Austenitic heat-resistant steel

Abstract

Provided is an austenitic heat-resistant steel having excellent creep rupture strength and excellent creep rupture ductility. The austenitic heat-resistant steel has a chemical composition comprising, in terms of mass%, 0.01-0.40% C, up to 3.00% Si, up to 3.00% Mn, up to 0.100% P, up to 0.100% S, 20.0-35.0% Cr, 20.0-48.0% Ni, 0.1-5.0% Mo, 0.01-1.50% Ti, 0.01-0.50% V, up to 0.030% N, up to 0.100% Al, 0.0001-0.0200% B, etc., and has an average austenite crystal grain size of 0.0-5.0 in terms of ASTM No. In the austenitic heat-resistant steel, the Ti, V, and Mo contents and a Ti amount, a V amount, and a Mo amount that are determined from an electrolytic extraction residue satisfy expression (i) and expression (ii).　(i): TiER+VER+MoER≤1.000 (ii): 1.00≤(Ti+V+Mo)-(TiER+VER+MoER)

Description

Austenitic heat-resistant steel

[0001] The present invention relates to an austenitic heat-resistant steel, and more particularly to an austenitic heat-resistant steel suitable as a raw material for steel materials (steel pipes, steel plates, steel bars, forged steel products, etc.) used in power generation boilers and heating furnaces for the chemical industry, or for steel materials used in hydrogen power generation using hydrogen gas and hydrogen reduction steelmaking.

[0002] In recent years, in order to reduce the environmental impact, there has been a trend toward higher temperatures and pressures in the operating conditions of power generation boilers and heating furnaces for the chemical industry, and the austenitic heat-resistant steels used as materials for heat exchanger tubes and cracking furnace tubes are being required to have superior creep rupture strength. Furthermore, the practical application of hydrogen power generation and hydrogen reduction ironmaking using hydrogen gas is being considered in the future, and high creep rupture strength will be required due to the higher temperature environments.

[0003] Against this technical background, various technologies relating to austenitic heat-resistant steels have been proposed. For example, Patent Document 1 (JP-B 1-52465) proposes a heat-resistant steel that is a forged pipe material but has higher high-temperature strength than conventional HK40 and Incoloy 800, and also has excellent workability, weldability, etc. Patent Document 2 (JP Patent No. 2760004) proposes a heat-resistant steel that has excellent high-temperature strength in high-temperature environments of approximately 700°C to 1150°C, and also has excellent workability.

[0004] Patent Document 3 (Japanese Patent No. 4007241) discloses an austenitic stainless steel suitable for use as a material for heat-resistant and pressure-resistant components, which exhibits excellent thermal fatigue resistance and structural stability at high temperatures of 700° C. or higher. Patent Document 4 (Japanese Patent No. 4442331) proposes a stainless steel that has excellent carburization resistance and coking resistance by being provided with the ability to form and regenerate a shielding scale against carburizing gases, for use in cracking furnace tubes for ethylene plants, etc.

[0005] Japanese Patent Publication No. 1-52465 Publication of Patent No. 2760004 Publication of Patent No. 4007241 Publication of Patent No. 4442331

[0006] Austenitic heat-resistant steels used in heat exchanger tubes and cracking furnace tubes in power generation boilers and chemical industry heating furnaces are required to have excellent creep rupture strength and creep rupture ductility, but it is generally difficult to achieve both excellent creep rupture strength and creep rupture ductility.

[0007] An object of the present invention is to provide an austenitic heat-resistant steel that is excellent in both creep rupture strength and creep rupture ductility.

[0008] The austenitic heat-resistant steel according to one embodiment of the present invention has a chemical composition, in mass %, of C: 0.01 to 0.40%, Si: 3.00% or less, Mn: 3.00% or less, P: 0.100% or less, S: 0.100% or less, Cr: 20.0 to 35.0%, Ni: 20.0 to 48.0%, Mo: 0.1 to 5.0%, Ti: 0.01 to 1.50%, V: 0.01 to 0.50%, N: 0.030% or less, Al: 0.100% or less, and B: 0.0001 to 0.0 200%, Ca: 0-0.0100%, Mg: 0-0.0500%, Zr: 0-0.100%, Co: 0-2.000%, Cu: 0-1.00%, Nb: 0-0.100%, W: 0-4.00%, Ta: 0-0.100%, Sn: 0-0.100%, Sb: 0-0.020%, As: 0-0.020%, Pb: 0-0.020%, Zn: 0-0.020%, balance: Fe and impurities, the average grain size of austenite grains is ASTM No. 0.0-5.0, and the Ti content, V content, and Mo content, and the Ti amount, V amount, and Mo amount analyzed as an electrolytic extraction residue, satisfy the following formulas (i) and (ii): Ti ＥＲ +V ＥＲ +Mo ＥＲ ≦1.000...(i) 1.00≦(Ti+V+Mo)-(Ti ＥＲ +V ＥＲ +Mo ＥＲ ) (ii) In the formula (i) and the formula (ii), Ti ＥＲ , V ＥＲ and Mo ＥＲ In formula (ii), the Ti content, V content, and Mo content in mass% are substituted for Ti, V, and Mo, respectively, which are analyzed as the electrolytic extraction residue.

[0009] An austenitic heat-resistant steel according to one embodiment of the present invention has a chemical composition, in mass %, of C: 0.01 to 0.40%, Si: 3.00% or less, Mn: 3.00% or less, P: 0.100% or less, S: 0.100% or less, Cr: 20.0 to 35.0%, Ni: 20.0 to 48.0%, Mo: 0.1 to 5.0%, Ti: 0.01 to 1.50%, V: 0.01 to 0.50%, N: 0.030% or less, Al: 0.100% or less, B: 0.0001 to 0.0200%, and the balance: Fe and impurities; and an average grain size of austenite grains in accordance with ASTM No. The Ti content, V content, and Mo content, and the Ti amount, V amount, and Mo amount analyzed as the electrolytic extraction residue satisfy the following formulas (i) and (ii): Ti ＥＲ +V ＥＲ +Mo ＥＲ ≦1.000...(i) 1.00≦(Ti+V+Mo)-(Ti ＥＲ +V ＥＲ +Mo ＥＲ ) (ii) In the formula (i) and the formula (ii), Ti ＥＲ , V ＥＲ and Mo ＥＲ In formula (ii), the Ti content, V content, and Mo content in mass% are substituted for Ti, V, and Mo, respectively, which are analyzed as the electrolytic extraction residue.

[0010] An austenitic heat-resistant steel according to one embodiment of the present invention has a chemical composition, in mass %, of C: 0.01 to 0.40%, Si: 3.00% or less, Mn: 3.00% or less, P: 0.100% or less, S: 0.100% or less, Cr: 20.0 to 35.0%, Ni: 20.0 to 48.0%, Mo: 0.1 to 5.0%, Ti: 0.01 to 1.50%, V: 0.01 to 0.50%, N: 0.030% or less, Al: 0.100% or less, B: 0.0001 to 0.0200%, and one or more elements selected from the group consisting of Group A, Group B, Group C, and Group D below, with the balance being Fe and impurities; and an average grain size of austenite grains satisfying ASTM No. The Ti content, V content, and Mo content, and the Ti amount, V amount, and Mo amount analyzed as an electrolytic extraction residue satisfy the following formulas (i) and (ii): [Group A] One or two selected from the group consisting of Ca: 0.0100% or less and Mg: 0.0500% or less [Group B] One or two or more selected from the group consisting of Zr: 0.100% or less, Co: 2.000% or less, Cu: 1.00% or less, Nb: 0.100% or less, W: 4.00% or less, and Ta: 0.100% or less [Group C] Sn: 0.100% or less [Group D] One or two or more selected from the group consisting of Sb: 0.020% or less, As: 0.020% or less, Pb: 0.020% or less, and Zn: 0.020% or less Ti ＥＲ +V ＥＲ +Mo ＥＲ ≦1.000...(i) 1.00≦(Ti+V+Mo)-(Ti ＥＲ +V ＥＲ +Mo ＥＲ ) (ii) In the formula (i) and the formula (ii), Ti ＥＲ , V ＥＲ and Mo ＥＲ In formula (ii), the Ti content, V content, and Mo content in mass% are substituted for Ti, V, and Mo, respectively, which are analyzed as the electrolytic extraction residue.

[0011] According to the present invention, it is possible to obtain an austenitic heat-resistant steel that is excellent in both creep rupture strength and creep rupture ductility.

[0012] In order to solve the above-mentioned problems, the present inventors investigated the creep rupture strength and creep rupture ductility of austenitic heat-resistant steels and obtained the following findings: (a) Excellent creep rupture strength can be obtained by adjusting the average grain size of austenite grains to ASTM No. 0.0 to 5.0. (b) Excellent creep rupture strength and creep rupture ductility can be obtained by adding Ti, V, and Mo in combination. (c) Excellent creep rupture ductility can be obtained by adjusting the sum of the Ti, V, and Mo contents analyzed as electrolytic extraction residue to 1.000% or less. (d) Excellent creep rupture strength can be obtained by adjusting the difference between the sum of the Ti, V, and Mo contents and the sum of the Ti, V, and Mo contents analyzed as electrolytic extraction residue to 1.00% or more.

[0013] The present invention has been completed based on the above findings. An austenitic heat-resistant steel according to one embodiment of the present invention will be described in detail below.

[0014] [Austenitic heat-resistant steel] [Chemical composition] The austenitic heat-resistant steel according to this embodiment has the chemical composition described below. In the following description, "%" for the content of an element means mass %.

[0015] C: 0.01 to 0.40% Carbon (C) stabilizes austenite and forms fine carbides at grain boundaries, improving creep strength at high temperatures. To fully achieve this effect, a C content of 0.01% or more is required. However, excessive C content causes the carbides to become coarse and precipitate in large quantities, reducing grain boundary ductility and further reducing hot workability, creep rupture strength, and creep rupture ductility. Therefore, an upper limit is set, with the C content being 0.01 to 0.40%. The lower limit of the C content is preferably 0.02%, more preferably 0.03%. The upper limit of the C content is preferably 0.35%, more preferably 0.30%.

[0016] Si: 3.00% or less When silicon (Si) is contained in excess, it reduces the stability of austenite, resulting in a decrease in the toughness and creep strength of the steel. Therefore, the upper limit of the Si content is set to 3.00% or less. The Si content is preferably 2.50% or less, and more preferably 2.00% or less. It is not necessary to set a lower limit for the Si content, but an extreme reduction increases the manufacturing cost. Therefore, the lower limit of the Si content is preferably 0.02%, and more preferably 0.05%.

[0017] Mn: 3.00% or less Excessive manganese (Mn) embrittles steel and also reduces the toughness and creep ductility of the steel. Therefore, the upper limit of the Mn content is set to 3.00% or less. The Mn content is preferably 2.50% or less, and more preferably 2.00% or less. There is no need to set a lower limit for the Mn content, but an extreme reduction increases manufacturing costs. Therefore, the lower limit of the Mn content is preferably 0.005%, more preferably 0.01%, even more preferably 0.05%, and even more preferably 0.10%.

[0018] P: 0.100% or less Phosphorus (P) is contained in steel as an impurity, and if present in large amounts, it significantly reduces hot workability and weldability, and also reduces creep ductility after long-term use. Therefore, the upper limit of the P content is set to 0.100% or less. The P content is preferably 0.090% or less, and more preferably 0.080% or less. It is preferable to reduce the P content as much as possible, but excessive reduction increases manufacturing costs. Therefore, the lower limit of the P content is preferably 0.0005%, and more preferably 0.0008%.

[0019] S: 0.100% or less Sulfur (S) is contained in steel as an impurity, and if present in large amounts, it significantly reduces hot workability and weldability, and also reduces creep ductility after long-term use. Therefore, the upper limit of the S content is set to 0.100% or less. The S content is preferably 0.095% or less, and more preferably 0.090% or less. On the other hand, a small amount of S has the effect of improving creep rupture properties. Therefore, the lower limit of the S content may be set to 0.0005%. A more preferable lower limit of the S content is 0.0008%.

[0020] Cr: 20.0 to 35.0% Chromium (Cr) is an essential element for ensuring carbide precipitation during creep deformation and for ensuring oxidation resistance and corrosion resistance at high temperatures. To achieve the above-mentioned effects of Cr, a Cr content of 20.0% or more is required. However, if the Cr content exceeds 35.0%, the stability of austenite at high temperatures deteriorates, resulting in a decrease in creep strength. Therefore, the Cr content is set to 20.0 to 35.0%. The lower limit of the Cr content is preferably 20.5%, more preferably 21.0%, even more preferably 23.0%, and even more preferably 24.0%. The upper limit of the Cr content is preferably 34.5%, even more preferably 34.0%, even more preferably 32.0%, even more preferably 30.0%, and even more preferably 28.0%.

[0021] Ni: 20.0 to 48.0% Nickel (Ni) is an effective element for obtaining austenite and is an essential element for ensuring structural stability during long-term use. To fully obtain the above-mentioned effects of Ni within the aforementioned Cr content range of 20.0 to 35.0%, a Ni content of 20.0% or more is required. However, Ni is an expensive element, and containing a large amount increases costs. Therefore, an upper limit is set, with the Ni content being 20.0 to 48.0%. The lower limit of the Ni content is preferably 21.0%, more preferably 22.0%, even more preferably 28.0%, even more preferably 30.0%, even more preferably 32.0%, even more preferably 33.0%, and even more preferably 34.0%. The upper limit of the Ni content is preferably 46.0%, even more preferably 44.0%, even more preferably 42.0%, and even more preferably 40.0%.

[0022] Mo: 0.1 to 5.0% Molybdenum (Mo) improves creep strength. That is, Mo dissolves in the matrix to improve creep strength at high temperatures. Furthermore, Mo dissolves in carbides to suppress carbide coarsening, thereby contributing to improved creep strength. If the Mo content is less than 0.1%, the amount of Mo dissolved in carbides is insufficient, causing carbide coarsening and preventing the desired creep strength from being achieved. However, excessive Mo content reduces the stability of austenite, resulting in a decrease in creep strength. Therefore, the Mo content is set to 0.1 to 5.0%. The lower limit of the Mo content is preferably 0.8%, more preferably 1.0%, even more preferably 1.2%, and even more preferably 1.5%. The upper limit of the Mo content is preferably 4.5%, even more preferably 4.0%, even more preferably 3.5%, and even more preferably 3.0%.

[0023] Ti: 0.01 to 1.50% Titanium (Ti) precipitates intragranularly as fine carbonitrides during use at high temperatures, contributing to high-temperature creep strength. To achieve this effect, a Ti content of 0.01% or more is required. However, excessive Ti content precipitates large amounts of carbonitrides, resulting in reduced creep ductility and toughness. For this reason, the Ti content is set to 0.01 to 1.50%. The lower limit of the Ti content is preferably 0.03%, more preferably 0.05%, even more preferably 0.10%, even more preferably 0.20%, even more preferably 0.30%, and even more preferably 0.40%. The upper limit of the Ti content is preferably 1.40%, even more preferably 1.30%, even more preferably 1.20%, even more preferably 1.10%, and even more preferably 1.00%.

[0024] V: 0.01 to 0.50% Vanadium (V) has the effect of improving creep strength. That is, V combines with C or N to form fine carbides or carbonitrides during use at high temperatures, improving creep strength. If the V content is less than 0.01%, the precipitation of fine carbides or carbonitrides is insufficient, making it impossible to obtain the desired creep strength. However, if V is contained in excess, large amounts of carbides or carbonitrides are precipitated, resulting in a decrease in creep ductility and toughness. Therefore, the V content is set to 0.01 to 0.50%. The lower limit of the V content is preferably 0.05%, more preferably 0.08%, even more preferably 0.10%, and even more preferably 0.12%. The upper limit of the V content is preferably 0.45%, even more preferably 0.40%, even more preferably 0.35%, and even more preferably 0.30%.

[0025] N: 0.030% or less Excessive N content causes intragranular precipitation of a large amount of fine nitrides during use at high temperatures, resulting in reduced creep ductility and toughness. Therefore, the upper limit of the N content is set to 0.030% or less. The N content is preferably 0.025% or less, more preferably 0.020% or less, even more preferably 0.018% or less, and even more preferably 0.015% or less. While there is no need to set a lower limit for the N content, excessive reduction increases manufacturing costs. Therefore, the lower limit of the N content is preferably 0.0005%, more preferably 0.0008%, even more preferably 0.001%, and even more preferably 0.003%.

[0026] Al: 0.100% or less Excessive Al content deteriorates the cleanliness of the steel, reducing its hot workability and ductility. Therefore, the upper limit of the Al content is set to 0.100% or less. The Al content is preferably 0.095% or less, more preferably 0.090% or less, even more preferably 0.070% or less, and even more preferably 0.050% or less. Although there is no need to set a lower limit for the Al content, an extreme reduction in the Al content increases manufacturing costs. Therefore, the lower limit of the Al content is preferably 0.0005%, more preferably 0.001%, even more preferably 0.002%, and even more preferably 0.005%.

[0027] B: 0.0001 to 0.0200% Boron (B) is an element necessary for improving creep strength by segregating to grain boundaries during use at high temperatures, strengthening the grain boundaries, and finely dispersing grain boundary carbides. To achieve this effect, a B content of 0.0001% or more is required. However, excessive B content degrades weldability and hot workability. Therefore, an upper limit is set, with the B content being 0.0001 to 0.0200%. The lower limit of the B content is preferably 0.0005%, more preferably 0.0010%, and even more preferably 0.0015%. The upper limit of the B content is preferably 0.0150%, more preferably 0.0100%, and even more preferably 0.0080%.

[0028] The remainder of the chemical composition of the austenitic heat-resistant steel according to this embodiment is Fe and impurities. Because iron (Fe) is an inexpensive raw material, it is preferably contained in an amount of 1.0 to 50.0%, more preferably 10.0 to 40.0%, and even more preferably 15.0 to 35.0%. The impurities referred to here refer to elements that are mixed in from ores or scrap used as raw materials for the steel, or from the environment during the manufacturing process, etc.

[0029] The chemical composition of the austenitic heat-resistant steel according to this embodiment may contain, in place of a portion of Fe, one or more elements selected from the group consisting of the following groups A, B, C, and D. The elements belonging to groups A, B, C, and D (Ca, Mg, Zr, Co, Cu, Nb, W, Ta, Sn, Sb, As, Pb, and Zn) are all optional elements, and the chemical composition of the austenitic heat-resistant steel according to this embodiment may not contain some or all of these elements.

[0030] [Group A] One or two elements selected from the group consisting of Ca: 0.0100% or less and Mg: 0.0500% or less The elements belonging to Group A have the effect of improving the hot workability of steel, and therefore may be added as necessary.

[0031] Ca: 0.0100% or less Calcium (Ca) forms a compound with S to reduce the amount of S in the steel and improve hot workability. This effect can be achieved even if even a small amount of Ca is contained. However, an excessive Ca content reduces cleanliness and, conversely, hot workability. Therefore, when Ca is contained, its content is set to 0.0100% or less. The lower limit of the Ca content is preferably 0.0001%, more preferably 0.0002%, and even more preferably 0.0003%. The upper limit of the Ca content is preferably 0.0090%, more preferably 0.0070%, and even more preferably 0.0060%.

[0032] Mg: 0.0500% or less Magnesium (Mg) forms a compound with S to reduce the amount of S in steel and improve hot workability. This effect can be achieved even if even a small amount of Mg is contained. However, an excessive Mg content reduces cleanliness and, conversely, hot workability. Therefore, if Mg is contained, its content should be 0.0500% or less. The lower limit of the Mg content is preferably 0.0001%, more preferably 0.0002%, and even more preferably 0.0003%. The upper limit of the Mg content is preferably 0.0450%, more preferably 0.0400%, and even more preferably 0.0300%.

[0033] [Group B] One or more elements selected from the group consisting of Zr: 0.100% or less, Co: 2.000% or less, Cu: 1.00% or less, Nb: 0.100% or less, W: 4.00% or less, and Ta: 0.100% or less. The elements belonging to Group B have the effect of further increasing the creep strength of steel, and therefore may be added as necessary.

[0034] Zr: 0.100% or less Zirconium (Zr) is a grain boundary strengthening element and has the effect of improving creep rupture strength. Zr also has the effect of improving rupture ductility. This effect can be achieved even if even a small amount of Zr is contained. However, an excessive Zr content may deteriorate hot workability. Therefore, if Zr is contained, the amount of Zr is set to 0.100% or less. The lower limit of the Zr content is preferably 0.001%, more preferably 0.002%. The upper limit of the Zr content is preferably 0.090%, more preferably 0.080%, even more preferably 0.060%, and even more preferably 0.040%.

[0035] Cobalt (Co) has the effect of improving creep strength. That is, Co, like Ni, is an austenite-forming element, and it contributes to improving creep strength by increasing phase stability. Even if even a small amount of Co is contained, this effect can be obtained. However, since Co is an extremely expensive element, excessive Co content significantly increases costs. Therefore, if Co is contained, its content should be 2.000% or less. The lower limit of the Co content is preferably 0.010%, more preferably 0.015%. The upper limit of the Co content is preferably 1.900%, more preferably 1.500%, and even more preferably 1.000%.

[0036] Cu: 1.00% or less Copper (Cu) has the effect of improving creep strength. That is, Cu, like Ni and Co, is an austenite-forming element and contributes to improving creep strength by increasing phase stability. Even a small amount of Cu can achieve this effect. However, excessive Cu content can lead to a decrease in hot workability. Therefore, when Cu is contained, its content is set to 1.00% or less. The lower limit of the Cu content is preferably 0.01%, more preferably 0.05%. The upper limit of the Cu content is preferably 0.90%, more preferably 0.80%, even more preferably 0.60%, and even more preferably 0.40%.

[0037] Nb: 0.100% or less Niobium (Nb) combines with C and N to precipitate intragranularly as fine carbides and carbonitrides, contributing to improved creep strength at high temperatures. Even a small amount of Nb can provide this effect. However, excessive Nb content precipitates large amounts of carbides and carbonitrides, resulting in reduced creep ductility and toughness. Therefore, when Nb is contained, its content is set to 0.100% or less. The lower limit of the Nb content is preferably 0.010%. The upper limit of the Nb content is preferably 0.080%, more preferably 0.060%, and even more preferably 0.040%.

[0038] W: 4.00% or less Tungsten (W) is an element that dissolves in the matrix and contributes to improving creep strength at high temperatures. This effect can be achieved even if even a small amount of W is contained. However, excessive W content actually reduces creep strength. Therefore, when W is contained, its content is set to 4.00% or less. The lower limit of the W content is preferably 0.10%, more preferably 0.50%. The upper limit of the W content is preferably 3.50%, more preferably 3.00%.

[0039] Ta: 0.100% or less Tantalum (Ta) is an element that contributes to improving creep strength at high temperatures. This effect can be achieved even if even a small amount of Ta is contained. However, excessive Ta content reduces creep ductility and toughness. Therefore, when Ta is contained, its content is set to 0.100% or less. The lower limit of the Ta content is preferably 0.010%, more preferably 0.050%. The upper limit of the Ta content is preferably 0.080%, more preferably 0.060%.

[0040] [Group C] Sn: 0.100% or less The element (Sn) belonging to Group C has the effect of further improving the corrosion resistance and high-temperature properties of steel, and therefore may be contained as required.

[0041] Sn: 0.100% or less Tin (Sn) is an element that enhances the corrosion resistance of steel. Even a small amount of Sn can provide this effect. However, excessive Sn content reduces hot workability. Therefore, when Sn is contained, its content is set to 0.100% or less. The lower limit of the Sn content is preferably 0.010%, more preferably 0.050%. The upper limit of the Sn content is preferably 0.080%, more preferably 0.060%.

[0042] [Group D] One or more elements selected from the group consisting of Sb: 0.020% or less, As: 0.020% or less, Pb: 0.020% or less, and Zn: 0.020% or less. The elements belonging to Group D (Sb, As, Pb, and Zn) are all impurities. These elements segregate at grain boundaries, lowering the melting point of the grain boundaries and reducing the bonding strength of the grain boundaries. Therefore, even if one or more of these elements are contained, their contents should each be 0.020% or less. The upper limit of the content of each of these elements is more preferably 0.019%, and even more preferably 0.018%.

[0043] [Structure] The austenitic heat-resistant steel according to this embodiment has a structure mainly composed of an austenite phase. The volume fraction of the austenite phase in the structure of the austenitic heat-resistant steel according to this embodiment is preferably 90% or more, and more preferably 95% or more.

[0044] [Average Grain Size] The austenitic heat-resistant steel according to this embodiment has an average grain size of austenite grains of ASTM No. 0.0 to 5.0. If the average grain size number is too small (if the grains are too large), excellent creep rupture ductility cannot be obtained. If the average grain size number is too large (if the grains are too fine), excellent creep rupture strength cannot be obtained. The lower limit of the average grain size number (ASTM No.) is preferably 0.5, and more preferably 1.0. The upper limit of the average grain size number (ASTM No.) is preferably 4.5, and more preferably 4.0.

[0045] The average grain size is measured in accordance with ASTM E112 (2013). Specifically, it is measured as follows. A test piece for microstructural observation is taken so that the cross section perpendicular to the longitudinal direction of the austenitic heat-resistant steel serves as the observation surface, and the observation surface is mirror-polished. After polishing, the specimen is etched with mixed acid and observed under an optical microscope. Ten fields of view are observed so that the center of the field of view is the center of the thickness of the austenitic heat-resistant steel. The grain size of each field of view is then determined using the comparison method specified in ASTM E112, and the average value is taken as the average grain size. The observation magnification is 100x, with 200x or 400x depending on the grain size. When the observation magnification is 200x or 400x, correction is performed in accordance with ASTM E112 (2013) using the correction value Q defined by the following formula: Q = 6.64 log １０ (M / 100) where M in the above formula is the observation magnification.

[0046] [Regarding Formula (i)] In the austenitic heat-resistant steel according to this embodiment, the Ti content, V content, and Mo content analyzed as the electrolytic extraction residue satisfy the following formula (i): Ti ＥＲ +V ＥＲ +Mo ＥＲ ≦1.000 (i) where the symbols in the above formula are defined as follows: Ti ＥＲ V: Ti content (mass%) in the precipitate obtained by extraction residue analysis ＥＲ : V content (mass%) in precipitates obtained by extraction residue analysis Mo ＥＲ : Mo content (mass%) in precipitates obtained by extraction residue analysis

[0047] Ti ＥＲ , V ＥＲ and Mo ＥＲ corresponds to the amount of Ti, V, and Mo present in the precipitates before the austenitic heat-resistant steel is used at high temperatures. ＥＲ , V ＥＲ and Mo ＥＲA high value of ρ / ρ / ρ means that there are many precipitates of Ti, V, and Mo formed during the manufacturing process (such as the solution heat treatment and subsequent cooling process described below). The temperature of the solution heat treatment is generally higher than the temperature during use, and the precipitates formed during the solution heat treatment tend to be coarser than those formed during use at high temperatures. Therefore, it is believed that the precipitates formed during the solution heat treatment and subsequent cooling process contribute less to improving creep rupture strength and creep rupture ductility than those formed during use at high temperatures. When there are many precipitates formed during the solution heat treatment and subsequent cooling process, specifically, when the value of the left side of formula (i) exceeds 1.000, excellent creep rupture strength and creep rupture ductility cannot be obtained. Therefore, the value of the left side of formula (i) is set to 1.000 or less.

[0048] The upper limit of the value on the left side of formula (i) is preferably 0.950, more preferably 0.900, even more preferably 0.700, even more preferably 0.600, and even more preferably 0.400. The lower limit of the value on the left side of formula (i) is not particularly limited, but is, for example, 0.010, preferably 0.050, and even more preferably 0.100.

[0049] The content (mass %) of each element in the precipitate analyzed as the electrolytic extraction residue in the above formula can be measured by the following procedure: using 10% acetylacetone-1% tetramethylammonium chloride / methanol at 20 mA / cm ２ Approximately 0.4 g of sample is electrolyzed at a current value of 1000 kJ / s. The electrolyzed sample solution is then filtered through a 0.2 μm filter, and the residue is subjected to acid decomposition. The amounts (mass%) of the above elements analyzed as electrolytic extraction residue are then calculated using an ICP (inductively coupled plasma) optical emission spectrometer.

[0050] [Regarding formula (ii)] In the austenitic heat-resistant steel according to this embodiment, the Ti content, V content, and Mo content, and the Ti amount, V amount, and Mo amount analyzed as the electrolytic extraction residue, satisfy the following formula (ii): 1.00≦(Ti+V+Mo)−(Ti ＥＲ +V ＥＲ +Mo ＥＲ) (ii) In formula (ii), Ti, V and Mo are substituted with the Ti content, V content and Mo content in mass %, respectively.

[0051] If the value of the right side of formula (ii) is less than 1.00, the amount of Ti, V, and Mo dissolved in solid solution is insufficient, and creep rupture strength and creep rupture ductility cannot be improved. Therefore, the value of the right side of formula (ii) is set to 1.00 or more. The lower limit of the value of the right side of formula (ii) is preferably 1.40, more preferably 1.60, even more preferably 2.00, even more preferably 2.10, and even more preferably 2.20. The upper limit of the value of the right side of formula (ii) is not particularly limited, but is, for example, 5.00, preferably 4.50.

[0052] [Dimensions, etc.] The austenitic heat-resistant steel according to this embodiment may be, for example, an alloy pipe or an alloy plate. When the austenitic heat-resistant steel is an alloy pipe, the wall thickness is preferably 1 mm or more, or 5 mm or more, and is preferably 100 mm or less, 80 mm or less, 65 mm or less, or 55 mm or less. When the austenitic heat-resistant steel is an alloy plate, the plate thickness is preferably 1 to 100 mm.

[0053] [Method for producing austenitic heat-resistant steel] There are no particular limitations on the method for producing the austenitic heat-resistant steel according to this embodiment, but the steel can be produced, for example, by hot working a steel ingot or slab having the above-mentioned chemical composition, and then, if necessary, further hot working by a different method such as hot extrusion, followed by solution heat treatment. Furthermore, if necessary, cold working may be performed before the solution heat treatment.

[0054] In order to achieve both excellent creep rupture strength and creep rupture ductility, it is necessary to control the solution heat treatment conditions. Specifically, although depending on the chemical composition, the solution heat treatment temperature must be 1100 to 1280°C and the solution heat treatment time must be 1 to 60 minutes. If the solution heat treatment temperature is less than 1100°C or the solution heat treatment time is less than 1 minute, Ti, V, and Mo, which contribute to improving creep strength, cannot be fully solid-dissolved, making it impossible to ensure good creep rupture strength. In addition, since recrystallization does not occur, strain due to working cannot be eliminated, and creep rupture ductility also deteriorates. On the other hand, if the solution heat treatment temperature is higher than 1280°C or the solution heat treatment time is longer than 60 minutes, austenite grains become coarse, resulting in deterioration of creep rupture ductility. Therefore, in order to achieve both excellent creep rupture strength and creep rupture ductility, the solution heat treatment temperature must be 1100 to 1280°C and the solution heat treatment time must be 1 to 60 minutes. The solution heat treatment temperature is preferably 1150 to 1250°C, and the solution heat treatment time is preferably 3 to 40 minutes.

[0055] The cooling rate after solution heat treatment also needs to be controlled to achieve both excellent creep rupture strength and creep rupture ductility. Specifically, the cooling rate from the solution heat treatment temperature to 500°C needs to be 2°C / s or more. If the cooling rate is less than 2°C / s, coarse carbonitrides containing Ti, V, and Mo precipitate during cooling, making it impossible to ensure good creep rupture strength. Therefore, the cooling rate from the solution heat treatment temperature to 500°C needs to be 2°C / s or more.

[0056] The above describes an example of an austenitic heat-resistant steel and a method for producing the same according to this embodiment. According to this embodiment, an austenitic heat-resistant steel excellent in both creep rupture strength and creep rupture ductility can be obtained.

[0057] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0058] [Example 1] Steels having the chemical compositions shown in Tables 1 and 2 were melted in a laboratory to prepare ingots. The ingots were then formed by hot forging and rolling, and then subjected to solution heat treatment under the conditions shown in Tables 3 and 4 to obtain austenitic heat-resistant steel plates.

[0059]

[0060]

[0061] A round bar creep rupture test specimen with a diameter of 6 mm and a gauge length of 30 mm was taken from the center of each steel plate's wall thickness and subjected to creep rupture tests at 900°C and 20 MPa. Specimens with a creep rupture time of more than 1000 hours were rated as passing, indicating good creep rupture strength. Specimens with a creep rupture reduction of more than 10.0% were also rated as passing, indicating good creep rupture ductility.

[0062] The results are shown in Tables 3 and 4. The "cooling rate" in Tables 3 and 4 is the cooling rate from the solution heat treatment temperature to 500°C. Note that the "-" in the average grain size column in Table 4 means that recrystallization did not occur, making it difficult to determine the grain size.

[0063]

[0064]

[0065] As shown in Table 3, the steel plates of Test Nos. 1 to 17 were good in both creep rupture strength and creep rupture ductility.

[0066] As shown in Table 4, the steel plate of Test No. 18 was inferior in both creep rupture strength and creep rupture ductility. This is thought to be because the grain size was inappropriate (recrystallization did not occur). The inappropriate grain size is thought to be due to the solution heat treatment temperature being too low.

[0067] The steel plate of Test No. 19 had poor creep rupture ductility. This is thought to be because the grain size number was too small (the grains were too large). The small grain size number is thought to be due to the solution heat treatment temperature being too high.

[0068] The steel plate of Test No. 20 was inferior in both creep rupture strength and creep rupture ductility. This is thought to be because the grain size was inappropriate (recrystallization did not occur). The inappropriate grain size is thought to be due to the solution heat treatment time being too short.

[0069] The steel plate of Test No. 21 had poor creep rupture ductility. This is thought to be because the grain size number was too small (the grains were too large). The small grain size number is thought to be due to the solution heat treatment time being too long.

[0070] The steel plate of Test No. 22 was inferior in both creep rupture strength and creep rupture ductility. ＥＲ +V ＥＲ +Mo ＥＲ was too large, or the amount of Ti, V and Mo dissolved in solid solution ((Ti + V + Mo) - (Ti ＥＲ +V ＥＲ +Mo ＥＲ This is thought to be because the cooling rate after the solution heat treatment was too slow.

[0071] The steel plate of Test No. 23 had poor creep rupture ductility, which is thought to be due to the Ti content being too high.

[0072] The steel plate of Test No. 24 had poor creep rupture ductility, which is thought to be due to the V content being too high.

[0073] The steel plate of Test No. 25 had poor creep rupture strength, which is thought to be due to the Mo content being too low.

[0074] The steel plate of Test No. 26 had poor creep rupture strength, which is thought to be due to the V content being too low.

[0075] The steel plate of Test No. 27 had poor creep rupture ductility. This was due to the Ti ＥＲ +V ＥＲ +Mo ＥＲ This is thought to be because the value was too large. ＥＲ +V ＥＲ +Mo ＥＲThe reason why the values ​​were large is thought to be that the contents of Ti, V and Mo were relatively high, while the temperature of the solution heat treatment was relatively low.

[0076] The steel plate of Test No. 28 had poor creep rupture strength. This was due to the amount of Ti, V, and Mo dissolved in solid solution ((Ti + V + Mo) - (Ti ＥＲ +V ＥＲ +Mo ＥＲ The reason why the amounts of Ti, V, and Mo dissolved in solid solution were small is thought to be because the total content of Ti, V, and Mo was relatively low.

[0077] The steel plate of Test No. 29 had poor creep rupture strength, which is thought to be due to the W content being too high.

[0078] Although the embodiments of the present invention have been described above, the above-described embodiments are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiments, and the above-described embodiments can be appropriately modified and carried out within the scope of the invention.

[0079] The austenitic heat-resistant steel according to the present invention is excellent in both long-term creep rupture strength and creep rupture ductility, and is therefore suitable for use as steel materials (such as steel pipes, steel plates, steel bars, and forgings) for power generation boilers and heating furnaces in the chemical industry, or for use in hydrogen power generation using hydrogen gas and hydrogen reduction steelmaking.

Claims

1. Austenitic heat-resistant steel having a chemical composition, in mass%, of C: 0.01 to 0.40%, Si: 3.00% or less, Mn: 3.00% or less, P: 0.100% or less, S: 0.100% or less, Cr: 20.0 to 35.0%, Ni: 20.0 to 48.0%, Mo: 0.1 to 5.0%, Ti: 0.01 to 1.50%, V: 0.01 to 0.50%, N: 0.030% or less, Al: 0.100% or less, B: 0.0001 to 0.0200%, Ca: 0 to 0.0100%, Mg: 0 to 0.0500%, Zr: 0 to 0.100%, An austenitic heat-resistant steel having the following composition: Co: 0 to 2.000%, Cu: 0 to 1.00%, Nb: 0 to 0.100%, W: 0 to 4.00%, Ta: 0 to 0.100%, Sn: 0 to 0.100%, Sb: 0 to 0.020%, As: 0 to 0.020%, Pb: 0 to 0.020%, Zn: 0 to 0.020%, balance: Fe and impurities, the average grain size of austenite grains is ASTM No. 0.0 to 5.0, and the Ti content, V content, and Mo content, and the Ti amount, V amount, and Mo amount analyzed as an electrolytic extraction residue, satisfy the following formulas (i) and (ii). ＥＲ +V ＥＲ +Mo ＥＲ ≦1.000...(i) 1.00≦(Ti+V+Mo)-(Ti ＥＲ +V ＥＲ +Mo ＥＲ ) (ii) In the formula (i) and the formula (ii), Ti ＥＲ , V ＥＲ and Mo ＥＲ In formula (ii), the Ti content, V content, and Mo content in mass% are substituted for Ti, V, and Mo, respectively, which are analyzed as the electrolytic extraction residue.

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