KR20190022723A - Austenitic stainless steel - Google Patents

Abstract

Also provided is an austenitic stainless steel having excellent hot-rolled elasticity and excellent hot workability at the time of production even in a high-temperature carburizing environment. The austenitic stainless steel according to the present embodiment contains, by mass%, C: less than 0.03 to less than 0.25%, Si: 0.01 to 2.0%, Mn: less than 2.0%, Cr: less than 10 to 22% Of Al, more than 2.5% to less than 4.5% of Al, 0.01 to 3.5% of Nb, 0.0005 to 0.05% of Ca, 0.0005 to 0.05% of Mg and 0.03% or less of N and the balance of Fe and impurities Lt; / RTI > The Cr concentration C _Cr 'of the surface layer of the austenitic stainless steel and the Al concentration C _Al ' of the surface layer satisfy the formula (1) with respect to the Cr concentration C _Cr other than the surface layer and the Al concentration C _Al other than the surface layer.
(C _Cr '/ C _Al ') / (C _Cr / C _Al )? 0.80 (1)

Description

Austenitic stainless steel

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

Conventionally, austenitic stainless steel or Ni-based alloy having a high Cr content or a high Cr content is used as a heat resistant steel in facilities such as thermal power generation boilers and chemical plants used under a high temperature carburizing environment. 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.

Pipes for facilities such as thermal power boilers and chemical plants are manufactured from steel pipes. The steel tube is manufactured by hot working after the above-described austenitic stainless steel or Ni-based alloy is dissolved. Therefore, heat resistant steel is required to have high hot workability. However, austenitic stainless steels generally have high deformation resistance at high temperatures and low ductility. Therefore, there is a demand for an austenitic stainless steel having excellent hot workability.

Recently, however, low-priced shale gas is being produced by the so-called shale revolution. In the case of using a shale gas as a raw material gas in a facility such as a chemical plant, carbon (C) derived from a raw material gas is used as a raw material gas in comparison with conventional raw materials such as naphtha, The reaction tube) is liable to cause carburization, which is a corrosion phenomenon. Therefore, steels used in facilities such as chemical plants are required to have excellent rebound resilience.

Stainless steel having improved impact resistance and coking resistance has been proposed in, for example, Japanese Patent Application Laid-Open No. 2005-48284 (Patent Document 1).

The stainless steel disclosed in Patent Document 1 contains, by mass%, 0.01 to 0.6% of C, 0.1 to 5% of Si, 0.1 to 10% of Mn, 0.08% or less of P, (Fe) and inevitable impurities in an amount of 0.05 to 55%, Ni: 10 to 70%, N: 0.001 to 0.25%, O (oxygen): 0.02% or less. This stainless steel has a Cr depletion layer in the surface layer portion, and the Cr concentration in the Cr depletion layer is 10% or more and less than the Cr concentration of the base material, and the thickness of the Cr depletion layer is within 20 占 퐉. Patent Document 1 discloses that the anti-caking property and the anti-coking property are improved by forming a protective coating mainly composed of a Cr ₂ O ₃ coating.

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

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

The casting product disclosed in Patent Document 2 contains, by mass%, C: 0.05 to 0.7%, Si: 0 to 2.5%, Mn: 0 to 3.0%, Cr: 15 to 50% Of Co: 18 to 70%, Al: 2 to 4%, rare earth elements: 0.005 to 0.4%, W: 0.5 to 10% and / or Mo: 0.1 to 5%, and the balance Fe and inevitable impurities And has a cast body of a heat resistant alloy. A barrier layer is formed on the surface of the casting body in contact with a high temperature atmosphere, and the barrier layer is an Al ₂ O ₃ layer having a thickness of 0.5 탆 or more, wherein 80% or more of the outermost surface of the barrier layer is Al ₂ O ₃ , Cr base 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 cast body. In Patent Document 2, it is described that by adding Al, a protective coating mainly composed of an Al ₂ O ₃ coating is formed to increase the sinking elasticity.

The nickel-chrome cast alloy disclosed in Patent Document 3 has a composition of up to 0.8% carbon, up to 1% silicon, up to 0.2% manganese, from 15% to 40% chromium, from 0.5% to 13% % Aluminum, up to 2.5% niobium, up to 1.5% titanium, 0.01% to 0.4% zirconium, up to 0.06% nitrogen, up to 12% cobalt, up to 5% molybdenum, up to 6% tungsten, To 0.089% of yttrium, and the balance of nickel. In Patent Document 3, it is described that a nickel-chrome cast alloy is obtained in which the peeling resistance of Al ₂ O _{3 as} a protective coating is enhanced by adding REM in addition to Al.

The austenitic stainless steels disclosed in Patent Document 4 contain 0.15% or less of C, 0.9% or less of Si, 0.2 to 2% of Mn, 0.04% or less of P, 0.005% or less of S and 0.005% or less of S ( % Of Cr, 12 to 30% of Cr, 10 to 35% of Ni, 1.5 to 5.5% of Al, 0.001 to 0.01% of B, 0.025% of N or less, Ca of 0% 0 to 2% of at least one of Ti, Nb, Zr, V and Hf as a total of at least one of W, Mo, Co and Re in an amount of 0 to 3% And 0 to 0.05% of a total of one kind or more of the rare earth elements, and the balance of Fe and inevitable impurities. Patent Document 4 discloses that by adding Al, a protective coating mainly composed of an Al ₂ O ₃ coating is formed and oxidation resistance is improved.

Japanese Patent Application Laid-Open No. 2005-48284 International Publication No. 2010/113830 International Publication No. 2004/067788 Japanese Patent Application Laid-Open No. 10-140296

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

In the castings and nickel-chrome cast alloys disclosed in Patent Documents 2 and 3, since the C content is high, the hot workability is remarkably lowered.

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

In Patent Document 4, no consideration is given to the pushing elasticity. For this reason, there is a case where the slip resistance is low.

It is an object of the present invention to provide an austenitic stainless steel having excellent hot pressing resistance even in a high temperature carburizing environment such as a hydrocarbon gas atmosphere and having excellent hot workability at the time of production.

The austenitic stainless steel according to the present embodiment contains, by mass%, less than 0.03 to 0.25% of C, 0.01 to 2.0% of Si, 2.0% or less of Mn, 0.04% or less of P, : Less than 10 to 22%, Ni: more than 30.0 to less than 40.0%, more than 2.5 to less than 4.5% Nb: 0.01 to 3.5%, N: 0.03 to less than 0.20 to 0.205, The steel sheet contains 0.05 to 0.05%, Ti to 0 to less than 0.2%, Mo to 0 to 0.5%, W 0 to 0.5%, Cu 0 to 0.5%, V 0 to 0.2% and B 0 to 0.01% , The balance being Fe and impurities, and satisfies the formula (1).

(C _Cr '/ C _Al ') / (C _Cr / C _Al )? 0.80 (1)

Here, C _Cr 'in the formula (1) is substituted with the Cr concentration (mass%) in the surface layer of the austenitic stainless steel. C _Al 'is substituted for the Al concentration (mass%) in the surface layer of the austenitic stainless steel. In addition, Cr concentration (mass%) other than the surface layer of the austenitic stainless steel is substituted for C _Cr . C _Al is substituted with Al concentration (mass%) other than the surface layer of the austenitic stainless steel.

The austenitic stainless steels according to the present embodiment have excellent rebound resilience even in a high temperature carburizing environment such as a hydrocarbon gas atmosphere and have excellent hot workability at the time of production.

The inventors of the present invention investigated and examined the hot pressing workability at the time of manufacturing and the stiffness of the austenitic stainless steel in a high temperature carburizing environment, and the following findings were obtained. The high-temperature carburizing environment refers to an environment of 1000 ° C or higher in a hydrocarbon gas atmosphere.

(A) When Cr is contained in the austenitic stainless steel or the Ni-based alloy, Cr ₂ O ₃ , which is a protective coating, is formed on the surface of the steel, thereby increasing the sinking elasticity. However, as mentioned above, Cr ₂ O ₃ is thermodynamically unstable. Therefore, in the present invention, an Al ₂ O ₃ coating is formed on the surface of the steel. Al ₂ O ₃ acts as a protective coating. Al ₂ O ₃ is more thermodynamically stable than Cr ₂ O ₃ in a high temperature carburizing environment. That is, in the case of an Al ₂ O ₃ coating film, even when the steel sheet is in an environment of 1000 ° C or higher, the austenitic stainless steel can be improved in the sinking elasticity.

(B) When Cr is excessively contained in an Al-containing austenitic stainless steel or a Ni-based alloy, Cr binds to C derived from atmospheric gas in a high temperature carburizing environment. Cr combined with C forms Cr carbide on the surface of the steel. Cr carbide physically inhibits the uniform formation of the Al ₂ O ₃ coating on the surface of the steel. As a result, the rebound resilience of the steel decreases. Therefore, the Cr content needs to be constantly limited.

On the other hand, Cr promotes the uniform formation of the Al ₂ O ₃ coating. This effect is hereinafter referred to as a Third Element Effect (hereinafter referred to as TEE effect) of Cr. The mechanism of the TEE effect is as follows. In the very early stage of the heat treatment step to be described later, on the surface of the steel, Cr is preferentially oxidized to form Cr ₂ O ₃ . Therefore, the oxygen partial pressure of the surface of the steel is locally lowered. Thereby, Al is formed as a uniform Al ₂ O ₃ film in the vicinity of the surface without internal oxidation. Then, the oxygen was used as a Cr ₂ O ₃ is taken to be Al ₂ O _3. At the end of the heat treatment process, only Al ₂ O ₃ protective film is formed. Cr also has a TEE effect under a high temperature carburizing environment. That is, Cr promotes the uniform formation of the Al ₂ O ₃ film even in a high-temperature carburizing environment. Therefore, in order to form a uniform Al ₂ O ₃ film, it is necessary to contain Cr of a certain amount or more.

Therefore, in order to suppress the formation of Cr carbide under a high temperature carburizing environment and promote the formation of the Al ₂ O ₃ film, the Cr content is set to be less than 10 to 22% in the present invention.

(C) In the austenitic stainless steel, it is effective to make the ratio of the Cr concentration in the surface layer and the Al concentration in the surface layer smaller than the ratio of the Cr concentration other than the surface layer and the Al concentration other than the surface layer. That is, when the austenitic stainless steel satisfies the formula (1), the sand elasticity in a high-temperature carburizing environment becomes high.

(C _Cr '/ C _Al ') / (C _Cr / C _Al )? 0.80 (1)

Here, C _Cr 'in the formula (1) is substituted with the Cr concentration (mass%) in the surface layer of the austenitic stainless steel. C _Al 'is substituted for the Al concentration (mass%) in the surface layer of the austenitic stainless steel. In addition, Cr concentration (mass%) other than the surface layer of the austenitic stainless steel is substituted for C _Cr . C _Al is substituted with Al concentration (mass%) other than the surface layer of the austenitic stainless steel.

F1 = (C _Cr '/ C _Al ') / (C _Cr / C _Al ). If F1 is 0.40 or more, a TEE effect due to Cr can be sufficiently obtained on the steel surface in a high temperature carburizing environment. In this case, the formation of the Al ₂ O ₃ coating film is promoted. When F1 is 0.80 or less, the formation of Cr carbide on the surface of the steel is suppressed in a high temperature carburizing environment. Therefore, a uniform Al ₂ O ₃ coating film is formed. As a result, the puncture elasticity becomes high.

When the chemical composition of (D) austenitic stainless steel contains calcium (Ca) of 0.0005% or more and magnesium (Mg) of 0.0005% or more, hot workability is enhanced. On the other hand, when the content of these elements is too high, the toughness and ductility of the austenitic stainless steel at high temperature are lowered and the hot workability is lowered. Therefore, Ca: 0.0005 to 0.05% and Mg: 0.0005 to 0.05% are contained.

The austenitic stainless steel according to the present embodiment, which is completed on the basis of the above knowledge, contains, by mass%, less than 0.03 to less than 0.25% of C, 0.01 to 2.0% of Si, less than or equal to 2.0% : Less than 0.01%, Cr: less than 10 to 22%, Ni: more than 30.0 to less than 40.0%, Al: more than 2.5 to less than 4.5%, Nb: 0.01 to 3.5% % Of Mo, 0.0005 to 0.05% of Mg, 0 to less than 0.2% of Ti, 0 to 0.5% of Mo, 0 to 0.5% of W, 0 to 0.5% of Cu, 0 to 0.2% of V, To 0.01%, and the balance of Fe and impurities, and satisfies the formula (1).

(C _Cr '/ C _Al ') / (C _Cr / C _Al )? 0.80 (1)

Here, C _Cr 'in the formula (1) is substituted with the Cr concentration (mass%) in the surface layer of the austenitic stainless steel. C _Al 'is substituted for the Al concentration (mass%) in the surface layer of the austenitic stainless steel. In addition, Cr concentration (mass%) other than the surface layer of the austenitic stainless steel is substituted for C _Cr . C _Al is substituted with Al concentration (mass%) other than the surface layer of the austenitic stainless steel.

Wherein the chemical composition is less than 0.005 to 0.2% of Ti, 0.01 to 0.5% of Mo, 0.01 to 0.5% of W, 0.005 to 0.5% of Cu, 0.005 to 0.2% of V and 0.0001 to 0.01 of B Or one or more selected from the group consisting of

Hereinafter, the austenitic stainless steel of the present embodiment will be described in detail. &Quot;% " of the 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: less than 0.03 to less than 0.25%

Carbon (C) mainly combines with Cr to form Cr carbide in the steel, thereby increasing the creep strength at the time of use in a high temperature carburizing environment. If the C content is too low, this effect can not be obtained. On the other hand, if the C content is too high, a large number of coarse process carbides are formed in the solidification structure after casting of the steel, thereby lowering the toughness of the steel. Therefore, the C content is less than 0.03 to 0.25%. The lower limit of the C content is preferably 0.05%, more preferably 0.08%. The preferred upper limit of the C content is 0.23%, more preferably 0.20%.

Si: 0.01 to 2.0%

Silicon (Si) deoxidizes the steel. When deoxidation can be sufficiently carried out with other elements, the Si content is at least as small as possible. On the other hand, if the Si content is too high, the hot workability deteriorates. Therefore, the Si content is 0.01 to 2.0%. The lower limit of the Si content is preferably 0.02%, more preferably 0.03%. The preferable 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 the steel to form MnS, thereby improving the hot workability of the steel. However, if the Mn content is too high, the steel becomes too hard and the hot workability and weldability are deteriorated. Therefore, the Mn content is 2.0% or less. The lower limit of the Mn content is preferably 0.1%, more preferably 0.2%. The preferable upper limit of the Mn content is 1.2%.

P: not more than 0.04%

Phosphorus (P) is an impurity. P lowers the weldability and hot workability of steel. Therefore, the P content is 0.04% or less. The preferable upper limit of the P content is 0.03%. The P content is preferably as low as possible. The lower limit of the P content is, for example, 0.0005%.

S: not more than 0.01%

Sulfur (S) is an impurity. S lowers the weldability and hot workability of steel. Therefore, the S content is 0.01% or less. The preferred upper limit of the S content is 0.008%. The S content is preferably as low as possible. The lower limit of the S content is, for example, 0.001%.

Cr: less than 10 ~ 22%

Chromium (Cr) promotes the formation of Al ₂ O ₃ coating film during the heat treatment process and in the high-temperature carburization environment due to the TEE effect described above. Cr also combines with C in the steel to form Cr carbide in the steel, thereby increasing the creep strength. If the Cr content is too low, this effect can not be obtained. On the other hand, when the Cr content is too high, Cr bonds with C derived from atmospheric gas (hydrocarbon gas) under a high-temperature carburizing environment to form Cr carbide on the surface of the steel. When Cr carbide is formed on the surface of the river, the Cr on the surface of the steel is locally deficient. As a result, the TEE effect is lowered and a uniform Al ₂ O ₃ coating film is not formed. If the Cr content is too high, the Cr carbide on the steel surface physically inhibits the formation of a uniform Al ₂ O ₃ coating. Therefore, the Cr content is less than 10 to 22%. The lower limit of the Cr content is preferably 11%, more preferably 12%. The upper limit of the Cr content is preferably 21%, more preferably 20%. In the present specification, the Cr carbide is divided into Cr carbide formed in the steel and Cr carbide formed on the steel surface. In the austenitic stainless steel of the present embodiment, Cr carbide is formed in the steel and Cr carbide in the steel surface is suppressed.

Ni: more than 30.0% to 40.0%

Nickel (Ni) stabilizes the austenite and increases the creep strength. Ni also improves the rebound resilience of the steel. If the Ni content is too low, such effects can not be obtained. On the other hand, if the Ni content is too high, this effect is not only saturated but also the raw material cost is increased. Therefore, the Ni content is more than 30.0% to 40.0%. The lower limit of the Ni content is preferably 31.0%, more preferably 32.0%. The preferable upper limit of the Ni content is 39.0%, and more preferably 38.0%.

Al: more than 2.5% to less than 4.5%

Aluminum (Al) forms an Al ₂ O ₃ coating on the surface of the steel during the heat treatment process and in a high-temperature carburizing environment, thereby increasing the slip resistance of the steel. Particularly, in the high temperature carburizing environment assumed in the present invention, the Al ₂ O ₃ coating film is thermodynamically stable as compared with the Cr ₂ O ₃ coating film conventionally used. If the Al content is too low, such an effect can not be obtained. On the other hand, if the Al content is too high, the structure stability is lowered and the creep strength remarkably decreases. Therefore, the Al content is more than 2.5% to less than 4.5%. The lower limit of the Al content is preferably 2.55%, more preferably 2.6%. The preferable upper limit of the Al content is 4.2%, more preferably 4.0%. In the austenitic stainless steel according to the present invention, the Al content means all the Al content contained in the steel material.

Nb: 0.01 to 3.5%

Niobium (Nb) forms an intermetallic compound (Laves phase and Ni ₃ Nb phase) as a precipitation strengthening phase and precipitates and strengthens grain boundaries and crystal grains, thereby increasing the creep strength of the steel. On the other hand, if the Nb content is too high, an intermetallic compound is formed excessively and the toughness of the steel is lowered. If the Nb content is too high, the toughness after aging also deteriorates. Therefore, the Nb content is 0.01 to 3.5%. The lower limit of the Nb content is preferably 0.05%, more preferably 0.1%. The preferred upper limit of the Nb content is less than 3.2%, more preferably 3.0%.

N: 0.03% or less

Nitrogen (N) stabilizes austenite and is inevitably contained. On the other hand, if the N content is too high, coarse nitrides and / or carbonitrides still remain in the solid solution after the heat treatment. Coarse nitrides and / or carbonitrides degrade the toughness of the steel. Therefore, the N content is 0.03% or less. The upper limit of the preferable N content is 0.01%. The lower limit of the N content is, for example, 0.0005%.

Ca: 0.0005 to 0.05%

Calcium (Ca) fixes S as a sulfide and enhances hot workability. On the other hand, if the Ca content is too high, the toughness and ductility deteriorate. As a result, the hot workability is lowered. If the Ca content is too high, the cleanliness also deteriorates. Therefore, the Ca content is 0.0005 to 0.05%. The lower limit of Ca is preferably 0.0006%, more preferably 0.0008%. The upper limit of the Ca content is preferably 0.01%, more preferably 0.008%.

Mg: 0.0005 to 0.05%

Magnesium (Mg) fixes S as a sulfide and enhances hot workability of the steel. On the other hand, if the Mg content is too high, the toughness and ductility are lowered. As a result, the hot workability is lowered. If the Mg content is too high, the cleanliness also deteriorates. Therefore, the Mg content is 0.0005 to 0.05%. The lower limit of Mg is preferably 0.0006%, more preferably 0.0008%. The upper limit of the Mg content is preferably 0.01%, more preferably 0.008%.

The balance of the chemical composition of the austenitic stainless steel of the present embodiment is composed of Fe and impurities. Here, the impurity means that the austenitic stainless steel is incorporated from an ore, a scrap, or a manufacturing environment as a raw material when the austenitic stainless steel is produced industrially, and is allowed within a range not adversely affecting the present invention.

[For arbitrary element]

The chemical composition of the above-described austenitic stainless steel may also contain Ti instead of a part of Fe.

Ti: 0 to less than 0.2%

Titanium (Ti) is an arbitrary element and may not be contained. When contained, Ti forms an intermetallic compound (Laves phase and Ni ₃ Ti phase) as a precipitation strengthening phase, and increases creep strength by precipitation strengthening. On the other hand, if the Ti content is too high, the intermetallic compound is excessively produced, and the high temperature ductility and hot workability are deteriorated. If the Ti content is too high, the toughness after aging for a long time is lowered. Therefore, the Ti content is less than 0 to 0.2%. The lower limit of the Ti content is preferably 0.005%, more preferably 0.01%. The upper limit of the Ti content is preferably 0.15%, more preferably 0.1%.

The chemical composition of the above-described austenitic stainless steel may also contain one or two kinds selected from the group consisting of Mo and W instead of a part of Fe. All of these elements are arbitrary elements and increase the creep strength of the steel.

Mo: 0 to 0.5%

Molybdenum (Mo) is an arbitrary element and may not be contained. When it is contained, Mo is solid-dissolved in austenite which is a parent phase. The molten 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 to 0.5%. The lower limit of the Mo content is preferably 0.01%, more preferably 0.05%. The upper limit of the Mo content is preferably 0.4%, more preferably 0.3%.

W: 0 to 0.5%

Tungsten (W) is an arbitrary element and may not be contained. When contained, W is dissolved in the parent phase austenite. W employed increases the strength of creep by strengthening employment. On the other hand, if the W content is too high, the hot workability deteriorates. Therefore, the W content is 0 to 0.5%. The lower limit of the W content is preferably 0.01%, more preferably 0.05%. The upper limit of the W content is preferably 0.4%, more preferably 0.3%.

The chemical composition of the above-described austenitic stainless steel may also contain Cu instead of a part of Fe.

Cu: 0 to 0.5%

Copper (Cu) is an arbitrary element and may be omitted. If contained, Cu stabilizes the austenite. Cu also increases the strength of steel by precipitation strengthening. On the other hand, if the Cu content is too high, ductility and hot workability of the steel decrease. Therefore, the Cu content is 0 to 0.5%. The lower limit of the Cu content is preferably 0.005%, more preferably 0.01%. The upper limit of the Cu content is preferably 0.3%, more preferably 0.1%.

The chemical composition of the above-described austenitic stainless steel may also contain V instead of a part of Fe.

V: 0 to 0.2%

Vanadium (V) is an arbitrary element and may not be contained. When contained, V forms an intermetallic compound like Ti and increases the creep strength of the steel. On the other hand, if the V content is too high, the volume ratio of the intermetallic compound in the steel becomes excessively high, and the hot workability is lowered. Therefore, the V content is 0 to 0.2%. The lower limit of the V content is preferably 0.005%, more preferably 0.01%. The preferred upper limit of the V content is 0.15%, more preferably 0.1%.

The chemical composition of the above-described austenitic stainless steel may also contain B instead of a part of Fe.

B: 0 to 0.01%

Boron (B) is an arbitrary element, and may not be contained. When contained, B is segregated in the grain boundary and promotes the precipitation of the intermetallic compound in the grain boundary. 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 steel are deteriorated. Therefore, the B content is 0 to 0.01%. The lower limit of the B content is preferably 0.0001%, more preferably 0.0005%. The upper limit of the B content is preferably 0.008%, more preferably 0.006%.

[About Equation (1)]

The austenitic stainless steel of the present embodiment also satisfies the formula (1).

(C _Cr '/ C _Al ') / (C _Cr / C _Al )? 0.80 (1)

Here, C _Cr 'in the formula (1) is substituted with the Cr concentration (mass%) in the surface layer of the austenitic stainless steel. C _Al 'is substituted for the Al concentration (mass%) in the surface layer of the austenitic stainless steel. In addition, Cr concentration (mass%) other than the surface layer of the austenitic stainless steel is substituted for C _Cr . C _Al is substituted with Al concentration (mass%) other than the surface layer of the austenitic stainless steel.

In this specification, the surface layer of the austenitic stainless steel means a range from the surface of the austenitic stainless steel to the depth of 2 탆. A depth of 2 mu m from the surface means a depth of 2 mu m from the surface of the base material. When an austenitic stainless steel has an Al ₂ O ₃ coating on its surface, a depth of 2 μm from the surface of the base material means a depth of 2 μm from the surface of the base material after removal of the Al ₂ O ₃ coating by descaling treatment. That is, in the case of C _Cr 'in the formula (1), the surface of the austenitic stainless steel (when the austenitic stainless steel has the Al ₂ O ₃ coating on the surface, the Al ₂ O ₃ coating is removed by descaling The Cr concentration (mass%) in the range from the surface of the base material to the depth of 2 mu m is substituted. C _Al 'in the formula (1) indicates the surface of the austenitic stainless steel (when an austenitic stainless steel has an Al ₂ O ₃ coating on its surface, the Al ₂ O ₃ coating is removed by a descaling process, (Mass%) in the range from the surface (the surface) to the depth of 2 mu m is substituted. The Cr concentration (mass%) other than the surface layer means the average Cr concentration (mass%) of the base material in the region other than the surface layer. The Al concentration (mass%) other than the surface layer means the average Al concentration (mass%) of the base material in the region other than the surface layer.

As shown in the formula (1), in the austenitic stainless steel of the present embodiment, the ratio of the Cr concentration in the surface layer to the Al concentration in the surface layer is suitably smaller than the ratio of the Cr concentration in the base material to the Al concentration in the base material. In this case, as described above, the formation of the Al ₂ O ₃ coating film is promoted. As a result, in a high temperature carburizing environment, the sinking elasticity becomes high.

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

When F1 exceeds 0.80, the ratio of the Cr concentration in the surface layer to the Al concentration in the surface layer is excessively larger than the ratio of the Cr concentration in the base material to the Al concentration in the base material. That is, the Cr concentration _{Cr Cr} 'in the surface layer is too high. In this case, Cr carbide is formed on the surface of the steel in a high-temperature carburizing environment to physically inhibit the formation of a uniform Al ₂ O ₃ coating.

If F1 is less than 0.40, the ratio of the Cr concentration in the surface layer to the Al concentration in the surface layer is too small as compared with the ratio of the Cr concentration in the base material to the Al concentration in the base material. That is, the Cr concentration _{Cr Cr} 'in the surface layer is too small. In this case, the TEE effect of Cr can not be obtained in a high-temperature carburizing environment. Therefore, a uniform Al ₂ O ₃ coating film is not formed on the surface of the steel.

Therefore, F1 is 0.40 to 0.80. The preferable lower limit of F1 is 0.42, and more preferably 0.44. The preferred upper limit of F1 is 0.79, more preferably 0.78.

The Cr concentration C _Cr 'of the surface layer and the Al concentration C _A1 ' of the surface layer are obtained by the following method. The austenitic stainless steel is cut perpendicular to the surface. From an austenitic stainless steel surface cutting (if having an Al ₂ O ₃ film on the surface of austenitic stainless steel, the surface of the base material after the removal of the Al ₂ O ₃ film by the descaling process) of up to 2μm depth In the range, arbitrary five points (measurement points) are selected. The Cr concentration and the Al concentration at each measurement point 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 ' (%).

When the austenitic stainless steel has an Al ₂ O ₃ coating on its surface, the Cr concentration C _Cr 'of the surface layer and the Al concentration C _Al ' of the surface layer are measured after descaling treatment. The conditions for descaling an austenitic stainless steel are in accordance with JIS Z 2290 (2004).

The Cr concentration C _Cr other than the above-described surface layer and the Al concentration C _Al other than the surface layer can be analyzed by a well-known component analysis method. Specifically, it is obtained by the following method. The austenitic stainless steel is cut vertically with respect to the longitudinal direction (steel pipe back-axis direction) to prepare a measurement surface. The thick central part of the measurement surface is drilled using a drill. Generate the cut powder by punching and collect the cut powder. Sections are taken from 4 places on the same measurement surface. When the austenitic stainless steel is a steel pipe, collect powder from four places at a pitch of 45 °. ICP-OES (Inductively Coupled Plasma Optical Emission Spectrometry) is performed on the collected fractions, and elemental analysis of the chemical composition is performed. The order of analysis by ICP-OES is in accordance with JIS G 1258 (2007). The average of the four measured values is defined as the Cr concentration C _Cr (%) other than the surface layer and the Al concentration C _Al (%) other than the surface layer.

The austenitic stainless steel of the present embodiment has an Al ₂ O ₃ coating on the surface thereof after the heat treatment step to be described later. Therefore, the austenitic stainless steel of the present embodiment may have an Al ₂ O ₃ coating on its surface. However, the Al ₂ O ₃ coating film can be removed by a known method such as pickling treatment after heat treatment or shot peening. Therefore, the austenitic stainless steel of the present embodiment may be in a state where the Al ₂ O ₃ coating on the surface is removed.

[Crystal grain size]

Preferably, the austenitic stainless steel of the present embodiment has a grain size of 30 to 80 占 퐉. If the crystal grain size is 30 탆 or more, the creep strength of the steel is further increased. When the crystal grain size is 80 탆 or less, the grain boundary diffusion of Al is promoted and the formation of the Al ₂ O ₃ coating is further promoted. The crystal grain size is obtained by a microscopic test method of crystal grain size stipulated in JIS G 0551 (2013).

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. The austenitic stainless steel pipe is used as a reaction pipe for a chemical plant. The austenitic stainless steel may be a plate material, a bar material, or a wire material.

[Manufacturing method]

A method of manufacturing a steel pipe will be described as an example of a method of manufacturing an austenitic stainless steel in this embodiment.

[Preparation process]

Molten steel having the above-mentioned chemical composition is produced. The molten steel is subjected to a known degassing treatment as necessary. Molten steel is used and the material is produced by casting. The material may be an ingot by the roughing method, or a cast slab such as a slab, a bloom, or a bill by a continuous casting method. In addition, a tubular cast body may be produced by a centrifugal casting method.

[Hot Forging Process]

The produced material may be subjected to hot forging to produce a cylindrical material. When the hot forging is performed, the internal structure of the molten steel produced in the preparation step can be changed from the solidified structure to a homogeneous sizing structure. The temperature of the hot forging is, for example, 900 to 1200 占 폚.

[Hot working step]

Hot working is performed on the material prepared in the preparation process or the hot forged material (cylindrical material), and a steel pipe is manufactured. For example, a through hole is formed in the center of a cylindrical material by machining. The cylindrical material having the through holes is subjected to hot extrusion to manufacture a steel pipe. The processing temperature of the hot extrusion is, for example, 900 to 1200 占 폚. A steel pipe may be manufactured by perforating the cylindrical material (Mann's method or the like).

[Cold working process]

The steel pipe after the hot working is subjected to cold working to produce an intermediate material. Cold working is, for example, cold drawing. When deformation is imparted to the surface of the steel in the cold working process, elements such as Al and Cr are likely to move to the steel surface. In this case, the TEE effect is sufficiently obtained. As a result, it is possible to obtain an austenitic stainless steel satisfying the formula (1) because Cr of the steel surface layer is adequately deficient. If the processing rate of the cold working is too low, this effect can not be obtained. The upper limit of the processing rate of the cold working is not particularly set, but it is difficult to carry out the cold working with a too high processing rate. Therefore, the processing rate of cold working is 10 to 90%.

[Heat treatment process]

The produced intermediate material is subjected to heat treatment in an air atmosphere. By the heat treatment in the air atmosphere, a uniform Al ₂ O ₃ coating film is formed on the surface of the steel. At this time, Cr of the steel surface layer is adequately deficient due to the TEE effect. As a result, an austenitic stainless steel satisfying the formula (1) can be obtained.

The heat treatment temperature is less than 900 to 1100 ° C, and the heat treatment time is 3.0 to 30.0 minutes.

If the heat treatment temperature is less than 900 占 폚 or the heat treatment time is less than 3.0 minutes, the TEE effect can not be sufficiently obtained. In this case, the Cr concentration _{Cr Cr} 'in the surface layer of the steel becomes too high to satisfy the equation (1). For this reason, Cr carbide is formed on the surface of the steel in a high-temperature carburizing environment, and a uniform Al ₂ O ₃ coating film is not formed sufficiently. As a result, the slip resistance decreases. Therefore, the heat treatment temperature is 900 占 폚 or higher and the heat treatment time is 3.0 minutes or higher. When the heat treatment temperature is 900 DEG C or more and the heat treatment time is 3.0 minutes or more, the crystal grains become 30 mu m or more.

On the other hand, if the heat treatment temperature is higher than or equal to 1100 캜, a scale mainly composed of Cr ₂ O ₃ is excessively formed on the steel surface. For this reason, Cr in the steel surface layer is excessively deficient. In this case, the Cr concentration _{Cr Cr} 'in the surface layer of the steel becomes too low to satisfy the equation (1). Therefore, under the high temperature carburizing environment, the TEE effect of Cr is lowered, and a uniform Al ₂ O ₃ coating film is not formed sufficiently. As a result, the slip resistance decreases. When the heat treatment time exceeds 30.0 minutes, a scale mainly composed of Al ₂ O ₃ is excessively formed on the surface of the steel. For this reason, Al in the steel surface layer is excessively deficient. In this case, the Al concentration C _Al 'of the steel surface layer is too low to satisfy the formula (1). Therefore, a uniform Al ₂ O ₃ coating film can not be sufficiently formed under a high-temperature carburizing environment. As a result, the slip resistance decreases. Therefore, the heat treatment temperature is less than 1100 占 폚, and the heat treatment time is 30.0 minutes or less. When the heat treatment temperature is less than 1100 占 폚 and the heat treatment time is 30.0 minutes or less, the crystal grains become 80 占 퐉 or less.

When the heat treatment temperature is less than 900 to 1100 占 폚 and the heat treatment time is 3.0 to 30.0 minutes, a steel having a chemical composition satisfying the formula (1) is obtained with sufficient and satisfactory TEE effect. As a result, the sand elasticity under high temperature carburizing environment becomes high.

The pickling treatment may be performed on the intermediate material after the heat treatment for the purpose of removing the 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, for the purpose of scaling off the surface of the steel and imparting deformation to the surface of the steel, the intermediate material after the pickling treatment may be subjected to a short machining on the steel surface. The material, shape, and treatment conditions of the shot particles in the short machining are not specified, but sufficient material, shape, and processing conditions are sufficient for scaling off the steel surface or imparting deformation to the steel surface. The scale is, for example, Al ₂ O ₃ . The Al ₂ O ₃ coating can be removed by well-known methods such as pickling and short processing.

The austenitic stainless steel of the present embodiment is produced by the above-described manufacturing method. In the above description, a method of manufacturing a steel pipe has been described. However, the plate material, the bar material, the wire material, and the like may be produced by the same manufacturing method (preparing process, hot forging process, hot working process, cold working process, and heat treatment process). The austenitic stainless steel of the present embodiment is particularly preferably applied to a steel pipe. Therefore, preferably, the austenitic stainless steel of the present embodiment is an austenitic stainless steel pipe.

Example

[Manufacturing method]

Molten steel having the chemical composition shown in Table 1 was produced by using a vacuum melting furnace.

[Table 1]

Using the molten steel, a cylindrical ingot (30 kg) having an outer diameter of 120 mm was produced. The ingot was subjected to hot forging and hot rolling. After hot rolling, cold rolling was carried out under the conditions shown in Table 2 to prepare an intermediate member having a thickness of 15 mm. From the obtained intermediate material, a plate material of 8 mm x 20 mm x 30 mm was produced by machining two sheets of each steel material. The plate material was subjected to heat treatment at the temperature and time shown in Table 2. After the heat treatment, the plate was water-cooled to prepare a test steel sheet.

[Table 2]

[Measurement of austenite grain size]

Test specimens for observing the microscope were prepared from the central portion of the cross section perpendicular to the rolling direction of the steel sheet of each test number. Among the surfaces of the test pieces, a microscopic test method specified in ASTM E 112 was carried out by using a surface (referred to as an observation surface) equivalent to the above cross section, and the crystal grain size was measured. Concretely, after the observation surface was mechanically polished, it was corroded by using a corrosive liquid, and crystal grain boundaries on the observation surface appeared. The average crystal grain size of each field of view was obtained at 10 fields of view on the corroded surface. The area of each field of view is about 0.75 mm ² .

[Measurement of Cr concentration _{Cr Cr} 'in the surface layer and Al concentration C _A1 ' in the surface layer]

The steel sheets of the respective test numbers were subjected to descaling treatment under the conditions in accordance with JIS Z 2290 (2004). The steel sheet after descaling treatment was cut perpendicularly to the rolling direction, and a sample including the surface was taken. The sample was embedded in the resin, and the observation surface including the section near the surface was polished. The Cr concentration C _Cr 'and the Al concentration C _Al ' of the surface layer (the range from the surface to the depth of 2 μm) were obtained for the observation surface after polishing by the above-described method.

[Cr concentration C _Cr other than the surface layer and Al concentration C _Al measurement other than the surface layer]

The Cr concentration C _Cr other than the surface layer and the Al concentration C _Al other than the surface layer were determined by the above-described method.

[Carburization test]

The steel for each test number, H ₄ -CH ₂ -CO ₂ atmosphere and maintained at 1100 ℃ × 96 hours. The surface of the steel sheet after carburizing was dry-hand polished with a # 600 abrasive paper to remove the scale of the surface and the like. The analytical cuts for four layers were collected at a pitch of 0.5 mm from the surface of the steel sheet. The C concentration was measured by the high-frequency burning infrared absorption method for the obtained cut section. From the measurement results, the C concentration originally contained in the steel was subtracted, and the C concentration increase was determined. The average of the amount of increase in the concentration of C in the four layers was defined as the penetration C amount.

[High temperature tensile test]

For the manufactured ingot, a cylindrical tensile test piece having a diameter of 10 mm and a length of 130 mm was cut out from the thick central portion. Each tensile test piece was subjected to a tensile test at a tensile rate (strain rate) of 10 / s to evaluate the hot workability. In the present invention, 60% or more of the shrinkage after the tensile test at 900 占 폚 was accepted as? (?) And less than 60% was rejected (占).

[Test result]

The test results are shown in Table 2.

Referring to Table 2, since the chemical compositions of Test Nos. 1 to 12 were appropriate and the production conditions were appropriate, F1 satisfied Formula (1). As a result, the penetration C amount was 0.4% or less and exhibited excellent sinking elasticity. Further, the shrinkage value of the high temperature tensile test was 60% or more, and excellent hot workability was exhibited.

On the other hand, in Test No. 13, the processing rate at the time of cold rolling was too low. Therefore, F1 was 0.35 and the equation (1) was not satisfied. As a result, the penetration C amount was 0.51%, and the lowering elasticity was low.

In test no. 14, the heat treatment temperature was too low. Therefore, F1 was 1.00 and the equation (1) was not satisfied. As a result, the penetration C amount was 0.65%, and the low elasticity was low. Test No. 14 also had a crystal grain size of 21 mu m.

In test no. 15, the heat treatment temperature was too high. Therefore, F1 was 0.39, and the formula (1) was not satisfied. As a result, the penetration C amount was 0.58%, and the low elasticity was low. Test No. 15 also had a crystal grain size of 131 탆.

In test No. 16, the heat treatment time was too short. Therefore, F1 was 1.06, and the formula (1) was not satisfied. As a result, the penetration C amount was 0.69%, and the low elasticity was low. Test No. 16 also had a crystal grain size of 22 mu m.

In Test No. 17, the heat treatment time was too long. Therefore, F1 was 0.95, and the formula (1) was not satisfied. As a result, the penetration C amount was 0.54%, and the lowering elasticity was low. In Test No. 17, the crystal grain size was also 95 m.

In Test No. 18, the Cr content was too low. Therefore, the TEE effect due to Cr was lowered. As a result, the penetration C amount was 0.75% and the low elasticity was low.

In Test No. 19, the Cr content was too high. For this reason, the formation of Al ₂ O ₃ coating was inhibited by Cr carbide. As a result, the penetration C amount was 0.60%, and the lowering elasticity was low.

In Test No. 20, the Al content was too low. Therefore, the Al ₂ O ₃ coating film was not sufficiently formed. As a result, the penetration C amount was 0.83%, and the puncture elasticity was low.

In Test No. 21, the Ni content was too low. For this reason, the penetration C amount was 0.52%, and the lowering elasticity was low.

In Test No. 22, the Mg content was too low. Therefore, the shrinkage value was less than 60% and the hot workability was low.

In Test No. 23, the Mg content was too high. Therefore, the shrinkage value was less than 60% and the hot workability was low.

The embodiments of the present invention have been described above. However, the above-described embodiments are merely examples for practicing the present invention. Therefore, the present invention is not limited to the above-described embodiment, and the above-described embodiment can be appropriately changed and carried out within a scope not departing from the spirit of the present invention.

Industrial availability

The austenitic stainless steel of the present invention can also be used in a high temperature carburizing environment where carburization and caulking are feared, such as hydrocarbon gas atmosphere. Particularly, it is particularly suitable for use as a reactor steel in a chemical industrial plant such as an ethylene production plant.

Claims (2)

In terms of% by mass,
C: less than 0.03 to less than 0.25%
Si: 0.01 to 2.0%
Mn: 2.0% or less,
P: 0.04% or less,
S: 0.01% or less,
Cr: less than 10 to 22%
Ni: more than 30.0% to 40.0%
Al: more than 2.5% to less than 4.5%
Nb: 0.01 to 3.5%,
N: 0.03% or less,
Ca: 0.0005 to 0.05%
Mg: 0.0005 to 0.05%
Ti: 0 to less than 0.2%
Mo: 0 to 0.5%,
W: 0 to 0.5%,
Cu: 0 to 0.5%,
V: 0 to 0.2%, and
B: 0 to 0.01%
The balance being Fe and impurities,
An austenitic stainless steel satisfying the formula (1).
(C _Cr '/ C _Al ') / (C _Cr / C _Al )? 0.80 (1)
Here, C _Cr 'in the formula (1) is substituted with the Cr concentration (mass%) in the surface layer of the austenitic stainless steel. C _Al 'is substituted for the Al concentration (mass%) in the surface layer of the austenitic stainless steel. In addition, Cr concentration (mass%) other than the surface layer of the austenitic stainless steel is substituted for C _Cr . C _Al is substituted with Al concentration (mass%) other than the surface layer of the austenitic stainless steel. The method according to claim 1,
The chemical composition,
Ti: 0.005 to less than 0.2%
Mo: 0.01 to 0.5%
W: 0.01 to 0.5%,
Cu: 0.005 to 0.5%
V: 0.005 to 0.2%, and
And B: 0.0001 to 0.01. The austenitic stainless steel according to claim 1,