Austenitic stainless steel and steel pipe

Abstract

An austenitic stainless steel, wherein a chemical composition thereof is C: 0.002 to 0.020%, Si: 0.10 to 0.60%, Mn: 0.2 to 2.0%, P: 0.035% or less, S: 0.010% or less, Cu: 2.50 to 4.50%, Ni: 9.00 to 16.00%, Cr: 15.00 to 20.00%, Mo: 0.20 to 1.50%, Nb: 0.15 to 0.60%, N: 0.05 to 0.15%, B: 0.0010 to 0.0060%, optional elements, the balance: Fe and impurities, and satisfies [0.010 ≤ V ER + Ti ER + Nb ER ], [27.0 ≤ 1.13(Ni - Ni ER ) + (Cr - Cr ER ) + 1.85(Mo - Mo ER ) + 1.79(Nb - Nb ER ) < 40.5] and [Nb ER < 0.052].

Description

TECHNICAL FIELD

[0001] The present application relates to an austenitic stainless steel and a steel pipe. BACKGROUND

[0002] A boiler provided in a plant such as a thermal power plant or a chemical plant is exposed to a high temperature. Therefore, a heat transfer pipe for a boiler (hereinafter, simply referred to as "a boiler heat transfer pipe") used for the boiler is required to have a good high-temperature strength, specifically, a good creep strength.

[0003] Further, for the boiler heat transfer pipe, from the viewpoint of corrosion resistance, an austenitic stainless steel is sometimes used. For example, in Patent Literature 1 and Patent Literature 2, an austenitic stainless steel having a good creep strength is disclosed.

[0004] PRIOR ART DOCUMENTS

[0005] PATENT LITERATURE

[0006] Patent Literature 1: Japanese Patent Application Laid-Open No. 2003-268503

[0007] Patent Literature 2: Japanese Patent Application Laid-Open No. 2021-21093 SUMMARY

[0008] PROBLEMS TO BE SOLVED BY THE INVENTION

[0009] Further, in recent years, the environment in which the boiler heat transfer pipe is used in a plant has become more severe, and stress corrosion cracking (hereinafter, referred to as "SCC") sometimes occurs. Therefore, for the raw material of the boiler heat transfer pipe, in addition to a good creep strength, the ability to suppress SCC is also required. However, in Patent Literature 1 and Patent Literature 2, SCC is not studied. Thus, the austenitic stainless steel disclosed in the above-described documents has room for improvement in terms of SCC resistance.

[0010] In view of the above, an object of the present application is to provide an austenitic stainless steel having a good creep strength and SCC resistance.

[0011] MEANS FOR SOLVING THE PROBLEMS

[0012] The present application is made in order to solve the above-described problems, and the gist thereof is an austenitic stainless steel and a steel pipe described below.

[0013] (1) An austenitic stainless steel, wherein a chemical composition thereof is, in terms of mass%, Cr: 15 to 25%

[0014] C: 0.002 to 0.020%,

[0015] Si: 0.10 to 0.60%,

[0016] Mn: 0.2 to 2.0%,

[0017] P: 0.035% or less,

[0018] S: 0.010% or less,

[0019] Cu: 2.50 to 4.50%,

[0020] Ni: 9.00 to 16.00%,

[0021] Cr: 15.00 to 20.00%,

[0022] Mo: 0.20 to 1.50%,

[0023] Nb: 0.15 to 0.60%,

[0024] N: 0.05 to 0.15%,

[0025] B: 0.0010 to 0.0060%,

[0026] V: 0 to 0.50%,

[0027] Ti: 0 to 0.500%,

[0028] Co: 0 to 1.00%,

[0029] W: 0 to 1.00%,

[0030] Ta: 0 to 0.40%,

[0031] Sn: 0 to 0.0300%,

[0032] Ca: 0 to 0.0100%,

[0033] Mg: 0 to 0.0100%,

[0034] REM: 0 to 0.0800%,

[0035] balance: Fe and impurities,

[0036] and satisfy the following (i) to (iii) formulas,

[0037] 0.010 ≤ V ER + Ti ER + Nb ER (i)

[0038] 27.0 ≤ 1.13 (Ni - Ni ER ) + (Cr - Cr ER ) + 1.85 (Mo - Mo ER ) + 1.79 (Nb - Nb ER) < 40.5 (ii)

[0039] Nb ER < 0.052 (iii)

[0040] wherein each symbol in the above formula is defined as follows, each element symbol in the above formula represents the content (mass%) of each element contained in the steel, and is set to zero in the case of not containing,

[0041] V ER : V content (mass%) in the precipitate obtained by the extraction residue analysis

[0042] Ti ER : Ti content (mass%) in the precipitate obtained by the extraction residue analysis

[0043] Nb ER : Nb content (mass%) in the precipitate obtained by the extraction residue analysis

[0044] Ni ER : Ni content (mass%) in the precipitate obtained by the extraction residue analysis

[0045] Cr ER : Cr content (mass%) in the precipitate obtained by the extraction residue analysis

[0046] Mo ER : Mo content (mass%) in the precipitate obtained by the extraction residue analysis.

[0047] (2) The austenitic stainless steel according to the above (1), wherein the chemical composition contains one or more elements selected from the group consisting of, in mass%,

[0048] V: 0.01 to 0.50%,

[0049] Ti: 0.001 to 0.500%.

[0050] (3) The austenitic stainless steel according to the above (1) or (2), wherein the chemical composition contains one or more elements selected from the group consisting of, in mass%,

[0051] Co: 0.02 to 1.00%,

[0052] W: 0.01 to 1.00%,

[0053] Ta: 0.01 to 0.40%,

[0054] Sn: 0.0010 to 0.0300%,

[0055] Ca: 0.0010 to 0.0100%,

[0056] Mg: 0.0002 to 0.0100%,

[0057] REM: 0.0010 to 0.0800%.

[0058] (4) A steel pipe, wherein the steel pipe is made of the austenitic stainless steel according to any one of (1) to (3).

[0059] Effects of the Invention

[0060] According to the present application, an austenitic stainless steel having good creep strength and SCC resistance can be obtained. BRIEF DESCRIPTION OF DRAWINGS

[0061] Figure 1 is a microstructure photograph obtained by observing a Nb compound. DETAILED DESCRIPTION

[0062] The present inventors have researched a method for improving the creep strength and SCC resistance of an austenitic stainless steel, and have obtained the following insights of (a) to (c).

[0063] (a) C has an effect of improving the creep strength. Therefore, the austenitic stainless steel disclosed in Patent Literature 1 contains 0.03% or more of C. However, in the austenitic stainless steel, C sometimes causes the generation of SCC. For example, when the austenitic stainless steel is heated to around 600 to 700°C, Cr in the steel combines with C to form Cr carbide. As a result, a Cr-depleted layer is generated, which causes the generation of SCC in a corrosive environment in the presence of a chloride or the like. Thus, from the viewpoint of suppressing SCC, it is necessary to reduce the C content.

[0064] (b) In addition, in order to suppress SCC, not only reducing the C content, but also controlling the amount of C (hereinafter, referred to as "solid-solved C") solid-solved in the matrix phase within an appropriate range is also effective. Solid-solved C, when heated in a use environment or the like, is separated from the solid-solved state, combines with Cr, and forms Cr carbide. This Cr carbide reduces the SCC resistance. Thus, it is desirable to reduce the solid-solved C by causing C to combine with elements such as V, Ti, and Nb in advance to perform pre-precipitation in a stage before use.

[0065] (c) On the other hand, in the case where the C content is reduced, it is difficult to ensure the creep strength. Therefore, in order to improve the creep strength, it is effective to contain elements such as Ni, Cr, Mo, and Nb. By causing these elements to form precipitates in the use environment exposed to high temperatures, the creep strength is improved. However, in the case where the above elements form precipitates before use at high temperatures, the creep strength cannot be sufficiently improved. Thus, in order to obtain good creep strength, it is desirable to cause these elements to be as much as possible solid-solved in the parent phase at the stage before use, and to cause these elements to form precipitates in the use environment. Therefore, at the stage before use, it is necessary to previously adjust the solid-solution amount of the above elements to an appropriate range.

[0066] In view of the above, it is desirable to adjust the balance of solid-solution and precipitation of each element in the steel by appropriately controlling the chemical composition, the manufacturing conditions.

[0067] The present application is made on the basis of the above insight. Hereinafter, each element of the present embodiment is explained in detail.

[0068] 1. Chemical composition

[0069] The reasons for limiting each element are as follows. In addition, in the following explanation, "%" with respect to the content means "mass %".

[0070] C: 0.002 to 0.020 %

[0071] C is an element necessary to ensure high-temperature strength, particularly creep strength. Therefore, the C content is made to be 0.002% or more. The C content is preferably 0.003% or more, and more preferably 0.004% or more. However, when C is contained in excess, the SCC resistance is reduced. Therefore, the C content is made to be 0.020% or less. The C content is preferably 0.015% or less, and more preferably 0.010% or less.

[0072] Si: 0.10 to 0.60 %

[0073] Si is an element having a deoxidizing effect. Therefore, the Si content is made to be 0.10% or more. The Si content is preferably 0.12% or more, and the Si content is preferably 0.14% or more. However, when Si is contained in excess, the workability is reduced. Therefore, the Si content is made to be 0.60% or less. The Si content is preferably 0.50% or less, and more preferably 0.40% or less.

[0074] Mn: 0.2 to 2.0 %

[0075] Mn combines with S, which is an impurity contained in the steel, to form MnS, which has the effect of improving hot workability. Therefore, the content of Mn is made to be 0.2% or more. The content of Mn is preferably 0.4% or more, and more preferably 0.6% or more. However, when Mn is contained in excess, the steel becomes hard and brittle, and the workability and weldability are reduced. Therefore, the content of Mn is made to be 2.0% or less. The content of Mn is preferably 1.5% or less, and more preferably 1.3% or less.

[0076] P: 0.035% or less

[0077] P is an element contained in the steel as an impurity, which reduces the resistance to SCC. In addition, P reduces the hot workability and toughness of the steel. Therefore, the content of P is made to be 0.035% or less. The content of P is preferably 0.030% or less, and more preferably 0.025% or less. The content of P is preferably as low as possible, but when the content thereof is reduced in excess, the manufacturing cost increases. Therefore, the content of P is preferably 0.010% or more.

[0078] S: 0.010% or less

[0079] S is an element contained in the steel as an impurity, which reduces the resistance to SCC. In addition, S reduces the hot workability and creep ductility of the steel. Therefore, the content of S is made to be 0.010% or less. The content of S is preferably 0.009% or less, and more preferably 0.008% or less. The content of S is preferably as low as possible, but when the content thereof is reduced in excess, the manufacturing cost increases. Therefore, the content of S is preferably 0.0001% or more.

[0080] Cu: 2.50 to 4.50%

[0081] Cu is precipitated as a Cu phase within the grains, and improves the creep strength and creep ductility of the steel through precipitation strengthening. Therefore, the content of Cu is made to be 2.50% or more. The content of Cu is preferably 2.70% or more, and more preferably 2.90% or more. However, when Cu is contained in excess, the hot workability and weldability are reduced. Therefore, the content of Cu is made to be 4.50% or less. The content of Cu is preferably 4.00% or less, and more preferably 3.50% or less.

[0082] Ni: 9.00 to 16.00%

[0083] Ni is an element that stabilizes the austenitic structure and improves resistance to SCC and corrosion. Additionally, Ni also improves creep strength. Therefore, the Ni content is 9.00% or more. The Ni content is preferably 10.00% or more, more preferably 10.50% or more. However, when Ni is present in excess, manufacturing costs increase. Furthermore, it reduces creep strength. Therefore, the Ni content is 16.00% or less. The Ni content is preferably 15.00% or less, more preferably 14.00% or less.

[0084] Cr: 15.00~20.00%

[0085] Cr is an element required to ensure corrosion resistance. In addition, Cr has the effect of improving creep strength. Therefore, the Cr content is 15.00% or more. The Cr content is preferably 15.50% or more, more preferably 16.00% or more. However, when Cr is present in excess, the stability of the austenitic structure decreases, and weldability also decreases. Therefore, the Cr content is 20.00% or less. The Cr content is preferably 19.75% or less, more preferably 19.50% or less.

[0086] Mo: 0.20–1.50%

[0087] Mo has the effect of improving creep strength. Therefore, the Mo content is 0.20% or more. The Mo content is preferably 0.35% or more, more preferably 0.50% or more. However, when Mo is present in excess, the stability of the austenitic structure decreases. Therefore, the Mo content is 1.50% or less. The Mo content is preferably 1.25% or less, more preferably 1.00% or less.

[0088] Nb: 0.15–0.60%

[0089] Nb has the effect of improving creep strength and resistance to SCC. Therefore, the Nb content is 0.15% or more. The Nb content is preferably 0.20% or more, more preferably 0.25% or more. However, when Nb is present in excess, coarse precipitates are significantly generated, which reduces creep strength. Therefore, the Nb content is 0.60% or less. The Nb content is preferably 0.55% or less, more preferably 0.50% or less.

[0090] N: 0.05~0.15%

[0091] Nitrogen (N) has the effect of improving strength through solid solution strengthening and precipitation strengthening based on Nb carbonitrides. Therefore, the N content is 0.05% or more. The N content is preferably 0.06% or more, more preferably 0.07% or more. However, when N is present in excess, blocky nitrides are formed, which reduces the quality of the steel. As a result, the strength is reduced. Therefore, the N content is 0.15% or less. The N content is preferably 0.13% or less, more preferably 0.12% or less.

[0092] B: 0.0010~0.0060%

[0093] Boron (B) has the effect of improving creep ductility. Therefore, the B content is 0.0010% or more. The B content is preferably 0.0015% or more, more preferably 0.0020% or more. However, when B is present in excess, weldability and hot workability at high temperatures decrease. Therefore, the B content is 0.0060% or less. The B content is preferably 0.0050% or less, more preferably 0.0045% or less.

[0094] In the chemical composition of the austenitic stainless steel of this embodiment, in addition to the elements described above, one or more elements selected from V and Ti may be further contained within the range shown below. The reasons for limiting each element are explained.

[0095] V: 0~0.50%

[0096] V has the effect of reducing solid solution carbon, thereby improving resistance to solid corrosion cracking (SCC). In addition, V also improves creep strength. Therefore, V can be included as needed. However, when V is present in excess, δ-ferrite is formed, which reduces the creep strength, toughness, and weldability of the steel. Therefore, the V content is kept to be 0.50% or less. The V content is preferably 0.40% or less, more preferably 0.30% or less. On the other hand, to obtain the above-mentioned effects, the V content is preferably 0.01% or more, more preferably 0.02% or more.

[0097] Ti: 0~0.500%

[0098] Similar to V, Ti has the effect of reducing solid solution C and thus improving SCC resistance. In addition, Ti also has the effect of improving creep strength. Therefore, Ti can be included as needed. However, when Ti is present in excess, it actually reduces creep strength. Therefore, the Ti content is kept to be 0.500% or less. The Ti content is preferably 0.400% or less, more preferably 0.100% or less, and even more preferably 0.050% or less. On the other hand, to obtain the above-mentioned effects, the Ti content is preferably 0.001% or more, more preferably 0.002% or more.

[0099] In addition to the elements described above, the steel of this embodiment may further contain one or more elements selected from Co, W, Ta, Sn, Ca, Mg, and REM within the range shown below in its chemical composition. The reasons for these limitations on each element are explained.

[0100] Co: 0~1.00%

[0101] Co has the effect of stabilizing the austenitic structure and improving creep strength. Therefore, Co can be included as needed. However, when Co is included in excess, the manufacturing cost increases. Therefore, the Co content is kept to be 1.00% or less. The Co content is preferably 0.50% or less, more preferably 0.30% or less. On the other hand, in order to obtain the above-mentioned effect, the Co content is preferably 0.02% or more.

[0102] W: 0~1.00%

[0103] W has the effect of improving the creep strength of steel by dissolving in the parent phase. Therefore, W can be included as needed. However, when W is present in excess, the stability of the austenite phase decreases, which in turn reduces creep strength and toughness. Therefore, the W content is kept to be 1.00% or less. The W content is preferably 0.50% or less, more preferably 0.30% or less. On the other hand, in order to obtain the above-mentioned effect, the W content is preferably 0.01% or more.

[0104] Ta: 0~0.40%

[0105] Ta combines with C to form carbonitrides, thus reducing the solid solution C. As a result, Ta improves resistance to solid cracking (SCC). Additionally, Ta also improves creep strength. Therefore, Ta can be included as needed. However, when Ta is present in excess, δ-ferrite is formed, reducing the creep strength, toughness, and weldability of the steel. Therefore, the Ta content is kept to 0.40% or less. The Ta content is preferably 0.30% or less, more preferably 0.10% or less. On the other hand, to obtain the above-mentioned effects, the Ta content is preferably 0.01% or more.

[0106] Sn: 0~0.0300%

[0107] Sn has the effect of improving corrosion resistance and high-temperature properties. Therefore, Sn can be included as needed. However, when Sn is present in excess, weldability and manufacturability decrease. Therefore, the Sn content is kept to be 0.0300% or less. The Sn content is preferably 0.0200% or less, more preferably 0.0100% or less. On the other hand, in order to obtain the above-mentioned effects, the Sn content is preferably 0.0010% or more.

[0108] Ca: 0~0.0100%

[0109] Ca has the effect of fixing S and O as inclusions, thereby improving the hot workability and creep ductility of steel. Therefore, Ca can be included as needed. However, when Ca is present in excess, it reduces both hot workability and creep ductility. Therefore, the Ca content is kept to be 0.0100% or less. The Ca content is preferably 0.0050% or less, more preferably 0.0030% or less. On the other hand, in order to obtain the above-mentioned effect, the Ca content is preferably 0.0010% or more.

[0110] Mg: 0~0.0100%

[0111] Similar to Ca, Mg has the effect of fixing S and O as inclusions, thereby improving the hot workability and creep ductility of steel. Therefore, Mg can be included as needed. However, when Mg is present in excess, it reduces both hot workability and long-term creep ductility. Therefore, the Mg content is kept to be 0.0100% or less. The Mg content is preferably 0.0050% or less, more preferably 0.0030% or less. On the other hand, in order to obtain the above-mentioned effects, the Mg content is preferably 0.0002% or more.

[0112] REM: 0~0.0800%

[0113] Similar to Ca and Mg, REM has the effect of fixing S and O as inclusions, thereby improving the hot workability and creep ductility of steel. Therefore, REM can be included as needed. However, when REM is present in excess, it reduces hot workability and long-term creep ductility. Therefore, the REM content is kept to be 0.0800% or less. The REM content is preferably 0.0600% or less, more preferably 0.0400% or less. On the other hand, in order to obtain the above-mentioned effect, the REM content is preferably 0.0010% or more.

[0114] Furthermore, REM refers to Sc, Y, and the lanthanides, a total of 17 elements. The REM content mentioned above refers to the total content of these elements. Industrially, REM is mostly added in the form of mixed rare earth elements.

[0115] In the chemical composition of the austenitic stainless steel of this embodiment, the balance is Fe and impurities. Here, "impurities" refers to components that may be introduced into the steel during industrial manufacturing from raw materials such as ores and waste, or due to various factors in the manufacturing process, and refers to substances that are permissible within the range that do not adversely affect the properties of the austenitic stainless steel.

[0116] 2.(i)Formula

[0117] As described above, dissolved C combines with Cr in high-temperature operating environments to form Cr carbides. This results in reduced SCC resistance. Therefore, it is desirable to reduce the amount of dissolved C before using austenitic stainless steel as heat transfer tubes for boilers. Specifically, it is preferable to pre-fix C as a precipitate (compound) using V, Ti, and Nb. Thus, the amounts of V, Ti, and Nb present as precipitates combined with C, i.e., the values ​​on the right-hand side of equation (i), are controlled.

[0118] 0.010≤V ER +Ti ER +Nb ER ···(i)

[0119] The symbols in the above formula are defined as follows.

[0120] V ER V content (mass%) in the precipitate obtained by analyzing the extracted residue.

[0121] Ti ER Ti content (mass%) in the precipitate obtained by analyzing the extracted residue.

[0122] Nb ER The Nb content (mass%) in the precipitate obtained by analyzing the extracted residue.

[0123] When the value on the right side of equation (i) is less than 0.010, the amount of dissolved C increases, and in the operating environment, the dissolved C combines with Cr to form Cr carbides. As a result, a chromium-depleted (Cr) layer is formed, and the SCC resistance decreases. Therefore, the value on the right side of equation (i) is 0.010 or more. The value on the right side of equation (i) is preferably 0.012 or more, more preferably 0.015 or more, and even more preferably 0.020 or more.

[0124] Furthermore, from the viewpoint of weldability, the value on the right side of equation (i) is preferably 0.120 or less. Additionally, from the viewpoint that excessive fixation of N leads to a decrease in strength, the value on the right side of equation (i) is preferably 0.100 or less.

[0125] 3.(ii)

[0126] In the austenitic stainless steel of this embodiment, the solid solubility of Ni, Cr, Mo, and Nb in the parent phase is controlled. This is because, while reducing C, which is effective in creep strength, ensuring the solid solubility of these elements allows for an increase in creep strength. The solid solubility in the parent phase can be calculated as the difference between the content (mass%) of each element and the content (mass%) of each element in the precipitates obtained by analysis of the extraction residue. Furthermore, the austenitic stainless steel of this embodiment needs to satisfy equation (ii).

[0127] 27.0≤1.13(Ni-Ni ER )+(Cr-Cr ER )+1.85(Mo-Mo ER )+1.79(Nb-Nb ER <40.5···(ii)

[0128] In the above formula, each symbol is defined as follows: each element symbol in the above formula represents the content (mass%) of each element contained in the steel, and is set to zero if the element is not contained.

[0129] Nb ER The Nb content (mass%) in the precipitate obtained by analyzing the extracted residue.

[0130] Ni ER The Ni content (mass%) in the precipitate obtained by analyzing the extracted residue.

[0131] Cr ER Cr content (mass%) in the precipitate obtained by analyzing the extracted residue.

[0132] Mo ER Mo content (mass%) in the precipitate obtained by analyzing the extracted residue.

[0133] When the boundary value in equation (ii) is less than 27.0, Ni, Cr, Mo, and Nb are not sufficiently dissolved, and the creep strength cannot be improved. Therefore, the boundary value in equation (ii) is 27.0 or more. The boundary value in equation (ii) is preferably 29.0 or more, and more preferably 31.0 or more.

[0134] On the other hand, when the boundary value in equation (ii) is 40.5 or higher, the following situation arises: Nb is too soluble in the above elements, thus failing to sufficiently ensure the amount of Nb that combines with the soluble C to form precipitates, and equation (i) cannot be satisfied. Therefore, the boundary value in equation (ii) is made less than 40.5. The boundary value in equation (ii) is preferably 39.0 or less, more preferably 37.0 or less, and even more preferably 35.0 or less.

[0135] 4. (iii)

[0136] Nb ER <0.052···(iii)

[0137] The symbols in the above formula are defined as follows.

[0138] Nb ER The Nb content (mass%) in the precipitate obtained by analyzing the extracted residue.

[0139] For the austenitic stainless steel of this embodiment, as described above, in the stage before use, C is pre-precipitated as a precipitate by combining with elements such as V, Ti, and Nb. In particular, it is preferable to precipitate Nb as a Ti-containing Nb carbonitride. Figure 1 The microstructure photographs were obtained by collecting Nb compounds from the precipitates obtained through residue analysis of the austenitic stainless steel of this embodiment and observing them. Analysis of these microstructure photographs revealed that the Nb compounds were Nb carbonitrides containing Ti.

[0140] Here, Nb sometimes combines with N, Cr, etc. in steel to form fine NbCr nitrides. When these fine NbCr nitrides are formed, Nb... ER The value of increases, and the solid solution content of Cr decreases. As a result, it is difficult for austenitic stainless steels to satisfy equation (ii). Therefore, it is necessary to control Nb so that it does not become NbCr nitride, but rather the aforementioned Nb carbonitride.

[0141] In view of the above, the austenitic stainless steel of this embodiment satisfies equation (iii). The reason is that austenitic stainless steel does not satisfy equation (iii), that is, when Nb... ER When the concentration is above 0.052, NbCr nitrides will form, making it difficult to form the desired Nb carbonitrides. ER Preferably, it is 0.050 or less, more preferably 0.045 or less, even more preferably 0.040 or less, and most preferably 0.035 or less.

[0142] Furthermore, the content (mass%) of each element in the precipitates obtained by residue analysis as described in items 2-4 above can be measured using the following steps. Specifically, 10% acetylacetone-1% tetramethylammonium chloride / methanol is used at 20 mA / cm². 2 Approximately 0.4 g of sample was electrolyzed using a given current. The electrolyzed sample solution was then filtered through a 0.2 μm filter, and the residue was subjected to acid decomposition. The amount (mass%) of the elements analyzed as electrolytic residue was then calculated using an ICP emission spectrophotometer.

[0143] 5. Thickness

[0144] Regarding the thickness of the austenitic stainless steel in this embodiment, considering its application, it is preferably set to be in the range of 2 to 100 mm. Furthermore, when the austenitic stainless steel is in the shape of a tube, the wall thickness is preferably in the range of 2 to 95 mm. Additionally, when the austenitic stainless steel is in the shape of a steel plate, the plate thickness is preferably in the range of 2 to 35 mm.

[0145] 6. Manufacturing method

[0146] This invention describes a method for manufacturing the austenitic stainless steel of this embodiment. Based on previous research, the inventors have confirmed that the austenitic stainless steel of this embodiment can be manufactured using the following method.

[0147] 6-1. Smelting

[0148] Steel with the above chemical composition is smelted, and ingots are produced by continuous casting or similar methods. The ingots are then pre-rolled to produce steel billets. The conditions for manufacturing the steel billets only need to meet commonly used methods.

[0149] 6-2. Hot working

[0150] Next, the obtained steel billet is hot-worked. There are no particular limitations on the hot-working conditions, but to avoid harmful defects during pipe manufacturing, it is recommended to heat the billet to a temperature range of 900–1300°C before hot-working. Furthermore, there are no particular restrictions on the type of hot work performed. Hot rolling can be used when manufacturing steel plates. Additionally, hot extrusion can be used to form the pipe shape when manufacturing steel pipes.

[0151] Here, after hot working, the steel is rapidly cooled under the following conditions.

[0152] Time from the end of hot working to the start of quenching: less than 5.0 minutes

[0153] The initial temperature of the steel at the start of rapid cooling: above 700℃

[0154] Cooling rate from the end of hot working to the start of quenching: 15°C or more per minute

[0155] The time (in minutes) from the end of hot working to the start of quenching is called the "setting time." After hot working, when steel is quenched, it is typically done using a water-cooling system. In other words, the "setting time" refers to the time from the completion of hot working until it is transported to the water-cooling system and water cooling begins. Setting time exceeding 5 minutes will result in the formation of coarse precipitates. Furthermore, NbCr nitrides are more likely to form than the desired Nb carbonitrides.

[0156] Furthermore, the precipitates cannot be dissolved during the softening treatment described later. As a result, coarse precipitates such as nitrides, Cr carbides, and NbCr nitrides are formed, where the boundary value in equation (ii) is smaller than the value on the left-hand side. Consequently, the creep strength decreases. Therefore, the standing time is kept to be 5.0 minutes or less. The standing time is preferably 4.5 minutes or less, more preferably 4.0 minutes or less, and even more preferably 3.5 minutes or less.

[0157] The temperature (°C) at which the steel begins quenching is referred to as the "quenching start temperature". When the quenching start temperature is less than 700°C, precipitates such as Cr carbides are formed coarsely. In addition, NbCr nitrides are easily formed instead of the desired Nb carbonitrides. Furthermore, even with softening treatment, the precipitates cannot be dissolved in the parent phase, and the boundary value in equation (ii) is less than the value on the left. As a result, the creep strength decreases. Therefore, the quenching start temperature is 700°C or higher. The quenching start temperature is preferably 750°C or higher, more preferably 780°C or higher, even more preferably over 790°C, and particularly preferably 800°C or higher.

[0158] When the cooling rate (°C / min) from the end of hot working to the start of quenching is less than 15°C / min, precipitates such as nitrides and Cr carbides are coarsely formed. Furthermore, the boundary value in equation (ii) is smaller than the value on the left-hand side. As a result, creep strength (or SCC resistance) decreases. Therefore, the cooling rate from the end of hot working to the start of quenching should be 15°C / min or more, preferably 18°C / min or more, and more preferably 20°C / min or more. Moreover, the above cooling rate is obtained by dividing the difference between the surface temperature of the steel immediately after hot working and the surface temperature of the steel just before quenching begins by the dwell time.

[0159] 6-3. Softening treatment

[0160] Softening treatment reduces deviations in the steel quality along the length and wall thickness directions that occur during hot working. Without softening treatment, coarse precipitates formed during hot working do not fully dissolve in the parent phase. Furthermore, during the solution heat treatment described later, portions of Ti, V, and Nb are not fully precipitated. As a result, the value on the right side of equation (i) is smaller than the value on the left side, leading to reduced SCC resistance. Therefore, softening treatment is performed as a post-hot working step to homogenize the steel. The holding temperature during softening treatment (hereinafter referred to as "softening treatment temperature T1") should be above the recrystallization temperature and below the grain boundary melting temperature. During softening treatment, a homogenization holding temperature of 1040–1300°C is preferable.

[0161] When the softening treatment temperature T1 is less than 1040°C, equation (i) is no longer satisfied, and the SCC resistance decreases. Therefore, the softening treatment temperature T1 is set to 1040°C or higher. The softening treatment temperature T1 is preferably 1100°C or higher, more preferably 1150°C or higher. On the other hand, when the softening treatment temperature T1 exceeds 1300°C, the grains tend to become coarse. Therefore, the softening treatment temperature T1 is set to 1300°C or lower. The softening treatment temperature T1 is preferably 1290°C or lower, more preferably 1280°C or lower.

[0162] Furthermore, there is no particular time limit for the softening treatment, but from the perspective of recrystallization in the wall thickness direction and manufacturing cost, it is expected to be 1 to 10 minutes.

[0163] 6-4. First Cooling

[0164] After softening treatment, the steel is cooled. This cooling is called the first cooling. In the first cooling, the steel is cooled at a rate equal to or greater than water cooling. If cooling is not performed at a rate equal to or greater than water cooling, inhomogeneous structures will form due to the formation and growth of precipitates. As a result, inhomogeneity will persist and product quality will be inconsistent during subsequent cold working and solution heat treatment. Therefore, cooling is performed at a rate equal to or greater than water cooling in the first cooling. This cooling is usually carried out up to 700°C or below. Furthermore, the cooling rate in water cooling is typically 2–8°C / s.

[0165] 6-5. Cold working

[0166] After the initial cooling, the steel undergoes cold working. Cold working is a process required to achieve the dimensional accuracy specified in the specifications. Similar to hot working, no particular restrictions are set on the type of processing. The reduction rate of the steel's cross-section accompanying the processing is not specifically defined, but in typical processing, the reduction rate is approximately 90% or less.

[0167] Furthermore, in the case of manufacturing steel pipes, for example, cold working can be set as drawing and formed into a predetermined steel pipe shape.

[0168] 6-6. Solution heat treatment

[0169] After the aforementioned cold working, the steel is subjected to solution heat treatment at 1100–1200°C. In the following description, the homogenization holding temperature during solution heat treatment is referred to as the solution temperature T2. This is because when the solution temperature T2 is less than 1100°C, the solution of elements effective in improving creep strength becomes insufficient, failing to satisfy equation (ii), and creep strength decreases. Furthermore, when the solution temperature T2 exceeds 1200°C, the elements to be precipitated will dissolve excessively, thus failing to satisfy equations (i) and / or (ii).

[0170] Adjustments are made so that when comparing the softening treatment temperature T1 and the solution treatment temperature T2, the softening treatment temperature T1 is higher than the solution treatment temperature T2. That is, the control is made to satisfy the following equation (a). The reason is that when the solution treatment temperature T2 is higher than the softening treatment temperature T1, it is difficult to sufficiently improve the SCC resistance.

[0171] T1>T2···(a)

[0172] The symbols in equation (a) above are defined as follows.

[0173] T1: Softening treatment temperature

[0174] T2: Solution temperature

[0175] 6-7. Second Cooling

[0176] After solution heat treatment, the steel is cooled. This cooling is called the second cooling. In the second cooling, the steel is cooled at a rate equal to or greater than that of water cooling, similar to the first cooling. This is because if cooling is not performed at a rate equal to or greater than that of water cooling, in addition to a decrease in creep strength due to the formation and growth of precipitates, inconsistent product quality may also occur. Furthermore, this cooling is generally preferably performed below 600°C.

[0177] The following embodiments will be used to illustrate this implementation in more detail, but this implementation is not limited to these embodiments.

[0178] Example

[0179] Steel with the chemical composition shown in Table 1 was melted and first rolled to produce hollow billets. These billets were hot-worked to form tubes of dimensions (outer diameter 56 mm × wall thickness 10.5 mm × length 6684 mm) and quenched under the conditions shown in Table 2. Subsequently, a softening treatment with a 3-minute homogenization holding period was performed under the conditions shown in Table 2, followed by water cooling and cold working with a 35.7% reduction in cross-section. That is, cold working was performed to achieve a wall thickness of 8.35 mm. The softening treatment temperatures T1 were all above the recrystallization temperature and below the grain boundary melting temperature. Next, a solution heat treatment with a 2-minute holding period was performed under the conditions shown in Table 2, followed by water cooling to obtain the steel tube.

[0180] [Table 1]

[0181]

[0182] [Table 2]

[0183] Table 2

[0184]

[0185] ＊ This refers to anything outside the scope defined in this invention.

[0186] Underlined: refers to conditions other than the preferred manufacturing conditions in this embodiment (except for the examples of double underlines below).

[0187] Double underline: This means that although the preferred temperature range for heat treatment is met, the condition T1>T2 is not met, which is outside the preferred manufacturing conditions of this embodiment.

[0188] The obtained steel pipes were subjected to electrolytic extraction residue measurement, SCC test and creep test according to the steps described below.

[0189] (Electrolytic extraction residue)

[0190] For V, Ti, Nb, Ni, Cr and Mo, calculate the content (mass%) of each element in the precipitate obtained by analysis of the extraction residue by following the steps below, and calculate the right-hand side of equation (i), the boundary value in equation (ii) and the left-hand side of equation (iii).

[0191] Specifically, 10% acetylacetone-1% tetramethylammonium chloride / methanol is used at 20 mA / cm. 2 Approximately 0.4 g of sample was electrolyzed using a given current. The electrolyzed sample solution was then filtered through a 0.2 μm filter, and the residue was acid-decomposed using a mixed acid solution of sulfuric acid, phosphoric acid, nitric acid, and perchloric acid. The amount (mass%) of the elements analyzed as electrolytic residue was then calculated using an ICP emission spectrophotometer.

[0192] (SCC test)

[0193] SCC resistance was evaluated based on ASTM A262 Method E. Specifically, a 15mm × 2mm × 70mm test piece was taken from one end of the obtained austenitic stainless steel. The test piece was subjected to a sensitization heat treatment at 700°C for 30 minutes, followed by immersion in a boiling solution of approximately 70g of copper shavings in the sulfuric acid / copper sulfate aqueous solution specified in the above specifications for 24 hours, and then a bending test was performed.

[0194] (Creep Test)

[0195] Creep tests were conducted to evaluate creep strength. Specifically, round rod-shaped creep test pieces were collected and creep fracture tests were performed. In the evaluation, test pieces with a fracture strength of ≥210 MPa after a creep test at 600℃ for 10000 h, and test pieces with a fracture strength of ≥150 MPa after a creep test at 650℃ for 10000 h, were rated as "excellent".

[0196] In addition to the test pieces rated "Excellent" as described above, those that only meet either the following criteria—a fracture strength of 210 MPa or higher after a creep test at 600°C for 10,000 hours, or a fracture strength of 150 MPa or higher after a creep test at 650°C for 10,000 hours—are rated "Good". Furthermore, in cases other than "Excellent" and "Good", those with poor creep characteristics are recorded as "Inferior". The results are summarized in Table 3 below.

[0197] [Table 3]

[0198] Table 3

[0199]

[0200] ＊ This refers to anything outside the scope defined in this implementation method.

[0201] 0.010≤V ER +Ti ER +Nb ER …(i)

[0202] 27.0≤1.13(Ni-Ni ER )+(Cr-Cr ER )+1.85(Mo-Mo ER )+1.79(Nb-Nb ER )<40.5…(ii)

[0203] Nb ER <0.052…(iii)

[0204] Tests No. 1 to 17, which meet the requirements of this invention, exhibit good SCC resistance and creep strength. On the other hand, Tests No. 18 to 29, which do not meet the requirements of this invention, show poor creep strength and SCC resistance, or one of them is poor.

[0205] Tests No. 1–18, 23, and 25–27 did not exhibit cracking in the SCC resistance test. Since these examples satisfy equation (i), it is believed that after sensitization heat treatment, C is also fixed due to the precipitation of V, Ti, and Nb, which inhibits the formation of grain boundary Cr carbides and can prevent cracking.

[0206] On the other hand, tests Nos. 19–22, 24, 28, and 29 exhibited cracking during the SCC resistance test. Since these examples did not satisfy equation (i), it is believed that V, Ti, and Nb were not sufficiently precipitated, leading to the formation of Cr carbides at the grain boundaries after the sensitization heat treatment, resulting in a chromium-depleted (Cr) layer around them, thus causing cracking during the SCC resistance test. The reason for this is believed to be that the chemical composition, softening treatment temperature, or solution temperature were outside the preferred range.

[0207] Tests No. 1, 3, 8, 10, 12-17, and 19-21 were rated "Excellent" in the creep test. Since the boundary values ​​in equation (ii) of these examples are above the more preferred lower limit, their creep strength is considered excellent. Furthermore, Tests No. 2, 4-7, 9, 11, 22, 24, 28, and 29 were rated "Good" in the creep test. Although the boundary values ​​in equation (ii) of these examples are not above the more preferred lower limit, they satisfy equation (ii), therefore it is considered that the elements contributing to creep strength are sufficiently dissolved, resulting in good creep strength suitable for the present invention.

[0208] On the other hand, tests No. 18, 23, and 25–27 were rated as “poor” in the creep test. In these examples, the boundary value in equation (ii) was less than 27.0, indicating that Ni, Cr, Mo, and Nb, which improve creep strength, were not sufficiently dissolved, and therefore, sufficient creep strength could not be obtained. Furthermore, since No. 23, 25, and 26 did not satisfy equation (iii), it was believed that NbCr nitrides were formed, resulting in a decrease in creep strength.

Claims

1. An austenitic stainless steel, wherein, Its chemical composition, by mass%, is C: 0.002–0.020%. Si: 0.10–0.60% Mn: 0.2-2.0% P: below 0.035% S: less than 0.010% Cu: 2.50–4.50% Ni: 9.00~16.00% Cr：15.00～20.00％、 Mo: 0.20–1.50% Nb: 0.15–0.60% N：0.05～0.15％、 B：0.0010～0.0060％、 V：0～0.50％、 Ti: 0~0.500% Co: 0-1.00% W：0～1.00％、 Ta: 0-0.40% Sn: 0~0.0300% Ca: 0~0.0100% Mg: 0~0.0100% REM: 0~0.0800% Balance: Fe and impurities, And satisfy the following equations (i) to (iii), 0.010≤V ER +If ER +Nb ER ···(i) 27.0≤1.13(Ni-Ni ER )+(Cr-Cr ER )+1.85(I-I ER )+1.79(Nb-Nb ER )<40.5···(ii) Nb ER <0.052···(iii) In the above formula, each symbol is defined as follows: each element symbol represents the content of each element contained in the steel, and is set to zero if the element is not present. The unit of content of each element is mass%. V ER V content in the precipitate obtained by analyzing the extracted residue Ti ER Ti content in the precipitate obtained by analyzing the extracted residue Nb ER The Nb content in the precipitate obtained by analyzing the extracted residue. Ni ER The Ni content in the precipitate obtained by analyzing the extracted residue. Cr ER Cr content in the precipitate obtained by analyzing the extracted residue Mo ER Mo content in the precipitate obtained by analyzing the extracted residue The units for the V content, Ti content, Nb content, Ni content, Cr content, and Mo content are by mass.

2. The austenitic stainless steel according to claim 1, wherein, The chemical composition, expressed as a percentage by mass, contains one or more elements selected from the following elements. V：0.01～0.50％、 Ti: 0.001~0.500%.

3. The austenitic stainless steel according to claim 1, wherein, The chemical composition, expressed as a percentage by mass, contains one or more elements selected from the following elements. Co: 0.02~1.00% W：0.01～1.00％、 Ta: 0.01~0.40% Sn: 0.0010~0.0300% Ca: 0.0010~0.0100% Mg: 0.0002~0.0100% REM: 0.0010~0.0800%.

4. The austenitic stainless steel according to claim 2, wherein, The chemical composition, by mass percent, contains one or more elements selected from the following: Co: 0.02–1.00%. W：0.01～1.00％、 Ta: 0.01~0.40% Sn: 0.0010~0.0300% Ca: 0.0010~0.0100% Mg: 0.0002~0.0100% REM: 0.0010~0.0800%.

5. A steel pipe, wherein, The steel pipe is made of austenitic stainless steel as described in any one of claims 1 to 4.

Images