Austenitic stainless steel sheet and method for manufacturing the same

Abstract

The present invention provides an austenitic stainless steel sheet having a predetermined chemical composition, a volume fraction of the δ-ferrite phase in the center of the sheet thickness of 1.00% or less, and satisfying the average grain size of the austenite phase (μm) ≥ -40 × volume fraction of the δ-ferrite phase (%) + 80, thereby providing an austenitic stainless steel sheet with excellent creep fracture life.

Description

Technical Field

[0001] The present invention relates to austenitic stainless steel plates.

Background Art

[0002] Austenitic stainless steel plates are applied to structures in high-temperature environments because of their excellent high-temperature strength. On the other hand, it is known that when there is component segregation in austenitic stainless steel, the creep rupture life at high temperatures deteriorates, and an improvement in the creep rupture life in high-temperature environments is required.

[0003] In Patent Document 1, it is stated that considering the creep strength in a high-temperature environment (300 to 700°C), larger austenite crystal grains are preferable.

[0004] In Patent Document 2, it is stated that the presence of the δ-ferrite phase, which is a form of component segregation, suppresses the coarsening of austenite grains.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0006] An object of the present invention is to improve the creep rupture life at high temperatures in austenitic stainless steel plates.

Means for Solving the Problems

[0007] The inventors of the present invention investigated means to improve the creep rupture life (hereinafter sometimes simply referred to as creep rupture life) at high temperatures in austenitic stainless steel sheets and obtained the following findings.

[0008] (a) Regarding the chemical composition, solid solution strengthening is achieved by adding small amounts of Nb and N to austenitic stainless steel sheets, improving creep rupture life.

[0009] Furthermore, both component segregation and grain size influence the creep rupture life of austenitic stainless steel at high temperatures. Therefore, the inventors considered that controlling these factors is important for further improving the creep rupture life, and conducted a detailed investigation into the effects of the δ-ferrite phase, a form of component segregation, and grain size on the creep rupture life at high temperatures, obtaining the following findings.

[0010] (b) It was found that there is a certain relationship between the volume fraction and grain size of the δ-ferrite phase and the creep rupture lifetime. Specifically, it was found that if the grain size is greater than or equal to the value calculated from the relationship using the volume fraction of the δ-ferrite phase, an excellent creep rupture lifetime can be obtained.

[0011] (c) In the manufacturing process, the δ-ferrite phase generated during solidification gradually decreases through heating and hot rolling. The less δ-ferrite phase there is in the steel, the slower the rate of reduction becomes, and the harder it is for the δ-ferrite phase to decrease. For this reason, the subsequent solution heat treatment is generally a low-temperature and short-duration process compared to slab heating and soaking heat treatment, and therefore its effect on reducing the δ-ferrite phase is limited. Furthermore, since solution heat treatment is also a grain size adjustment process that is affected by the presence of the δ-ferrite phase, it is most effective to reduce the δ-ferrite phase before the solution heat treatment.

[0012] (d) We found that by controlling the total heating time in slab heating and soaking heat treatment before the solution heat treatment, and by performing the soaking heat treatment between primary and secondary hot rolling rather than before primary hot rolling, the segregation of components including the δ-ferrite phase can be efficiently mitigated. This is because primary hot rolling causes the segregation to spread perpendicular to the thickness direction of the sheet, increasing the surface area of ​​the δ-ferrite phase / austenite phase interface, and promoting the dissolution of the alloy components of the δ-ferrite phase into the austenite phase. In addition, the diffusion distance of the alloy components in the thickness direction during dissolution is shortened, and the concentration gradient of the alloy components in the thickness direction within the austenite phase becomes larger. Primary hot rolling is the process of hot rolling the slab to break down the slab's structure, and as mentioned above, it also plays a role in spreading the segregation of components. It is usually performed with a cumulative reduction ratio of 30-70%. Furthermore, soaking heat treatment is performed separately from the slab heating for hot rolling, with the aim of mitigating segregation of components including the δ-ferrite phase. It is a process of heating the slab to diffuse the alloy components. Secondary hot rolling is a process of breaking down the slab's structure through hot rolling and shaping it to the specified product dimensions. It is usually performed with a cumulative reduction ratio of 50-95%.

[0013] (e) The greater the plate thickness, the more difficult it becomes for the temperature at the center of the plate to reach the target temperature, and the less likely crystal grains are to grow at the center of the plate. Also, the greater the plate thickness during solution heat treatment, the greater the amount of δ-ferrite phase at the center of the plate, making it more susceptible to the suppression of crystal grain growth by the δ-ferrite phase. For these reasons, it is effective to adjust the heating time of the solution heat treatment for crystal grain growth so that the heating time increases as the plate thickness increases. Therefore, the inventors investigated the heating time according to the plate thickness.

[0014] It has been found that there is a corresponding relationship among the crystal grain size on the center side of the plate thickness, the plate thickness, and the heating time. At this time, grain growth progresses more on the surface layer side, but as the crystal grain size increases, its growth rate gradually slows down, and the difference in crystal grain size in the plate thickness direction can be reduced. By controlling the conditions of the above solution heat treatment, an excellent creep rupture life can be obtained.

[0015] (f) By controlling the above-described chemical composition and manufacturing method, in addition to solid solution strengthening due to the inclusion of trace amounts of Nb and N, the crystal grain size can be increased along with the alleviation of component segregation including the δ-ferrite phase, and an austenitic stainless steel plate excellent in creep rupture life at high temperatures can be obtained.

[0016] The present invention has been made based on the above findings, and the gist thereof is as follows.

[0017] (1) The chemical composition is, in mass%, C: 0.100% or less, Si: 2.00% or less, Mn: 2.50% or less, P: 0.0450% or less, S: 0.0300% or less, Ni: 9.00 - 13.00%, Cr: 19.00 - 24.00%, Mo: 0.001 - 1.00%, Nb: 0.005 - 0.100%, N: 0.100 - 0.300%, Al: 0.200% or less, B: 0.0001 - 0.0100%, Ti: from 0 to 0.300%, Cu: from 0 to 1.00%, V: from 0 to 0.500%, Co: from 0 to 1.000%, Ca: from 0 to 0.1000%, Mg: from 0 to 0.0050%, Sb: from 0 to 0.200%, Sn: from 0 to 0.100%, Se: from 0 to 0.080%, W: 0 to 0.50%, Ta: 0 to 0.50%, Hf: 0 to 0.100%, Zr: 0 to 0.100%, Bi: 0 to 0.300%, Pb: 0 to 0.100%, REM: 0 to 0.200%, The balance: Fe and impurities and the volume fraction of the δ-ferrite phase at the center of the plate thickness is 1.00% or less, the average crystal grain size (μm) of the austenite phase satisfies Equation 1 Austenitic stainless steel sheet characterized by the above. Average crystal grain size (μm) of austenite phase ≧ -40 × volume fraction (%) of δ-ferrite phase + 80 ····· Equation 1

[0018] (2) The chemical composition is in mass%, Ti: 0.001 to 0.300%, Cu: 0.01 to 1.00%, V: 0.010 to 0.500%, Co: 0.010 to 1.000%, Ca: 0.0010 to 0.0100%, Mg: 0.0001 to 0.0050%, Sb: 0.020 to 0.200%, Sn: 0.001 to 0.100%, Se: 0.005 to 0.080%, W: 0.01 to 0.50%, Ta: 0.005 to 0.50%, Hf: 0.010 to 0.100%, Zr: 0.001 to 0.100%, Bi: 0.030 to 0.300%, Pb: 0.010 to 0.100%, and REM: 0.005 to 0.200% The austenitic stainless steel sheet according to (1) above, containing one or more selected from the above.

[0019] (3) A method for manufacturing an austenitic stainless steel sheet as described in (1) above, The chemical composition is expressed in mass percent. C: 0.100% or less, Si: 2.00% or less, Mn: 2.50% or less, P:0.0450% or less, S: 0.0300% or less, Ni: 9.00~13.00%, Cr: 19.00~24.00%, Mo: 0.001~1.00%, Nb: 0.005~0.100%, N: 0.100~0.300%, Al: 0.200% or less, B: 0.0001~0.0100%, Ti: 0~0.300%, Cu: 0~1.00%, V: 0~0.500%, Co: 0~1.000%, Ca: 0~0.1000%, Mg: 0~0.0050%, Sb: 0~0.200%, Sn: 0~0.100%, Se: 0~0.080%, W: 0~0.50%, Ta: 0~0.50%, Hf: 0~0.100%, Zr: 0~0.100%, Bi: 0~0.300%, Pb: 0~0.100%, REM: 0~0.200%, Remainder: Fe and impurities The slab is heated to a temperature range of 1100-1300°C, and then subjected to a primary hot rolling process. A soaking heat treatment step is performed in which the slab that has undergone the primary hot rolling step is heated at a temperature range of 1100 to 1300°C for 3 hours or more. A secondary hot rolling process is performed to obtain a hot-rolled steel sheet by heating the slab that has undergone the soaking heat treatment process to a temperature range of 1100 to 1300°C, and then performing secondary hot rolling. Solution heat treatment process: The hot-rolled steel sheet is heated in a temperature range of 1000 to 1300°C for a time equal to or greater than the minimum heating time (hours) determined by the following formula 2 to perform a solution heat treatment. Equipped with, A method for manufacturing an austenitic stainless steel sheet, characterized in that the cumulative slab heating time, which is the sum of the heating time in the primary hot rolling step, the heating time in the soaking heat treatment step, and the heating time in the secondary hot rolling step, is 4.0 hours or more. Minimum heating time (hours) = 0.025 (hours / mm) × thickness of hot-rolled steel sheet (mm) ... Equation 2 [Effects of the Invention]

[0020] According to the present invention, an austenitic stainless steel sheet with excellent creep rupture life at high temperatures can be obtained. [Modes for carrying out the invention]

[0021] One embodiment of the present invention (hereinafter sometimes simply referred to as "this embodiment") will be described in detail below.

[0022] Herein, in this specification, "austenitic" refers to materials whose microstructure at room temperature is mainly composed of the austenite phase (γ phase). Therefore, materials containing other phases (δ ferrite phase, work-induced martensite phase, inclusions, etc.) are also included. For example, the austenite phase accounts for more than 70% of stainless steel sheets, and is typically 95% or more. Furthermore, the thickness (plate thickness) of the austenitic stainless steel sheet according to the embodiment of the present invention is not particularly limited, but may be, for example, 3 to 70 mm.

[0023] <Chemical composition> The reasons for limiting the content of each element are as follows. Unless otherwise specified, the "%" for content refers to the "mass %" of the steel.

[0024] (C: 0.100% or less) While carbon (C) promotes the stabilization of the austenite phase, a high C content may reduce the creep rupture life. Therefore, the C content should be 0.100% or less. The C content may be 0.095% or less, 0.090% or less, 0.085% or less, or 0.080% or less. A C content of 0.001% or more is preferable. The C content may be 0.005% or more, 0.007% or more, 0.015% or more, 0.030% or more, 0.045% or more, or 0.050% or more. The preferred range for the C content is 0.001 to 0.100%.

[0025] (Si:2.00% or less) Silicon (Si) is an effective element for deoxidation. However, if the Si content is too high, weldability and creep rupture life may deteriorate. For this reason, the Si content should be 2.00% or less. The Si content may be 1.85% or less, 1.65% or less, 1.30% or less, 1.10% or less, 1.00% or less, 0.80% or less, 0.65% or less, or 0.50% or less. A Si content of 0.001% or more is preferable. The Si content may be 0.01% or more, 0.03% or more, 0.05% or more, 0.10% or more, 0.15% or more, or 0.20% or more. The preferred range for Si content is 0.01% to 2.00%, and the more preferred range is 0.10% to 1.00%.

[0026] (Mn:2.50% or less) Manganese (Mn) is an effective element for deoxidation and also stabilizes the austenite phase. However, if the Mn content is too high, the oxidation resistance may deteriorate. For this reason, the Mn content should be 2.50% or less. The Mn content may be 2.30% or less, 2.10% or less, 1.75% or less, 1.40% or less, or 1.20% or less. A Mn content of 0.001% or more is preferable. The Mn content may be 0.01% or more, 0.05% or more, 0.10% or more, 0.15% or more, 0.25% or more, 0.40% or more, or 0.45% or more. The preferred range for the Mn content is 0.01% to 2.50%.

[0027] (P:0.0450% or less) Phosphorus (P) is an element present as an impurity that reduces toughness and hot workability. Therefore, the P content should be 0.0450% or less. It is preferable to reduce P as much as possible, and the P content may be 0.0430% or less, 0.0410% or less, 0.0380% or less, 0.0340% or less, or 0.0320% or less. On the other hand, excessive reduction of the P content increases manufacturing costs, so it is preferable that the P content be 0.0001% or more. The P content may be 0.0020% or more, 0.0050% or more, 0.0100% or more, 0.0160% or more, or 0.0190% or more. The preferred range for the P content is 0.0001% to 0.0450%.

[0028] (S:0.0300% or less) S (sulfur) is an element present as an impurity that reduces toughness and hot workability. Therefore, the sulfur content should be 0.0300% or less. It is preferable to reduce the sulfur content as much as possible, and the sulfur content may be 0.0270% or less, 0.0240% or less, 0.0180% or less, 0.0120% or less, or 0.0100% or less. On the other hand, excessive reduction of the sulfur content increases manufacturing costs, so it is preferable that the sulfur content be 0.0001% or more. The sulfur content may be 0.0003% or more, 0.0005% or more, or 0.0006% or more. The preferred range for sulfur content is 0.0001% to 0.0300%.

[0029] (Ni: 9.00~13.00%) Nickel (Ni) is effective in stabilizing the austenite phase, improving oxidation resistance, and enhancing creep fracture life. For this reason, the Ni content should be 9.00% or more. Preferably, the Ni content may be 9.20% or more, 9.40% or more, 9.60% or more, 9.80% or more, 10.00% or more, 10.20% or more, 10.40% or more, or 10.80% or more. If the Ni content is too high, it may impair weldability. For this reason, the Ni content should be 13.00% or less. Preferably, the Ni content may be 12.80% or less, 12.60% or less, 12.40% or less, 12.20% or less, 12.00% or less, 11.80% or less, or 11.60% or less.

[0030] (Cr: 19.00~24.00%) Chromium (Cr) is an effective element for improving oxidation resistance, high-temperature wear resistance, and creep rupture life. For this reason, the Cr content should be 19.00% or more. Preferably, the Cr content may be 19.30% or more, 19.70% or more, 20.10% or more, 20.50% or more, 20.90% or more, 21.20% or more, 21.60% or more, 22.00% or more, or 22.50% or more. If the Cr content is too high, the austenite phase may become unstable and the hot workability may deteriorate. For this reason, the Cr content should be 24.00% or less. Preferably, the Cr content may be 23.80% or less, 23.70% or less, 23.50% or less, 23.30% or less, 23.20% or less, or 23.10% or less.

[0031] (Mo: 0.001~1.00%) Mo (molybdenum) is an element that improves corrosion resistance and contributes to improved high-temperature strength and creep rupture life. For this reason, the Mo content is 0.001% or more. Preferably, the Mo content may be 0.01% or more, 0.03% or more, 0.05% or more, 0.10% or more, 0.15% or more, 0.20% or more, or 0.25% or more. A higher Mo content leads to increased raw material costs and also increases the amount of δ-ferrite phase present. For this reason, the Mo content is 1.00% or less. Preferably, the Mo content may be 0.95% or less, 0.85% or less, 0.75% or less, 0.70% or less, 0.60% or less, or 0.50% or less.

[0032] (Nb: 0.005~0.100%) Niobium (Nb) has the effect of improving high-temperature strength and creep rupture life. For this reason, the Nb content should be 0.005% or more. Preferably, the Nb content may be 0.006% or more, 0.007% or more, 0.008% or more, 0.010% or more, 0.012% or more, or 0.015% or more. If the Nb content is too high, a mixed granular structure will be formed, which will actually decrease the creep rupture life. For this reason, the Nb content should be 0.100% or less. Preferably, the Nb content may be 0.090% or less, 0.085% or less, 0.075% or less, 0.065% or less, 0.055% or less, 0.050% or less, or 0.040% or less.

[0033] (N: 0.100~0.300%) Nitrogen (N) is an effective element for stabilizing the austenite phase and improving the creep rupture life. For this reason, the N content should be 0.100% or more. Preferably, the N content may be 0.110% or more, 0.120% or more, 0.130% or more, 0.140% or more, 0.150% or more, 0.160% or more, 0.170% or more, or 0.180% or more. If the N content is too high, it promotes nitride deposition and actually reduces the creep rupture life. For this reason, the N content should be 0.300% or less. Preferably, the N content may be 0.290% or less, 0.280% or less, 0.270% or less, 0.260% or less, 0.250% or less, 0.240% or less, 0.230% or less, 0.220% or less, 0.210% or less, or 0.200% or less.

[0034] (Al: 0.200% or less) Aluminum (Al) is an effective element for deoxidation. However, if the Al content is too high, the toughness may decrease due to the formation of Al nitrides. For this reason, the Al content should be 0.200% or less. Preferably, the Al content may be 0.185% or less, 0.170% or less, 0.135% or less, 0.110% or less, or 0.090% or less. Preferably, the Al content is 0.001% or more. Preferably, the Al content is 0.004% or more, 0.008% or more, 0.015% or more, 0.020% or more, 0.025% or more, or 0.030% or more. The preferred range for the Al content is 0.001% to 0.200%.

[0035] (B: 0.0001~0.0100%) Boron (B) is an effective element for improving creep rupture life and hot workability. For this reason, the B content is 0.0001% or more. Preferably, the B content may be 0.0002% or more, 0.0005% or more, 0.0010% or more, 0.0015% or more, or 0.0020% or more. If the B content is too high, hot workability may be inhibited. For this reason, the B content should be 0.0100% or less. Preferably, the B content may be 0.0090% or less, 0.0080% or less, 0.0070% or less, 0.0060% or less, or 0.0050% or less.

[0036] In addition to the elements listed above, one or more elements selected from Ti, Cu, V, Co, Ca, Mg, Sb, Sn, Se, W, Ta, Hf, Zr, Bi, Pb, and REM may be included within the ranges shown below. That is, the lower limit for the above elements is 0%. The reasons for the limitations on each element are explained below.

[0037] (Ti: 0~0.300%) Titanium (Ti) is an element added to improve corrosion resistance and intergranular corrosion resistance by bonding with carbon (C) and nitrogen (N), but it is not an essential element, and the Ti content may be 0%. To obtain the above effects, the Ti content may be 0.001% or more, 0.002% or more, or 0.003% or more. If the Ti content is too high, nozzle clogging is more likely to occur during the casting stage, and ductility may deteriorate due to coarse Ti carbonitrides. For this reason, the Ti content should be 0.300% or less. Preferably, the Ti content may be 0.290% or less, 0.280% or less, 0.270% or less, 0.250% or less, 0.200% or less, 0.150% or less, or 0.100% or less.

[0038] (Cu: 0~1.00%) While copper (Cu) is an effective element for stabilizing and softening the austenite phase, it is not an essential element, and the Cu content may be 0%. To obtain the above effects, the Cu content may be 0.001% or more, 0.01% or more, 0.03% or more, 0.07% or more, 0.10% or more, or 0.12% or more. If the Cu content is too high, oxidation resistance and hot workability may deteriorate. For this reason, the Cu content should be 1.00% or less. Preferably, the Cu content may be 0.90% or less, 0.80% or less, 0.65% or less, 0.50% or less, or 0.40% or less.

[0039] (V: 0~0.500%) V (vanadium) is an element that improves corrosion resistance and promotes the formation of V carbides, thereby improving high-temperature strength. However, it is not an essential element, and the V content may be 0%. To obtain the above effects, the V content may be 0.001% or more, 0.005% or more, 0.010% or more, 0.015% or more, 0.030% or more, 0.045% or more, or 0.055% or more. If the V content is too high, the toughness may decrease due to V carbides. For this reason, the V content should be 0.500% or less. Preferably, the V content may be 0.450% or less, 0.400% or less, 0.350% or less, 0.300% or less, 0.250% or less, or 0.200% or less.

[0040] (Co: 0~1.000%) Co (cobalt) contributes to improving high-temperature strength through the stabilization of the austenite phase, but it is not an essential element, and the Co content may be 0%. To obtain the above effect, the Co content may be 0.001% or more, 0.010% or more, 0.015% or more, 0.030% or more, 0.060% or more, 0.080% or more, or 0.100% or more. If the Co content is too high, the effect saturates, leading to an increase in alloy costs. For this reason, the Co content should be 1.000% or less. Preferably, the Co content may be 0.900% or less, 0.800% or less, 0.650% or less, 0.500% or less, or 0.400% or less.

[0041] (Ca: 0~0.1000%) Calcium (Ca) is an element that improves the hot workability of steel, but it is not an essential element, and the Ca content may be 0%. To obtain the above effect, the Ca content may be 0.0001% or more, 0.0002% or more, or 0.0003% or more. If the Ca content is too high, relatively large oxides may be formed in the steel, which may reduce the toughness of the steel. For this reason, the Ca content should be 0.1000% or less. Preferably, the Ca content may be 0.0900% or less, 0.0800% or less, 0.0700% or less, 0.0600% or less, 0.0500% or less, 0.0400% or less, 0.0300% or less, 0.0200% or less, or 0.0100% or less.

[0042] (Mg: 0~0.0050%) Magnesium (Mg) is an element that improves the hot workability of steel, but it is not an essential element, and the Mg content may be 0%. To obtain the above effect, the Mg content may be 0.0001% or more, 0.0002% or more, 0.0005% or more, 0.0007% or more, or 0.0008% or more. If the Mg content is too high, relatively large oxides may be formed in the steel, which may reduce the toughness of the steel. For this reason, the Mg content should be 0.0050% or less. Preferably, the Mg content may be 0.0045% or less, 0.0040% or less, 0.0035% or less, 0.0030% or less, or 0.0025% or less.

[0043] (Sb: 0~0.200) Antimony (Sb) has the effect of improving high-temperature strength by segregating at grain boundaries, but it is not a particularly essential element, and the Sb content may be 0%. To obtain the above effect, the Sb content may be 0.001% or more, 0.010% or more, 0.020% or more, 0.040% or more, 0.080% or more, 0.120% or more, or 0.140% or more. If the Sb content is too high, Sb segregation may occur, which may cause cracking during welding. For this reason, the Sb content should be 0.200% or less. Preferably, the Sb content may be 0.190% or more, or 0.180% or less.

[0044] (Sn: 0~0.100%) While tin (Sn) contributes to improved corrosion resistance and high-temperature strength, it is not an essential element, and the Sn content may be 0%. To obtain the above effects, the Sn content may be 0.001% or more, 0.008% or more, 0.020% or more, 0.035% or more, 0.055% or more, or 0.060% or more. If the Sn content is too high, slab cracking may occur during steel sheet manufacturing. For this reason, the Sn content should be 0.100% or less. Preferably, the Sn content may be 0.095% or less, or 0.090% or less.

[0045] (Se: 0~0.080%) Selenium (Se) has the effect of improving machinability, but it is not an essential element, and the Se content may be 0%. To obtain the above effect, the Se content may be 0.001% or more, 0.005% or more, 0.007% or more, 0.015% or more, 0.030% or more, 0.045% or more, or 0.055% or more. If the Se content is too high, the hot workability and corrosion resistance may decrease. For this reason, the Se content should be 0.080% or less. Preferably, the Se content may be 0.075% or less, or 0.070% or less.

[0046] (W: 0~0.50%) Tungsten (W) is an element that improves high-temperature strength and corrosion resistance, but it is not an essential element, and the W content may be 0%. To obtain the above effects, the W content may be 0.001% or more, 0.01% or more, 0.02% or more, 0.03% or more, or 0.05% or more. If the W content is too high, the toughness may decrease. For this reason, the W content should be 0.50% or less. Preferably, the W content may be 0.45% or less, 0.40% or less, 0.30% or less, 0.20% or less, or 0.15% or less.

[0047] (Ta: 0~0.50%) While tantalum (Ta) has the effect of improving high-temperature strength, it is not a particularly essential element, and the Ta content may be 0%. To obtain the above effect, the Ta content may be 0.001% or more, 0.005% or more, 0.01% or more, 0.05% or more, 0.10% or more, 0.15% or more, 0.25% or more, or 0.30% or more. If the Ta content is too high, the toughness may decrease. For this reason, the Ta content should be 0.50% or less. Preferably, the Ta content may be 0.49% or less, 0.48% or less, 0.47% or less, 0.46% or less, or 0.45% or less.

[0048] (Hf: 0~0.100%) Hf (hafnium) has the effect of improving corrosion resistance, intergranular corrosion resistance, high-temperature strength, and oxidation resistance, but it is not an essential element, and the Hf content may be 0%. To obtain the above effects, the Hf content may be 0.001% or more, 0.005% or more, 0.010% or more, 0.020% or more, 0.040% or more, 0.060% or more, or 0.070% or more. If the Hf content is too high, the toughness may decrease. For this reason, the Hf content should be 0.100% or less. Preferably, the Hf content may be 0.099% or less, 0.098% or less, or 0.090% or less.

[0049] (Zr: 0~0.100%) Zr (zirconium) is an effective element for deoxidation and desulfurization, and also improves hot workability, but it is not an essential element, and the Zr content may be 0%. To obtain the above effects, the Zr content may be 0.001% or more, 0.002% or more, 0.003% or more, 0.004% or more, or 0.005% or more. If the Zr content is too high, the hot workability may deteriorate. For this reason, the Zr content should be 0.100% or less. Preferably, the Zr content may be 0.090% or less, 0.080% or less, 0.060% or less, 0.040% or less, or 0.030% or less.

[0050] (Bi: 0~0.300%) Bi (bismuth) has the effect of improving machinability, but it is not an essential element, and the Bi content may be 0%. To obtain the above effect, the Bi content may be 0.001% or more, 0.010% or more, 0.020% or more, 0.030% or more, 0.050% or more, 0.100% or more, 0.150% or more, or 0.190% or more. If the Bi content is too high, the hot workability may decrease. For this reason, the Bi content should be 0.300% or less. Preferably, the Bi content may be 0.295% or less, 0.290% or less, 0.285% or less, or 0.280% or less.

[0051] (Pb: 0~0.100%) While lead (Pb) has the effect of improving machinability, it is not a particularly essential element, and the Pb content may be 0%. To obtain the above effect, the Pb content may be 0.001% or more, 0.010% or more, 0.020% or more, 0.035% or more, 0.055% or more, or 0.065% or more. If the Pb content is high, it may lower the melting point of the grain boundaries and reduce the bonding force of the grain boundaries, potentially reducing hot workability, such as liquefaction cracking due to grain boundary melting. For this reason, the Pb content should be 0.100% or less. Preferably, the Pb content may be 0.097% or less, 0.095% or less, or 0.090% or less.

[0052] (REM: 0~0.200%) Rare earth elements (REMs) are effective in improving oxidation resistance and high-temperature wear resistance, but they are not particularly essential, and the REM content may be 0%. To obtain the above effects, the REM content may be 0.001% or more, 0.005% or more, 0.0010% or more, 0.020% or more, 0.025% or more, 0.030% or more, or 0.0050% or more. If the REM content is too high, hot workability and weldability may be impaired. For this reason, the REM content should be 0.200% or less. Preferably, the REM content may be 0.185% or less, 0.170% or less, 0.0150% or less, 0.130% or less, 0.100% or less, or 0.090% or less.

[0053] REM refers to a total of 17 elements, including Sc, Y, and lanthanide elements, and the REM content mentioned above refers to the total content of these elements. Industrially, REM is often added in the form of mischmetal.

[0054] In the chemical composition of this embodiment, the remainder is Fe and impurities. Here, "impurities" refers to components that are mixed in during the industrial production of austenitic stainless steel sheets due to various factors in the raw materials such as ore and scrap, and the manufacturing process, and which are acceptable as long as they do not adversely affect this embodiment.

[0055] <Relationship between the volume fraction of the δ-ferrite phase and the average grain size of the austenite phase> In this embodiment, the austenitic stainless steel sheet has a volume fraction of the δ-ferrite phase in the center of the sheet thickness (hereinafter sometimes referred to as "δ amount") of 1.00% or less, and the average grain size of the austenite phase (hereinafter sometimes simply referred to as "average grain size") satisfies the following equation 1.

[0056] Average grain size (μm) of the austenite phase ≥ -40 × δ ferrite phase volume fraction (%) + 80 ····· Equation 1

[0057] (Volume fraction of δ-ferrite phase) The δ-ferrite phase tends to form towards the center of the slab's thickness during solidification, and although it is gradually reduced through heating and hot rolling, it tends to remain more towards the center of the product's thickness. The more δ-ferrite phase there is, the worse the creep rupture life of the austenitic stainless steel sheet at high temperatures becomes. Therefore, in this embodiment, the volume fraction of the δ-ferrite phase at the center of the steel sheet's thickness is set to 1.00% or less. The volume fraction of the δ-ferrite phase at the center of the steel sheet's thickness is preferably 0.95% or less, 0.90% or less, 0.85% or less, 0.80% or less, 0.75% or less, 0.70% or less, 0.65% or less, 0.60% or less, 0.55% or less, or 0.50% or less.

[0058] The volume fraction of the δ-ferrite phase at the center of the plate thickness is measured using the following procedure. A ferrite meter is used as the measuring device. A ferrite meter measures the volume fraction of the ferrite phase near the measurement point using a magnetic induction method, and can directly measure the volume fraction of the δ-ferrite phase. An example of a ferrite meter is the Fischer MP30 manufactured by Fischer Instruments. The measurement is taken at the center of the plate thickness (a distance of 3 / 8 to 5 / 8 of the plate thickness from the surface of the steel plate) in a cross section perpendicular to the surface of the steel plate and perpendicular to the rolling direction (hereinafter referred to as the plate width cross section). At that time, to avoid overlapping measurement positions, the volume fraction of the δ-ferrite phase is measured at a total of 60 points, with 20 points each at 3 / 8, 4 / 8, and 5 / 8 of the plate thickness from the surface of the steel plate, separated by at least 5 mm in the direction perpendicular to the rolling direction (hereinafter referred to as the plate width direction). The average value (arithmetic mean) of the 60 measured values ​​is taken as the volume fraction of the δ-ferrite phase. However, if the plate thickness is less than 8 mm, measurements are taken at 20 points only, at a position 4 / 8 of the plate thickness from the surface of the steel plate, and the average value is obtained. Note that if no δ-ferrite phase is detected, the volume fraction of the δ-ferrite phase is considered to be 0.

[0059] (Average grain size of the austenite phase) The grain size is determined in accordance with JIS G0551:2023, and the grain size is converted from the grain size measured by the comparative method. The measurement is performed by electrolytic etching of the cross-section of the plate width with nitric acid, and observing five fields of view under a microscope in accordance with JIS G 0551:2023 at a position half the thickness from the surface of the steel plate, measuring the grain size in 0.5 increments, and calculating the average grain size number G of the five fields of view. At the level of this invention, the presence of the δ-ferrite phase is minute and is therefore ignored during measurement. Next, the average grain size G is converted to the average grain size [μm] using the following conversion formula obtained from the correspondence table between grain size and grain size described in JIS.

[0060]

number

[0061] <Average grain size of the austenite phase ≥ -40 × volume fraction of the δ-ferrite phase + 80> The inventors have found that if the average grain size of the austenite phase is greater than or equal to the value obtained by a function (Equation 1) with respect to the volume fraction of the δ-ferrite phase, then an excellent creep rupture life at high temperatures can be obtained. In other words, an excellent creep rupture life can be obtained not simply by reducing the amount of δ-ferrite phase and increasing the grain size, but by increasing the grain size to a value greater than the value obtained by Equation 1. For this reason, the austenitic stainless steel sheet of this embodiment has a volume fraction of δ-ferrite phase at the center of the sheet thickness of 1.00% or less, and satisfies the following Equation 1.

[0062] The average grain size of the austenite phase (μm) ≥ -40 × δ ferrite phase volume fraction (%) + 80 ... Equation 1

[0063] <Creep rupture life at high temperatures> In recent years, there has been a trend towards pursuing improved energy efficiency, leading to increased use of austenitic stainless steel at higher temperatures, and there is a demand for improved creep rupture life at the 900°C level. Therefore, this invention evaluates austenitic stainless steel sheets based on their creep rupture life at high temperatures (900°C). The austenitic stainless steel sheet of this embodiment achieves excellent creep rupture life by sufficiently reducing the volume fraction (δ amount) of the δ ferrite phase and promoting grain growth, thereby satisfying Equation 1.

[0064] The creep rupture life at high temperatures is measured according to the test method compliant with JIS Z 2271:2019. The test specimen is taken from the width direction of the steel plate, with the center located at half the thickness from the surface of the plate. Using the obtained test specimen, a constant load creep rupture test is performed in a creep testing machine equipped with a heating furnace at a stress of 35 MPa and a temperature of 900°C, and the time required until rupture is defined as the creep rupture life.

[0065] <Manufacturing method> The following describes an example of a method for manufacturing an austenitic stainless steel sheet according to this embodiment of the present invention. In this specification, "heating" means maintaining a temperature range for soaking, and "heating temperature" and "heating time" mean soaking temperature and soaking time, respectively.

[0066] The austenitic stainless steel sheet of this embodiment is, for example, A slab having the aforementioned chemical composition is heated to a temperature range of 1100-1300°C, followed by a primary hot rolling process. A soaking heat treatment process is performed in which the slab, which has undergone the primary hot rolling process, is heated at a temperature range of 1100-1300°C for more than 3 hours. A secondary hot rolling process is performed in which the slab that has undergone a soaking heat treatment process is heated to a temperature range of 1100 to 1300°C, and then secondary hot rolling is performed to obtain a hot-rolled steel sheet, and The system includes a solution heat treatment step in which a hot-rolled steel sheet is heated in a temperature range of 1000 to 1300°C for a time equal to or greater than the minimum heating time (hours) determined by the following formula 2, It can be manufactured by a manufacturing method in which the cumulative slab heating time, which is the sum of the heating time in the primary hot rolling process, the heating time in the soaking heat treatment process, and the heating time in the secondary hot rolling process, is 4.0 hours or more.

[0067] Minimum heating time (hours) = 0.025 (hours / mm) × thickness of hot-rolled steel sheet (mm) ...Formula 2

[0068] (Primary hot rolling process) In the primary hot rolling process, a slab having the aforementioned chemical composition is melted and cast, and the resulting slab is heated in a furnace in the temperature range of 1100 to 1300°C. After extraction from the furnace, primary hot rolling is performed. If the heating temperature of the slab is below 1100°C, the volume fraction (δ amount) of the δ ferrite phase cannot be sufficiently reduced. Furthermore, the hot workability decreases, and defects and edge cracks are more likely to occur. For this reason, the heating temperature of the slab should be 1100°C or higher, preferably 1130°C or higher, 1160°C or higher, 1180°C or higher, or 1200°C or higher. On the other hand, if the heating temperature of the slab exceeds 1300°C, the heating temperature is too high, making it prone to surface roughness due to high-temperature oxidation and reducing yield. For this reason, the heating temperature of the slab should be 1300°C or lower, preferably 1280°C or lower, 1260°C or lower, or 1250°C or lower. After primary hot rolling, the slab may be cooled to room temperature. The cumulative reduction ratio of primary hot rolling is not particularly limited, but may be between 30% and 70%.

[0069] (Soaking heat treatment process) In the soaking heat treatment process, the slab after primary hot rolling is heated at a temperature range of 1100 to 1300°C for 3 hours or more. If the heating temperature of the slab is too low, the effect of reducing the δ-ferrite phase will be small. For this reason, the heating temperature of the slab should be 1100°C or higher, preferably 1130°C or higher, 1160°C or higher, 1180°C or higher, or 1200°C or higher. On the other hand, if the heating temperature of the slab exceeds 1300°C, the heating temperature is too high, making it easy for surface roughness due to high-temperature oxidation to occur and reducing the yield. For this reason, the heating temperature of the slab should be 1300°C or lower, preferably 1280°C or lower, 1260°C or lower, or 1250°C or lower. After the soaking heat treatment, the slab may be cooled to room temperature.

[0070] In this invention, the soaking heat treatment process can be effectively reduced by performing the soaking heat treatment process after the primary hot rolling process and before the secondary hot rolling process.

[0071] The reason for this is that the δ-ferrite phase expands after primary hot rolling. Specifically, the surface area of ​​the δ-ferrite phase / austenite phase interface increases, which promotes the dissolution of the alloy components of the δ-ferrite phase into the austenite phase. In addition, after primary hot rolling, the diffusion distance of the alloy components in the thickness direction during dissolution is shortened, and the concentration gradient of the alloy components in the thickness direction within the austenite phase also increases.

[0072] (Secondary hot rolling process) The secondary hot rolling process is a process for breaking down the slab's structure through hot rolling and shaping it to the predetermined product dimensions. After primary hot rolling and soaking heat treatment, the slab is placed in a heating furnace and heated in the temperature range of 1100-1300°C. After extraction from the furnace, secondary hot rolling is performed. If the heating temperature of the slab is below 1100°C, the δ-ferrite phase cannot be sufficiently reduced. Furthermore, hot workability decreases, and defects and edge cracks are more likely to occur. For this reason, the heating temperature of the slab should be 1100°C or higher, preferably 1130°C or higher, 1160°C or higher, 1180°C or higher, or 1200°C or higher. On the other hand, if the heating temperature of the slab exceeds 1300°C, the heating temperature is too high, making it prone to surface roughness due to high-temperature oxidation and reducing yield. Therefore, the heating temperature of the slab should be 1300°C or lower, preferably 1280°C or lower, 1260°C or lower, or 1250°C or lower. After secondary hot rolling, the slab may be cooled to room temperature. The cumulative reduction ratio of secondary hot rolling is not particularly limited, but may be between 50% and 95%.

[0073] (Cumulative slab heating time) In the slab heating before primary hot rolling, slab heating before secondary hot rolling, and slab heating during soaking heat treatment described above, the amount of δ is controlled by the cumulative slab heating time. If the cumulative slab heating time is too short, the effect of reducing the δ ferrite phase cannot be sufficiently obtained. For this reason, the cumulative slab heating time should be 4.0 hours or more. The cumulative slab heating time may preferably be 4.2 hours or more, 4.5 hours or more, 4.8 hours or more, or 5.0 hours or more. There is no upper limit specified for the cumulative slab heating time. However, if the cumulative slab heating time is too long, the effect will saturate, so the cumulative slab heating time may preferably be 24.0 hours or less, 20.0 hours or less, 18.0 hours or less, or 15.0 hours or less.

[0074] Furthermore, the rolling line may have, for example, a 2Hi roughing mill and a 4Hi finishing mill installed on the same line, and the steel sheet is rolled as it passes between the rolls of each of these two mills. Primary hot rolling and secondary hot rolling may be performed using the same mill or different mills.

[0075] (Solid solution heat treatment process) Following secondary hot rolling, solution heat treatment is performed. The greater the plate thickness, the more difficult it becomes for the temperature at the center of the plate to reach the target temperature, and the less likely crystal grains are to grow at the center of the plate. In addition, the greater the plate thickness during solution heat treatment, the greater the amount of δ-ferrite phase at the center of the plate, making it more susceptible to the suppression of crystal grain growth by the δ-ferrite phase. For these reasons, it is effective to adjust the heating time of the solution heat treatment for crystal grain growth so that the heating time increases as the plate thickness increases. The inventors have found that by setting the heating time according to the plate thickness, it is possible to grow crystal grains at the center of the plate even when the plate thickness is large. Specifically, they derived a minimum heating time of 0.025 hours / mm per plate thickness, which allowed them to set the optimal heating time.

[0076] In the solution heat treatment process, the hot-rolled steel sheet obtained by secondary hot rolling is subjected to solution heat treatment by heating it in the temperature range of 1000 to 1300°C for a time equal to or greater than the minimum heating time calculated by the following formula 2, and then cooled to room temperature. This heat treatment allows for significant growth of the grain size. If the heating temperature is below 1000°C, grain growth will be slow, requiring longer heating times. Therefore, the heating temperature of the hot-rolled steel sheet should be 1000°C or higher, preferably 1030°C or higher, 1060°C or higher, 1080°C or higher, or 1100°C or higher. On the other hand, if the heating temperature of the hot-rolled steel sheet exceeds 1300°C, the heating temperature is too high, making it prone to surface roughness due to high-temperature oxidation and reducing yield. Therefore, the heating temperature of the hot-rolled steel sheet should be 1300°C or lower, preferably 1280°C or lower, 1260°C or lower, or 1250°C or lower. Furthermore, if the heating time is less than the heating time calculated by Equation 2, the crystals will not grow sufficiently, and the grain size will be small. If the δ amount has not been sufficiently reduced by this step, grain growth will be suppressed, requiring longer heating times. Normally, solution heat treatment does not involve long heating times compared to slab heating or soaking heat treatment before hot rolling due to operational issues, so it is desirable to reduce the δ amount by the stage before solution heat treatment (secondary hot rolling step).

[0077] Minimum heating time (hours) = 0.025 (hours / mm) × thickness of hot-rolled steel sheet (mm) ...Formula 2 [Examples]

[0078] The austenitic stainless steel sheet according to the present invention will be specifically described below with reference to examples. The present invention is not limited to these examples.

[0079] <Example 1> Slabs having the chemical compositions shown in Steel No. A-Z, AA-AD, and a-d in Tables 1-4 (Note that Tables 1-4 are divisions of a single table) were manufactured. The obtained slabs were heated in a furnace at 1180°C for 1 hour, and after extraction from the furnace, primary hot rolling was performed. Next, the slabs after primary hot rolling were subjected to soaking heat treatment by heating at 1250°C for 8 hours. Then, the slabs were heated in a furnace at 1180°C for 1 hour and subjected to secondary hot rolling to finish as hot-rolled steel sheets with a thickness of 25 mm. The cumulative slab heating time, which is the sum of the heating time in the primary hot rolling process (1 hour), the heating time in the soaking heat treatment process (8 hours), and the heating time in the secondary hot rolling process (1 hour), was 10 hours. Subsequently, solution heat treatment was performed by heating at 1120°C for 0.625 hours to obtain an austenitic stainless steel sheet.

[0080] The volume fraction of the δ-ferrite phase and the average grain size of the austenite phase were measured for the obtained austenitic stainless steel sheet using the method described above. The results are shown in Table 5. Here, "δ amount" in Table 5 indicates the volume fraction (%) of the δ-ferrite phase in the center of the steel sheet, "average grain size" indicates the average grain size (μm) of the austenite phase, and the "right-hand side of Equation 1" column indicates the value of -40 × volume fraction (%) of the δ-ferrite phase + 80, which is the right-hand side of Equation 1 described above. "Equation 1" was marked as "○ (good)" if it satisfied Equation 1 (average grain size (μm) of the austenite phase ≥ -40 × volume fraction (%) of the δ-ferrite phase + 80), and "× (unacceptable)" if it did not. In addition, "-" in Table 5 means that the grains were mixed, and therefore measurement was not possible using the measurement method described above.

[0081] Furthermore, a round bar creep test specimen with a gauge length of 30 mm was collected using the method described above. The diameter was set to 6 mmφ so that the parallel section would fall within the measurement range for the volume fraction of the δ-ferrite phase. In addition, a constant load creep rupture test was performed at a stress of 35 MPa and a temperature of 900°C, and the time required to rupture was measured. In this embodiment, a creep rupture life of 200 hours or more was considered to indicate excellent creep rupture life at high temperatures, and a creep rupture life of 250 hours or more was considered to indicate particularly excellent creep rupture life at high temperatures. In Table 5, the "Creep Rupture Life Evaluation" is rated as "◎ (Excellent)" if the creep rupture life is 250 hours or more, "○ (Good)" if it is 200 hours or more but less than 250 hours, and "× (Poor)" if it is less than 200 hours. In this embodiment, a "◎ (Excellent)" or "○ (Good)" evaluation of "Creep Rupture Life" indicates excellent creep rupture life at high temperatures. The evaluation results are shown in Table 5.

[0082] Steels No. A to Z and AA to AD have a volume fraction of δ-ferrite phase at the center of the steel sheet thickness of 1.00% or less, and a large average grain size of the austenite phase, satisfying Equation 1. As a result, austenitic stainless steel sheets with excellent creep rupture life at high temperatures were obtained. Furthermore, in examples with a Si content of 1.00% or less, austenitic stainless steel sheets with particularly excellent creep rupture life at high temperatures were obtained.

[0083] Steel No. a has a low Nb content, resulting in a shorter creep rupture life.

[0084] Steel No. b has a high Nb content, resulting in a mixed grain structure and a shortened creep rupture life.

[0085] Steel No. c has a low nitrogen content, resulting in a shorter creep rupture life.

[0086] Steel No. d has a high nitrogen content, resulting in a shorter creep rupture life.

[0087] [Table 1]

[0088] [Table 2]

[0089] [Table 3]

[0090] [Table 4]

[0091] [Table 5]

[0092] <Example 2> Austenitic stainless steel sheets were fabricated using steel slabs No. B, D, and G shown in Tables 1-4, under the manufacturing conditions shown in Table 6, with other manufacturing conditions being the same as in Example 1. In Table 6, time is denoted as "h". For the obtained austenitic stainless steel sheets, the volume fraction (δ amount) of the δ ferrite phase in the center of the sheet thickness, the average grain size of the austenite phase, and the creep rupture life of the steel sheet at a stress of 35 MPa and a temperature of 900°C were determined, as in Example 1. The evaluation results are shown in Table 7. In Table 7, "δ amount" refers to the volume fraction of the δ ferrite phase, and "average grain size" refers to the average grain size of the austenite phase. Furthermore, the "Creep Rupture Life Evaluation" in Table 7 was evaluated using the same criteria as in Example 1.

[0093] [Table 6]

[0094] [Table 7]

[0095] Examples of the invention in Tests No. 1, 8, 10, 16, and 19 that satisfy the requirements for the chemical composition and manufacturing method of the steel sheet of the present invention have a δ-ferrite phase volume fraction of 1.00% or less, and a large grain size that satisfies Equation 1. As a result, austenitic stainless steel sheets with excellent creep rupture life at high temperatures were obtained. Furthermore, in Tests No. 1, 8, and 10, where the Si content was 1.00% or less, austenitic stainless steel sheets with particularly excellent creep rupture life at high temperatures were obtained.

[0096] Tests No. 2, 7, and 9 are examples where soaking heat treatment was performed before primary hot rolling. As a result, a sufficient reduction in the δ-ferrite phase was not achieved, and the volume fraction of the δ-ferrite phase increased. Consequently, the creep rupture life was inferior.

[0097] Tests No. 3, 4, 5, and 6 were examples where soaking heat treatment was not performed and the cumulative slab heating time was short. As a result, a sufficient reduction effect of the δ-ferrite phase was not obtained, and the volume fraction of the δ-ferrite phase increased. Consequently, the creep rupture life was inferior.

[0098] Tests No. 11 and 12 are examples where the cumulative slab heating time was 4.0 hours or more, but soaking heat treatment was not performed. Test No. 15 is an example where the cumulative slab heating time was 4.0 hours or more, but neither soaking heat treatment nor secondary hot rolling was performed. As a result, a sufficient reduction effect of the δ-ferrite phase was not obtained, and the volume fraction of the δ-ferrite phase increased. Consequently, the creep rupture life was inferior.

[0099] Test No. 13 is an example where soaking heat treatment was performed before primary hot rolling, and both the heating time for soaking heat treatment and the cumulative slab heating time were short. As a result, a sufficient reduction effect on the δ-ferrite phase was not obtained, and the volume fraction of the δ-ferrite phase increased. Consequently, the creep rupture life was inferior.

[0100] Test No. 14 is an example where soaking heat treatment was performed after primary hot rolling and before secondary hot rolling, but the heating time for soaking heat treatment and the cumulative slab heating time were too short. As a result, a sufficient reduction effect on the δ-ferrite phase was not obtained, and the volume fraction of the δ-ferrite phase increased. Consequently, the creep rupture life was inferior.

[0101] Test No. 17 is an example where the heating time during solution heat treatment was too short. As a result, the crystal grains did not grow sufficiently, resulting in smaller grain sizes. Consequently, the creep rupture life was inferior. [Industrial applicability]

[0102] The austenitic stainless steel sheet of the present invention can be used in various plants such as the steel industry and thermal power generation, and can be used in structures used in high-temperature environments, such as heat-resistant retaining fittings, burner tubes, skid rollers, hoppers, cyclones, and pressure vessels.

Claims

1. The chemical composition is expressed in mass percent. C: 0.100% or less, Si: 2.00% or less, Mn: 2.50% or less, P: 0.0450% or less, S: 0.0300% or less, Ni: 9.00-13.00%, Cr: 19.00-24.00%, Mo: 0.001-1.00%, Nb: 0.005-0.100%, N: 0.100-0.300%, Al: 0.200% or less, B: 0.0001 to 0.0100%, Ti: 0-0.300%, Cu: 0 to 1.00%, V: 0 to 0.500%, Co: 0-1.000%, Ca: 0-0.1000%, Mg: 0 to 0.0050%, Sb: 0 to 0.200%, Sn: 0-0.100%, Se: 0 to 0.080%, W: 0 to 0.50%, Ta: 0 to 0.50%, Hf: 0-0.100%, Zr: 0 to 0.100%, Bi: 0-0.300%, Pb: 0 to 0.100%, REM: 0-0.200%, Remainder: Fe and impurities And, The volume fraction of the δ-ferrite phase in the center of the plate thickness is 1.00% or less. The average grain size (μm) of the austenite phase satisfies Equation 1. Austenitic stainless steel sheet characterized by the following features. The average grain size of the austenite phase (μm) ≥ -40 × δ volume fraction of the ferrite phase (%) + 80 .....Equation 1

2. The aforementioned chemical composition is, in mass%, Ti: 0.001 to 0.300%, Cu: 0.01 to 1.00%, V: 0.010 to 0.500%, Co: 0.010 to 1.000%, Ca: 0.0010-0.0100%, Mg: 0.0001 to 0.0050%, Sb: 0.020-0.200%, Sn: 0.001 to 0.100%, Se: 0.005-0.080%, W: 0.01-0.50%, Ta: 0.005-0.50%, Hf: 0.010-0.100%, Zr: 0.001 to 0.100%, Bi: 0.030-0.300%, Pb: 0.010–0.100%, and REM: 0.005-0.200% An austenitic stainless steel sheet according to claim 1, comprising one or more selected from the following.

3. A method for manufacturing an austenitic stainless steel sheet according to claim 1, The chemical composition is expressed in mass percent. C: 0.100% or less, Si: 2.00% or less, Mn: 2.50% or less, P: 0.0450% or less, S: 0.0300% or less, Ni: 9.00-13.00%, Cr: 19.00-24.00%, Mo: 0.001-1.00%, Nb: 0.005-0.100%, N: 0.100-0.300%, Al: 0.200% or less, B: 0.0001 to 0.0100%, Ti: 0-0.300%, Cu: 0 to 1.00%, V: 0 to 0.500%, Co: 0-1.000%, Ca: 0-0.1000%, Mg: 0 to 0.0050%, Sb: 0 to 0.200%, Sn: 0-0.100%, Se: 0 to 0.080%, W: 0 to 0.50%, Ta: 0 to 0.50%, Hf: 0-0.100%, Zr: 0 to 0.100%, Bi: 0-0.300%, Pb: 0 to 0.100%, REM: 0-0.200%, Remainder: Fe and impurities The slab is heated to a temperature range of 1100 to 1300°C, and then subjected to a primary hot rolling process. A soaking heat treatment step is performed in which the slab that has undergone the primary hot rolling step is heated at a temperature range of 1100 to 1300°C for 3 hours or more. A secondary hot rolling step is performed to obtain a hot-rolled steel sheet by heating the slab that has undergone the soaking heat treatment step to a temperature range of 1100 to 1300°C, and then performing secondary hot rolling. Solution heat treatment process: The hot-rolled steel sheet is heated in a temperature range of 1000 to 1300°C for a time equal to or greater than the minimum heating time (hours) determined by the following formula 2 to perform a solution heat treatment. Equipped with, A method for manufacturing an austenitic stainless steel sheet, characterized in that the cumulative slab heating time, which is the sum of the heating time in the primary hot rolling step, the heating time in the soaking heat treatment step, and the heating time in the secondary hot rolling step, is 4.0 hours or more. Minimum heating time (hours) = 0.025 (hours / mm) × thickness of hot-rolled steel sheet (mm) ...Formula 2