Austenitic stainless steel material and method for producing same

Abstract

The present invention provides an austenitic stainless steel material which achieves both excellent creep strength and fatty acid corrosion resistance. This austenitic stainless steel material has a chemical composition which contains, in mass%, 0.100% or less of C, 0.10-1.00% of Si, 0.20-2.00% of Mn, 0.045% or less of P, 0.030% or less of S, 15.0-25.0% of Cr, 9.0-18.0% of Ni, 1.0-5.0% of Mo, 0.20-2.00% of Nb, 0.05-0.18% of N, 0.001-0.080% of sol. Al and the like, wherein F1 represented by formula (1) is 18.5 or less, and F2 represented by formula (2) is less than 200.　(1): F1 = 27.3Si - 0.9Ni + 0.8Cr + Mo + 13.5N + 6.8Cu (2): F2 = [R] + 0.9 × [H] [R]: Crystal grain size (µm), [H]: Vickers hardness

Description

Austenitic stainless steel material and method for manufacturing the same

[0001] The present invention relates to an austenitic stainless steel material and a method for producing the same.

[0002] Steel materials used in chemical plant facilities such as oil refineries and petrochemical plants are required to have excellent creep strength. Austenitic stainless steel materials are used as the steel materials for these chemical plant facilities.

[0003] Japanese Patent No. 7339526 discloses an austenitic stainless steel material that exhibits both excellent creep strength and excellent creep ductility even when used for long periods in high-temperature environments. This austenitic stainless steel material has a specified chemical composition, a grain size number of 4.0 to 9.0, and a specified relationship between the B concentration at the austenite grain boundaries and the B concentration within the austenite grains.

[0004] WO 2012 / 133506 discloses a ferritic stainless steel for use in biofuel supply system components, characterized in that the ferritic stainless steel has a predetermined chemical composition and is coated on its surface with an oxide film containing Cr, Si, Nb, Ti, and Al in a total cation fraction of 30% or more.

[0005] Patent No. 7339526 International Publication No. 2012 / 133506

[0006] In high-temperature areas such as in renewable diesel, biofuel, and SAF (sustainable aviation fuel) manufacturing equipment, fatty acid corrosion resistance is required in addition to creep strength.

[0007] An object of the present invention is to provide an austenitic stainless steel material that combines excellent creep strength and fatty acid corrosion resistance.

[0008] An austenitic stainless steel material according to one embodiment of the present invention is an austenitic stainless steel material having a chemical composition, in mass%, of C: 0.100% or less, Si: 0.10 to 1.00%, Mn: 0.20 to 2.00%, P: 0.045% or less, S: 0.030% or less, Cr: 15.0 to 25.0%, Ni: 9.0 to 18.0%, Mo: 1.0 to 5.0%, Nb: 0.20 to 2.00%, N: 0.05 to 0.18%, sol. The alloy has an F1 value of 18.5 or less and an F2 value of less than 200. ... In formula (2), [R] is substituted with the grain size of the austenitic stainless steel material in μm, and [H] is substituted with the Vickers hardness of the austenitic stainless steel material.

[0009] An austenitic stainless steel material according to one embodiment of the present invention is an austenitic stainless steel material having a chemical composition, in mass%, of C: 0.100% or less, Si: 0.10 to 1.00%, Mn: 0.20 to 2.00%, P: 0.045% or less, S: 0.030% or less, Cr: 15.0 to 25.0%, Ni: 9.0 to 18.0%, Mo: 1.0 to 5.0%, Nb: 0.20 to 2.00%, N: 0.05 to 0.18%, sol. Al: 0.001 to 0.080%, the balance: Fe and impurities, and F1, represented by the following formula (1), is 18.5 or less, and F2, represented by the following formula (2), is less than 200. F1 = 27.3Si - 0.9Ni + 0.8Cr + Mo + 13.5N (1) F2 = [R] + 0.9 × [H] (2) In formula (1), the content of the corresponding element in mass% is substituted for each element symbol. In formula (2), the grain size of the austenitic stainless steel material in μm is substituted for [R], and the Vickers hardness of the austenitic stainless steel material is substituted for [H].

[0010] An austenitic stainless steel material according to one embodiment of the present invention is an austenitic stainless steel material having a chemical composition, in mass%, of C: 0.100% or less, Si: 0.10 to 1.00%, Mn: 0.20 to 2.00%, P: 0.045% or less, S: 0.030% or less, Cr: 15.0 to 25.0%, Ni: 9.0 to 18.0%, Mo: 1.0 to 5.0%, Nb: 0.20 to 2.00%, N: 0.05 to 0.18%, sol.Al: 0.001 to 0.080%, and further containing one or more elements selected from the group consisting of the following A group, B group, and C group, the balance being Fe and impurities, and F1 represented by the following formula (1) is 18.5 or less, and F2 represented by the following formula (2) is less than 200. [Group A] B: 0.0080% or less [Group B] One or more selected from the group consisting of Cu: 2.00% or less, V: 1.00% or less, Co: 1.00% or less, Ta: 0.20% or less, and W: 1.00% or less [Group C] One or more selected from the group consisting of Ca: 0.0100% or less, Mg: 0.0100% or less, and REM: 0.100% or less F1 = 27.3Si - 0.9Ni + 0.8Cr + Mo + 13.5N + 6.8Cu (1) F2 = [R] + 0.9 × [H] (2) In formula (1), the content of the corresponding element in mass% is substituted for each element symbol. In formula (2), [R] is substituted with the grain size of the austenitic stainless steel material in μm, and [H] is substituted with the Vickers hardness of the austenitic stainless steel material.

[0011] A method for producing an austenitic stainless steel material according to one embodiment of the present invention is a method for producing the above-mentioned austenitic stainless steel material, comprising the steps of hot working a material and solution heat treating the material, wherein, when the temperature (furnace temperature) of the heating furnace in the heating before the final hot working is T1 (°C), the residence time in the furnace is t1 (minutes), the temperature (furnace temperature) of the solution heat treatment is T2 (°C), and the soaking time is t2 (minutes), f1 expressed by the following formula (3) is 750 or more, and f2 expressed by the following formula (4) is 600 or more: f1 = 0.5 × T1 × log (t1) (3) f2 = 0.5 × T2 × log {(t2) + 13} (4)

[0012] According to the present invention, an austenitic stainless steel material having both excellent creep strength and fatty acid corrosion resistance can be obtained.

[0013] In order to give steel excellent resistance to fatty acid corrosion, it is necessary to increase the Cr content. When the Cr content is increased, if the C content is high, M will be present at the grain boundary. ２３ C ６ Therefore, by increasing the Cr content and decreasing the C content, M ２３ C ６ However, simply lowering the C content reduces creep strength.

[0014] After extensive investigation, it was found that excellent creep strength and fatty acid corrosion resistance can be achieved by setting F1, expressed in the following formula (1), to 18.5 or less, and F2, expressed in the following formula (2), to less than 200. F1 = 27.3Si - 0.9Ni + 0.8Cr + Mo + 13.5N + 6.8Cu (1) F2 = [R] + 0.9 × [H] (2) In formula (1), the content of the corresponding element in mass% is substituted for each element symbol. In formula (2), the grain size of the austenitic stainless steel material in μm is substituted for [R], and the Vickers hardness of the austenitic stainless steel material is substituted for [H].

[0015] Furthermore, in order to make F2 less than 200, it is necessary to appropriately control the heating conditions before hot working and the conditions for solution heat treatment. Specifically, when the temperature (furnace temperature) of the heating furnace in the heating before the final hot working is T1 (°C), the residence time is t1 (minutes), the temperature (furnace temperature) of the solution heat treatment is T2 (°C), and the soaking time is t2 (minutes), it is necessary to make f1 expressed by the following formula (3) 750 or more and f2 expressed by the following formula (4) 600 or more. f1 = 0.5 × T1 × log (t1) (3) f2 = 0.5 × T2 × log {(t2) + 13} (4) where log (x) is the common logarithm of x (logarithm with base 10).

[0016] The present invention has been completed based on the above findings. An austenitic stainless steel material according to one embodiment of the present invention will be described in detail below.

[0017] [Austenitic Stainless Steel Material] [Chemical Composition] The austenitic stainless steel material according to this embodiment has the chemical composition described below. In the following description, "%" for the content of an element means mass %.

[0018] C: 0.100% or less Carbon (C) is inevitably contained. C is present at grain boundaries. ２３ C ６ This generates Cr carbides of this type, creating a Cr-depleted layer near the grain boundaries and reducing the fatty acid corrosion resistance of the steel. Therefore, the C content is 0.100% or less. The upper limit of the C content is preferably 0.050%, more preferably 0.030%, and even more preferably 0.020%. From the viewpoint of fatty acid corrosion resistance, it is preferable that the C content is as low as possible, but excessive reduction increases the manufacturing cost. The lower limit of the C content is preferably 0.001%, and even more preferably 0.003%.

[0019] Si: 0.10 to 1.00% Silicon (Si) deoxidizes steel. Si also improves the oxidation resistance and steam oxidation resistance of steel in high-temperature environments. On the other hand, if the Si content is too high, a σ phase is formed in the steel, reducing the creep strength of the steel. Therefore, the Si content is 0.10 to 1.00%. The lower limit of the Si content is preferably 0.15%, more preferably 0.20%. The upper limit of the Si content is preferably 0.80%, more preferably 0.60%.

[0020] Mn: 0.20 to 2.00% Manganese (Mn) stabilizes austenite and increases the creep strength of steel. Mn also deoxidizes steel. On the other hand, if the Mn content is too high, a σ phase is formed in the steel, reducing the creep strength of the steel. Therefore, the Mn content is 0.20 to 2.00%. The lower limit of the Mn content is preferably 0.30%, more preferably 0.40%. The upper limit of the Mn content is preferably 1.80%, more preferably 1.60%.

[0021] P: 0.045% or less Phosphorus (P) reduces the hot workability and toughness of steel. Therefore, the P content is 0.045% or less. The upper limit of the P content is preferably 0.040%, more preferably 0.030%. The lower the P content, the better. However, excessive reduction of the P content increases the cost of the steel. Therefore, taking into account normal industrial production, the lower limit of the P content is preferably 0.002%, more preferably 0.005%.

[0022] S: 0.030% or less Sulfur (S) is an impurity. S reduces the hot workability of steel. Therefore, the S content is 0.030% or less. The upper limit of the S content is preferably 0.020%, more preferably 0.010%, even more preferably 0.008%, and even more preferably 0.006%. From the viewpoint of hot workability, it is preferable that the S content is as low as possible, but excessive reduction increases the manufacturing cost. The lower limit of the S content is preferably 0.0001%, and even more preferably 0.0005%.

[0023] Cr: 15.0 to 25.0% Chromium (Cr) increases the fatty acid corrosion resistance of steel. Cr also increases the oxidation resistance, steam oxidation resistance, and high-temperature corrosion resistance of steel. On the other hand, if the Cr content is too high, the stability of austenite decreases, and the creep strength of the steel decreases. Therefore, the Cr content is 15.0 to 25.0%. The lower limit of the Cr content is preferably 16.0%, more preferably 16.5%. The upper limit of the Cr content is preferably 22.0%, more preferably 20.0%.

[0024] Ni: 9.0 to 18.0% Nickel (Ni) stabilizes austenite and increases the creep strength of steel. However, even if the Ni content is excessively high, the manufacturing cost increases and the above effect saturates. Therefore, the Ni content is 9.0 to 18.0%. The lower limit of the Ni content is preferably 10.0%, more preferably 11.0%, and even more preferably 12.0%. The upper limit of the Ni content is preferably 17.0%, more preferably 16.0%, and even more preferably 15.0%.

[0025] Mo: 1.0 to 5.0% Molybdenum (Mo) acts as a metal at grain boundaries. ２３ C ６ Mo suppresses the formation and growth of Cr carbides. As a result, Mo enhances the fatty acid corrosion resistance of steel. Mo also forms a chi-phase during use of steel in high-temperature environments, enhancing the creep strength of steel through precipitation strengthening. On the other hand, if the Mo content is too high, the stability of austenite decreases. Therefore, the Mo content is 1.0 to 5.0%. The lower limit of the Mo content is preferably 1.5%, more preferably 2.0%, and even more preferably 2.5%. The upper limit of the Mo content is preferably 4.5%, more preferably 4.2%, even more preferably 4.0%, and even more preferably 3.8%.

[0026] Nb: 0.20 to 2.00% Niobium (Nb) forms carbonitrides to fix C, thereby reducing the amount of solute C and improving the fatty acid corrosion resistance of steel. Nb also increases the creep strength of steel by precipitating as carbonitrides. On the other hand, if the Nb content is too high, δ ferrite is formed, reducing creep strength, toughness, and weldability. Therefore, the Nb content is 0.20 to 2.00%. The lower limit of the Nb content is preferably 0.25%, more preferably 0.30%. The upper limit of the Nb content is preferably 1.50%, more preferably 1.00%, even more preferably 0.80%, and even more preferably 0.60%.

[0027] N: 0.05 to 0.18% Nitrogen (N) dissolves in the matrix, stabilizing austenite and improving the creep strength of steel. N also forms fine carbonitrides within grains, increasing the creep strength of steel. On the other hand, if the N content is too high, Cr nitrides are formed at grain boundaries, which reduces the fatty acid corrosion resistance of the weld heat-affected zone when the steel is welded. In addition, the hot workability of the steel is reduced. Therefore, the N content is 0.05 to 0.18%. The lower limit of the N content is preferably 0.06%, more preferably 0.08%. The upper limit of the N content is preferably 0.15%, more preferably 0.12%.

[0028] Sol. Al: 0.001 to 0.080% Aluminum (Al) deoxidizes steel. On the other hand, if the Al content is too high, the hot workability and toughness of the steel will decrease. Therefore, the Al content is 0.001 to 0.080%. The lower limit of the Al content is preferably 0.002%, more preferably 0.005%. The upper limit of the Al content is preferably 0.070%, more preferably 0.050%, and even more preferably 0.040%. In this embodiment, the Al content refers to the content of acid-soluble Al (sol. Al).

[0029] The balance of the chemical composition of the austenitic stainless steel material according to this embodiment is Fe and impurities, which refer to elements that are mixed in from ores or scrap used as raw materials for steel, or from the environment during the manufacturing process.

[0030] The chemical composition of the austenitic stainless steel material 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, and C. The elements belonging to groups A, B, and C (B, Cu, V, Co, Ta, W, Ca, Mg, and REM) are all optional elements, and the chemical composition of the austenitic stainless steel material according to this embodiment may not contain some or all of these elements.

[0031] [Group A] B: 0.0080% or less Elements belonging to Group A have the effect of further increasing the creep strength and creep ductility of steel, and therefore may be contained as required.

[0032] B: 0.0080% or less Boron (B) segregates at grain boundaries, increasing grain boundary strength and improving the creep strength and creep ductility of steel. This effect can be achieved even if even a small amount of B is contained. On the other hand, if the B content is too high, the weldability and hot workability of the steel will decrease. Therefore, the B content is 0.0080% or less. The lower limit of the B content is preferably 0.0001%, more preferably 0.0003%, and even more preferably 0.0005%. The upper limit of the B content is preferably 0.0060%, and even more preferably 0.0040%.

[0033] [Group B] One or more elements selected from the group consisting of Cu: 2.00% or less, V: 1.00% or less, Co: 1.00% or less, Ta: 0.20% or less, and W: 1.00% 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] Cu: 2.00% or less Copper (Cu) precipitates as a fine Cu phase within grains, increasing the creep strength of steel. This effect can be achieved even if even a small amount of Cu is contained. On the other hand, if the Cu content is too high, the creep ductility of steel decreases. Therefore, the Cu content is 2.00% or less. The lower limit of the Cu content is preferably 0.01%, more preferably 0.03%, even more preferably 0.05%, and even more preferably 0.20%. The upper limit of the Cu content is preferably 1.80%, more preferably 1.50%, and even more preferably 1.00%.

[0035] V: 1.00% or less Vanadium (V) forms carbonitrides to increase the creep strength of steel. This effect can be achieved even if even a small amount of V is contained. On the other hand, if the V content is too high, δ ferrite is formed, reducing the creep strength, toughness, and weldability of the steel. Therefore, the V content is 1.00% or less. The lower limit of the V content is preferably 0.01%, more preferably 0.05%, even more preferably 0.10%, and even more preferably 0.20%. The upper limit of the V content is preferably 0.80%, even more preferably 0.60%, and even more preferably 0.40%.

[0036] Co: 1.00% or less Cobalt (Co) stabilizes austenite and increases the creep strength of steel. This effect can be achieved even if even a small amount of Co is contained. On the other hand, excessively high Co content increases the manufacturing cost and the above effect saturates. Therefore, the Co content is 1.00% or less. The lower limit of the Co content is preferably 0.01%, more preferably 0.05%, even more preferably 0.10%, and even more preferably 0.20%. The upper limit of the Co content is preferably 0.80%, more preferably 0.60%, and even more preferably 0.40%.

[0037] Ta: 0.20% or less Tantalum (Ta) forms carbonitrides to increase the creep strength of steel. This effect can be achieved even if even a small amount of Ta is contained. On the other hand, if the Ta content is too high, δ ferrite is formed, reducing the creep strength, toughness, and weldability of the steel. Therefore, the Ta content is 0.20% or less. The lower limit of the Ta content is preferably 0.01%, more preferably 0.03%, even more preferably 0.05%, and even more preferably 0.10%. The upper limit of the Ta content is preferably 0.18%, even more preferably 0.16%.

[0038] W: 1.00% or less Tungsten (W) dissolves in steel to increase the creep strength of the steel. This effect can be achieved even if even a small amount of W is contained. On the other hand, excessively high W content increases the manufacturing cost and the above effect saturates. Therefore, the W content is 1.00% or less. The lower limit of the W content is preferably 0.01%, more preferably 0.05%, even more preferably 0.10%, and even more preferably 0.15%. The upper limit of the W content is preferably 0.80%, more preferably 0.60%, and even more preferably 0.40%.

[0039] [Group C] One or more elements selected from the group consisting of Ca: 0.0100% or less, Mg: 0.0100% or less, and REM: 0.100% or less. The elements belonging to Group C have the effect of improving the hot workability and creep ductility of steel, and therefore may be added as needed.

[0040] Ca: 0.0100% or less Calcium (Ca) improves the hot workability and creep ductility of steel. This effect can be achieved even if even a small amount of Ca is contained. On the other hand, if the Ca content is too high, the hot workability and creep ductility of the steel will decrease. Therefore, the Ca content is 0.0100% or less. The lower limit of the Ca content is preferably 0.0005%, more preferably 0.0010%. The upper limit of the Ca content is preferably 0.0080%, more preferably 0.0060%.

[0041] Mg: 0.0100% or less Magnesium (Mg) improves the hot workability and creep ductility of steel. This effect can be achieved even if even a small amount of Mg is contained. On the other hand, if the Mg content is too high, the hot workability and creep ductility of the steel will decrease. Therefore, the Mg content is 0.0100% or less. The lower limit of the Mg content is preferably 0.0005%, more preferably 0.0010%. The upper limit of the Mg content is preferably 0.0080%, more preferably 0.0060%.

[0042] REM: 0.100% or less Rare earth elements (REM) improve the hot workability and creep ductility of steel. This effect can be achieved even if even a small amount of REM is contained. On the other hand, if the REM content is too high, the hot workability and creep ductility of the steel will decrease. Therefore, the REM content is 0.100% or less. The lower limit of the REM content is preferably 0.005%, more preferably 0.010%. The upper limit of the REM content is preferably 0.080%, more preferably 0.060%.

[0043] In this specification, REM refers to one or more elements selected from the group consisting of scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanoids lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. In addition, in this specification, the REM content refers to the total content of the above elements.

[0044] [F1] In the austenitic stainless steel material according to this embodiment, F1, expressed by the following formula (1), is 18.5 or less: F1=27.3Si-0.9Ni+0.8Cr+Mo+13.5N+6.8Cu (1) In formula (1), the content of each element is substituted for the corresponding element in mass%.

[0045] If the austenitic stainless steel material does not contain a corresponding element, 0 is substituted for the symbol of that element in formula (1). For example, if the austenitic stainless steel material does not contain Cu, F1 is expressed by the following formula: F1 = 27.3Si - 0.9Ni + 0.8Cr + Mo + 13.5N

[0046] By setting F1 to 18.5 or less, excellent fatty acid corrosion resistance can be obtained. The upper limit of F1 is preferably 18.2, more preferably 18.0, and even more preferably 17.9. The lower limit of F1 is not particularly limited, but is, for example, 10.0, and preferably 11.0.

[0047] [F2] The austenitic stainless steel material according to this embodiment has F2, expressed by the following formula (2), of less than 200. F2 = [R] + 0.9 × [H] (2) In formula (2), [R] is substituted with the grain size of the austenitic stainless steel material in μm, and [H] is substituted with the Vickers hardness of the austenitic stainless steel material.

[0048] Here, the grain size and Vickers hardness of an austenitic stainless steel material refer to the grain size and Vickers hardness of the steel material after the solution heat treatment described below and before use at high temperatures (hereinafter sometimes referred to as "solution heat treated material"). The grain size and Vickers hardness are measured at the center in the plate thickness direction if the steel material is a plate, and at the center in the wall thickness direction if the steel material is a pipe material. If the steel material is a steel bar, they are measured at the center in the radial direction. For other shapes, they are also measured at the center in the thickness direction.

[0049] The creep strength is ３６ Cr １２ Mo １０ This is achieved by precipitation strengthening of CrNbN and Z phases (CrNbN). A high Vickers hardness at the time of solution heat treatment indicates the presence of many precipitates at the time of solution heat treatment. The more precipitates there are at the time of solution heat treatment, the fewer strengthening phases precipitate during creep (during use at high temperatures). The temperature of solution heat treatment is generally higher than the temperature at which the material will be used, and the strengthening phases formed during solution heat treatment tend to be coarser than the strengthening phases precipitated during use at high temperatures. Therefore, it is thought that the strengthening phases present at the time of solution heat treatment contribute less to improving creep strength than the strengthening phases precipitated during use at high temperatures.

[0050] Therefore, from the viewpoint of improving creep strength, it is preferable that the Vickers hardness of the solution heat treated material is low. However, the Vickers hardness also depends on the grain size, and the smaller the grain size, the higher the hardness. Therefore, in F2, the effect of the grain size is taken into account.

[0051] Excellent creep strength can be obtained by setting F2 to less than 200. The upper limit of F2 is preferably 195, more preferably 190. The lower limit of F2 is not particularly limited, but is, for example, 150, preferably 160.

[0052] [Structure, etc.] The austenitic stainless steel material 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 stainless steel material according to this embodiment is preferably 90% or more, and more preferably 95% or more.

[0053] The grain size [R] of the austenitic stainless steel material according to this embodiment is not particularly limited as long as it is a size that results in the above-mentioned F2 being less than 200, but is, for example, 5 to 100 μm. The lower limit of the grain size [R] is preferably 10 μm, more preferably 15 μm. The upper limit of the grain size [R] is preferably 80 μm, more preferably 70 μm.

[0054] The Vickers hardness [H] of the austenitic stainless steel material according to this embodiment is not particularly limited as long as the above-mentioned F2 is less than 200, but is, for example, 120 to 200 HV. The lower limit of the Vickers hardness [H] is preferably 130 HV, more preferably 140 HV. The upper limit of the Vickers hardness [H] is preferably 195 HV, more preferably 190 HV.

[0055] [Shape and Use] The shape of the austenitic stainless steel material according to this embodiment is not particularly limited. The austenitic stainless steel material according to this embodiment may be a steel pipe, a steel plate, or a steel bar. The austenitic stainless steel material according to this embodiment may also be a forged product.

[0056] The austenitic stainless steel material according to this embodiment is suitable for use in equipment used for long periods in high-temperature environments. In this specification, a high-temperature environment refers to an environment in which the average operating temperature is in the range of 300 to 800°C. The operating temperature may exceed 800°C. Examples of equipment in such high-temperature environments include chemical plants such as oil refineries and petrochemical plants. The austenitic stainless steel material according to this embodiment is particularly suitable as a steel material for use in equipment for producing renewable diesel, biofuels, and SAF.

[0057] [Method for manufacturing austenitic stainless steel material] An example of a method for manufacturing an austenitic stainless steel material according to this embodiment will be described below. The method for manufacturing an austenitic stainless steel material according to this embodiment is not limited to the method described below.

[0058] The method for manufacturing an austenitic stainless steel material according to this embodiment includes a step of preparing a raw material (preparation step), a hot working step of hot working the raw material, and, if necessary, a cold working step of cold working the hot worked raw material, and a solution heat treatment step.

[0059] [Preparation Step] A material having the above-described chemical composition is prepared. The material may be an ingot, slab, bloom, or billet.

[0060] [Hot Working Step] Hot working is performed on the raw material to produce an intermediate steel material. The intermediate steel material may be a steel pipe, a steel plate, or a steel bar. When the intermediate steel material is a steel pipe, the hot working may be, for example, hot extrusion, such as that typified by the Ugine-Séjournet method, hot punching, or piercing rolling by the Mannesmann method. When the intermediate steel material is a steel plate, the hot working may be, for example, hot rolling. When the intermediate steel material is a steel bar, for example, blooming may be performed as the rough rolling step, and hot rolling using a continuous rolling mill may be performed as the finish rolling.

[0061] In either case, before the final hot working, the material is held in a heating furnace at a temperature T1 (furnace temperature, in °C) for a time t1 (furnace time (the time from when the material is placed in the furnace until it is extracted), in minutes). The temperature T1 is, for example, 800 to 1300°C. The time t1 is, for example, 30 to 400 minutes.

[0062] At this time, f1 expressed by the following formula (3) is set to 750 or more: f1=0.5×T1×log(t1) (3)

[0063] By setting f1 to 750 or more, segregation is eliminated and precipitates are dissolved, allowing subsequent hot working to be performed uniformly. This allows a uniform structure to be obtained after solution heat treatment, and the formation of precipitates at the time of solution heat treatment to be suppressed. The lower limit of f1 is preferably 780, more preferably 800, and even more preferably 820. The upper limit of f1 is not particularly limited, but is, for example, 1300, preferably 1250.

[0064] When the heating before the final hot working is carried out in multiple stages, the sum of f1 calculated for each heating should be 750 or more. That is, when the heating before the final hot working is carried out in N stages, the temperature of the heating furnace in the kth stage should be set to T1 ｋ (unit: ° C.), the time spent in the k-th stage heating furnace is t1 ｋ (unit: minutes), f1 represented by the following formula (3') Ｎ is set to 750 or more.

[0065] In addition, f1 (or f1 Ｎ Even if multiple hot working processes are performed in the manufacturing process of austenitic stainless steel material, f1 (or f1) is calculated only from the heating conditions before the final hot working. In other words, the heating conditions before the hot working processes other than the final hot working are calculated based on f1 (or f1) Ｎ ) is not taken into account in the calculation.

[0066] [Cold Working Step] The cold working step is performed as needed. That is, the cold working step does not have to be performed. The cold working step may be performed after pickling treatment. When the intermediate steel material is a steel pipe or a steel bar, the cold working may be, for example, cold drawing. When the intermediate steel material is a steel plate, the cold working may be, for example, cold rolling.

[0067] [Solution Heat Treatment Step] The hot-worked or cold-worked material is held in a heating furnace at temperature T2 (furnace temperature, in °C) for time t2 (soaking time (the time from when the entire material placed in the furnace reaches temperature T2 until extraction), in minutes). The temperature T2 is, for example, 900 to 1250°C. The time t2 is, for example, 1 to 60 minutes.

[0068] At this time, f2 expressed by the following formula (4) is set to be 600 or more: f2=0.5×T2×log{(t2)+13} (4)

[0069] By setting f2 to 600 or more, a uniform structure can be obtained after solution heat treatment, and the formation of precipitates at the time of solution heat treatment can be suppressed. The lower limit of f2 is preferably 620, more preferably 640, and even more preferably 660. The upper limit of f2 is not particularly limited, but is, for example, 900, preferably 800, and even more preferably 750.

[0070] When the solution heat treatment is carried out in multiple stages, the sum of f2 calculated for each heat treatment should be 600 or more. That is, when the solution heat treatment is carried out in N stages, the heating temperature of the kth stage should be T2 ｋ (unit: °C), the heating temperature (T2 ｋ ) and the time from when the heating temperature is reached to when the heating temperature is changed again or when the extraction is performed is defined as t2 ｋ (unit: minutes), f2 expressed by the following formula (4') Ｎ is set to 600 or more.

[0071] By the above steps, the austenitic stainless steel material according to this embodiment can be manufactured. By setting f1 to 800 or more and f2 to 600 or more, the above-mentioned F2 can be set to less than 200.

[0072] The above describes an example of an austenitic stainless steel material and a method for producing the same according to this embodiment. According to this embodiment, an austenitic stainless steel material that combines excellent creep strength and fatty acid corrosion resistance can be obtained.

[0073] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0074] Steels having the chemical compositions shown in Table 1 were produced.

[0075]

[0076] A 30 kg ingot with an outer diameter of 120 mm was produced using the molten steel. The ingot was hot forged to produce a 40 mm thick steel plate. This 40 mm thick steel plate was further hot rolled to produce a 15 mm thick steel plate.

[0077] This 15 mm thick steel plate was cold rolled to produce a steel plate having a thickness of 10.5 mm, a width of 50 mm and a length of 100 mm. The cold rolled steel plate was subjected to a solution heat treatment.

[0078] Table 2 shows the heating conditions (f1) before the final hot working (hot rolling) and the solution heat treatment conditions (f2).

[0079]

[0080] [Grain size] The grain size [R] of the steel material after solution heat treatment was determined as follows. A sample was taken from the center of the steel sheet width and the center of the sheet thickness. The cross section including the longitudinal direction (rolling direction) and the thickness direction was used as the observation surface. The mirror-polished observation surface was corroded with 10% oxalic acid to reveal the austenite grain boundaries. Three arbitrary fields of view on the observation surface were observed, and the grain size number was determined based on the intercept method in accordance with JIS G 0551 (2013). The area of ​​each field was 0.75 mm ２The arithmetic mean value of the grain size numbers of the three fields of view was taken as the grain size number G. Furthermore, the grain size number G was converted into the grain size [R] (μm) based on the following formula: n=2 Ｇ＋３ [R]=1000 / n １／２

[0081] [Vickers Hardness] The Vickers hardness [H] of the steel material after solution heat treatment was determined as follows. A sample was taken from the center of the thickness and width of the steel material after solution heat treatment. The test was carried out on a cross section including the longitudinal direction (rolling direction) and the thickness direction. The Vickers hardness was determined in accordance with JIS Z2244 (2009). The test force was 10 kgf, and the average value of five points was taken as [H].

[0082] [Creep strength] Creep rupture test specimens according to JIS Z2271 (2010) were prepared from the center of the thickness and width of the steel material after solution heat treatment. The cross section perpendicular to the axial direction of the creep rupture test specimen was circular, the outer diameter of the creep rupture test specimen was 6 mm, and the length of the parallel portion was 30 mm. The parallel portion was parallel to the longitudinal direction (rolling direction) of the steel sheet.

[0083] Using this creep rupture test specimen, a creep rupture test was conducted in accordance with JIS Z2271 (2010). Specifically, the creep rupture test specimen was heated to 600°C and then subjected to a creep rupture test. The test force was set to 250 MPa, and the creep rupture time was determined. Test specimens with a creep rupture time of more than 1000 hours were evaluated as "good," and those with a creep rupture time of 1000 hours or less were evaluated as "unacceptable."

[0084] [Fatty Acid Corrosion Test] A test piece measuring 0.95 inches × 2 inches × 0.063 inches (24.1 mm × 50.8 mm × 1.6 mm) was taken from the center of the thickness and width of the steel material after solution heat treatment. The corrosion rate was evaluated under the following conditions: the concentration of corrosion components in the reagent, such as TAN (Total Acid Number), was maintained at a certain value or higher, the reagent was caused to flow by rotating the tester at 1500 rpm, and shear stress was applied to the surface of the test piece. The test started with a TAN value of 30, the temperature was 750°F (approximately 399°C), the test time was 48 hours, the atmosphere was nitrogen, and the oil used was DCO (Distillers Corn Oil). A corrosion rate of 2.0 mils / year (0.0508 mm / year) or less was evaluated as "good," and a corrosion rate of more than 2.0 mils / year was evaluated as "unacceptable."

[0085] The results are shown in Table 2. As shown in Table 2, the steels of test numbers 1 to 9 had excellent creep strength and fatty acid corrosion resistance. Specifically, these steels had creep rupture times of over 1000 hours and corrosion rates of 2.0 mils / year or less in the fatty acid corrosion test.

[0086] The steels of test numbers 10 to 12 were inferior in at least one of creep strength and fatty acid corrosion resistance, which is thought to be because the chemical compositions of these steels were inappropriate.

[0087] The steels of test numbers 13 and 16 had poor resistance to fatty acid corrosion, which is thought to be because F1 was too large.

[0088] The steel material of test number 14 had poor creep strength. This is thought to be because F2 was too large. The reason why F2 was large is thought to be because f1 was too small.

[0089] The steel material of test number 15 had poor creep strength. This is thought to be because F2 was too large. The reason why F2 was large is thought to be because f2 was too small.

[0090] 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.

Claims

1. An austenitic stainless steel material having a chemical composition, in mass%, of C: 0.100% or less, Si: 0.10 to 1.00%, Mn: 0.20 to 2.00%, P: 0.045% or less, S: 0.030% or less, Cr: 15.0 to 25.0%, Ni: 9.0 to 18.0%, Mo: 1.0 to 5.0%, Nb: 0.20 to 2.00%, N: 0.05 to 0.18%, sol. An austenitic stainless steel material having an F1 value of 18.5 or less, and an F2 value of less than 200, wherein the F1 value is represented by the following formula (1): F1=18.5 or less; and an F2 value is represented by the following formula (2): F1=18.5 or less; and an F2 value is represented by the following formula (2): F1=18.5 or less. F1 = 27.3Si - 0.9Ni + 0.8Cr + Mo + 13.5N + 6.8Cu (1) F2 = [R] + 0.9 × [H] (2) In formula (1), the content of the corresponding element in mass% is substituted for each element symbol. In formula (2), the grain size of the austenitic stainless steel material in μm is substituted for [R], and the Vickers hardness of the austenitic stainless steel material is substituted for [H].

2. An austenitic stainless steel material according to claim 1, wherein the chemical composition contains, in mass %, B: 0.0001 to 0.0080%.

3. An austenitic stainless steel material according to claim 1, wherein the chemical composition contains, in mass %, one or more elements selected from the group consisting of Cu: 0.01 to 2.00%, V: 0.01 to 1.00%, Co: 0.01 to 1.00%, Ta: 0.01 to 0.20%, and W: 0.01 to 1.00%.

4. An austenitic stainless steel material according to claim 1, wherein the chemical composition contains, in mass%, one or more elements selected from the group consisting of: Ca: 0.0005 to 0.0100%, Mg: 0.0005 to 0.0100%, and REM: 0.005 to 0.100%.

5. A method for producing the austenitic stainless steel material according to any one of claims 1 to 4, comprising the steps of hot working a material and solution heat treating the material, wherein, when the temperature (furnace temperature) of the heating furnace in the heating before the final hot working is T1 (°C), the time spent in the furnace is t1 (minutes), the temperature (furnace temperature) of the solution heat treatment is T2 (°C), and the soaking time is t2 (minutes), f1, expressed by the following formula (3), is 750 or more, and f2, expressed by the following formula (4), is 600 or more. f1 = 0.5 × T1 × log (t1) (3) f2 = 0.5 × T2 × log {(t2) + 13} (4)

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