface was mechanically polished, the surface was corroded using a corrosive liquid to reveal crystal grain boundaries on the observation surface. In 10 visual fields on the corroded surface, the average crystal grain size of each visual field was determined. The area of each field of view is about 0.75 mm ² .

[Measurement of Cr concentration C _Cr ′ and Al concentration C _Al ′ of surface layer]
The steel material of each test number was cut perpendicular to the rolling direction, and a sample including the surface was collected. The sample was embedded in resin, and the observation surface including the cross section near the surface was polished. The Cr concentration C _Cr ′ and the Al concentration C _Al ′ of the surface layer (range from the surface to a depth of 2 μm) of the polished observation surface were determined using the above-described method.

[Carburizing test]
The steel material of each test number was kept at 1100 ° C. for 96 hours in an H ₂ —CH ₄ —CO ₂ atmosphere. The surface of the steel material after carburization was dry-hand polished with # 600 abrasive paper to remove scale and the like on the surface. Analytical chips from four layers were collected from the steel surface at 0.5 mm pitch. About the obtained analysis chip, C concentration was measured by the high frequency combustion infrared absorption method. From the measurement results, the C concentration originally contained in the steel was subtracted to obtain an increase in the C concentration. The average of the increase in the C concentration for the four layers was defined as the amount of intrusion C.

[Creep rupture test]
A creep test piece was prepared from the manufactured intermediate material. The creep test specimen was taken from the center of the thickness of the steel material in parallel with the rolling direction. The test piece was a round bar test piece, the diameter of the parallel portion was 6 mm, and the distance between the gauges was 30 mm. A creep rupture test was performed using the test piece. 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 (○ in Table 2), and those with a rupture time of less than 1.0 × 10 ³ hours were evaluated as unacceptable (× in Table 2).

[Test results]
Table 2 shows the test results.

Referring to Table 2, the chemical compositions of Test Nos. 1 to 7 were appropriate, and F1 satisfied Formula (1). As a result, the infiltration C amount was 0.18% or less, indicating excellent carburization resistance. Further, the rupture time in the creep test was 1.0 × 10 ³ hours or more, indicating excellent creep strength.

On the other hand, in Test No. 8, the heat treatment temperature was too high. Therefore, Cr deficiency on the surface was excessively promoted, and did not satisfy the expression (1). In addition, the particle size was increased, and the formation of the Al ₂ O ₃ film was inhibited. As a result, the carburization resistance was low.

  In Test No. 9, the heat treatment temperature was too low. Therefore, scale formation was insufficient, and did not satisfy Expression (1). As a result, the carburization resistance was low.

  In Test No. 10, the heat treatment time was too short. Therefore, scale formation was insufficient, and did not satisfy Expression (1). As a result, the carburization resistance was low.

In test number 11, the heat treatment time was too long. Therefore, Cr deficiency on the surface was excessively promoted, and did not satisfy the expression (1). In addition, the particle size was increased, and the formation of the Al ₂ O ₃ film was inhibited. As a result, the carburization resistance was low.

In Test No. 12, the Cr content was too low. For this reason, the effect of TEE by Cr was reduced, and an Al ₂ O ₃ film could not be formed. As a result, the carburization resistance was low.

In Test No. 13, the Al content was too low. Therefore, the Al ₂ O ₃ film was not sufficiently formed, and the carburization resistance was low.

In Test No. 14, the Cr content was too high. Therefore, Cr was formed as a carbide. This also inhibited the formation of the Al ₂ O ₃ film. As a result, the carburization resistance was low.

  In Test No. 15, the Al content was too high. Therefore, the creep strength decreased.

  In Test No. 16, the C content was too low. Therefore, the creep strength decreased.

  The embodiments of the present invention have been described above. However, the above-described embodiment is merely an example 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 thereof.

  The austenitic stainless steel of the present invention can be used in a high-temperature environment where carburization and caulking are concerned, such as in a hydrocarbon gas atmosphere. In particular, it is particularly suitable for use as steel for reaction tubes in chemical industry plants such as ethylene production plants.

Claims (3)

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: 20-40%,
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: containing 0 to 0.05%,
The balance has a chemical composition consisting of Fe and impurities,
Austenitic stainless steel that satisfies formula (1).
0.4 ≦ (C _Cr ′ / C _Al ′) / (C _Cr / C _Al ) ≦ 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 the formula (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 ′. 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 . An austenitic stainless steel according to claim 1,
The chemical composition is
Ti: Austenitic stainless steel containing 0.005 to less than 0.2%. Austenitic stainless steel according to claim 1 or claim 2,
The chemical composition is
Ca: 0.0005 to 0.05%, and
Mg: an austenitic stainless steel containing at least one selected from the group consisting of 0.0005 to 0.05%.