Austenitic stainless steel sheet, method for producing same, and component

Abstract

Provided is an austenitic stainless steel sheet in which grain boundary cracks (nitridation resistance) and the generation of red scale (oxidation resistance) can be suppressed in a medium-high temperature region of 500-700 DEG C even in a gas atmosphere containing nitrogen and water vapor, such as ammonia combustion gas. The austenitic stainless steel sheet according to the present invention has a prescribed component composition having a nitridation tendency index of 15 or less, has an Si oxide coating film present in an area ratio of 5% or more on the surface of the steel sheet, and can form an internal oxide layer (Si oxide layer) by subjecting the steel sheet to final annealing. The Cr oxide layer and the Fe oxide layer on the surface layer are removed by subsequent pickling, thereby forming a Si oxide layer on the surface layer. And the nitriding tendency index is equal to 0.5 Cr + 10 Al + 2 Mo + 3 Ti + 0.5 Cu-1.5 Si.

Description

Technical Field

[0001] The present invention relates to an austenitic stainless steel sheet, a method for manufacturing the same, and a component using the austenitic stainless steel sheet. Background Art

[0002] Global warming has become an international environmental problem, and technological development for achieving a decarbonized society such as carbon neutrality and net-zero carbon emissions is actively underway. In this trend, ammonia is attracting attention as a fuel to replace carbon fuels. The combustion reaction formula of ammonia is 4NH 3 +3O 2 →2N 2 +6H 2 O, which generates water and nitrogen and has a small environmental load, and is expected to be a recyclable fuel. The combustion temperature of ammonia is 1750 °C according to the adiabatic flame thermometer, which is lower than that of hydrogen (2120 °C), methane (1970 °C), and gasoline (about 2000 °C). The combustion temperature in actual engines and gas turbines is also lower than that of these existing fuels. Therefore, when ammonia is used as a fuel, its exhaust gas temperature is also about 500 to 700 °C, which is lower than that of existing fuels. This temperature range of 500 to 700 °C is a temperature at which the steel used for exhaust pipes and the like is easily oxidized, and is a temperature range in which so-called red scale is easily generated.

[0003] In Patent Document 1, an austenitic stainless steel having good corrosion resistance even in an environment containing high sulfur (S) and high chlorine (Cl) such as boiler superheater tubes, waste incinerators, and ammonia synthesis apparatuses is proposed.

[0004] In Patent Document 2, an austenitic stainless steel having good corrosion resistance in an ammonia atmosphere even without adding chromic acid in an ammonia-water absorption heat exchanger or the like is proposed.

[0005] Prior Art Documents

[0006] Patent Documents

[0007] Patent Document 1: Japanese Patent Laid-Open No. 3-126842

[0008] Patent Document 2: Japanese Patent Laid-Open No. 10-280100 Summary of the Invention

[0009] The effective utilization of ammonia as a fuel is being developed not only for single combustion but also for co-combustion with other fuels (heavy oil, light oil, hydrogen, etc.). However, although it is co-combustion, since ammonia with a lower combustion temperature than existing fuels is added for combustion, the combustion exhaust gas temperature is lower than that of existing fuels, about 500 to 700 °C. Moreover, a large amount of nitrogen and water vapor are contained in the combustion gas of ammonia.

[0010] In the presence of water vapor, oxidation and red scale formation are likely to occur. Moreover, the combustion gas temperature is around 500 - 700 °C, which is also a temperature range prone to red scale formation. Therefore, steel materials used in ammonia combustion gas systems and the like are required to have oxidation resistance (red scale resistance).

[0011] Furthermore, due to the large amount of nitrogen contained in ammonia combustion exhaust gas, nitrogen penetrates into the surface layer of the steel material (nitriding), and embrittlement caused by intergranular cracks is significantly enhanced. Therefore, steel materials for ammonia combustion gas systems are also required to have nitriding resistance (intergranular crack resistance).

[0012] The stainless steel in Patent Document 1 is described as being applicable to ammonia synthesis plants, but it does not target ammonia combustion gas (combustion exhaust gas), and does not consider countermeasures against intergranular cracks caused by nitriding and red scale (oxidation) at a temperature of 500 - 700 °C.

[0013] The stainless steel in Patent Document 2 is premised on being applied to ammonia - water system absorption heat exchangers, i.e., contacting ammonia gas and ammonia solution, and does not consider countermeasures against intergranular cracks caused by nitriding and red scale (oxidation) at a temperature of 500 - 700 °C.

[0014] An object of the present invention is to provide a steel material (austenitic stainless steel) that can solve the problem of having red scale resistance (oxidation resistance) and intergranular crack resistance (nitriding resistance) even against gases containing a large amount of nitrogen and water (water vapor) at a temperature of around 500 - 700 °C such as ammonia combustion exhaust gas.

[0015] To solve the above problems, the inventors of the present invention conducted in - depth research and obtained the following insights.

[0016] (a) From the perspective of suppressing nitriding, it is considered preferable to optimize the contents of Cr, Mo, Ti, Al, and Cu, which are elements that promote nitriding.

[0017] (b) In addition, it was conceived to form a red scale - resistant and nitriding - resistant film on the surface layer of the steel material, and it was found that having a Si oxide film (SiO 2 film) on the steel surface is effective for red scale resistance and nitriding resistance. It was found that in order to ensure red scale resistance and nitriding resistance, it is preferable to form a Si oxide film of 5% or more on the surface of the steel plate.

[0018] (c) In addition, it was also found that Si not only has an inhibitory effect on red scale caused by water vapor oxidation, but also has an inhibitory effect on nitriding, although the reason is not yet clear. A nitriding tendency index indicating the nitriding tendency of the steel material was derived based on the contents of the nitriding - promoting elements Cr, Mo, Ti, Al, and Cu and the content of Si that inhibits nitriding, and it was found that it is preferable to make the nitriding tendency index 15.0 or less.

[0019] Nitriding tendency index = 0.5Cr + 10Al + 2Mo + 3Ti + 0.5Cu - 1.5Si ≤ 15.0…(Equation 1)

[0020] (d) A method for moderately forming SiO 2 film on the surface of steel has also been studied. As a result, an internal oxide layer of SiO 2 is formed under the surface of austenitic stainless steel with a specified composition, and then, a Cr oxide layer and an iron oxide layer are formed on the internal oxide layer of SiO 2 . Next, it was found that by pickling such a steel plate, the Cr oxide layer and the iron oxide layer are etched away, whereby an Si oxide film can be moderately left on the surface layer.

[0021] The present invention has been completed based on these insights, and its main idea is as follows.

[0022] [1] An austenitic stainless steel plate, characterized in that

[0023] it contains, by mass%,

[0024] C: 0 to 0.150%,

[0025] Si: 0.05 to 4.50%,

[0026] Mn: 0.05 to 3.00%,

[0027] P: 0.050% or less,

[0028] S: 0.0050% or less,

[0029] Ni: 8.00 to 21.00%,

[0030] Cr: 15.0 to 30.0%,

[0031] N: 0 to 0.350%,

[0032] Nb: 0 to 1.00%,

[0033] Mo: 0 to 3.00%,

[0034] Cu: 0 to 3.50%,

[0035] Al: 0.002 to 0.800%,

[0036] Ti: 0 to 0.600%,

[0037] V: 0 to 1.00%,

[0038] B: 0 to 0.0100%,

[0039] Ca: 0 to 0.0150%,

[0040] Sn: 0 to 1.00%,

[0041] Hf: 0 to 0.60%,

[0042] Zr: 0 to 0.60%,

[0043] Sb: 0 to 0.60%,

[0044] Co: 0 to 1.50%,

[0045] W: 0 to 2.00%,

[0046] Ta: 0 to 1.00%,

[0047] Ga: 0 to 0.50%,

[0048] Mg: 0 to 0.0050%, and

[0049] REM: 0 to 0.20%,

[0050] satisfying Formula 1,

[0051] the balance consists of Fe and impurities,

[0052] satisfying the following Formula 1,

[0053] when observing the surface of the steel sheet from directly above, there is a Si oxide film with an area ratio of 5.0% or more on the surface,

[0054] 0.5Cr + 10Al + 2Mo + 3Ti + 0.5Cu - 1.5Si ≤ 15.0... (Formula 1)

[0055] wherein, the element symbols in Formula 1 represent the content (mass %) of the element, and 0 is substituted when not contained.

[0056] [2] For the austenitic stainless steel sheet described in the above [1], in a cross-section perpendicular to the surface of the steel sheet, in a region with a width of 30 μm and up to 10 μm in the thickness direction of the steel sheet starting from the surface of the steel sheet, there is a Si-based oxide with a particle size of 1 μm or more with an area ratio of 3.0% or more.

[0057] [3] For the austenitic stainless steel sheet described in [1] or [2], in the cross-section in the thickness direction of the steel sheet, taking a range of 50 μm square as one field of view, the total length of the grain boundary cracks in any three fields of view is 15 μm or less.

[0058] [4] The austenitic stainless steel sheet according to any one of [1] to [3], wherein the Si oxide film is 50% or less in terms of area ratio.

[0059] [5] The austenitic stainless steel sheet according to any one of the above [1] to [4], which is for an ammonia combustion apparatus.

[0060] [6] A method for manufacturing an austenitic stainless steel sheet, which is a method for manufacturing the austenitic stainless steel sheet according to any one of the above [1] to [4], characterized in that

[0061] it has a pickling step, which is a step of heating and holding a steel sheet having the composition described in [1] above at 900 to 1100 °C after final cold rolling, then cooling to a temperature of 50 °C or less, and immersing it in a pickling solution containing 2.0% or less of hydrofluoric acid and 6 to 15% of nitric acid at a temperature of 50 to 60 °C for 40 to 60 seconds.

[0062] [7] According to the method for manufacturing an austenitic stainless steel sheet described in the above [6], after the acid treatment step, at least a part of the surface of the steel sheet is brushed.

[0063] [8] A component having the austenitic stainless steel sheet according to any one of the above [1] to [4] in at least a part thereof.

[0064] [9] The component according to the above [8], which is a component for an ammonia combustion apparatus.

[0065] With the austenitic stainless steel according to the present invention, it is possible to obtain a stainless steel having good corrosion resistance and abrasion resistance even when contacted with a gas containing a large amount of nitrogen and water (water vapor) at a temperature of about 500 to 700 °C such as ammonia combustion exhaust gas. Moreover, the austenitic stainless steel has a smaller coefficient of thermal expansion than ferritic stainless steel, so thermal fatigue is small, and in addition, high-temperature strength and high-temperature corrosion resistance are also good. Therefore, in an environment where corrosion resistance and strength at high temperatures are required and a large amount of nitrogen and water vapor are contained such as ammonia combustion exhaust gas, the austenitic stainless steel of the present invention is a very effective material. Detailed Description

[0066] Hereinafter, an embodiment of the present invention (hereinafter simply referred to as the present invention) will be described. Unless otherwise specified, "%" related to the composition means mass% in steel. The case where there is no particular lower limit specified and the case where the lower limit is 0% also include the case of not containing (0%).

[0067] <Regarding Steel Composition>

[0068] C: 0 to 0.150%

[0069] C is an element that reduces formability (r value), so it is preferably less, and the upper limit is set to 0.150%. From the perspective of formability, it is preferably 0.140% or less, 0.120% or less, or 0.100% or less. The lower limit is not particularly limited, but excessive reduction will lead to an increase in refining cost, so it is preferably 0.001% or more, and more preferably 0.002% or more.

[0070] Si: 0.05 - 4.50%

[0071] Si is an element effective in suppressing oxidation, especially water vapor oxidation, and is also effective in suppressing nitridation. Furthermore, from the perspective of generating an internal oxide layer of SiO 2 beneath the steel plate surface, it contains 0.05% or more. The lower limit of Si is preferably 0.10%, 0.20%, 0.30%, 0.50%, 0.80%, 1.00%, 1.25%, 1.50%, 1.70%, 1.90%, 2.00%, 2.20%, 2.40%, 2.50%, or 2.60%. On the other hand, if the Si content is high, the area ratio of the Si oxide layer (or SiO 2 internal oxide layer) increases, deteriorating workability and weldability, so the upper limit is set to 4.50%. The upper limit of Si is preferably 4.30%, 4.10%, or 4.00%.

[0072] Mn: 0.05 - 3.00%

[0073] Mn, like Si, is an element effective in oxidation resistance, so it is preferably contained at 0.05% or more. The lower limit of Mn is preferably 0.07%, 0.10%, 0.13%, or 0.15%. On the other hand, if Mn is contained in a large amount, workability deteriorates, so it is preferably contained at 3.00% or less. The upper limit of Mn is preferably 2.80%, 2.60%, 2.50%, or 2.40%.

[0074] P: 0.050% or less

[0075] P reduces toughness, hot workability, corrosion resistance, etc., and is harmful to stainless steel, so the less the better, preferably 0.040% or less. However, excessive reduction will increase the load during refining or require the use of high - price raw materials, so in reality, it can also contain 0.001% or more.

[0076] S: 0.0050% or less

[0077] S reduces toughness, hot workability, corrosion resistance, etc., which is harmful to stainless steel. Therefore, the less the better, and the upper limit is preferably 0.0050% or less, more preferably 0.0030% or less. However, excessive reduction will increase the load during refining or require the use of high-cost raw materials. Therefore, in reality, it can also contain 0.0001% or more.

[0078] Ni: 8.00 - 21.00%

[0079] Ni is an element that stabilizes the austenite phase, improves corrosion resistance against various acids, and also improves low-temperature toughness. Therefore, it is preferably contained at 8.00% or more, more preferably 9.00% or more, 10.00% or more, or 11.00% or more. On the other hand, since it is a high-cost element, even if contained in large amounts, the effect is not commensurate with the increase in alloy cost. Therefore, it is preferably 21.00% or less, more preferably 20.00% or less, or 18.00% or less.

[0080] Cr: 15.0 - 30.0%

[0081] Cr is an important element that gives stainless steel corrosion resistance. It is preferably contained at 15.0% or more, more preferably 15.5% or more, 16.0% or more, 17.0% or more, 18.0% or more, 19.0% or more, or 20.0% or more. On the other hand, containing a large amount will lead to a reduction in workability. Therefore, it is preferably 30.0% or less, more preferably 29.0% or less, 28.0% or less, 27.0% or less, or 26.0% or less.

[0082] N: 0 - 0.350%

[0083] From the perspective of suppressing grain boundary cracks caused by N on the surface, the N originally contained in the steel is preferably less. Moreover, N reduces workability and reduces corrosion resistance in combination with Cr. Therefore, it is preferably less, and the upper limit is preferably 0.350% or less, more preferably 0.300% or less, 0.280% or less, 0.260% or less, 0.240% or less, 0.220% or less, or 0.200% or less. On the other hand, since excessive reduction will increase the load in the refining process, it can preferably contain 0.001% or more, 0.005% or more, or 0.010% or more.

[0084] Nb: 0 - 1.00%

[0085] Nb has the effect of improving formability and corrosion resistance. On the other hand, if the addition exceeds 1.00%, recrystallization becomes difficult and the structure becomes coarser. Therefore, it is preferably 1.00% or less, more preferably 0.90% or less, 0.80% or less, or 0.70% or less. The lower limit of the Nb content is not particularly limited, but in order to effectively obtain the effect, it is preferably contained at 0.01% or more.

[0086] Mo: 0 to 3.00%

[0087] Mo has the effect of further improving the high corrosion resistance of stainless steel by adding Mo. On the other hand, it is not only an element that promotes nitriding, but also forms a brittle σ phase under high Cr, resulting in embrittlement and a decrease in corrosion resistance. Therefore, it is preferably set to 3.00% or less, more preferably 2.50% or less or 2.20% or less. The lower limit of the Mo content is not particularly limited, but in order to effectively obtain the corrosion resistance effect, it is preferably contained in an amount of 0.01% or more.

[0088] Cu: 0 to 3.50%

[0089] Cu has the effect of further improving the high corrosion resistance of stainless steel by adding Cu. On the other hand, excessive addition cannot achieve a performance improvement commensurate with the manufacturing cost. Therefore, it is preferably set to 3.50% or less, more preferably 3.20% or less, 3.00% or less or 2.80% or less. The lower limit of the Cu content is not particularly limited, but in order to effectively obtain the effect, it is preferably contained in an amount of 0.01% or more.

[0090] Al: 0.002 to 0.800%

[0091] Al is an element that combines with N to form AlN and promotes nitriding. Moreover, excessive addition will reduce the workability. Therefore, it is preferably that the Al content is 0.800% or less, more preferably 0.750% or less, 0.700% or less, 0.600% or less, 0.500% or less, 0.400% or less, 0.300% or less or 0.200% or less. On the other hand, since it has the effect of desulfurizing and thus improving the corrosion resistance, it is preferably that the Al content is 0.002% or more, more preferably 0.004% or more, 0.006% or more or 0.008% or more.

[0092] Ti: 0 to 0.600%

[0093] Ti ensures corrosion resistance through the stabilization effect of C and N. On the other hand, Ti is an element that promotes nitriding. If added in excess, TiN is significantly generated, resulting in nozzle blockage during manufacturing and surface defects of the product. Therefore, it is preferably set to 0.600% or less, more preferably 0.500% or less, 0.400% or less, or 0.300% or less. The lower limit of the Ti content is not particularly limited, but in order to effectively obtain the effect, it is preferably contained in an amount of 0.001% or more.

[0094] V: 0 to 1.00%

[0095] V has the effect of further improving the high corrosion resistance of stainless steel by adding V. On the other hand, if it is contained at a high concentration, it will cause a decrease in toughness. Therefore, it is preferably set to 1.00% or less, more preferably 0.90% or less, 0.70% or less, or 0.50% or less. The lower limit of the V content is not particularly limited, but in order to effectively obtain the effect, it is preferably contained at 0.01% or more or 0.05% or more.

[0096] B: 0 to 0.0100%

[0097] B is an element that improves the strength of the grain boundary and contributes to the improvement of workability. On the other hand, excessive addition will instead cause a decrease in workability due to a decrease in elongation. Therefore, it is preferably set to 0.0100% or less, more preferably 0.0090% or less, 0.0070% or less, or 0.0050% or less. The lower limit of the B content is not particularly limited, but in order to effectively obtain the effect, it is preferably contained at 0.0001% or more or 0.0005% or more.

[0098] Ca: 0 to 0.0150%

[0099] If Ca is contained in a large amount, the concentration in the oxide used to promote TiN formation will increase, causing its ability to be lost. Therefore, it is preferably contained at 0.0150% or less, more preferably 0.0120% or less, 0.0090% or less, 0.0070% or less, or 0.0050% or less. The lower limit is not particularly limited, but Ca is the main component of the slag and it is inevitable to be involved to some extent. In addition, it is difficult to completely remove it, and excessive reduction will increase the load during refining. Therefore, in actual operation, it can also be contained at 0.0001% or more or 0.0002% or more.

[0100] Sn: 0 to 1.00%

[0101] Sn has the effect of further improving the high corrosion resistance of stainless steel by adding Sn. On the other hand, excessive addition will cause a decrease in workability. Therefore, it is preferably set to 1.00% or less, more preferably 0.70% or less, 0.50% or less, or 0.30% or less. The lower limit of the Sn content is not particularly limited, but in order to effectively obtain the effect, it is preferably contained at 0.001% or more or 0.002% or more.

[0102] In addition, it may further contain Hf: 0 to 0.600%, Zr: 0 to 0.600%, Sb: 0 to 0.600%, Co: 0 to 1.500%, W: 0 to 2.000%, Ta: 0 to 1.000%, Ga: 0 to 0.500%, Mg: 0 to 0.0050%, REM: 0 to 0.200% in terms of mass%. These elements have the effect of improving the corrosion resistance of stainless steel by addition. On the other hand, since they are high-price elements, even if contained in excess, no effect commensurate with the increase in cost can be obtained, so an upper limit is set. The lower limit of the content of these elements is not particularly limited, but in order to surely obtain the effect when contained, it is preferred that Mg contains 0.0001% or more and the elements other than Mg contain 0.001% or more respectively.

[0103] The remainder (balance) of the above steel composition is Fe and impurities. Here, the so-called impurities mean components that are mixed in due to various factors in the manufacturing process such as raw materials like ores and scraps and are allowed within the range that does not give an adverse effect to the present invention.

[0104] <Nitriding tendency index>

[0105] From the viewpoint of suppressing nitriding of steel, when optimizing the steel composition, the relationship of the content of elements affecting nitriding is considered. As elements promoting nitriding, Cr, Mo, Ti, Al, and Cu are known, but a certain amount can also be contained in terms of ensuring functions such as corrosion resistance of stainless steel. Moreover, the present inventors found that Si, which is an important contained element in the steel of the present invention, not only has an inhibitory effect on red scale caused by steam oxidation, but also has an inhibitory effect on nitriding, although the reason is not yet certain. In addition, as described later, it is also effective to form a Si oxide film (SiO 2 film) on the steel surface. Then, it was thought to combine these elements promoting nitriding, namely Cr, Mo, Ti, Al, and Si, which has an effect in suppressing nitriding, in a well-balanced manner. It was found that in the case of austenitic stainless steel, as an index indicating the nitriding tendency, it can be evaluated by 0.5Cr + 10Al + 2Mo + 3Ti + 0.5Cu - 1.5Si. It was found that from the viewpoint of suppressing nitriding, it is preferable that this nitriding tendency index is set to 15.0 or less.

[0106] Nitriding tendency index = 0.5Cr + 10Al + 2Mo + 3Ti + 0.5Cu - 1.5Si ≤ 15.0... (Equation 1)

[0107] Among them, the element symbols in Equation 1 represent the content (mass%) of the element, and 0 is substituted when not contained.

[0108] The nitriding tendency index is, simply put, an indicator of the ease of nitriding, and a smaller preferred value is better. Therefore, the upper limit value of the nitriding tendency index is preferably 14.5, 14.0, 13.5, 13.0, 12.5, 12.0, 11.5, 11.0, or 10.0.

[0109] <Surface Si oxide film (SiO 2 film) area ratio>

[0110] It is preferable that there is a Si oxide film (SiO 2 film) on the steel plate surface. This is because even if the part where the Si oxide film exists comes into contact with nitrogen (N), nitrogen will not penetrate into the steel, and nitriding of this part can be suppressed. Therefore, it is preferable that there is 5.0% or more of the Si oxide film on the steel plate surface in terms of area ratio. It is preferably 6.0% or more, 7.0% or more, 8.0% or more, 9.0% or more, 10.0% or more, 11.0% or more, 12.0% or more, 13.0% or more, 14.0% or more, or 15.0% or more. There is no particular limitation on the upper limit of the area ratio of the Si oxide film. However, the Si oxide film hinders the gloss and design of stainless steel, and also deteriorates workability and weldability. Therefore, on the steel plate surface, it is preferable that the Si oxide film is 50.0% or less in terms of area ratio. It is preferably 45.0% or less, 40.0% or less, 35.0% or less, 30.0% or less, 25.0% or less, or 20.0% or less. The Si oxide film of the present invention is a film in which the Si oxide internally oxidized during the manufacturing process is exposed on the steel plate surface by removing the surface Fe-based and Cr-based oxides. Therefore, it is different from the amorphous Si oxide in the FeCr-based oxide layer (passive film).

[0111] The area ratio of the Si oxide film on the steel plate surface can be measured as follows. On the surface of the stainless steel plate to be measured, a square observation field of 30 μm square is set, and EPMA analysis is performed on this observation surface. Then, the part with a Si content of 5 wt% or more among the oxides generated on the surface is regarded as the Si oxide film, its area is measured, and the area ratio in the observation field is calculated, thereby obtaining it. Three or more observation fields can be arbitrarily selected in the same stainless steel plate, and the area ratios of the Si oxide films obtained in each observation field are arithmetically averaged to obtain the result. The method for measuring the area is not particularly limited. The photograph obtained by EPMA can be taken into a photo editing software (such as ImageJ), the photo can be binarized, and the measurement can be performed using image processing software.

[0112] <Area ratio of the surface Si oxide>

[0113] There is also Si oxide (SiO 2)It is preferable. The so-called surface layer of the steel plate refers to the region starting from the surface of the steel plate and extending up to 10 μm in the plate thickness direction. This is because the presence of Si-based oxides in the surface layer of the steel plate can prevent the intrusion of nitrogen (N) into the steel and inhibit nitriding. Therefore, in the surface layer of the steel plate, it is preferable that Si oxides with a particle size of 1 μm or more exist at an area ratio of 3.0% or more within an observation surface with a width of 30 μm. It is preferably 4.0% or more, 5.0% or more, 6.0% or more, 7.0% or more, 8.0% or more, 9.0% or more, or 10.0% or more. There is no particular limitation on the upper limit of the number of Si oxides in the observation surface with a width of 30 μm. However, the Si oxides in the surface layer of the steel plate deteriorate the workability and weldability. Therefore, in the surface layer of the steel plate, within an observation surface with a width of 30 μm, it is preferable that the Si oxides with a particle size of 1 μm or more are 20.0% or less in terms of area ratio. It is preferably 19.0% or less, 18.0% or less, 17.0% or less, 16.0% or less, or 15.0% or less. The Si oxides in the surface layer of the steel plate are Si oxides formed by internal oxidation during the manufacturing process. Therefore, they are different from the amorphous Si oxides in the FeCr-based oxide layer (passive film).

[0114] The area ratio of the Si oxides in the surface layer of the steel plate can be measured as follows. In a cross-section perpendicular to the steel plate surface of the stainless steel plate to be measured, an observation surface in the shape of a rectangle with a width of 30 μm and a thickness of 10 μm starting from the steel plate surface is arbitrarily selected, and EPMA analysis is performed on this observation surface. Then, among the observed oxides, the part with a Si content of 5 wt% or more is regarded as Si oxide, its shape (especially the major axis and minor axis) is measured, Si oxides with an average particle size of 1 μm or more are determined, and the area ratio in the observation surface is calculated, thereby obtaining it. Here, the average particle size is the equivalent area circle diameter (equivalent area circle diameter). Three or more observation surfaces can be arbitrarily selected in the same stainless steel plate, and the area ratios of the Si oxides obtained in each observation surface are arithmetically averaged to obtain the result. There is no particular limitation on the method for measuring the number. The photograph obtained by EPMA can be taken into a photo editing software (such as ImageJ), the photo can be binarized, and measurement can be performed using image processing software.

[0115] <Grain boundary crack length>

[0116] As a result of suppressing nitridation of the steel sheet surface, intergranular cracks caused by intrusion of nitrogen (N) are suppressed. The length of the intergranular cracks can be measured by observing the grain boundaries. It is preferable that in an arbitrarily selected range of 50 μm square in the surface layer part (a part including at least the nitrided part) of the steel sheet cross section at three locations, the total length of the intergranular cracks is 15 μm or less. If the total length of the intergranular cracks in the three observation surfaces is 15 μm or less, embrittlement of the steel sheet surface can be suppressed, and the strength of the steel sheet in the temperature range of 500 to 700 °C can be ensured. The shorter the total length of the intergranular cracks, the more preferable it is, and more preferably it is 14 μm or less, 13 μm or less, 12 μm or less, 11 μm or less, or 10 μm or less.

[0117] The length of the intergranular cracks in the surface layer part of the steel sheet can be measured as follows. The cross section of the steel sheet as a specimen is observed with a square observation field of 50 μm square under an optical microscope, and the length of the intergranular cracks is measured. At this time, since the part closer to the steel sheet surface is more easily affected by nitrogen, it is preferable to use the part immediately below the steel sheet surface as the observation field. It is preferable to measure by image processing during the measurement. For example, the intergranular crack part can be marked on the measurement image, and its length can be measured by image processing.

[0118] <Nitriding depth>

[0119] In the steel sheet of the present invention, the composition is adjusted to suppress nitridation, and a Si oxide film is present on the surface. Therefore, the nitriding depth becomes shallower on average. In particular, the smaller the value of the nitriding tendency index (including negative values), the shallower the nitriding depth tends to be. The nitriding depth varies somewhat depending on the nitrogen (N) content of the contacting gas, but it is confirmed that if it is approximately 100 μm or less, surface embrittlement is suppressed. The nitriding depth is preferably 95 μm or less, 90 μm or less, 85 μm or less, or 80 μm or less.

[0120] <Manufacturing method>

[0121] Next, the manufacturing method will be described. The manufacturing method described below is one embodiment for obtaining the steel sheet of the present invention, and is not limited to this manufacturing method. As long as the steel sheet of the present invention can be obtained, the manufacturing method is not limited.

[0122] One embodiment of the manufacturing method of the steel sheet of the present invention is a manufacturing method in which after manufacturing a steel sheet by a conventional method, a Si oxide layer is formed under the surface layer of the steel sheet by internal oxidation during final annealing, a Cr oxide layer and an Fe oxide layer are formed on the upper part (the steel sheet surface side), and after final annealing, the Cr oxide layer and the Fe oxide layer are etched and removed by pickling. Thus, the Si oxide layer appears on the surface and is formed into a Si oxide film, and thus the steel sheet of the present invention can be obtained.

[0123] The steel sheet before final annealing can be manufactured by using a manufacturing method as a conventional method. For example, it can be manufactured by the processes of steelmaking - hot rolling, steelmaking - hot rolling - annealing, or steelmaking - hot rolling - pickling - cold rolling.

[0124] However, in steelmaking, it is preferable to melt steel containing components adjusted to be the components described above in a converter or an electric furnace, and then perform a secondary refining method. The molten steel adjusted to a specified composition is made into a slab by a known casting method (e.g., continuous casting method). The slab is heated to a specified temperature and hot - rolled to a specified plate thickness. After hot rolling, cold rolling (cold rolling process) can also be performed as needed. Cold rolling can also be carried out by a conventional method.

[0125] Various conditions in the manufacturing process can be appropriately selected. For example, the slab thickness, hot - rolled plate thickness, etc. can be appropriately set. It is also possible to immerse the hot - rolled coil in a water - cooling bath after coiling. Regarding the pickling process after hot rolling or hot - rolled annealing, there is no particular limitation. Regarding mechanical descaling methods such as shot blasting, bending, and brushing, they can be appropriately selected. There is no particular limitation on the pickling solution after hot rolling either. For example, it can be existing conditions such as sulfuric acid, nitriding acid (nitric acid - hydrofluoric acid), etc. Furthermore, surface coil grinding can also be performed thereafter.

[0126] The hot - rolled steel sheet, hot - rolled annealed steel sheet, and cold - rolled steel sheet obtained in this way are subjected to final annealing. The annealing atmosphere is not particularly limited and can be an atmospheric atmosphere. Annealing is preferably carried out in a temperature range of 900 to 1100 °C. The holding time is not particularly limited, but it is preferably 30 seconds to 5 minutes. By annealing in this temperature range, a Si oxide layer (internal oxidation) is formed under the steel sheet surface. Moreover, on the upper layer (steel sheet surface side) of the Si oxide layer, a Cr oxide layer and an Fe oxide layer based on Fe in the steel sheet are formed by the diffusion of Cr in the steel sheet.

[0127] Cool the finally annealed steel sheet to below 60°C and perform pickling to etch away the upper Cr oxide layer and Fe oxide layer. For this purpose, the pickling solution is a pickling solution containing 2.0% or less of hydrofluoric acid (HF) and 6 - 15% of nitric acid, and it is preferably adjusted so that the immersion time is 50 - 80 seconds at a temperature of 50 - 60°C. Thus, the upper Fe oxide and Cr oxide are removed, and the Si oxide layer formed by internal oxidation appears on the surface layer, forming a Si oxide film with a moderate area ratio. Although it is also possible not to contain hydrofluoric acid, if too much Si oxide remains, not only will the design be deteriorated due to surface coloring, but also the workability and weldability will be deteriorated. Therefore, the pickling solution may contain preferably 0.1% or more, more preferably 0.2% or more, 0.3% or more, 0.4% or more, or 0.5% or more of hydrofluoric acid to dissolve a part of the Si oxide and make the Si oxide film remain moderately. If there is too much HF, the Si oxide will be removed excessively, so it is preferably 2.0% or less, more preferably 1.5% or less or 1.0% or less.

[0128] Furthermore, a brushing process for brushing the surface of the steel sheet may be added after pickling. By brushing the surface of the steel sheet, the upper Fe oxide and Cr oxide can be effectively removed, and the removal amount can also be adjusted. Therefore, the Si oxide film can be exposed on the surface of the steel sheet in a manner that achieves the desired area ratio. Regarding brushing, it is sufficient to brush at least a part of the surface of the steel sheet, and the entire surface of the steel sheet may also be brushed. In addition, either the front surface or the back surface of the steel sheet may be brushed, or both may be brushed.

[0129] The type of brush for brushing is not particularly limited. It is preferable to select a brush according to the hardness difference between the Fe oxide and Cr oxide to be removed and the Si oxide to be left. This is because, by doing so, the Fe oxide and Cr oxide can be selectively removed without removing the surface Si oxide. For example, an abrasive brush with abrasive grains whose roughness has been adjusted is applied.

[0130] <Use>

[0131] Even when the steel sheet of the present invention is used in a gas environment with a high nitrogen (N) content, from the perspective of good nitridation resistance, the nitrogen infiltration into the surface layer of the steel sheet is small, and intergranular cracking is suppressed. Moreover, it also has oxidation resistance. Especially in the medium to high temperature range of about 500 - 700°C, it is effective against the generation of red scale, which has been a problem in conventional stainless steels. Therefore, it can be used, for example, for ammonia combustion equipment in a medium to high temperature region with a high nitrogen content and a gas temperature of 500 - 700°C. In particular, it can be used for exhaust components of ammonia combustion equipment.

[0132] Of course, in terms of the properties of nitridation resistance and oxidation resistance, even when used in containers, piping components, etc. that come into direct contact with, for example, ammonia, urea, etc., it is possible to inhibit the intrusion of nitrogen ions, ammonia, urea in the solution, and nitrogen in the evaporation gas into the steel plate surface layer, and inhibit intergranular cracks.

[0133] In addition, if the steel plate of the present invention is applied to components that require nitridation resistance and oxidation resistance, the effects can be obtained.

[0134] Examples

[0135] The present invention will be described more specifically below by way of examples, but the present invention is not limited by these examples.

[0136] Steel with the composition shown in Table 1 of melting was cast into a slab, and the slab was hot-rolled to obtain a hot-rolled steel plate with a thickness of 4 mm. Then, annealing of the hot-rolled plate was carried out at a temperature of 900 to 1100 °C, pickling was carried out, and cold rolling was carried out to obtain a cold-rolled steel plate with a thickness of 1.5 mm. The obtained cold-rolled steel plate was annealed (final annealing) at a temperature of 900 to 1100 °C, and then, it was immersed in a pickling solution (2% hydrofluoric acid + 10% nitric acid + water) at 50 to 60 °C for 50 to 80 seconds (final pickling), and then washed with water to obtain a test material. The liquid temperature conditions and immersion time of the final pickling are shown in Table 2.

[0137] After the final pickling, the surface was finished by brushing. Regarding the brush, a brush with SiC abrasive grains was used, and it was carried out under the conditions of a load current of 80 to 120 A, a rotation speed of 1000 rpm, and a reduction amount of 0.5 to 1.0 mm.

[0138] Four test pieces of 20 mm × 25 mm were cut out from the obtained test material. One was used for the determination of the area ratio of the Si oxide film on the surface, and the remaining three were subjected to nitridation and oxidation treatments assuming ammonia combustion gas.

[0139] The determination of the area ratio of the Si oxide film on the steel plate surface was carried out by EPMA. It was carried out under the conditions of an acceleration voltage: 15 kV, an irradiation current: 2.0×10 -7 A, an analysis area: 30 μm × 30 μm, and a measurement time: 50 ms. The Si oxide film (SiO 2 ) was discriminated from the obtained image, the image was binarized using photo editing software (Image J), and the area ratio with respect to the observed field area was obtained using image processing software.

[0140] Regarding the nitriding-oxidation treatment, a gas containing 10 vol% ammonia, 10 vol% water vapor, and the remaining nitrogen (N) as the atmosphere gas is introduced into an annealing furnace. The remaining test pieces are placed in the furnace, heated to 600 °C and held for 50 hours, then cooled, taken out, and the intergranular crack length and nitriding depth are measured.

[0141] Regarding the intergranular crack length, the test piece after nitriding-oxidation treatment is cut in a manner that enables observation of the cross-section in the plate thickness direction, and the cross-section of the test piece is observed using an optical microscope. For the observation, a range of 50 μm × 50 μm directly below the steel plate surface is regarded as one field of view, and three randomly selected locations in the cross-section of the specimen are observed to measure the generated length of intergranular cracks. If the total length of the intergranular cracks in the three observed surfaces is 15 μm or less, it is considered good.

[0142] Regarding the nitriding depth, the test piece after nitriding-oxidation treatment is cut, and electrolytic etching is performed for 5 seconds at a voltage of 6 V using a 10% oxalic acid aqueous solution, and then observed using an optical microscope. The measurement of the nitriding depth is carried out using photographs.

[0143] The red scale property is confirmed visually. A specimen in which no red scale is found is rated as qualified (○), and a specimen in which even a little red scale is found is rated as unqualified (×). These measurement results are shown in Table 2. From the data in Table 2, it can be seen that the intergranular crack length of the steel plate of the present invention is reduced.

[0144]

[0145]

[0146] Industrial Applicability

[0147] The present invention can be utilized in all industries such as the automotive industry and general machinery industry.

Claims

1. An austenitic stainless steel plate, characterized in that it contains, by mass%, C：0～0.150％、 Si: 0.05 - 4.50%, Mn: 0.05 - 3.00%, P: 0.050% or less, S: 0.0050% or less, Ni: 8.00 - 21.00%, Cr：15.0～30.0％、 N：0～0.350％、 Nb: 0 - 1.00%, Mo: 0 - 3.00%, Cu: 0 - 3.50%, Al：0.002～0.800％、 Ti: 0 - 0.600%, V：0～1.00％、 B：0～0.0100％、 Ca: 0 - 0.0150%, Sn: 0 - 1.00%, Hf: 0 - 0.60%, Zr：0～0.60％、 Sb: 0 - 0.60%, Co: 0 - 1.50%, W：0～2.00％、 Ta: 0 - 1.00%, Ga: 0 - 0.50%, Mg: 0 - 0.0050%, and REM: 0 - 0.20%, satisfies formula 1, the balance is composed of Fe and impurities, satisfies the following formula 1, when observing the surface of the steel plate vertically from above, there is a Si oxide film of 5.0% or more by area on the surface, 0.5Cr + 10Al + 2Mo + 3Ti + 0.5Cu - 1.5Si ≤ 15.0... (Formula 1) wherein, the element symbols in Formula 1 represent the content of the element by mass%, and 0 is substituted when not contained.

2. The austenitic stainless steel plate according to claim 1, in a cross-section perpendicular to the surface of the steel plate, there are Si-based oxides with a particle size of 1 μm or more and an area ratio of 3.0% or more in a region with a width of 30 μm and extending 10 μm in the thickness direction of the steel plate starting from the surface of the steel plate.

3. The austenitic stainless steel plate according to claim 1 or 2, in the cross-section in the thickness direction of the steel plate, taking a range of 50 μm square as one field of view, the total length of grain boundary cracks in any three fields of view is 15 μm or less.

4. The austenitic stainless steel plate according to any one of claims 1 to 3, there is a Si oxide film of 50% or less by area ratio.

5. The austenitic stainless steel plate according to any one of claims 1 to 4, which is for an ammonia combustion device.

6. A method for manufacturing an austenitic stainless steel plate, which is a method for manufacturing the austenitic stainless steel plate according to any one of claims 1 to 4, characterized in that it has a pickling process, and the pickling process is to heat and hold the steel plate having the composition described in claim 1 at 900 - 1100 °C after final cold rolling, then cool it to a temperature of 50 °C or less, and immerse it in a pickling solution containing 2.0% or less of hydrofluoric acid and 6 - 15% of nitric acid and having a temperature of 50 - 60 °C for 50 - 80 seconds.

7. The method for manufacturing an austenitic stainless steel plate according to claim 6, after the pickling process, at least a part of the surface of the steel plate is brushed.

8. A component having the austenitic stainless steel plate according to any one of claims 1 to 4 in at least a part thereof.

9. The component according to claim 8, which is a component for an ammonia combustion device.

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