Austenitic stainless steel materials, steel plates, and steel pipes, and methods for manufacturing the same.

Abstract

To provide an austenitic stainless steel with a reduced content of expensive Ni and improved fatigue resistance in a hydrogen environment.SOLUTION: An austenitic stainless steel has a predetermined chemical composition. The f value is more than 29.5 and less than 32.5. In a cross section parallel with a process direction and a thickness direction, when inclusions with a long side of 5 μm or more are termed "coarse inclusions", the number of coarse inclusions is 10 or less per 0.05 mm2, and the number of coarse inclusions that include 30 mol% or more of CaO constitutes 50% or more of the total number of coarse inclusions.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to austenitic stainless steel materials, steel plates, and steel pipes, as well as methods for manufacturing the same. [Background technology]

[0002] In recent years, hydrogen energy has attracted attention as a clean energy source that does not emit greenhouse gases such as carbon dioxide. To utilize hydrogen energy, the establishment of hydrogen-related technologies, such as hydrogen production, storage, and transportation, is required.

[0003] On the other hand, there are various problems in establishing hydrogen-related technologies. One of these is the problem of hydrogen embrittlement, in which the strength and ductility of materials decrease in a hydrogen environment. Austenitic stainless steel is one material used in a hydrogen environment, and for example, Patent Documents 1 and 2 disclose austenitic stainless steel with improved resistance to hydrogen embrittlement.

[0004] Furthermore, in a hydrogen environment, the components used are subjected to a gas pressure above a certain level, i.e., continuous stress. Repeated stress can lead to fatigue fracture, even at stress levels lower than those normally causing fracture. Therefore, with the intention of using these components, Patent Documents 3 and 4 disclose austenitic stainless steel that improves not only hydrogen embrittlement resistance but also fatigue resistance in a hydrogen environment. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2019-143228 [Patent Document 2] Japanese Patent Publication No. 2021-109998 [Patent Document 3] International Publication No. 2016 / 68009 [Patent Document 4] Japanese Patent Publication No. 2019-194357 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] In the technologies disclosed in Patent Documents 1 and 2, the fatigue resistance properties of austenitic stainless steel under hydrogen conditions have not been investigated. Therefore, there is room for further improvement in the fatigue resistance properties of the austenitic stainless steel disclosed in the above documents under hydrogen conditions. Furthermore, while the technologies disclosed in Patent Documents 3 and 4 have investigated the fatigue resistance properties of austenitic stainless steel under hydrogen conditions, they contain a large amount of expensive Ni, so there is room for further improvement in terms of alloy cost.

[0007] Based on the above, the present invention aims to solve the above problems and provide an austenitic stainless steel material that improves fatigue resistance in a hydrogen environment while reducing the amount of expensive Ni. [Means for solving the problem]

[0008] This invention was made to solve the above-mentioned problems, and its gist is the austenitic stainless steel material, steel plate, and steel pipe described below, as well as a method for manufacturing them.

[0009] (1) The chemical composition is expressed in mass%, C: 0.1% or less, Si: 2.0% or less, Mn: 6.0~12.0%, P:0.030% or less, S: 0.003% or less, Cr: 13.0~18.0%, Ni: 5.0-9.0%, N: 0.15~0.25%, Al: 0.005~0.080%, Ca: 0.0005~0.01%, B: 0.0001~0.01%, Ga: 0.0010 to 0.020%, Cu: less than 1.0%, Mo: less than 2.0%, Nb: 0 to 0.5%, Ti: 0 to 0.5%, V: 0 to 0.50%, W: 0 to 0.50%, Zr: 0 to 0.50%, Co: 0 to 0.50%, Mg: 0 to 0.005%, Hf: 0 to 0.10%, REM: 0 to 0.1%, The balance: Fe and impurities, The f value calculated by the following formula (i) is greater than 29.5 and less than 32.5, In a cross-section parallel to the processing direction and the thickness direction, When inclusions with a long side of 5 μm or more are defined as coarse inclusions, the number of the coarse inclusions is 10 or less per 0,05 mm , , ,

[0011] , , ,

[0010] , , ,

[0012] , , and The ratio of the number of coarse inclusions containing 30 mol% or more of CaO to the number of the coarse inclusions is 50% or more. An austenitic stainless steel material. f value = Ni + 0.72Cr + 0.88Mo + 1.11Mn - 0.27Si + 0.53Cu + 12.93C + 7.55N - 1.1Nb - 2.2Ti ···(i) <00000八十九>However, each element symbol in the above formula (i) represents the content (mass%) of each element contained in the steel, and is zero if not contained.

[0010] (2) In the cross-section, The negative segregation degree of Ni is 0.80 or more, The negative segregation degree of Mn is 0.80 or more. The austenitic stainless steel material according to (1) above.

[0011] (3) A steel plate. The austenitic stainless steel material according to (1) or (2) above.

[0012] (4) A steel pipe. The austenitic stainless steel material according to (1) or (2) above.

[0013] (5) A manufacturing method for producing an austenitic stainless steel material as described in (1) or (2) above, A cold working process is performed to obtain a cold-worked material by cold working a hot-worked material having the chemical composition described in (1) above. A method for manufacturing austenitic stainless steel, comprising a bright annealing step in which bright annealing is performed at a temperature range of 950 to 1150°C with a dew point of -45°C or lower.

[0014] (6) A manufacturing method for producing the austenitic stainless steel material described in (3) above, A method for producing an austenitic stainless steel material according to (5) above, wherein cold rolling is performed in the cold working step.

[0015] (7) A manufacturing method for producing the austenitic stainless steel material described in (4) above, A method for manufacturing an austenitic stainless steel material as described in (5) above, wherein drawing is performed in the cold working step.

[0016] (8) A method for producing the austenitic stainless steel material described in (4) above, A method for producing an austenitic stainless steel material as described in (7) above, wherein the drawing process is performed two or more times. [Effects of the Invention]

[0017] According to the present invention, it is possible to obtain an austenitic stainless steel material that has improved fatigue resistance in a hydrogen environment while reducing the amount of expensive Ni. [Brief explanation of the drawing]

[0018] [Figure 1] Figure 1 shows the observation locations for inclusions and the sampling locations for steel plates. [Figure 2] Figure 2 shows the observation locations for inclusions and the sampling locations for samples taken in the case of steel pipes. [Modes for carrying out the invention]

[0019] The inventors of the present invention investigated an austenitic stainless steel material that improves fatigue resistance in a hydrogen environment (hereinafter also simply referred to as "fatigue resistance") while reducing the amount of expensive Ni, and obtained the following findings (a) to (c).

[0020] (a) When repeated stress is applied and fracture occurs due to fatigue, inclusions often serve as the starting point for fracture. Therefore, it is desirable to reduce the number of inclusions that form. Thus, it is effective to include trace amounts of Ga and Ca, which suppress the formation of inclusions.

[0021] (b) Furthermore, if the inclusions are large or hard, they are more likely to become the starting point for fracture. For this reason, it is effective to suppress the formation of coarse inclusions and to actively form soft CaO as inclusions. In normal manufacturing processes, hard inclusions such as Al2O3 and MgO tend to form. The inventors have shown that bright annealing performed under predetermined conditions is effective in reducing hard Al2O3 and MgO while leaving behind CaO, which is difficult to reduce.

[0022] (c) Furthermore, since large segregation of Ni and Mn can also be a starting point for fracture, it is desirable to reduce the negative segregation of these elements to 0.80 or higher and eliminate segregation as much as possible. In order to reduce the negative segregation of Ni and Mn to 0.80 or higher, it is desirable to perform cold working multiple times, followed by bright annealing in the final step.

[0023] One embodiment of this invention is based on the above findings. The requirements of this embodiment will be described in detail below.

[0024] 1.Chemical composition The reasons for the limitations on each element are as follows. Note that in the following explanation, "%" for content refers to "mass%".

[0025] C: 0.1% or less Carbon (C) is an effective element for stabilizing the austenite phase and improving hydrogen embrittlement resistance. However, if carbon is included in excess, excessive chromium-based carbides precipitate, reducing fatigue resistance. For this reason, the carbon content should be 0.1% or less. Preferably, the carbon content should be 0.08% or less, and more preferably 0.07% or less. On the other hand, to obtain the above effects, it is preferable that the carbon content be 0.01% or more.

[0026] Si:2.0% or less Si is an effective element for deoxidation and also contributes to improved resistance to hydrogen embrittlement. However, excessive Si content promotes the formation of hard inclusions such as SiO2 and intermetallic compounds such as the sigma phase, leading to a decrease in fatigue resistance. For this reason, the Si content should be 2.0% or less. Preferably, the Si content should be 1.0% or less. On the other hand, to obtain the above effects, it is preferable that the Si content be 0.5% or more.

[0027] Mn: 6.0~12.0% Mn is an effective element for stabilizing the austenite phase and contributes to improving hydrogen embrittlement resistance and fatigue resistance. Furthermore, by increasing the solid solubility limit of N, it indirectly contributes to saving expensive Ni. Therefore, the Mn content should be 6.0% or higher. Preferably, the Mn content is 8.0% or higher, and more preferably 9.0% or higher. However, excessive Mn content promotes the formation of the ε phase, which is highly susceptible to hydrogen embrittlement, thus reducing hydrogen embrittlement resistance. In addition, it promotes the formation of inclusions, reducing fatigue resistance. Therefore, the Mn content should be 12.0% or less, and preferably 10% or less.

[0028] P:0.030% or less P is an element present in steel as an impurity, but it forms phosphides, which reduce fatigue resistance. For this reason, the P content should be 0.030% or less. Preferably, the P content should be 0.025% or less, and more preferably 0.015% or less. On the other hand, excessively reducing the P content increases manufacturing costs. For this reason, it is preferable that the P content be 0.005% or more.

[0029] S: 0.003% or less S is an element present in steel as an impurity, and it reduces corrosion resistance, especially weather resistance. Therefore, the S content should be 0.003% or less. Preferably, the S content should be 0.002% or less, and more preferably 0.001% or less. However, excessively reducing the S content increases manufacturing costs. Therefore, it is preferable that the S content be 0.0001% or more.

[0030] Cr: 13.0~18.0% Cr is an element present in a certain amount in stainless steel and has the effect of improving corrosion resistance, especially weather resistance. For this reason, the Cr content should be 13.0% or more. Preferably, the Cr content should be 15.0% or more. However, Cr is a ferrite-forming element. Therefore, if Cr is present in excess, it destabilizes the austenite phase and reduces hydrogen embrittlement resistance. It also promotes the formation of hard MnCr2O4 inclusions and reduces fatigue resistance. For this reason, the Cr content should be 18.0% or less. Preferably, the Cr content should be 17.0% or less, and more preferably 16.0% or less.

[0031] Ni: 5.0~9.0% Ni is an essential element for ensuring hydrogen embrittlement resistance and fatigue resistance. Therefore, the Ni content should be 5.0% or more. Preferably, the Ni content should be 7.0% or more. However, excessive Ni content increases alloy costs. Therefore, the Ni content should be 9.0% or less. Preferably, the Ni content should be 8.5% or less, and more preferably 8.0% or less.

[0032] N: 0.15~0.25% N is an effective element for improving hydrogen embrittlement resistance. For this reason, the N content should be 0.15% or more. However, excessive N content promotes the formation of AlN, reducing fatigue resistance. It can also cause internal defects such as blowholes during welding, reducing the manufacturability of welded steel pipes. For this reason, the N content should be 0.25% or less. Preferably, the N content should be 0.22% or less, and more preferably 0.20% or less.

[0033] Al: 0.005~0.080% Al is an effective deoxidizing element and has the effect of improving fatigue resistance. For this reason, the Al content should be 0.005% or more. Preferably, the Al content should be 0.010% or more. However, if Al is included in excess, hard Al2O3 inclusions and AlN are more likely to form, and the fatigue resistance will decrease. For this reason, the Al content should be 0.080% or less. Preferably, the Al content should be 0.060% or less.

[0034] Ca: 0.0005~0.01% Ca has a deoxidizing effect and reduces the O content, thereby suppressing the formation of inclusions and improving fatigue resistance. It also has the effect of promoting the formation of CaO, which is effective in improving fatigue resistance. For this reason, the Ca content should be 0.0005% or more. Preferably, the Ca content should be 0.0020% or more, and preferably 0.0025% or more. However, if Ca is included in excess, coarse CaO inclusions will be excessively formed, which will actually decrease fatigue resistance. For this reason, the Ca content should be 0.01% or less. Preferably, the Ca content should be 0.0050% or less.

[0035] B: 0.0001~0.01% B has the effect of improving fatigue resistance by reinforcing grain boundaries, thereby suppressing the propagation of fracture at grain boundaries. For this reason, the B content should be 0.0001% or more. Preferably, the B content should be 0.0003% or more, and preferably 0.0005% or more. However, if B is included in excess, not only will its effect saturate, but it will also promote the precipitation of boron compounds (BN, BC, Cr2B) at grain boundaries, thus actually reducing fatigue resistance. For this reason, the B content should be 0.01% or less. Preferably, the B content should be 0.0050% or less.

[0036] Furthermore, it is preferable that the total content of Ca and B be 0.005% or less. This is because if the total content of Ca and B exceeds 0.005%, Ca and B will form compounds at the grain boundaries, making it difficult to obtain a sufficient improvement in fatigue resistance.

[0037] Ga: 0.0010~0.020% Ga is an important element in the steel material of this embodiment because it forms oxides and indirectly contributes to the formation of CaO, which is effective in improving fatigue resistance. For this reason, the Ga content should be 0.0010% or more. Preferably, the Ga content should be 0.0050% or more, and more preferably 0.0090% or more. However, excessive Ga content reduces manufacturability. For this reason, the Ga content should be 0.020% or less. Preferably, the Ga content should be 0.0110% or less.

[0038] Cu: Less than 1.0% Cu is an element that is introduced from raw materials such as scrap, and is effective in stabilizing the austenite phase and improving resistance to hydrogen embrittlement. On the other hand, Cu is a low-melting-point element, segregates at grain boundaries, promotes high-temperature cracking by P and S, and makes the material more prone to cracking. For this reason, the Cu content should be less than 1.0%. Preferably, the Cu content should be 0.5% or less. However, excessively reducing the Cu content leads to limitations on the raw materials that can be dissolved, increasing manufacturing costs. For this reason, it is preferable that the Cu content be 0.01% or more.

[0039] Mo: Less than 2.0% Mo is an element that is mixed in from raw materials such as scrap, and it has the effect of improving strength and corrosion resistance. On the other hand, if it is included in excess, it promotes the formation of the δ-ferrite phase and reduces resistance to hydrogen embrittlement. For this reason, the Mo content should be less than 2.0%. Preferably, the Mo content should be 0.5% or less. On the other hand, if the Mo content is reduced too much, it leads to restrictions on the raw materials that can be dissolved and increases manufacturing costs. For this reason, it is preferable that the Mo content be 0.01% or more.

[0040] In addition to the elements listed above, one or more elements selected from Nb, Ti, V, W, Zr, Co, Mg, Hf, and REM may be included within the ranges shown below. The reasons for limiting each element are explained below.

[0041] Nb: 0~0.5% Nb has the effect of forming carbonitrides, refining crystal grains, and strengthening grain boundaries. For this reason, it may be included as needed. However, if Nb is included in excess, carbonitrides will form and the fatigue resistance will decrease. For this reason, the Nb content should be 0.5% or less. Preferably, the Nb content should be 0.3% or less. On the other hand, in order to obtain the above effects, it is preferable that the Nb content be 0.01% or more.

[0042] Ti: 0~0.5% Ti forms carbonitrides, refines crystal grains, and strengthens grain boundaries. As a result, Ti suppresses crack formation during welding. Therefore, it may be included as needed. However, excessive Ti content leads to carbonitride formation and a decrease in fatigue resistance. For this reason, the Ti content should be 0.5% or less. Preferably, the Ti content should be 0.3% or less. On the other hand, to obtain the above effects, it is preferable that the Ti content be 0.01% or more.

[0043] V: 0~0.50% V precipitates in steel as a solid solution or carbonitride, improving its strength. For this reason, it may be included as needed. However, excessive V content leads to carbonitride formation and a decrease in fatigue resistance. Therefore, the V content should be 0.50% or less. Preferably, the V content should be 0.30% or less. On the other hand, to obtain the above effect, it is preferable that the V content be 0.01% or more.

[0044] W: 0~0.50% W has the effect of improving strength and corrosion resistance. For this reason, it may be included as needed. However, if W is included in excess, the manufacturing cost will increase. For this reason, the W content should be 0.50% or less. Preferably, the W content should be 0.30% or less. On the other hand, in order to obtain the above effect, it is preferable that the W content be 0.001% or more.

[0045] Zr: 0~0.50% Zr has a deoxidizing effect and improves weldability. Therefore, it may be included as needed. However, if Zr is included in excess, carbonitrides will be formed in excess, and the fatigue resistance will decrease. For this reason, the Zr content should be 0.50% or less. Preferably, the Zr content should be 0.30% or less. On the other hand, in order to obtain the above effects, it is preferable that the Zr content be 0.01% or more.

[0046] Co: 0~0.50% Co has the effect of improving corrosion resistance and stabilizing the austenite phase. For this reason, it may be included as needed. However, excessive co content increases manufacturing costs. Therefore, the co content should be 0.50% or less. Preferably, the co content should be 0.30% or less. On the other hand, to obtain the above effects, it is preferable that the co content be 0.01% or more.

[0047] Mg: 0~0.005% Mg is an effective element for deoxidation and improves the weldability of steel pipes. Therefore, it may be included as needed. However, if Mg is included in excess, excessive MgO inclusions will form, reducing fatigue resistance. For this reason, the Mg content should be 0.005% or less. Considering the balance between effectiveness and manufacturability, the Mg content should preferably be 0.002% or less. On the other hand, to obtain the above effects, the Mg content should preferably be 0.0001% or more.

[0048] Hf: 0~0.10% Hf has a deoxidizing effect and improves weldability. For this reason, it may be included as needed. However, if Hf is included in excess, inclusions will form excessively, and the fatigue resistance will decrease. For this reason, the Hf content should be 0.10% or less. Preferably, the Hf content should be 0.05% or less. On the other hand, in order to obtain the above effects, it is preferable that the Hf content be 0.01% or more.

[0049] REM: 0~0.1% REM has a deoxidizing effect and improves weldability. It also improves corrosion resistance. For this reason, it may be included as needed. However, if REM is included in excess, not only will its effect saturate, but excessive inclusions will form, and the fatigue resistance will decrease. For this reason, the REM content should be 0.1% or less. Preferably, the REM content should be 0.05% or less. On the other hand, in order to obtain the above effects, it is preferable that the REM content be 0.01% or more.

[0050] REM refers to the 17 elements totaling Sc, Y, and lanthanides, and the REM content mentioned above means the total content of these elements. Industrially, REM may be added in the form of mischmetal.

[0051] In the chemical composition of the steel sheet according to the present invention, the remainder is Fe and impurities. Here, "impurities" refers to components that are mixed in during the industrial production of austenitic stainless steel materials 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 the present invention.

[0052] f-number: In the austenitic stainless steel material according to this embodiment, the f value shown below is limited to a predetermined range as an indicator of the stability of the austenite phase. Specifically, the f value calculated by equation (i) below is set to be greater than 29.5 and less than 32.5.

[0053] f value=Ni+0.72Cr+0.88Mo+1.11Mn-0.27Si+0.53Cu+12.93C+7.55N-1.1Nb-2.2Ti...(i) However, each element symbol in formula (i) above represents the mass %) of each element contained in the steel, and zero is used if the element is not present.

[0054] Here, if the f-value is 29.5 or less, the stability of the austenite phase is low, and hydrogen embrittlement resistance and fatigue resistance decrease. For this reason, the f-value should be greater than 29.5. It is preferable that the f-value be 30.0 or higher. However, if the f-value is 32.5 or higher, the fatigue resistance actually decreases due to the increased alloying. In addition, raw material costs and manufacturing costs increase. For this reason, the f-value should be less than 32.5. From the viewpoint of manufacturability, weldability, and economics, it is preferable that the f-value be 31.5 or less.

[0055] 2. Coarse inclusions 2-1. Number of bulky inclusions In the austenitic stainless steel material of this embodiment, it is necessary to reduce the number of coarse inclusions in order to improve fatigue resistance. In other words, when inclusions with a long side of 5 μm or more in a cross section parallel to the processing direction and thickness direction (hereinafter also referred to as "L cross section") are defined as coarse inclusions, the number of coarse inclusions should be 0.05 mm. 2The number of pieces per item should be 10 or less. Note that the processing direction is synonymous with the rolling direction in the case of steel plates, and the drawing direction in the case of steel pipes. Furthermore, the thickness direction refers to the plate thickness direction in the case of steel plates, and the wall thickness direction in the case of steel pipes.

[0056] The number of coarse inclusions mentioned above is 0.05 mm. 2 When the number of coarse inclusions exceeds 10 per unit area, the fatigue resistance in a hydrogen environment decreases. Therefore, the number of coarse inclusions is 0.05 mm. 2 The number of inclusions per container should be 10 or less, preferably 7 or less. While there is no specific lower limit for the number of these coarse inclusions, it is preferable to reduce them as much as possible, with 0 being the most preferable.

[0057] Here, the above 0.05 mm 2 This section describes the method for measuring the number of coarse inclusions per unit area. A sample for observation is taken from an L-shaped cross-section of the steel material. From the perspective of observing the average metal structure, for example, in the case of a steel plate, an L-shaped cross-section near the center of the plate width is selected, as shown in Figure 1. The selected L-shaped cross-section includes five fields of view (indicated by black circles in Figure 1) in the thickness direction: the surface, 3t / 4, t / 2, t / 4, and the front and back surfaces. The sample is taken so that observation can be performed in these fields of view.

[0058] In the case of steel pipes, as shown in Figure 2, an L-shaped section is selected near the circumferential center of the weld. The selected L-shaped section includes five fields of view (indicated by black circles in Figure 2) in the thickness direction: the surface, 3t / 4, t / 2, t / 4, and the front and back surfaces. A sample is taken so that observation can be performed in these fields of view.

[0059] The collected sample is polished using an etching solution or the like to allow for clear observation. The sample, with an L-shaped cross-section, is then observed using a scanning electron microscope (SEM). For SEM settings during observation, it is preferable to set the acceleration voltage to 15kV. The observation field of view is 100μm × 100μm, and by observing five of these fields, the total observation area is 0.05mm. 2The surface is adjusted to achieve the following. Then, inclusions with a longest side of 5 μm or more are identified as coarse inclusions, and the number of coarse inclusions is counted within the total observation area. The longest side refers to the longest line when connecting two points on the outer circumference of the inclusion.

[0060] 2-2. Composition of Coarse Inclusions In the austenitic stainless steel material of this embodiment, it is necessary to form coarse inclusions containing a large amount of soft CaO. This is because hard inclusions such as Al2O3 and MgO, which are usually abundant in these inclusions, tend to become fracture initiation points and reduce fatigue resistance.

[0061] Therefore, in the L section, the ratio of coarse inclusions containing 30 mol% or more of CaO (hereinafter simply referred to as "CaO-based coarse inclusions") to the total number of coarse inclusions shall be 50% or more. If the ratio of CaO-based coarse inclusions is less than 50%, the inclusions become hard, and the fatigue resistance characteristics cannot be sufficiently improved.

[0062] Therefore, the ratio of CaO-based coarse inclusions to the total number of coarse inclusions should be 50% or more, preferably 60% or more, and more preferably 70% or more. There is no specific upper limit set for the ratio of CaO-based coarse inclusions, but it is usually 90% or less. Furthermore, in the following, the ratio of CaO-based coarse inclusions to the total number of coarse inclusions will also be simply referred to as the number ratio.

[0063] Here, we will explain the method for measuring the above-mentioned ratio of inclusions. For inclusions identified as coarse inclusions using the method described above, point analysis is performed at three points within the inclusion using EDX attached to the SEM. If the average value of the three points analyzed is the mol% ratio in terms of oxide, and CaO is 30 mol% or more, it is identified as a CaO-based coarse inclusion. The ratio (%) of inclusions identified as CaO-based coarse inclusions to all observed coarse inclusions is calculated. Note that the inclusions are oxides, and each inclusion is defined as one or more of MgO, Al2O3, MnO, Cr2O3, and CaO, or a composite oxide thereof, and other oxides, such as ZrO2, are not considered.

[0064] 3. Negative segregation degree If the segregation of Ni and Mn is large, the segregated areas are more likely to become fracture initiation points. As a result, the fatigue resistance is reduced. For this reason, in the austenitic stainless steel material of this embodiment, it is preferable that the negative segregation degree, which is an indicator of the degree of segregation of Ni and Mn, is within the following range. Specifically, in the L section, it is preferable that the negative segregation degree of Ni be 0.80 or higher, and that the negative segregation degree of Mn be 0.80 or higher. If the negative segregation degree of Ni is less than 0.80 in the L section, or if the negative segregation degree of Mn is less than 0.80, it means that coarse inclusions remain in the steel material, and the fatigue resistance cannot be sufficiently improved.

[0065] Therefore, it is preferable that the degree of negative segregation of Ni and the degree of negative segregation of Mn be 0.80 or higher in the L-section. Here, negative segregation refers to the distribution of an element at a lower concentration than the average concentration, and the degree of negative segregation refers to the degree of this distribution. For example, the degree of negative segregation of Ni is calculated as (Ni concentration in the negatively segregated Ni area) / (average Ni content of the steel), and the degree of negative segregation of Mn is calculated as (Mn concentration in the negatively segregated Mn area) / (average Mn content of the steel).

[0066] This section describes the method for measuring negative segregation. For negative segregation, the negative segregation of Ni and Mn is measured within a 250 μm × 250 μm observation field. The beam diameter is 1 μm, step size is 0.5 μm, acceleration voltage is 15 kV, and irradiation current is 2.0 × 10⁻¹⁰. -9 Under condition A, EPMA analysis is performed, and when the average value of the Ni or Mn concentration at all analysis points is set to 1, the ratio of Ni and Mn concentrations in the negative segregation area is defined as the degree of negative segregation. The sample for analysis is collected at the same location as when observing coarse inclusions.

[0067] 4. Types of steel In the austenitic stainless steel material of this embodiment, the type of steel material is not particularly limited. Examples include steel plates and steel pipes. Preferred applications include high-pressure hydrogen gas piping. In the case of steel pipes, the welded joint consists of weld metal and a heat-affected zone, and it is sufficient if this welded joint satisfies the above requirements.

[0068] In the steel pipe of this embodiment, it is preferable that the grain size at the center of the wall thickness of the welded joint be 5.0 or higher, and more preferably 6.0 or higher. This is because if the grain size is less than 5.0, the segregation of Ni and Mn may not be sufficiently resolved. The grain size should be determined by the cutting method in accordance with JIS G 0551:2013.

[0069] 5. Manufacturing method A preferred manufacturing method for the austenitic stainless steel material according to this embodiment will be described. The austenitic stainless steel material according to this embodiment can be stably manufactured by, for example, the following manufacturing method.

[0070] 5-1. Manufacturing method of steel plates 5-1-1. Casting Steel having the above chemical composition is melted and cast. Below, we will explain using the cases where the cast slab is made into a slab and then into a steel plate, and the case where the steel plate is manufactured into a steel pipe, as examples.

[0071] 5-1-2. Hot Rolling Process The obtained slab is hot-rolled to produce hot-rolled steel sheet. The heating temperature during hot rolling is not particularly limited, but is often set in the range of 1150 to 1250°C. Furthermore, the other conditions for hot rolling are not particularly limited, but for example, the hot-rolling temperature is often set in the range of 800 to 1200°C, the total reduction ratio is often set to more than 90%, and the coiling temperature is often adjusted to 900°C or lower. These conditions may be adjusted as appropriate, taking into account the composition of the slab and other conditions. Note that hot working includes hot rolling. Also, hot working material includes hot-rolled steel sheet.

[0072] 5-1-3. Hot-rolled sheet annealing The hot-rolled steel sheet obtained in the above hot-rolling process may be subjected to hot-rolled sheet annealing as needed. The temperature for hot-rolled sheet annealing is not particularly limited, but is often in the range of 1000 to 1200°C. Similarly, the annealing time for hot-rolled sheet annealing is not particularly limited, but is often in the range of 1 to 10 minutes, for example, from a manufacturing standpoint. Pickling may also be performed after hot-rolled sheet annealing to remove scale. In the case of materials other than steel sheets, for example, the hot-worked material described above may be annealed.

[0073] 5-1-4. Cold Rolling Process Next, the hot-rolled steel sheet (or, if necessary, the hot-rolled steel sheet that has undergone annealing) is cold-rolled to produce a cold-rolled steel sheet. The reduction ratio during cold rolling is not particularly limited. It is sufficient to crush and reduce coarse inclusions, and is often in the range of 30-60%. Here, cold rolling may be performed two or more times, with one roll at the above reduction ratio being considered as one roll. By performing cold rolling two or more times, coarse inclusions can be finely crushed and their number can be further reduced. In addition, segregation of Ni and Mn can be reduced and the homogenization of these elements can be promoted. In other words, the negative segregation degree of Ni and Mn can be made 0.80 or higher, improving fatigue resistance.

[0074] If cold rolling is performed two or more times, intermediate annealing may be performed between cold rolling cycles. The intermediate annealing between cold rolling cycles may be performed using either conventional annealing in an oxidizing atmosphere of combustion gases, or bright annealing in a non-oxidizing atmosphere, as described later. If conventional annealing is performed, pickling is recommended afterward. The annealing temperature and time for intermediate annealing are not particularly limited, but for example, the annealing temperature is often in the range of 950 to 1150°C, and the annealing time is often 1 minute or more but less than 1 hour. When performing bright annealing, it is desirable to use the atmosphere and dew point described later. After intermediate annealing, the material is cooled to room temperature and cold rolling is performed again. Note that cold working includes cold rolling. Cold-worked material includes cold-rolled steel sheets.

[0075] 5-1-5. Bright Annealing Next, the cold-rolled steel sheet, which has undergone the cold-rolling process, is subjected to bright annealing. Bright annealing is usually performed on materials where aesthetics are important, and is not typically done to improve fatigue resistance. However, in the steel material of this embodiment, bright annealing is performed to control inclusions.

[0076] Bright annealing is an annealing method performed in a non-oxidizing atmosphere using hydrogen gas or the like. During this process, hard inclusions such as Al2O3 and MgO, as well as composite inclusions formed by their combined formation, are reduced, while relatively soft CaO, which is effective in improving fatigue properties, remains. As a result, the proportion of CaO in the coarse inclusions increases, making it possible to achieve a number ratio of CaO-based coarse inclusions of 50% or more.

[0077] In the bright annealing process, a reducing atmosphere is created using hydrogen or a mixture of hydrogen and nitrogen gas, and the dew point during bright annealing is set to -45°C or lower. If the dew point is above -45°C, the aforementioned inclusions will not be sufficiently reduced, and the proportion of CaO in the coarse inclusions cannot be increased. For this reason, the dew point is set to -45°C or lower, and preferably to -50°C or lower.

[0078] Furthermore, the annealing temperature in the bright annealing process should be in the range of 950 to 1150°C. If the annealing temperature in the bright annealing process is below 950°C, annealing will not be sufficient, the desired metal structure cannot be obtained, and the hydrogen embrittlement resistance will decrease. In addition, segregation of Ni and Mn will not be sufficiently resolved, and the fatigue resistance will also decrease. For this reason, the annealing temperature in the bright annealing process should be 950°C or higher, preferably 980°C or higher, and more preferably 1000°C or higher.

[0079] On the other hand, if the annealing temperature in the bright annealing process exceeds 1150°C, the crystal grains become coarser, making the material more susceptible to hydrogen embrittlement and high-temperature cracking. For this reason, the annealing temperature in the bright annealing process should be 1150°C or lower, preferably 1120°C or lower. Furthermore, there are no particular limitations on the annealing time in bright annealing. It can be adjusted as needed, but the annealing time in bright annealing is usually in the range of 1 minute or more and less than 1 hour.

[0080] Bright annealing eliminates the need for subsequent pickling; therefore, when manufacturing steel sheets, bright annealing should be performed as the final step. After bright annealing, the sheet should be cooled to become an austenitic stainless steel sheet.

[0081] 5-2. Manufacturing method of steel pipes 5-2-1. Molding process Next, the method for manufacturing steel pipes will be explained. It is preferable to manufacture steel pipes using austenitic stainless steel sheets obtained through the process described in 5-1 above. The steel sheets are formed into a tubular shape. The forming method is not particularly limited, but it is common to use roll forming, which involves bending the sheets using rolls with various curvatures to form a tubular shape. The outer diameter of the steel pipe is not particularly limited, but it is generally in the range of 1 / 8 to 1 / 2 inch, or approximately 2 to 14 mm, in accordance with the ASTM A269 standard.

[0082] 5-2-2. Welding Process Next, it is preferable to weld the ends of the steel plate in the width direction, which have been formed into the shape of a pipe, to form a steel pipe. The welding method is not particularly limited, but for example, high-frequency electric resistance welding (also called "ERW"), inert gas arc welding (also called "TIG welding"), or laser welding may be used. Other welding conditions may be adjusted as appropriate. When pipes are formed by welding, welding discoloration occurs on the pipe, so it is preferable to perform pickling to remove the welding discoloration.

[0083] 5-2-3. Annealing process before drawing. Next, after the welding process, the welded pipes (hereinafter also referred to as "welded pipes") are preferably annealed in a temperature range of 950 to 1150°C as needed. If the annealing temperature of the welded pipes is less than 950°C, residual processing strain and segregation cannot be sufficiently eliminated. For this reason, it is preferable to anneal the welded pipes at 950°C or higher. On the other hand, if the annealing temperature of the welded pipes exceeds 1150°C, the yield decreases and the crystal grains do not form a uniform structure. For this reason, it is preferable to anneal the welded pipes at 1150°C or lower.

[0084] In the annealing process for welded pipes, annealing may be performed in normal air or an oxidizing atmosphere, but bright annealing as described above may also be performed. The conditions for bright annealing are the same as those for steel plates described above. After the above annealing, the pipes are cooled at an appropriate cooling rate to form austenitic stainless steel pipes. The annealing time is not particularly limited, but for example, it is usually in the range of 1 min to less than 1 hour. Furthermore, if the pre-drawing annealing process is carried out in combustion gas or an oxidizing atmosphere, it is preferable to perform pickling after the annealing process to remove the oxide film formed during annealing along with the weld burn.

[0085] 5-2-4.Drawing process Next, if annealing is performed after the welding process, or before drawing as needed, cold drawing is performed after annealing. The reduction ratio for each drawing step can be adjusted as appropriate.

[0086] In this case, it is preferable to perform the drawing process two or more times. Performing the drawing process two or more times reduces the number of coarse inclusions and eliminates negative segregation of Ni and Mn, resulting in a negative segregation degree of 0.80 or higher. As a result, the fatigue resistance is improved. Heat treatment may be performed between drawing processes as needed. The heat treatment should be performed under the same conditions as the annealing process for welded pipes described above.

[0087] 5-2-5. Bright Annealing Process It is preferable to perform bright annealing on steel pipes that have undergone the drawing process. However, if bright annealing is performed before the drawing process, it is not necessarily required to perform bright annealing after the drawing process. Bright annealing should be performed at least once before and after the drawing process, but it is preferable to perform it at least twice. The various conditions for bright annealing should be those described in 5-1-5. That is, the dew point should be -45°C or lower, and the annealing temperature should be 950 to 1150°C. The same applies to the annealing time. In bright annealing of steel pipes, as in the case of steel plates, it is possible to reduce the hard Al2O3 and MgO and increase the proportion of soft CaO in the inclusions. After that, appropriate cooling is performed to produce an austenitic stainless steel pipe.

[0088] The austenitic stainless steel sheets and pipes according to the present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. [Examples]

[0089] Steel having the chemical composition shown in Table 1 was melted to obtain a slab. The obtained slab was heated to 1230°C, hot-rolled in a temperature range of 900°C or higher, and coiled at 850°C to obtain a 5.0 mm thick hot-rolled steel sheet. Subsequently, the obtained hot-rolled steel sheet was annealed at 1100°C for 3 minutes, then pickled and descaled.

[0090] [Table 1]

[0091] Subsequently, cold rolling was performed on the annealed hot-rolled steel sheet. The cold rolling was carried out under the conditions shown in Table 2. For some examples, cold rolling was performed multiple times, and annealing was carried out in between. For annealing, it was carried out in the range of 1050 to 1120 °C for 30 seconds to 1 minute. Also, for the annealing between cold rollings, in some cases, bright annealing was performed, while in other cases, annealing was carried out in a combustion gas atmosphere. Note that the number of annealing times in Table 2 indicates the number of bright annealing times performed in all processes. After the final annealing, it was cooled to obtain an austenitic stainless steel sheet. In the case of bright annealing, hydrogen gas was used, the dew point was set to -45 °C or lower, and annealing was carried out in the above-mentioned temperature range.

[0092] For the obtained austenitic stainless steel sheet, the state of inclusions, the degree of negative segregation of Ni and Mn, and the fatigue resistance characteristics were examined.

[0093] (State of inclusions) Regarding the state of inclusions in the steel sheet, the number of coarse inclusions per 0.05 mm 2 and the ratio of the number of CaO-based coarse inclusions were examined and evaluated. The observation sample was taken as described above, and the length of the longitudinal direction of the L cross-section of the taken sample was adjusted to be 20 mm. The taken sample was polished while using a corrosion liquid or the like so that the L cross-section became the observation surface. This sample was observed with a SEM (scanning electron microscope). The setting conditions of the SEM during observation were an acceleration voltage of 15 k. The observation field was set to 100 μm × 100 μm per field, and by observing 5 fields at 5 positions in the above-mentioned plate thickness direction in this area, the total observation area was adjusted to be 0.05 mm 2 as described above. And inclusions with a major axis length of 5 μm or more were identified as coarse inclusions, and the number of them was counted. For other matters, it was as described above.

[0094] Next, the method for measuring the ratio of the number of CaO-based coarse inclusions will be explained. For the above-mentioned 0.05 mm 2Coarse inclusions were identified within the specified region, and their number was counted. The composition of the coarse inclusions observed within this region was then analyzed using EDX attached to the SEM. Point analysis was performed at three points within each inclusion. The average value of these three points, expressed as a mol% ratio in oxide terms, was used to identify CaO-based coarse inclusions if the CaO content was 30 mol% or higher. The ratio of CaO-based coarse inclusions to all observed coarse inclusions was calculated. Note that the inclusions were oxides, and each inclusion consisted of one or more of MgO, Al2O3, MnO, Cr2O3, and CaO, or a composite oxide thereof. Other oxides, such as ZrO2, were not considered.

[0095] (Negative segregation degree) The negative segregation of Ni and Mn was measured using the following procedure. Specifically, the negative segregation of Ni and Mn was measured for a 250 μm × 250 μm observation field. Beam diameter 1 μm, step size 0.5 μm, acceleration voltage 15 kV, irradiation current 2.0 × 10⁻¹⁰ -9 Under condition A, EPMA analysis was performed, and the ratio of Ni and Mn concentrations in the negative segregation region was defined as the degree of negative segregation, with the average value of Ni or Mn concentrations at all analysis points set to 1. The sample for analysis was taken from the same location as when observing coarse inclusions, and the length of the L-section of the sample in the longitudinal direction was 20 mm. The analysis was performed on the L-section.

[0096] (Fatigue resistance characteristics) The fatigue resistance of the steel plates was evaluated using a planar bending fatigue test. The test specimens were held in a 45 MPa hydrogen gas atmosphere at 300°C for 200 hours to charge the steel with hydrogen. The stress ratio was set to -1 and the frequency to 10 Hz. The maximum test stress at which fracture occurred after 1 × 10^7 cycles was defined as the fatigue limit. A difference of 0-20 MPa between the fatigue limit of the hydrogen-charged material and the uncharged material was marked with ◎, a difference between 20 MPa and 30 MPa was marked with ○, and a difference of over 30 MPa was marked with ×. Table 2 is shown below.

[0097] [Table 2]

[0098] Samples No. 1 to 17 satisfied the requirements of this embodiment and exhibited good fatigue resistance. On the other hand, samples No. 18 to 25, which did not satisfy the requirements of this embodiment, exhibited poor fatigue resistance. [Examples]

[0099] Cold-rolled steel sheets having the chemical composition of Example 1 were welded or otherwise formed into the shape of steel pipes, and then drawn the number of times shown in Table 3. The specific chemical composition of each steel sheet is indicated by the symbol in Table 3. In the cases where drawing was performed multiple times, annealing was performed between drawing processes. The annealing temperature was in the range of 1000 to 1100°C. After that, the pipes were cooled to obtain various types of austenitic stainless steel pipes. The annealing between drawing processes was performed using either normal combustion gas atmosphere annealing or bright annealing. Finally, bright annealing was performed, and the pipes were cooled to obtain austenitic stainless steel pipes. The number of bright annealing processes in Table 3 refers to the total number of bright annealing processes performed between drawing processes and after drawing. In the case of bright annealing, hydrogen gas was used, the dew point was set to -45°C or lower, and annealing was performed within the temperature range described above.

[0100] The obtained austenitic stainless steel tubes were examined for grain size, degree of negative segregation of Ni and Mn, state of inclusions, and fatigue resistance.

[0101] (Grain size) The grain size of the steel pipe was measured using the following procedure. The weld was observed in a cross-section perpendicular to the drawing direction of the steel pipe, and the grain size was measured at the center of the wall thickness in the center of the weld. The measurement method was performed using the cutting method in accordance with JIS G 0551:2013, and for the observation during grain size measurement, five fields of view were measured with observation fields of view ranging from 200 to 1000 times, and the grain size was calculated.

[0102] (State of inclusions) Similar to the case of steel plates, the state of inclusions was also examined in steel pipes. The measurement method involved taking samples and observing them as described above. Other observation conditions were the same as in Example 1.

[0103] (Negative segregation degree) As described above, the degree of negative segregation was measured using EPMA, similar to the method used when observing the state of the inclusions. The sample for analysis was taken from the same location as when observing the coarse inclusions. Other observation conditions were the same as in Example 1.

[0104] (Fatigue resistance characteristics) The fatigue resistance characteristics of the steel pipe were measured using the following procedure. Hydrogen gas was filled into the inside of the steel pipe, and a cycle test was performed with a gas pressure limit of 10 MPa (+0 to 3 MPa) and an upper limit of 70 MPa (+0 to 3 MPa), or a lower limit of 40 MPa (+0 to 3 MPa) and an upper limit of 100 MPa (+0 to 5 MPa). The pressure was increased and decreased in 20 to 30 seconds per cycle, and 10,000 cycles were performed. No cracking was indicated by ◎ under the upper limit of 100 MPa, no cracking was indicated by ○ under the upper limit of 70 MPa, and × was indicated if cracking occurred. The results are shown in Table 3 below.

[0105] [Table 3]

[0106] Numbers No. 1 to 17, which satisfied the requirements of this embodiment, showed good fatigue resistance. On the other hand, numbers No. 18 to 25, which did not satisfy the requirements of this embodiment, had poor fatigue resistance.

Claims

1. The chemical composition is expressed in mass percent. C: 0.1% or less, Si: 2.0% or less, Mn: 6.0 to 12.0%, P: 0.030% or less, S: 0.003% or less, Cr: 13.0-18.0%, Ni: 5.0 to 9.0%, N: 0.15-0.25%, Al: 0.005-0.080%, Ca: 0.0005-0.01%, B: 0.0001-0.01%, Ga: 0.0010-0.020%, Cu: Less than 1.0% Mo: Less than 2.0% Nb: 0 to 0.5%, Ti: 0 to 0.5%, V: 0 to 0.50%, W: 0-0.50%, Zr: 0 to 0.50%, Co: 0 to 0.50%, Mg: 0 to 0.005%, Hf: 0-0.10%, REM: 0-0.1%, The remainder consists of Fe and impurities. The f-value calculated by the following formula (i) is greater than 29.5 and less than 32.5, In a cross-section parallel to the machining direction and the thickness direction, When inclusions with a long side of 5 μm or more are defined as coarse inclusions, the number of such coarse inclusions is 0.05 mm. ２ The number of winning items is 10 or less. An austenitic stainless steel material in which the ratio of the number of coarse inclusions containing 30 mol% or more of CaO to the total number of coarse inclusions is 50% or more. f value=Ni+0.72Cr+0.88Mo+1.11Mn-0.27Si+0.53Cu+12.93C+7.55N...(i) However, each element symbol in formula (i) above represents the mass percentage of each element contained in the steel, and zero is used if the element is not present.

2. In the cross-section, The degree of negative segregation of Ni is 0.80 or higher. The austenitic stainless steel material according to claim 1, wherein the degree of negative segregation of Mn is 0.80 or higher.

3. An austenitic stainless steel material according to claim 1 or 2, which is a steel plate.

4. An austenitic stainless steel material according to claim 1 or 2, which is a steel pipe.

5. A method for producing an austenitic stainless steel material according to claim 1 or 2, A cold working step comprising cold working a hot-workable material having the chemical composition described in claim 1 to obtain a cold-worked material, A method for manufacturing austenitic stainless steel, comprising a bright annealing step in which bright annealing is performed at a temperature range of 950 to 1150°C with a dew point of -45°C or lower.

6. A method for producing an austenitic stainless steel material according to claim 3, A method for producing an austenitic stainless steel material according to claim 5, wherein cold rolling is performed in the cold working step.

7. A method for producing an austenitic stainless steel material according to claim 4, A method for manufacturing an austenitic stainless steel material according to claim 5, wherein drawing is performed in the cold working step.

8. A method for producing an austenitic stainless steel material according to claim 4, The method for manufacturing an austenitic stainless steel material according to claim 7, wherein the drawing process is performed two or more times.

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