Austenitic stainless steel and method for producing austenitic stainless steel

Abstract

Provided is an austenitic stainless steel having excellent strength and ductility. The austenitic stainless steel has: a chemical composition containing 0.005-0.060% of C, 0.20-1.20% of Si, 4.0-8.0% of Mn, 12.0-15.0% of Ni, 19.0-24.0% of Cr, 1.0-4.0% of Mo, 0.05-0.40% of Nb, 0.05-0.40% of V, 0.20-0.50% of N, and 0.050% or less of Al, etc.; a tensile strength of 800 MPa or above; and an elongation at break of 35% or above. The austenitic stainless steel satisfies the following relationships (1) and (2).　Relationship 1: 0.7 × Nb ≤ [Nb] ≤ 0.30 Relationship 2: 20 × [Nb] / D ≥ 0.050 In relationships (1) and (2), a value in which the Nb content of the chemical composition is represented in mass%, a value in which the amount of Nb analyzed as an electrolytic extraction residue is represented in mass%, and a value in which the crystal grain diameter is represented in μm, are respectively substituted into "Nb", "[Nb]", and "D".

Description

[Technical Field]

[0001] The present invention relates to an austenitic stainless steel and a method for producing an austenitic stainless steel. [Background technology]

[0002] In recent years, research has been progressing on the practical application of transportation equipment that uses hydrogen as an energy source instead of fossil fuels. For example, fuel cell vehicles that run on hydrogen and hydrogen stations that supply hydrogen to fuel cell vehicles are being developed.

[0003] When stainless steel is used in fuel cell vehicles and hydrogen stations, it is used in a high-pressure hydrogen gas environment. Furthermore, there is a demand for lightweight fuel cell vehicles, compact hydrogen stations, and high-pressure operation. Therefore, the stainless steel used in fuel cell vehicles and hydrogen stations must have high strength.

[0004] International Publication No. 2012 / 132992 discloses a high-strength austenitic stainless steel for high-pressure hydrogen gas. International Publication No. 2017 / 056619 discloses an austenitic stainless steel with excellent strength, ductility, and weldability. Japanese Patent Application Laid-Open No. 2014-47420 describes an austenitic stainless steel for nuclear reactors that exhibits excellent strength in the reactor water temperature range and also has excellent SCC resistance and weldability. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2012 / 132992 [Patent Document 2] International Publication No. 2017 / 056619 [Patent Document 3] Japanese Patent Application Laid-Open No. 2014-47420 Summary of the Invention [Problem to be solved by the invention]

[0006] A variety of components are used in hydrogen stations. Traditionally, piping made from steel pipes has been widely used for hydrogen station components. However, in recent years, high strength is required not only for piping but also for various other components, and the application of austenitic stainless steel to components other than piping is being considered. These components include solid components. Therefore, it is desirable for solid materials to be able to combine excellent strength and ductility similar to steel pipes.

[0007] WO 2012 / 132992 discloses that austenitic stainless steel with a tensile strength of 800 MPa or more can be obtained by cold working with a cross-sectional area reduction of 20% or more after solution heat treatment and then heat treating again. However, in the production of solid materials, it is difficult to perform cold working with a large cross-sectional area reduction.

[0008] An object of the present invention is to provide an austenitic stainless steel having excellent strength and ductility, and a method for producing the austenitic stainless steel. [Means for solving the problem]

[0009] An austenitic stainless steel according to one embodiment of the present invention has a chemical composition, in mass%, of C: 0.005 to 0.060%, Si: 0.20 to 1.20%, Mn: 4.0 to 8.0%, Ni: 12.0 to 15.0%, Cr: 19.0 to 24.0%, Mo: 1.0 to 4.0%, Nb: 0.05 to 0.40%, V: 0.05 to 0.40%, N: 0.20 to 0.50%, Al: 0.050% or less, Cu: 0 to 3.0%, Co: 0 to 0.50%, Ca: 0 to 0.0050%, The alloy is composed of B: 0 to 0.0050%, W: 0 to 0.10%, Ta: 0 to 0.10%, and the balance: Fe and impurities, of which P, S, and O are each 0.030% or less, S: 0.010% or less, and O: 0.015% or less, respectively; the tensile strength is 800 MPa or more; the elongation at break is 35% or more; the amount of Nb analyzed as an electrolytic extraction residue satisfies the following formula (1); and the amount of Nb analyzed as an electrolytic extraction residue and the crystal grain size satisfy the following formula (2). 0.7×Nb≦[Nb]≦0.30 (1) 20×[Nb] / D≧0.050 (2) In formulas (1) and (2), "Nb", "[Nb]", and "D" are respectively substituted with the Nb content in the chemical composition expressed in mass%, the amount of Nb analyzed as the electrolytic extraction residue expressed in mass%, and the crystal grain size expressed in μm.

[0010] A method for producing austenitic stainless steel according to one embodiment of the present invention is a method for producing the above-mentioned austenitic stainless steel, and includes the steps of: subjecting a steel material having the above-mentioned chemical composition to a primary heat treatment at 1180 to 1280°C; after the primary heat treatment, subjecting the steel material to cold working with a cross-sectional area reduction rate of 5% or more and less than 20%; and after the cold working, subjecting the steel material to a secondary heat treatment at a temperature of 1000 to 1180°C. [Effects of the Invention]

[0011] According to the present invention, an austenitic stainless steel having excellent strength and ductility can be obtained. DETAILED DESCRIPTION OF THE INVENTION

[0012] The present inventors have conducted various studies to solve the above problems and have obtained the following findings (a) to (f).

[0013] (a) Methods for increasing the strength of steel materials include solid solution strengthening and grain refinement using N. In the austenitic stainless steel of this embodiment, strength is increased by solid solution strengthening by adding 0.20 to 0.50 mass% of N. Refining the crystal grains further increases the strength.

[0014] (b) One method for refining grains is to suppress grain growth through the pinning effect by precipitating fine alloy carbonitrides or nitrides during the final solution treatment. Adding Nb is effective for refining the grains of high-nitrogen steel by forming fine carbonitrides or nitrides. However, with conventional methods, Nb carbonitrides and Nb nitrides precipitate, but they are coarse, and the pinning effect cannot be fully achieved.

[0015] (c) To solve this problem, WO 2012 / 132992 proposes a method of performing solution heat treatment, cold working, and secondary heat treatment in this order. However, in the case of solid materials for which it is difficult to perform cold working with a large reduction in area, it is not possible to ensure strength and ductility using this method.

[0016] (d) Therefore, as a primary heat treatment, heat treatment before cold working is performed at a temperature higher than the conventional solution heat treatment temperature. This allows the alloying elements to dissolve more, and even with strain caused by cold working with a low cross-sectional reduction, it is possible to increase the number of precipitation nuclei for Nb carbonitrides and Nb nitrides that precipitate during secondary heat treatment. Therefore, even with cold working with a low cross-sectional reduction, it is possible to precipitate fine and abundant Nb carbonitrides and Nb nitrides.

[0017] (e) If the effect of pinning in suppressing grain growth is the same, increasing the temperature of the primary heat treatment and secondary heat treatment will result in relatively larger grains. However, even if the grains are larger than those described in WO 2012 / 132992, high strength and ductility can be achieved by ensuring that the amount of Nb carbonitrides or Nb nitrides and the grain size satisfy a specific relationship. Specifically, by ensuring that the amount of Nb analyzed as the electrolytic extraction residue satisfies the following formula (1), and that the amount of Nb analyzed as the electrolytic extraction residue and the grain size satisfy the following formula (2), the combined effect of grain refinement and precipitation strengthening can achieve both high strength and ductility. 0.7×Nb≦[Nb]≦0.30 (1) 20×[Nb] / D≧0.050 (2) In formulas (1) and (2), "Nb", "[Nb]", and "D" are respectively substituted with the Nb content expressed in mass%, the amount of Nb analyzed as the electrolytic extraction residue expressed in mass%, and the crystal grain size expressed in μm.

[0018] (f) More preferably, the secondary heat treatment is carried out at a temperature of 1050 to 1160° C. By carrying out the secondary heat treatment in a temperature range near the precipitation nose of Nb carbonitrides or Nb nitrides, it is possible to precipitate more Nb carbonitrides or Nb nitrides in a short period of time.

[0019] The present invention was completed based on the above findings. An austenitic stainless steel and a method for producing an austenitic stainless steel according to one embodiment of the present invention will now be described in detail.

[0020] [Chemical composition] An austenitic stainless steel according to one embodiment of the present invention has the chemical composition described below. In the following description, "%" in the content of an element means mass %.

[0021] C: 0.005 to 0.060% Carbon (C) is an element effective in stabilizing austenite. Furthermore, C forms Nb carbonitrides, contributing to ensuring strength. To fully obtain this effect, the C content must be 0.005% or more. However, if the C content is too high, excessive Nb carbonitrides are formed, reducing the ductility of the steel. Therefore, the C content must be 0.060% or less. The lower limit of the C content is preferably 0.010%, more preferably 0.020%. The upper limit of the C content is preferably 0.050%, more preferably 0.040%.

[0022] Si: 0.20 to 1.20% Silicon (Si) is an element effective as a deoxidizer and also effective in improving corrosion resistance. To fully obtain this effect, the Si content needs to be 0.20% or more. However, if the Si content is too high, the stability of the austenitic structure decreases and the ductility of the steel decreases. Therefore, the Si content is 0.20 to 1.20%. The lower limit of the Si content is preferably 0.25%, more preferably 0.30%. The upper limit of the Si content is preferably 1.10%, more preferably 1.00%.

[0023] Mn: 4.0 to 8.0% Manganese (Mn) contributes to deoxidation during manufacturing and is also an effective element for stabilizing austenite. Mn also indirectly contributes to increasing the strength by increasing the solubility of N. If the Mn content is too low, these effects cannot be obtained. On the other hand, if the Mn content is too high, the ductility and hot workability of the steel decrease. Therefore, the Mn content is 4.0 to 8.0%. The lower limit of the Mn content is preferably 4.5%, more preferably 5.0%. The upper limit of the Mn content is preferably 7.5%, more preferably 7.0%.

[0024] Ni: 12.0 to 15.0% Nickel (Ni) is an essential element for obtaining stable austenite, and it increases stacking fault energy and reduces embrittlement susceptibility in a hydrogen environment. To fully obtain this effect, the Ni content needs to be 12.0% or more. However, since Ni is an expensive element, adding a large amount increases costs. Therefore, the Ni content is 12.0 to 15.0%. The lower limit of the Ni content is preferably 12.3%, more preferably 12.5%. The upper limit of the Ni content is preferably 14.8%, more preferably 14.5%.

[0025] Cr: 19.0~24.0% Chromium (Cr) is an essential element for ensuring corrosion resistance in the usage environment. Cr also indirectly contributes to increasing the strength by increasing the solubility of N during manufacturing. To fully obtain this effect, the Cr content must be 19.0% or more. However, if the Cr content is too high, the austenitic structure becomes unstable. Therefore, the Cr content is 19.0 to 24.0%. The lower limit of the Cr content is preferably 19.5%, more preferably 20.0%. The upper limit of the Cr content is preferably 23.5%, more preferably 23.0%.

[0026] Mo: 1.0-4.0% Molybdenum (Mo) is an element effective in improving corrosion resistance and increasing strength in the usage environment. To fully obtain this effect, the Mo content needs to be 1.0% or more. However, since Mo is an expensive element, adding a large amount results in increased costs. Furthermore, if the Mo content is too high, the austenite structure becomes unstable. Therefore, the Mo content is 1.0 to 4.0%. The lower limit of the Mo content is preferably 1.2%, more preferably 1.5%. The upper limit of the Mo content is preferably 3.8%, more preferably 3.5%.

[0027] Nb: 0.05 to 0.40% Niobium (Nb) is an element that precipitates in the matrix as fine carbonitrides or nitrides and is effective in increasing strength. To fully obtain this effect, the Nb content needs to be 0.05% or more. However, if the Nb content is too high, the ductility of the steel decreases. Therefore, the Nb content is 0.05 to 0.40%. The lower limit of the Nb content is preferably 0.10%, more preferably 0.15%. The upper limit of the Nb content is preferably 0.35%, more preferably 0.30%.

[0028] The Nb content here means the total amount of Nb contained in the austenitic stainless steel. That is, it means the sum of the amount of Nb dissolved in the matrix and the amount of Nb present as precipitates. In this embodiment, in addition to the Nb content, the amount of Nb present as precipitates, i.e., the amount of Nb analyzed as electrolytic extraction residue, must also be within a specified range.

[0029] V: 0.05 to 0.40% Vanadium (V) is an element that precipitates in the matrix as fine carbonitrides or nitrides and is effective in increasing strength. To fully obtain this effect, the V content needs to be 0.05% or more. However, if the V content is too high, the ductility of the steel decreases. Therefore, the V content is 0.05 to 0.40%. The lower limit of the V content is preferably 0.10%, more preferably 0.15%. The upper limit of the V content is preferably 0.35%, more preferably 0.30%.

[0030] N: 0.20 to 0.50% Nitrogen (N) dissolves in the matrix, contributing to increased strength, and also forms fine carbonitrides and nitrides with Nb and other elements, contributing to increased strength. N is also an element that is effective in stabilizing the austenite structure. To fully obtain these effects, the N content must be 0.20% or more. However, if the N content is too high, the hot workability during manufacturing decreases, and excessive Nb precipitates occur, reducing the ductility of the steel. Therefore, the N content is 0.20 to 0.50%. The lower limit of the N content is preferably 0.23%, and more preferably 0.25%. The upper limit of the N content is preferably 0.48%, and more preferably 0.45%.

[0031] Al: 0.050% or less Aluminum (Al), like Si, is contained as a deoxidizer. However, if the Al content is too high, the cleanliness of the steel decreases, resulting in a decrease in hot workability. Therefore, the Al content is 0.050% or less. The Al content is preferably 0.040% or less, and more preferably 0.030% or less. There is no need to set a lower limit for the Al content, but an extreme reduction leads to an increase in steelmaking costs. Therefore, the lower limit for the Al content is preferably 0.005%, and more preferably 0.010%.

[0032] Cu: 0 to 3.0% Copper (Cu) is an optional element and does not necessarily need to be included. Cu stabilizes the austenite structure. Cu also increases the strength of steel through solid solution strengthening. Even if even a small amount of Cu is included, this effect can be achieved to some extent. However, since Cu is an expensive element, adding a large amount of Cu increases costs. Furthermore, if the Cu content is too high, the ductility of the steel decreases. Therefore, the Cu content is 0 to 3.0%. The lower limit of the Cu content is preferably 0.01%, more preferably 0.10%. The upper limit of the Cu content is preferably 2.5%, more preferably 2.0%.

[0033] Co: 0 to 0.50% Cobalt (Co) is an optional element and does not necessarily need to be contained. Co stabilizes the austenite structure. Co also increases the strength of steel through solid solution strengthening. Even if even a small amount of Co is contained, this effect can be obtained to some extent. However, since Co is an expensive element, the inclusion of a large amount increases costs. Therefore, the Co content is 0 to 0.50%. The lower limit of the Co content is preferably 0.01%, and more preferably 0.10%. The upper limit of the Co content is preferably 0.30%, and more preferably 0.20%.

[0034] Ca: 0 to 0.0050% Calcium (Ca) is an optional element and does not necessarily need to be contained. Ca suppresses solidification cracking when steel is cast. Even if even a small amount of Ca is contained, this effect can be obtained to some extent. However, if the Ca content is excessive, the hot workability of the steel decreases. Therefore, the Ca content is 0 to 0.0050%. The lower limit of the Ca content is preferably 0.0001%, and more preferably 0.0005%. The upper limit of the Ca content is preferably 0.0040%, and more preferably 0.0030%.

[0035] B: 0 to 0.0050% Boron (B) is an optional element and does not necessarily need to be present. B refines precipitates, thereby refining crystal grains and increasing the strength of steel. Even if even a small amount of B is present, this effect can be achieved to some extent. However, if the B content is excessive, the hot workability of the steel decreases. Therefore, the B content is 0 to 0.0050%. The lower limit of the B content is preferably 0.0001%, and more preferably 0.0005%. The upper limit of the B content is preferably 0.0030%, and more preferably 0.0020%.

[0036] W: 0 to 0.10% Tungsten (W) is an optional element and does not necessarily need to be contained. W increases the strength of steel. Even if even a small amount of W is contained, this effect can be obtained to a certain extent. However, even if W is contained in excess, the effect saturates. Therefore, the W content is 0 to 0.10%. The lower limit of the W content is preferably 0.01%, more preferably 0.02%. The upper limit of the W content is preferably 0.08%, more preferably 0.06%.

[0037] Ta: 0 to 0.10% Tantalum (Ta) is an optional element and does not necessarily need to be contained. Ta increases the strength of steel. Even if even a small amount of Ta is contained, this effect can be obtained to some extent. However, even if an excessive amount is contained, the effect saturates. Therefore, the Ta content is 0 to 0.10%. The lower limit of the Ta content is preferably 0.001%, and more preferably 0.002%. The upper limit of the Ta content is preferably 0.08%, and more preferably 0.06%.

[0038] The balance of the chemical composition of the austenitic stainless steel according to this embodiment is Fe and impurities. The impurities referred to here are elements that are mixed in from the ore or scrap used as raw materials for the steel, or elements that are mixed in from the environment during the manufacturing process. In this embodiment, the contents of P, S, and O among the impurities are particularly limited as follows.

[0039] P:0.030% or less Phosphorus (P) is contained in steel as an impurity. If the P content is too high, hot workability during manufacturing deteriorates. Therefore, the P content is 0.030% or less. The P content is preferably 0.025% or less, and more preferably 0.020% or less.

[0040] S: 0.010% or less Sulfur (S) is contained in steel as an impurity. If the S content is too high, hot workability during manufacturing deteriorates. Therefore, the S content is 0.010% or less. The S content is preferably 0.005% or less, and more preferably 0.002% or less.

[0041] O: 0.015% or less Oxygen (O) is contained in steel as an impurity. If the O content is too high, the hot workability during manufacturing decreases, and the cleanliness of the steel deteriorates, resulting in a decrease in ductility. Therefore, the O content is 0.015% or less. The O content is preferably 0.010% or less, and more preferably 0.005% or less.

[0042] [Nb content and steel structure analyzed as electrolytic extraction residue] In the austenitic stainless steel according to this embodiment, the amount of Nb analyzed as the electrolytic extraction residue satisfies the following formula (1), and the amount of Nb analyzed as the electrolytic extraction residue and the grain size satisfy the following formula (2). 0.7×Nb≦[Nb]≦0.30 (1) 20×[Nb] / D≧0.050 (2) In formulas (1) and (2), "Nb", "[Nb]", and "D" are respectively substituted with the Nb content expressed in mass%, the amount of Nb analyzed as the electrolytic extraction residue expressed in mass%, and the crystal grain size expressed in μm.

[0043] In the austenitic stainless steel according to this embodiment, the amount of Nb analyzed as electrowinning residue is 0.7 times or more and 0.30 mass% or less of the Nb content of the base material. The Nb contained in the base material precipitates as fine carbonitrides or nitrides during solution heat treatment. The finely precipitated Nb carbonitrides and Nb nitrides improve the strength of the steel. To achieve this effect, the amount of Nb precipitated as carbonitrides or nitrides, i.e., the amount of Nb analyzed as electrowinning residue, must be 0.7 times or more of the Nb content of the base material. However, excessive Nb content in the electrowinning residue reduces the ductility of the steel. Therefore, the upper limit of the amount of Nb analyzed as electrowinning residue is 0.30 mass%. The lower limit of the amount of Nb analyzed as electrowinning residue is more preferably 0.8 times the amount of Nb in the base material. The upper limit of the amount of Nb analyzed as electrowinning residue is even more preferably 0.28 mass%, and even more preferably 0.25 mass%.

[0044] In order to achieve both strength and excellent ductility, it is effective to use Nb carbonitrides and Nb nitrides to refine the crystal grains. To achieve both strength and ductility, the amount of Nb analyzed as the extraction residue and the crystal grain size must satisfy formula (2). The value of the left side of formula (2) is more preferably 0.060 or more, and even more preferably 0.080 or more.

[0045] The amount of Nb analyzed as the electrowinning residue can be adjusted by the Nb and N contents of the raw material, as well as manufacturing conditions such as heat treatment. Specifically, the higher the Nb and N contents of the raw material, the higher the amount of Nb analyzed as the electrowinning residue. The manufacturing conditions will be described later.

[0046] The amount of Nb analyzed as electrolytic extraction residue is measured as follows: A test piece of a specified size is taken from the austenitic stainless steel. A constant current electrolysis method is carried out using a 10% acetylacetone-1% tetramethylammonium chloride / methanol solution as the electrolyte, at a current density of 20-25 mA / cm. 2The test piece is anodically dissolved, and carbonitrides and nitrides are extracted as residue. The extracted residue is then acid-decomposed, and then ICP (inductively coupled plasma) emission spectrometry is performed to measure the mass of Nb in the residue.

[0047] The grain size can be measured by an intercept method.

[0048] The austenitic stainless steel according to this embodiment preferably has a grain size of 20.0 μm or more. Even when it is difficult to reduce the grain size below 20.0 μm, the austenitic stainless steel according to this embodiment can achieve both strength and ductility. The lower limit of the grain size is more preferably 25.0 μm, and even more preferably 30.0 μm. The upper limit of the grain size is preferably 60.0 μm.

[0049] [Mechanical properties] The austenitic stainless steel according to this embodiment has a tensile strength of 800 MPa or more, preferably 830 MPa or more, and more preferably 850 MPa or more.

[0050] The austenitic stainless steel according to this embodiment has a breaking elongation of 35% or more, preferably 38% or more, and more preferably 40% or more.

[0051] The austenitic stainless steel according to this embodiment is preferably a solid material (solid steel). Here, "solid material" refers to a material other than a hollow material such as a steel pipe. The austenitic stainless steel according to this embodiment is more preferably a round bar, and even more preferably a round bar with an outer diameter of 25 mm or more. Solid materials, particularly round bars with an outer diameter of 25 mm or more, are difficult to manufacture by cold working with a large cross-sectional reduction. The austenitic stainless steel according to this embodiment can achieve both strength and ductility even when cold working with a large cross-sectional reduction is difficult. When the austenitic stainless steel according to this embodiment is a round bar, the lower limit of the outer diameter is more preferably 28 mm, even more preferably 30 mm, and even more preferably 33 mm.

[0052] The austenitic stainless steel according to this embodiment is preferably used as a material for equipment that stores or transports high-pressure hydrogen gas or liquid hydrogen.

[0053] [Manufacturing method] In this embodiment, in order to precipitate fine Nb carbonitrides and Nb nitrides, the following primary heat treatment, cold working, and secondary heat treatment are carried out in this order.

[0054] The primary heat treatment is carried out at a temperature of 1180°C or higher in order to fully dissolve Nb carbonitrides and Nb nitrides formed in processes prior to the primary heat treatment, such as hot working. On the other hand, if the temperature of the primary heat treatment exceeds 1280°C, the crystal grains become extremely coarse. The lower limit of the temperature of the primary heat treatment is preferably 1200°C. The upper limit of the temperature of the primary heat treatment is preferably 1260°C. Hereinafter, for convenience, the temperature of the primary heat treatment will be referred to as "T1 (°C)". The holding time of the primary heat treatment is preferably 1 to 20 minutes, more preferably 1 to 10 minutes. After the primary heat treatment, the steel is desirably rapidly cooled, and preferably water-cooled.

[0055] After Nb carbonitrides and Nb nitrides are sufficiently dissolved by primary heat treatment, cold working and secondary heat treatment can be performed to precipitate fine Nb carbonitrides and Nb nitrides. In this embodiment, the area reduction rate of cold working is low, so it is necessary to dissolve more Nb carbonitrides and Nb nitrides in the primary heat treatment. The Nb carbonitrides and Nb nitrides generated by hot working or the like are coarse and do not contribute to grain refinement by the pinning effect. Furthermore, if the Nb carbonitrides and Nb nitrides generated by hot working or the like remain without being sufficiently dissolved, it becomes more difficult to introduce strain by cold working.

[0056] Next, in order to increase the precipitation nuclei of Nb carbonitrides or Nb nitrides, cold working is performed with a cross-sectional area reduction of 5% or more but less than 20%. The higher the cross-sectional area reduction, the more precipitation nuclei there are, and finer Nb carbonitrides or Nb nitrides precipitate during secondary heat treatment. If the cross-sectional area reduction of the cold working is too low, fine Nb carbonitrides or Nb nitrides do not precipitate during secondary heat treatment, and the effect of grain refinement due to the pinning effect cannot be fully achieved. On the other hand, when the austenitic stainless steel is a thick solid material, it is difficult to perform cold working with a cross-sectional area reduction of 20% or more. The lower limit of the cross-sectional area reduction of the cold working is preferably 8%, more preferably 10%. The upper limit of the cross-sectional area reduction of the cold working is preferably 18%, more preferably 15%.

[0057] Finally, in order to remove strain due to cold working and to precipitate fine Nb carbonitrides and Nb nitrides, a secondary heat treatment is performed at a temperature of 1000 to 1180°C. For convenience, the heat treatment temperature in the secondary heat treatment is hereinafter referred to as "T2 (°C)".

[0058] T2 is preferably 1050 to 1160°C. By carrying out the secondary heat treatment in a temperature range near the precipitation nose of Nb carbonitrides or Nb nitrides, it is possible to precipitate a larger amount of Nb carbonitrides or Nb nitrides in a short time. The lower limit of T2 is more preferably 1080°C. The upper limit of T2 is more preferably 1120°C. The holding time of the secondary heat treatment is preferably 1 to 20 minutes, more preferably 1 to 15 minutes. After the secondary heat treatment, the steel is preferably rapidly cooled, and more preferably water-cooled.

[0059] The above describes an austenitic stainless steel and a method for producing an austenitic stainless steel according to one embodiment of the present invention. According to this embodiment, an austenitic stainless steel having excellent strength and ductility can be obtained. [Example]

[0060] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0061] Steel types A to J having the chemical compositions shown in Table 1 were melted in a laboratory and cast into ingots, which were then hot forged and hot rolled to produce plates with a thickness of 20 mm. The plates were then subjected to a primary heat treatment at the temperature T1 (°C) shown in Table 2. Cooling after the primary heat treatment was by water cooling. Then, cold working was performed with the cross-sectional area reduction ratio shown in Table 2. Then, secondary heat treatment was performed at the temperature T2 (°C) shown in Table 2. The holding time for the primary heat treatment was 1 to 10 minutes, and the holding time for the secondary heat treatment was 1 to 15 minutes.

[0062] [Residue analysis] A test piece measuring 10 mm square and 30 mm long was taken from each test material, and the amount of Nb analyzed as an electrolytic extraction residue was measured by the method described above.

[0063] [Crystal grain size] Five test pieces were cut out from each test material so that the cross section perpendicular to the rolling direction (T cross section) served as the observation surface, mirror polished, etched, and observed under an optical microscope. Each test piece was observed in three fields at 100x magnification, and the average grain size for each of the five test pieces was determined by the intercept method. The average of these was then calculated to determine the grain size.

[0064] [Tensile test] Round bar tensile test specimens were prepared from each test material in accordance with JIS Z2241 (2011) with a parallel section diameter of 6 mm and a parallel section length of 30 mm, and tensile tests were conducted at room temperature. The tensile test specimens were prepared from the center of the thickness direction of the plate material so that the longitudinal direction of the specimen was parallel to the rolling direction. Tensile tests were conducted according to JIS Z2241 (2011) to determine the tensile strength (MPa) and elongation at break (%).

[0065] The results are shown in Table 2. In Table 2, "R" is the area reduction rate after cold working, "[Nb]" is the amount of Nb analyzed as an electrolytic extraction residue, "D" is the grain size, "TS" is the tensile strength, "EL" is the elongation at break, F1 is the value of the left side of formula (1) (0.7 × Nb), and F2 is the value of the left side of formula (2) (20 × [Nb] / D).

[0066] [Table 1]

[0067] [Table 2]

[0068] As shown in Table 2, the test materials with test numbers 1 to 25 had a tensile strength of 800 MPa or more and a breaking elongation of 35% or more.

[0069] Test Nos. 26 and 27 are specimens that were not subjected to primary heat treatment. Test No. 26 had sufficient ductility, but the tensile strength was less than 800 MPa. This is thought to be because it did not satisfy formula (1). On the other hand, Test No. 27 had sufficient tensile strength, but the breaking elongation was less than 35%. This is thought to be because it did not satisfy formula (2).

[0070] The specimen No. 28 had sufficient ductility, but the tensile strength was less than 800 MPa. This is thought to be because it did not satisfy formula (1). The reason it did not satisfy formula (1) is thought to be because the temperature of the primary heat treatment was too low. The specimen No. 29 had low tensile strength and ductility. This is thought to be because it did not satisfy formula (2). The reason it did not satisfy formula (2) is thought to be because the temperature of the primary heat treatment was too high.

[0071] The specimen No. 30 had low tensile strength and ductility. This is thought to be because it did not satisfy formulas (1) and (2). The reason for not satisfying formulas (1) and (2) is thought to be because the temperature of the secondary heat treatment was too high.

[0072] The specimen No. 31 had sufficient tensile strength, but the elongation at break was less than 35%. This is thought to be because the amount of Nb analyzed as an electrolytic extraction residue was too high. The reason why the amount of Nb analyzed as an electrolytic extraction residue was too high is thought to be because the temperature of the primary heat treatment was low compared to the Nb content of the raw material, and the carbides and carbonitrides formed during hot working, etc., did not form a solid solution sufficiently.

[0073] The specimen No. 32 had low tensile strength and ductility. This is thought to be due to the low Nb content of the material. The specimen No. 33 had low tensile strength and ductility. This is thought to be due to the low N content of the material.

[0074] Test numbers 34 to 36 are specimens that were not subjected to primary heat treatment and cold working. The specimens of test numbers 34 and 35 had low tensile strength. The specimen of test number 36 had low ductility. These specimens did not satisfy either formula (1) or (2).

[0075] The above evaluation tests were performed using plate materials, but for steel type A, round steel specimens were manufactured and similar evaluations were performed. Specifically, round steel specimens hot extruded from ingots with an outer diameter of 150 mm were subjected to a primary heat treatment at the temperature T1 (°C) shown in Table 3. Cooling after the primary heat treatment was performed by water cooling. Then, cold tensile straightening was performed to achieve the cross-sectional area reduction rate shown in Table 3, and solid materials in the shape of round steel with an outer diameter of 55 mm were produced. Then, secondary heat treatment was performed at the temperature T2 (°C) shown in Table 2. The holding time for the primary heat treatment was 1 to 10 minutes, and the holding time for the secondary heat treatment was 1 to 15 minutes. The results are shown in Table 3.

[0076] [Table 3]

[0077] As shown in Table 3, the test materials of test numbers 51 to 56 had a tensile strength of 800 MPa or more and a breaking elongation of 35% or more. On the other hand, the test material of test number 57 had a tensile strength of less than 800 MPa. This is thought to be because formula (1) was not satisfied. The reason why formula (1) was not satisfied is thought to be because the primary heat treatment was not performed.

[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.

Claims

1. The chemical composition, in mass%, is C: 0.005-0.060%, Si: 0.20-1.20%, Mn: 4.0 to 8.0%, Ni: 12.0-15.0%, Cr: 19.0-24.0%, Mo: 1.0 to 4.0%, Nb: 0.05-0.40%, V: 0.05-0.40%, N: 0.20-0.50%, Al: 0.050% or less, Cu: 0-3.0%, Co: 0-0.50% Ca: 0-0.0050%, B: 0 to 0.0050%, W: 0 to 0.10%, Ta: 0-0.10%, The balance is Fe and impurities. Among the impurities, P, S and O are P: 0.030% or less, S: 0.010% or less, and O: 0.015% or less; The tensile strength is 800 MPa or more, The elongation at break is 35% or more, The amount of Nb analyzed as the electrolytic extraction residue satisfies the following formula (1): The austenitic stainless steel has an Nb content and a crystal grain size analyzed as the electrolytic extraction residue that satisfy the following formula (2): 0.7×Nb≦[Nb]≦0.30 (1) 20×[Nb] / D≧0.050 (2) In formulas (1) and (2), "Nb", "[Nb]", and "D" are respectively substituted with the value of the Nb content in the chemical composition expressed in mass%, the value of the amount of Nb analyzed as the electrolytic extraction residue expressed in mass%, and the value of the crystal grain size expressed in μm.

2. 2. The austenitic stainless steel according to claim 1, The austenitic stainless steel is a solid material.

3. 3. The austenitic stainless steel according to claim 2, The solid material is a round bar, and the austenitic stainless steel.

4. 4. The austenitic stainless steel according to claim 3, An austenitic stainless steel having an outer diameter of 25 mm or more.

5. The austenitic stainless steel according to any one of claims 1 to 4, The chemical composition is, in mass %, Cu: 0.01 to 3.0%, Co: 0.01 to 0.50%, Ca: 0.0001-0.0050%, B: 0.0001 to 0.0050%, W: 0.01 to 0.10%, and Ta: 0.001 to 0.10%, An austenitic stainless steel containing one or more elements selected from the group consisting of:

6. The austenitic stainless steel according to any one of claims 1 to 4, Austenitic stainless steel used as a material for equipment that stores or transports high-pressure hydrogen gas or liquid hydrogen.

7. A method for producing the austenitic stainless steel according to any one of claims 1 to 4, comprising the steps of: A step of subjecting a steel material having the above chemical composition to a primary heat treatment at 1180 to 1280°C; After the primary heat treatment, a step of cold working the steel material to a cross-sectional area reduction rate of 5% or more and less than 20%; and a step of subjecting the steel material to secondary heat treatment at a temperature of 1000 to 1180°C after the cold working.

8. 8. A method for producing an austenitic stainless steel according to claim 7, The method for producing austenitic stainless steel, wherein the temperature of the secondary heat treatment is 1050 to 1160°C.

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