Austenitic stainless steel sheet

Abstract

An austenitic stainless steel sheet having a chemical composition comprising, in mass%, 0.150% or less of C, 2.0% or less of Si, 3.00% or less of Mn, 0.060% or less of P, 0.0080% or less of S, 16.0-22.0% of Cr, 3.00% or less of Mo, 8.0-15.0% of Ni, 2.0% or less of Cu, 0.080% or less of Al, and 0.250% or less of N, the remainder being Fe and impurities, and having a maximum A value [= Cr + 2Si + 5Mo + 10Al] as measured by GDS from the outermost surface to a depth of 20 nm of 45 or more, the minimum value of the B value [= Cr + Ni + Mn + Cu + 8N] from the outermost surface to a depth of 50 nm is 25 or more, and the thickness of the austenitic stainless steel sheet is 4.5 mm or more.

Description

Technical Field

[0001] The invention relates to an austenitic stainless steel plate. Background Art

[0002] In recent years, hydrogen has attracted attention as a new energy source to replace fossil fuels. Hydrogen is a clean energy source that does not emit CO2. On the other hand, hydrogen can cause hydrogen embrittlement, which can embrittle the billet, for example. Therefore, Patent Document 1 discloses an austenitic stainless steel with improved hydrogen embrittlement resistance.

[0003] Prior art literature

[0004] Patent Literature

[0005] Patent Document 1: Japanese Patent Application Publication No. 2015-196842

[0006] However, in order to actually use hydrogen as an energy source, a large amount of liquefied hydrogen compressed at ultra-low temperatures needs to be stored. For this reason, large storage tanks capable of accommodating liquefied hydrogen (hereinafter also referred to as "liquefied hydrogen storage tanks") are being planned and constructed. Here, thick plates of austenitic stainless steel plates, which are billets with excellent resistance to hydrogen embrittlement, are preferred for use in liquefied hydrogen storage tanks.

[0007] On the other hand, for such austenitic stainless steel plates, not only hydrogen embrittlement resistance is required, but also fracture resistance is required as a building structure. For example, fractures are required to be suppressed even in the event of an earthquake. It is believed that, especially at ultra-low temperatures such as when storing liquefied hydrogen, trace amounts of hydrogen intruding into the steel can easily promote the occurrence and development of fractures. Therefore, impact resistance that can suppress fractures even in such harsh environments is required.

[0008] However, the austenitic stainless steel disclosed in Patent Document 1 has not studied the above-mentioned impact resistance characteristics. Therefore, there is still room for improvement in the above-mentioned impact resistance characteristics. Summary of the invention

[0009] Problem that the invention aims to solve

[0010] The present invention aims to solve the above-mentioned problems and to provide an austenitic stainless steel sheet having excellent impact resistance and capable of suppressing fracture even when exposed to an environment where hydrogen easily penetrates into the steel, such as when storing liquefied hydrogen at ultra-low temperatures.

[0011] Solutions for solving problems

[0012] The present invention has been made to solve the above-mentioned technical problems, and the gist of the present invention is the following austenitic stainless steel sheet.

[0013] (1) An austenitic stainless steel sheet having a chemical composition in mass % as follows:

[0014] C: 0.150% or less,

[0015] Si: 2.0% or less,

[0016] Mn: 3.00% or less,

[0017] P: 0.060% or less,

[0018] S: 0.0080% or less,

[0019] Cr: 16.0~22.0%,

[0020] Mo: 3.00% or less,

[0021] Ni: 8.0-15.0%,

[0022] Cu: 2.0% or less,

[0023] Al: 0.080% or less,

[0024] N: 0.250% or less,

[0025] Nb: 0~0.10%,

[0026] Ti: 0~0.10%,

[0027] B: 0~0.0050%,

[0028] V: 0~0.50%,

[0029] W: 0~0.50%,

[0030] Ca: 0~0.0100%,

[0031] Mg: 0~0.0100%,

[0032] Zr: 0~0.50%,

[0033] Co: 0-1.0%,

[0034] Ga: 0~0.010%,

[0035] Hf: 0~0.10%,

[0036] REM: 0~0.10%,

[0037] Balance: Fe and impurities,

[0038] The concentration changes of O, Fe, Cr, Mn, Ni, Mo, Cu, Si, Al and N were measured from the outermost surface of the steel plate along the depth direction by glow discharge emission spectroscopy. When the total amount of other elements except O was converted to 100% in mass %,

[0039] In the region from the outermost surface to a depth of 20 nm, the maximum value of the A value calculated by the following formula (i) is 45 or more,

[0040] In the region from the outermost surface to a depth of 50 nm, the minimum value of the B value calculated by the following formula (ii) is 25 or more,

[0041] The thickness of the austenitic stainless steel plate is 4.5 mm or more.

[0042] A value = Cr + 2Si + 5Mo + 10Al (i)

[0043] B value = Cr + Ni + Mn + Cu + 8N (ii)

[0044] The symbols of the elements in the above formulae (i) and (ii) represent the content (mass %) of the elements measured at the respective depth positions in glow discharge emission spectrometry, and are set to zero when not contained.

[0045] (2) The austenitic stainless steel sheet according to (1) above, wherein the chemical composition contains, in mass %, selected from

[0046] Nb: 0.01-0.10%,

[0047] Ti: 0.01-0.10%,

[0048] B: 0.0002~0.0050%,

[0049] V: 0.05~0.50%,

[0050] W: 0.05~0.50%,

[0051] Ca: 0.0002~0.0100%,

[0052] Mg: 0.0002~0.0100%,

[0053] Zr: 0.01~0.50%,

[0054] Co: 0.01-1.0%,

[0055] Ga: 0.001~0.010%,

[0056] Hf: 0.01 to 0.10%, and

[0057] REM: 0.01~0.10%

[0058] One or more of the following.

[0059] (3) The austenitic stainless steel sheet according to (1) or (2) above, wherein in the chemical composition, the M value calculated by the following formula (iii) is -100 or less.

[0060] M value = 551-462 (C + N) -9.2Si-8.1Mn-13.7Cr-29 (Ni + Cu) -18.2Mo (iii)

[0061] The symbol of each element in the above formula (iii) represents the content (mass %) of each element contained in the steel, and is set to zero when not contained.

[0062] Effects of the Invention

[0063] According to the present invention, an austenitic stainless steel sheet having excellent impact resistance can be obtained, and fracture can be suppressed even when exposed to an environment where hydrogen easily penetrates into the steel, such as when storing liquefied hydrogen at ultra-low temperatures. DETAILED DESCRIPTION

[0064] The present inventors have studied the impact resistance characteristics in an environment where liquefied hydrogen is stored, and have obtained the following findings.

[0065] (a) Large liquefied hydrogen storage tanks can have a capacity of tens of thousands of m 3 Therefore, it is necessary to use billets that will not break even in earthquakes. It is believed that even at ultra-low temperatures where liquefied hydrogen is stored, specifically below -235°C, even a small amount of hydrogen intruding from the environment into the steel will cause so-called hydrogen embrittlement and easily cause fracture.

[0066] (b) The higher the temperature, the easier it is for hydrogen to intrude into the steel from the environment. However, during long-term use, even at ultra-low temperatures, hydrogen will intrude into the steel and cause hydrogen embrittlement. From the perspective of improving hydrogen embrittlement resistance, it is expected that liquefied hydrogen storage tanks will use austenitic stainless steel plates that can inhibit hydrogen from invading the steel.

[0067] (c) The present inventors have studied methods for inhibiting hydrogen intrusion into steel and found that forming a passive film mainly composed of Cr2O3 on the surface of a steel plate is effective in inhibiting hydrogen intrusion. In addition, solid solution of Si, Mo and Al in the passive film is effective. It is believed that these elements improve the compactness of the passive film and inhibit hydrogen intrusion.

[0068] (d) In addition, the following insight was obtained: by controlling the chemical composition of Cr, Ni, Mn, Cu and N near the surface of the base material, the toughness reduction can be suppressed. Cr, Ni, Mn, Cu and N elements that stabilize austenite are used to form oxide scales during annealing, and thus decrease near the surface. It is believed that the area where these elements are reduced will become embrittled with only a small amount of hydrogen intrusion, and act as a starting point for fracture.

[0069] One embodiment of the present invention has been completed based on the above findings. Hereinafter, each condition of this embodiment will be described in detail.

[0070] 1. Chemical composition

[0071] The reasons for limiting the content of each element are as follows. In the following description, "%" about the content means "mass %".

[0072] C: 0.150% or less

[0073] C is an element effective in stabilizing the austenite phase and also helps improve hydrogen embrittlement resistance. However, excessive C content promotes the precipitation of Cr-based carbides at the grain boundaries, which easily forms the starting point of fracture. As a result, the impact resistance is reduced. Therefore, the C content is set to 0.150% or less. The C content is preferably set to 0.100% or less, and more preferably set to 0.050% or less. On the other hand, in order to obtain the above-mentioned effect, the C content is preferably set to 0.010% or more.

[0074] Si: 2.0% or less

[0075] Si is an element effective for deoxidation and also helps improve hydrogen embrittlement resistance. However, when Si is contained in excess, it promotes the formation of intermetallic compounds such as σ phases, which easily form the starting point of fracture. As a result, the impact resistance is reduced. Therefore, the Si content is set to 2.0% or less. The Si content is preferably set to 1.5% or less, and more preferably set to 1.0% or less. On the other hand, in order to obtain the above-mentioned effect, the Si content is preferably set to 0.30% or more.

[0076] Mn: 3.00% or less

[0077] Mn is an element effective in stabilizing the austenite phase, and helps improve hydrogen embrittlement resistance. However, when Mn is contained in excess, it promotes the formation of the ε phase with high hydrogen embrittlement sensitivity, which in turn reduces hydrogen embrittlement resistance. In addition, excessive precipitation of MnS reduces impact resistance. Therefore, the Mn content is set to 3.00% or less. The Mn content is preferably set to 2.00% or less, and more preferably set to 1.00% or less. On the other hand, in order to obtain the above-mentioned effect, the Mn content is preferably set to 0.30% or more.

[0078] P: 0.060% or less

[0079] P is an element contained in steel as an impurity and is an element that is easily segregated at grain boundaries. Therefore, the P element sometimes forms a starting point for fracture, reducing impact resistance. Therefore, the P content is set to 0.060% or less. The P content is preferably set to 0.050% or less, and more preferably set to 0.040% or less. On the other hand, excessive reduction of P will lead to an increase in manufacturing costs, so the P content is preferably set to 0.010% or more.

[0080] S: 0.0080% or less

[0081] S is an element contained in steel as an impurity. It may form MnS, which forms the starting point of fracture and is an element that reduces impact resistance. Therefore, the S content is set to 0.0080% or less. The S content is preferably set to 0.0050% or less, and more preferably set to 0.0030% or less. However, when the S content is excessively reduced, the manufacturing cost increases. Therefore, the S content is preferably contained at 0.0003% or more.

[0082] Cr: 16.0~22.0%

[0083] Cr is an element contained in a certain amount in stainless steel, and has the effect of improving corrosion resistance. Therefore, the Cr content is set to 16.0% or more. However, Cr is a ferrite-forming element. Therefore, when Cr is contained in excess, the austenite phase is unstable and the hydrogen embrittlement resistance is reduced. In addition, the impact resistance is also reduced. Therefore, the Cr content is set to 22.0% or less. The Cr content is preferably set to 21.0% or less, and more preferably set to 20.0% or less.

[0084] Mo: 3.00% or less

[0085] Mo has the effect of improving strength. However, when contained in excessive amounts, it promotes the formation of δ-ferrite phase and reduces the resistance to hydrogen embrittlement. Therefore, the Mo content is set to 3.00% or less. The Mo content is preferably set to 2.50% or less, and more preferably set to 2.20% or less. On the other hand, when the Mo content is reduced excessively, the molten raw material is limited and the manufacturing cost increases. Therefore, the Mo content is preferably set to 0.05% or more.

[0086] Ni: 8.0~15.0%

[0087] Ni and Mn are elements necessary to ensure hydrogen embrittlement resistance and impact resistance. Therefore, the Ni content is set to 8.0% or more. However, when Ni is contained excessively, the manufacturing cost increases. In addition, segregation is likely to occur. Therefore, the Ni content is set to 15.0% or less. The Ni content is preferably set to 14.0% or less, more preferably set to 13.0% or less, and further preferably set to 12.5% ​​or less.

[0088] Cu: 2.0% or less

[0089] Cu is an element mixed from raw materials such as scraps, and is an effective element for stabilizing the austenite phase. On the other hand, Cu is a low melting point element, and segregates at the grain boundary, which is easy to produce the starting point of fracture. Therefore, the Cu content is set to 2.0% or less. The Cu content is preferably set to 1.0% or less, and more preferably set to 0.70% or less. However, when the Cu content is excessively reduced, the molten raw material will be limited, and the manufacturing cost will increase. Therefore, the Cu content is preferably set to 0.05% or more.

[0090] Al: 0.080% or less

[0091] Al is an effective deoxidizing element and has the effect of suppressing the grain boundary segregation of low melting point elements, thereby strengthening the grain boundaries. As a result, it also has the effect of improving impact resistance. However, Al is a ferrite-forming element, so when Al is contained excessively, the austenite phase is unstable, which reduces the resistance to hydrogen embrittlement. Therefore, the Al content is set to 0.080% or less. The Al content is preferably set to 0.050% or less. On the other hand, in order to obtain the above-mentioned effect, the Al content is preferably set to 0.005% or more, and more preferably set to 0.010% or more.

[0092] N: 0.250% or less

[0093] N, like Mn and Ni, is an element effective in improving hydrogen embrittlement resistance. However, when N is contained excessively, internal defects such as pores may occur during smelting, which may easily become the starting point of fracture. As a result, the impact resistance is reduced. Therefore, the N content is set to 0.250% or less. The N content is preferably set to 0.200% or less, and more preferably set to 0.100% or less. On the other hand, in order to obtain the above-mentioned effect, the N content is preferably set to 0.010% or more.

[0094] In addition to the above elements, one or more selected from Nb, Ti, B, V, W, Ca, Mg, Zr, Co, Ga, Hf and REM may be further contained within the range shown below. The reasons for limiting each element are explained below.

[0095] Nb: 0~0.10%

[0096] Nb has the effect of forming carbonitrides, refining grains, and strengthening grain boundaries. As a result, it has the effect of improving impact resistance. Therefore, it can be contained as needed. However, when Nb is contained excessively, the manufacturability and processability during hot rolling are reduced. In addition, a large number of inclusions may be formed, and the impact resistance is reduced. Therefore, the Nb content is set to less than 0.10%. The Nb content is preferably set to less than 0.07%. On the other hand, in order to obtain the above-mentioned effects, the Nb content is preferably set to more than 0.01%.

[0097] Ti: 0~0.10%

[0098] Ti has the effects of forming carbonitrides, refining grains, and strengthening grain boundaries. As a result, impact resistance is improved. Therefore, it can be contained as needed. However, when Ti is contained excessively, the manufacturability during hot rolling is reduced. In addition, a large number of inclusions may be formed, and the impact resistance is reduced. Therefore, the Ti content is set to less than 0.10%. The Ti content is preferably set to less than 0.070%, more preferably set to less than 0.050%, and further preferably set to less than 0.020%. On the other hand, in order to obtain the above-mentioned effects, the Ti content is preferably set to more than 0.001%.

[0099] B: 0~0.0050%

[0100] B has the effect of strengthening grain boundaries, improving strength, and improving impact resistance. Therefore, it can be contained as needed. However, even if B is contained excessively, not only will its effect be saturated, but sometimes the impact resistance will be reduced. Therefore, the B content is set to 0.0050% or less. The B content is preferably set to 0.0030% or less. On the other hand, in order to obtain the above-mentioned effects, the B content is preferably set to 0.0002% or more.

[0101] V: 0~0.50%

[0102] V is dissolved in steel or precipitated in the form of carbonitrides, which has the effect of improving strength. Therefore, it can be contained as needed. However, when V is contained excessively, excessive carbonitrides will be formed, thereby reducing the manufacturability during hot rolling. In addition, the impact resistance may be reduced. Therefore, the V content is preferably set to less than 0.50%. The V content is preferably set to less than 0.30%. On the other hand, in order to obtain the above-mentioned effect, the V content is preferably set to more than 0.05%.

[0103] W: 0~0.50%

[0104] W has the effect of improving strength and corrosion resistance. Therefore, it can be contained as needed. However, when W is contained excessively, the manufacturing cost increases. Therefore, the W content is set to 0.50% or less. The W content is preferably set to 0.30% or less. On the other hand, in order to obtain the above-mentioned effect, the W content is preferably set to 0.05% or more.

[0105] Ca: 0~0.0100%

[0106] Ca has the effect of suppressing the grain boundary segregation of low melting point elements and strengthening the grain boundaries. As a result, the impact resistance is improved. Therefore, it can be contained as needed. However, when Ca is contained excessively, segregation is likely to occur and it is easy to become the starting point of fracture. As a result, the impact resistance may be reduced. Therefore, the Ca content is set to less than 0.0100%. The Ca content is preferably set to less than 0.0050%. On the other hand, in order to obtain the above-mentioned effect, the Ca content is preferably set to more than 0.0002%.

[0107] Mg: 0~0.0100%

[0108] Mg has the effect of suppressing the grain boundary segregation of low melting point elements and strengthening the grain boundaries. As a result, the impact resistance is improved. Therefore, it can be contained as needed. However, when Mg is contained excessively, a large number of inclusions will be formed, which can easily become the starting point of fracture, and the impact resistance may be reduced as a result. Therefore, the Mg content is set to 0.0100% or less. The Mg content is preferably set to 0.0050% or less. On the other hand, in order to obtain the above-mentioned effect, the Mg content is preferably set to 0.0002% or more.

[0109] Zr: 0~0.50%

[0110] Zr has a deoxidizing effect. In addition, it has the effect of improving corrosion resistance. Therefore, it can be contained as needed. However, when Zr is contained excessively, toughness and processability are reduced. In addition, a large number of inclusions will be formed, which can easily become the starting point of fracture, and as a result, the impact resistance may be reduced. Therefore, the Zr content is set to 0.50% or less. The Zr content is preferably set to 0.30% or less. On the other hand, in order to obtain the above-mentioned effect, the Zr content is preferably set to 0.01% or more.

[0111] Co: 0~1.0%

[0112] Co has the effect of improving impact resistance. In addition, it also has the effect of improving corrosion resistance and stabilizing the austenite phase. Therefore, it can be contained as needed. However, when Co is contained excessively, toughness and processability will decrease. Therefore, the Co content is set to 1.0% or less. The Co content is preferably set to 0.70% or less. On the other hand, in order to obtain the above-mentioned effect, the Co content is preferably set to 0.01% or more, more preferably to 0.05% or more, and further preferably to 0.10% or more.

[0113] Ga: 0~0.010%

[0114] Ga has the effect of improving hot workability. Therefore, it can be contained as needed. However, when Ga is contained excessively, the manufacturability is reduced. Therefore, the Ga content is set to 0.010% or less. The Ga content is preferably set to 0.009% or less. On the other hand, in order to obtain the above-mentioned effect, the Ga content is preferably set to 0.001% or more.

[0115] Hf: 0~0.10%

[0116] Hf has the effect of improving strength and hydrogen embrittlement resistance. Therefore, it can be contained as needed. However, when Hf is contained excessively, the workability will be reduced. Therefore, the Hf content is set to 0.10% or less. The Hf content is preferably set to 0.07% or less. On the other hand, in order to obtain the above-mentioned effect, the Hf content is preferably set to 0.01% or more.

[0117] REM: 0~0.10%

[0118] REM has the effect of improving hot workability. In addition, it also has the effect of improving corrosion resistance. Therefore, it can be contained as needed. However, when REM is contained excessively, not only will its effect be saturated, but hot workability will also be reduced. Therefore, the REM content is set to 0.10% or less. The REM content is preferably set to 0.07% or less. On the other hand, in order to obtain the above-mentioned effects, the REM content is preferably set to 0.01% or more.

[0119] REM refers to a total of 17 elements including Sc, Y and lanthanide elements, and the above REM content refers to the total content of these elements. In industry, REM is often added in the form of mixed rare earth metals.

[0120] In the chemical composition of this embodiment, the balance is Fe and impurities. Here, "impurities" refer to components mixed in due to various factors such as raw materials such as ores and wastes and manufacturing processes when manufacturing austenitic stainless steel sheets in industry, and are allowed within the range that does not adversely affect this embodiment.

[0121] M-value

[0122] The M value calculated by the following formula (iii) is an index showing the stability of the γ phase in the austenitic stainless steel sheet. In the austenitic stainless steel sheet of the present embodiment, the M value is preferably set to -100 or less in order to further improve the impact resistance.

[0123] M value = 551-462 (C + N) -9.2Si-8.1Mn-13.7Cr-29 (Ni + Cu) -18.2Mo (iii)

[0124] The symbol of each element in the above formula (iii) represents the content (mass %) of each element contained in the steel, and is set to zero when not contained.

[0125] By setting the M value to be -100 or less, the stability of the γ phase can be ensured, the phase transition to the α' phase can be suppressed, and the impact resistance can be further improved. The M value is more preferably -110 or less, and even more preferably -120 or less.

[0126] The M value does not need to have a lower limit, but if it is less than -220, a large amount of additive elements will be required, which will increase the alloy cost. Therefore, the M value is preferably set to -220 or more, more preferably -210 or more, and further preferably -200 or more.

[0127] 2. Chemical composition of the surface area

[0128] As described above, in order to suppress hydrogen intrusion into steel, it is important to form a passive film mainly composed of Cr2O3 and containing Si, Mo and Al as solid solutions. In addition, by controlling the chemical composition of Cr, Ni, Mn, Cu and N near the surface of the base material, it is possible to suppress the reduction of toughness. In other words, in order to improve the impact resistance in a liquefied hydrogen storage environment, it is important to control the chemical composition of the surface area of ​​the steel plate.

[0129] In this embodiment, the chemical composition of the surface layer of the steel plate is measured by glow discharge emission spectroscopy (GDS). Specifically, the concentration changes of O, Fe, Cr, Mn, Ni, Mo, Cu, Si, Al and N are measured from the outermost surface of the steel plate along the depth direction using GDS, and the total amount of other elements except O is converted to 100% in mass %.

[0130] From the above perspective, in the present embodiment, in the region from the outermost surface of the steel plate to a depth of 20 nm, the maximum value of the A value calculated by formula (i) is set to be greater than 45, and in the region from the outermost surface of the steel plate to a depth of 50 nm, the minimum value of the B value calculated by formula (ii) is set to be greater than 25.

[0131] A Value

[0132] The A value is an indicator of the ability to suppress hydrogen intrusion into the passive film. In the austenitic stainless steel sheet of the present embodiment, in order to improve the impact resistance, the maximum value of the A value calculated at each depth position in the region from the outermost surface of the steel sheet to a depth of 20 nm is set to 45 or more.

[0133] A value = Cr + 2Si + 5Mo + 10Al (i)

[0134] The symbol of each element in the above formula (i) represents the content (mass %) of each element measured at each depth position in glow discharge emission spectrometry, and is set to zero when not contained.

[0135] The passivation film is a thinner film less than 20nm, which is usually composed of Fe and Cr oxides corresponding to the chemical composition of the base material. The inventors of the present invention have found that by optimizing the manufacturing conditions, the ratio of Cr oxides can be improved, and Si, Mo and Al can be dissolved in the passivation film.

[0136] If the maximum value of the A value is less than 45, the ratio of Fe oxides is excessive, and the effect of suppressing hydrogen intrusion into the steel cannot be sufficiently obtained. The maximum value of the A value is preferably set to 50 or more, and more preferably set to 55 or more. Since the maximum value of the A value is more preferable as it is higher, there is no need to set an upper limit, but 80 is the upper limit that can be actually manufactured.

[0137] B value

[0138] The B value is an index of the toughness of the base material. In the austenitic stainless steel plate of the present embodiment, the minimum value of the B value calculated at each depth position in the region from the outermost surface of the steel plate to a depth of 50 nm is set to 25 or more from the perspective of suppressing the decrease in toughness of the base material surface.

[0139] B value = Cr + Ni + Mn + Cu + 8N (ii)

[0140] The symbol of each element in the above formula (ii) represents the content (mass %) of each element measured at each depth position in glow discharge emission spectrometry, and is set to zero when not contained.

[0141] Cr, Ni, Mn, Cu and N are elements that stabilize austenite. If these elements are reduced near the surface of the base material, the toughness will decrease, and a small amount of hydrogen intrusion will also cause embrittlement, acting as a starting point for fracture. As a result of the research conducted by the present inventors, it was found that the reduction of the B value can be suppressed, especially by controlling the annealing conditions.

[0142] If the minimum value of the B value is less than 25, the reduction in toughness becomes significant, and it is difficult to ensure the impact resistance under the liquefied hydrogen storage environment. The minimum value of the B value is preferably set to 27 or more, and more preferably set to 29 or more. The higher the minimum value of the B value, the more preferred it is, so there is no need to set an upper limit, but from the perspective of the relationship with the chemical composition of the base material, 44 is the actual upper limit.

[0143] The analysis using GDS is performed at any point selected from an area with few surface defects. At the measurement point, sputtering is performed from the outermost surface of the steel plate to a depth of 50nm, and the concentrations of each element of O, Fe, Cr, Mn, Ni, Mo, Cu, Si, Al and N are measured at intervals of 0.8 to 1.2nm. Thus, the content (mass %) of the above elements at each depth position is calculated respectively. At this time, the total amount of other elements except O is converted to 100% in mass %. Then, the above A value and B value are calculated at each depth position, and the maximum value of A value and the minimum value of B value are calculated from all the calculated measured values.

[0144] As a measuring device for GDS, for example, a GD-Profiler 2 device manufactured by Kuba Manufacturing Co., Ltd. can be used, and the measuring conditions can be set to 35 W, argon gas pressure 600 Pa, frequency 100 Hz, and measuring diameter 4 mmφ.

[0145] 3.Plate thickness

[0146] The thickness of the austenitic stainless steel plate of the present embodiment is set to be 4.5 mm or more. This is because by setting the thickness to the above range, the strength required by the liquefied hydrogen storage tank can be ensured. The thickness is preferably set to be 10 mm or more, and more preferably set to be 20 mm or more. When used for a larger liquefied hydrogen storage tank, the thickness is preferably set to be 30 mm or more. It should be noted that the upper limit of the thickness is not particularly limited, but is generally 100 mm.

[0147] 4. Purpose

[0148] The austenitic stainless steel sheet of this embodiment is excellent in hydrogen embrittlement resistance and ultra-low temperature impact resistance, and is therefore preferably used in liquefied hydrogen storage tanks. In particular, it is suitable for use in liquefied hydrogen storage tanks with a capacity of 10,000 m 3 Above, or 50,000 m 3 Large onshore liquefied hydrogen storage tanks as above.

[0149] 5. Manufacturing method

[0150] A preferred method for producing the austenitic stainless steel sheet of the present embodiment will be described. The austenitic stainless steel sheet of the present embodiment can be stably produced by, for example, the following production method.

[0151] The stainless steel having the above chemical composition is melted to produce steel sheets such as slabs. Next, the steel sheet is heated to a predetermined temperature and hot rolled (hot rolling process). The heating temperature in hot rolling is preferably set in the range of 1050 to 1250°C, and the reduction ratio is preferably set to 40% or more. This is because by setting the heating temperature and reduction ratio during hot rolling to the above range, it is easy to control the desired plate thickness.

[0152] After the hot rolling process, an annealing process and a descaling process are performed. In the annealing process, the temperature is maintained in the range of 1000-1100°C for 1-30 minutes. When the heating temperature in the annealing process is less than 1000°C or the holding time is less than 1 minute, the annealing effect cannot be fully obtained. On the other hand, when the heating temperature in the annealing process exceeds 1100°C or the holding time exceeds 30 minutes, Cr, Ni, Mn, Cu and N near the surface of the base material are excessively used in the formation of the oxide scale, making it difficult to make the minimum value of the above-mentioned B value be more than 25.

[0153] In the descaling process after the annealing process, the scale generated by the annealing is immersed in a nitric acid-hydrofluoric acid aqueous solution at 40 to 80°C for 1 to 30 minutes to be pickled and removed. As a result, the maximum value of the A value can be made 45 or more. In addition, before pickling, it is preferred to remove the scale generated by the annealing by grinding, and after confirming the metal surface, further remove the base material with a thickness of more than 100 μm. By grinding, the composition of the passivation film formed after the descaling process becomes more suitable, and specifically, the maximum value of the above-mentioned A value can be made 55 or more.

[0154] Hereinafter, the austenitic stainless steel sheet of the present invention will be described in more detail with reference to Examples, but the present embodiment is not limited to these Examples.

[0155] Example

[0156] Stainless steel with the chemical composition shown in Table 1 is melted to produce a slab. Thereafter, the slab is heated to a range of 1050 to 1250°C and hot rolled. After hot rolling, annealing is performed according to the conditions shown in Table 2, and then pickling is performed to obtain an austenitic stainless steel plate. In addition, in some examples, the oxide scale generated by annealing is removed by grinding before pickling, and after confirming the metal surface, the base material in the range of 100 to 200 μm in thickness is further removed. It should be noted that the implementation status of each process is shown in Table 2.

[0157] [Table 1]

[0158]

[0159] [Table 2]

[0160] Table 2

[0161]

[0162] A value = Cr + 2Si + 5Mo + 10AI... (i)

[0163] B value = Cr + Ni + Mn + Cu + 8N... (ii)

[0164] Underline: indicates conditions that deviate from the chemical composition of this embodiment or do not satisfy the target characteristics.

[0165] ※For grinding No.15 only, stop grinding after confirming the metal surface.

[0166] Determination of the chemical composition of the surface area

[0167] For each obtained austenitic stainless steel plate, GDS analysis was performed at any one point selected from a range with few surface defects. At the measurement point, sputtering was performed from the outermost surface of the steel plate to a depth of 50 nm, and the concentrations of each element of O, Fe, Cr, Mn, Ni, Mo, Cu, Si, Al and N were measured at a spacing of about 1 nm, and the content (mass %) of the above elements at each depth position was respectively calculated. At this time, the total amount of other elements except O was converted to 100% in mass %. Then, the above A value and B value were calculated at each depth position, and the maximum value of the A value and the minimum value of the B value were calculated from all the calculated measured values.

[0168] As a GDS measuring device, a GD-Profiler 2 device manufactured by Kuba Manufacturing Co., Ltd. was used, and the measuring conditions were set to 35 W, argon gas pressure 600 Pa, frequency 100 Hz, and measuring diameter 4 mmφ.

[0169] Evaluation of impact resistance

[0170] The impact resistance at ultra-low temperature is measured by the following steps. Specifically, a small-sized Charpy test piece with a V-notch of 55 mm in length, 10 mm in width and 5 mm in thickness is collected. The direction of the Charpy test piece is set to the L direction, and the sample is collected from the center of the plate thickness. The obtained Charpy test piece is arranged in a pressure vessel, and after the pressure vessel is replaced with hydrogen, it is heated and pressurized, and maintained at 10 MPa and 300°C for 300 hours.

[0171] Then, a thermocouple is attached to the Charpy test piece, fixed to a jig, and inserted into a polyurethane tubular cold-keeping capsule. Liquid helium is flowed into the cold-keeping capsule while measuring the temperature of the test piece. After the temperature reaches -253°C and is maintained for 10 seconds, the entire cold-keeping capsule is impacted from the C direction to perform the Charpy test. The absorbed energy is adjusted by subtracting the energy of the cold-keeping capsule (3.53 J). It should be noted that for conditions other than the above, JIS Z 2242:2018 is followed.

[0172] Based on the above results, the Charpy impact value is less than 100J / cm 2 When the impact property is determined as F, the Charpy impact value is 100 J / cm 2 Above and less than 120J / cm 2 When the impact property is judged as B, the Charpy impact value is 120 J / cm 2When the above values ​​were obtained, the impact property was judged as A. The obtained results are summarized in Table 2 below.

[0173] Test No. 1 to 13 satisfy the conditions of this embodiment and therefore have good impact resistance at ultra-low temperatures. In particular, in Test No. 2, 3, 5, 6, 8, 10 and 13 where grinding was performed before pickling, the maximum value of the A value was 55 or more, and the impact value was improved compared to the case where grinding was not performed.

[0174] On the other hand, Test Nos. 14 to 22 did not meet the conditions of the present embodiment, and thus had poor impact resistance at ultra-low temperatures. Specifically, in Test Nos. 14 and 15, the minimum value of the B value was less than the specified value due to the excessively high annealing temperature. In addition, in Test No. 14, since grinding before pickling was not performed, the maximum value of the A value was also less than the specified value. As a result, the impact resistance was deteriorated.

[0175] In Test No. 16, due to excessive C content, carbides precipitated excessively, and the impact resistance deteriorated. In Test No. 17, due to excessive Mn content, MnS precipitated excessively, and the impact resistance deteriorated. In Test No. 18, due to excessive P content, grain boundary segregation occurred, and the impact resistance deteriorated. In Test No. 19, due to excessive S content, MnS precipitated excessively, and the impact resistance deteriorated.

[0176] In Test No. 20, the Cr content was excessive, resulting in reduced hydrogen embrittlement resistance and deteriorated impact resistance. In Test No. 21, the Ni content was insufficient, resulting in deteriorated hydrogen embrittlement resistance and impact resistance. In Test No. 22, the N content was excessive, resulting in the occurrence of fracture starting points and deterioration of impact resistance.

[0177] Industrial Applicability

[0178] According to the present invention, an austenitic stainless steel sheet having excellent impact resistance can be obtained, and fracture can be suppressed even when exposed to an environment where hydrogen easily penetrates into steel, such as when storing liquefied hydrogen at ultra-low temperatures.

Claims

1. An austenitic stainless steel sheet, the chemical composition of which is, by mass%, C: 0.150% or less, Si: 2.0% or less, Mn: 3.00% or less, P: 0.060% or less, S: 0.0080% or less, Cr：16.0～22.0％、 Mo: 3.00% or less, Ni: 8.0-15.0%, Cu: 2.0% or less, Al: 0.080% or less, N: 0.250% or less, Nb: 0-0.10%, Ti: 0~0.10%, B：0～0.0050％、 V：0～0.50％、 W：0～0.50％、 Ca: 0~0.0100%, Mg: 0~0.0100%, Zr：0～0.50％、 Co: 0-1.0%, Ga: 0~0.010%, Hf: 0~0.10%, REM: 0~0.10%, Balance: Fe and impurities, The concentration changes of O, Fe, Cr, Mn, Ni, Mo, Si, Cu, Al and N were measured from the outermost surface of the steel plate along the depth direction by glow discharge emission spectrometry. When the total amount of other elements except O was converted to 100% in mass %, In the region from the outermost surface to a depth of 20 nm, the maximum value of the A value calculated by the following formula (i) is 45 or more, In the region from the outermost surface to a depth of 50 nm, the minimum value of the B value calculated by the following formula (ii) is 25 or more, The thickness of the austenitic stainless steel plate is 4.5 mm or more. A value = Cr + 2Si + 5Mo + 10Al (i) B value = Cr + Ni + Mn + Cu + 8N (ii) in, The symbol of each element in the above formula (i) and formula (ii) represents the mass % content of each element measured at each depth position in glow discharge emission spectrometry, and is set to zero when not contained.

2. The austenitic stainless steel sheet according to claim 1, wherein: The chemical composition contains, in mass%, a Nb: 0.01-0.10%, Ti: 0.01-0.10%, B：0.0002～0.0050％、 V：0.05～0.50％、 W：0.05～0.50％、 Ca: 0.0002~0.0100%, Mg: 0.0002~0.0100%, Zr：0.01～0.50％、 Co: 0.01-1.0%, Ga: 0.001~0.010%, Hf: 0.01 to 0.10%, and REM: 0.01~0.10% One or more of the following.

3. The austenitic stainless steel sheet according to claim 1 or claim 2, wherein: In the chemical composition, the M value calculated by the following formula (iii) is -100 or less. M value = 551-462 (C + N) -9.2Si-8.1Mn-13.7Cr-29 (Ni + Cu) -18.2Mo (iii) The symbol of each element in the above formula (iii) represents the mass % content of each element contained in the steel, and is set to zero when not contained.

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