WO2024096109A1 - Ferritic stainless steel sheet, production method therefor, and parts - Google Patents

Ferritic stainless steel sheet, production method therefor, and parts Download PDF

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Publication number
WO2024096109A1
WO2024096109A1 PCT/JP2023/039683 JP2023039683W WO2024096109A1 WO 2024096109 A1 WO2024096109 A1 WO 2024096109A1 JP 2023039683 W JP2023039683 W JP 2023039683W WO 2024096109 A1 WO2024096109 A1 WO 2024096109A1
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Prior art keywords
steel sheet
less
stainless steel
ferritic stainless
oxide layer
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PCT/JP2023/039683
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French (fr)
Japanese (ja)
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佳幸 藤村
純一 濱田
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日鉄ステンレス株式会社
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Publication of WO2024096109A1 publication Critical patent/WO2024096109A1/en

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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
    • C23G1/00—Cleaning or pickling metallic material with solutions or molten salts
    • C23G1/02—Cleaning or pickling metallic material with solutions or molten salts with acid solutions
    • C23G1/08—Iron or steel

Definitions

  • the present invention relates to ferritic stainless steel, its manufacturing method, and parts made from said ferritic stainless steel sheet.
  • ammonia is attracting attention as a fuel to replace carbon fuels.
  • the combustion reaction of ammonia is 4NH 3 + 3O 2 ⁇ 2N 2 + 6H 2 O, and it is expected to be a circulable fuel because it produces water and nitrogen and has a small environmental impact.
  • the combustion temperature of ammonia is 1750°C in adiabatic flame temperature, which is lower than the 2120°C of hydrogen, the 1970°C of methane, and the approximately 2000°C of gasoline, and the combustion temperature in actual engines and gas turbines is also lower than these existing fuels.
  • the exhaust gas temperature is about 500 to 700°C, which is lower than that of existing fuels.
  • This temperature range of 500 to 700°C is a temperature range at which steel materials used in exhaust pipes, etc. are easily oxidized, and is a temperature range at which so-called red scale is easily generated.
  • Patent Document 1 proposes a ferritic stainless steel for fuel cell reformers that is resistant to oxidation and red scale even at high temperatures of around 600°C.
  • Patent Document 2 proposes a ferritic stainless steel for use in the exhaust pipes of internal combustion engines equipped with a pollution control system that contains urea or ammonia to reduce nitrogen oxides.
  • ammonia which has a lower combustion temperature than existing fuels, is added and burned, so the combustion exhaust gas temperature is lower than that of existing fuels, at around 500 to 700°C.
  • ammonia combustion gas contains a large amount of nitrogen and water vapor. The inclusion of water vapor makes it easy for steam oxidation and red scale to occur.
  • the combustion gas temperature is about 500 to 700°C, which is also a temperature range in which red scale is likely to occur. For this reason, oxidation resistance (red scale resistance) is required for steel materials used in ammonia combustion gas systems, etc.
  • Patent Document 1 has high-temperature oxidation resistance and red scale resistance as a steel material for exhaust gas system parts, but does not take into consideration measures against grain boundary cracking caused by surface nitriding in gases that contain large amounts of nitrogen, such as ammonia combustion gas.
  • Patent Document 2 does have the effect of suppressing surface grain boundary cracking caused by urea and ammonia, but it cannot be said to have sufficient nitriding resistance against gases containing large amounts of nitrogen, such as the exhaust gas from ammonia combustion.
  • the present invention aims to provide a ferritic stainless steel that has red scale resistance (oxidation resistance) and intergranular cracking resistance (nitridation resistance) even when exposed to gases containing large amounts of nitrogen and water (water vapor), such as ammonia combustion exhaust gas, at temperatures of about 500 to 700°C.
  • Nitriding tendency index 10Al + 2Mo + 3Ti + 0.5Cu - 1.5Si ⁇ 5.0 .... (Equation 1)
  • the ferritic stainless steel of the present invention makes it possible to obtain stainless steel with good corrosion resistance and wear resistance, even when it comes into contact with gases containing large amounts of nitrogen and water (water vapor) at temperatures of about 500 to 700°C, such as ammonia combustion exhaust gas.
  • % indicates mass% in the steel. When no lower limit is specified or the lower limit is 0%, this also includes cases where the component is not contained (0%).
  • ⁇ Steel composition> C 0 to 0.030%
  • C is an element that reduces formability (r value), so the less C the better, with the upper limit set at 0.030%. From the viewpoint of formability, 0.020% or less, or 0.010% or less is preferred. There is no particular lower limit, but an excessive reduction leads to an increase in refining costs, so 0.001% or more is preferred, and 0.002% or more is even more preferred.
  • Si 0.05 to 3.00% Si is an element that is effective in suppressing oxidation, particularly steam oxidation, and is also effective in suppressing nitridation. Furthermore, from the viewpoint of generating an SiO 2 internal oxidation layer just below the steel sheet surface, the content is 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%.
  • the upper limit is set to 3.00%.
  • the upper limit of Si is preferably 2.95% or 2.90%.
  • Mn 0.05 to 1.20% Mn, like Si, is an element effective for oxidation resistance, so it is preferable to contain 0.05% or more.
  • the lower limit of Mn is preferably 0.07%, 0.09%, 0.11%, 0.13%, or 0.15%.
  • the upper limit of Mn is preferably 1.10%, 1.00%, 0.90%, or 0.80%.
  • P 0.050% or less P is harmful to stainless steels, as it reduces toughness, hot workability, and corrosion resistance, so the less the better, and it is better to keep it at 0.050% or less, and preferably at 0.040% or less.
  • an excessive decrease in P content increases the load during refining or requires the use of expensive raw materials, so in reality it may contain 0.001% or more.
  • S 0.005% or less S is harmful to stainless steels, as it reduces toughness, hot workability, and corrosion resistance, so the less the better, and the upper limit should be 0.005% or less, preferably 0.003% or less.
  • an excessive reduction in S content increases the load during refining or requires the use of expensive raw materials, so in reality, a content of 0.0001% or more is acceptable.
  • Ni 0 to 1.00% or less
  • Ni has the effect of further enhancing the high corrosion resistance of stainless steel by adding it.
  • Ni is an expensive element, even if it is contained in a large amount, the effect is not commensurate with the increase in alloy cost, so it is good to make it 1.00% or less, and preferably 0.80% or less, 0.60% or less, or 0.50% or less.
  • Cr 12.0 to 31.0% Cr is an important element that provides corrosion resistance to stainless steel, and should be contained at 12.0% or more, preferably 12.5% or more, 13.0% or more, 14.0% or more, 15.0% or more, 18.0% or more, or 20.0% or more. On the other hand, since a large content of Cr leads to a decrease in workability, it should be contained at 31.0% or less, preferably 30.0% or less, 29.0% or less, 28.0% or less, 26.0% or less, or 24.0% or less.
  • N 0 to 0.030%
  • the N content in the steel material is small.
  • N reduces workability and reduces corrosion resistance by combining with Cr, it is preferable that the content is small, and it is good to set it to 0.030% or less, preferably 0.025% or less, 0.020% or less, 0.015% or less, or 0.010% or less.
  • excessive reduction places a large load on the refining process, it may be contained at 0.001% or more.
  • Nb 0 to 1.00% Nb has the effect of improving formability and corrosion resistance. On the other hand, if it is added in excess of 1.00%, recrystallization becomes difficult and the structure becomes coarse, so it is preferable to set the content at 1.00% or less, and preferably at 0.90% or less, 0.80% or less, or 0.70% or less. There is no particular lower limit for the Nb content, but it is preferable to include 0.01% or more in order to ensure the effect.
  • Mo 0 to 2.50%
  • Mo is not only an element that promotes nitriding, but also forms a brittle sigma phase with high Cr, which leads to embrittlement and a decrease in corrosion resistance, so it is recommended that the content be 2.50% or less, preferably 2.20% or less, or 2.00% or less.
  • Mo content is no particular lower limit for the Mo content, but it is preferable to contain 0.01% or more in order to reliably obtain the effect of corrosion resistance.
  • Cu 0 to 3.00%
  • the addition of Cu has the effect of further enhancing the high corrosion resistance of stainless steel.
  • excessive addition does not improve performance to justify the manufacturing cost, so it is preferable to set it to 3.00% or less, and preferably to 2.50% or less, 2.20% or less, or 1.90% or less.
  • Al 0.002 to 0.500%
  • Al is an element that combines with N to form AlN and promotes nitriding, and since excessive addition reduces workability, the Al content should be 0.500% or less, and preferably 0.450% or less, 0.400% or less, 0.350% or less, 0.300% or less, 0.250% or less, or 0.200% or less.
  • the Al content should be 0.002% or more, and preferably 0.004% or more, 0.007% or more, or 0.010% or more.
  • Ti 0 to 0.600% Ti ensures corrosion resistance by stabilizing C and N.
  • Ti is an element that promotes nitriding, and if added in excess, TiN is significantly generated, causing nozzle blockage during production and surface defects in the product, so it is preferable to make the content 0.600% or less, and preferably 0.500% or less, 0.400% or less, or 0.300% or less. There is no particular lower limit for the Ti content, but it is preferable to contain 0.001% or more to ensure the effect.
  • V 0 to 1.00%
  • the addition of V has the effect of further enhancing the high corrosion resistance of stainless steel.
  • the upper limit of V should be set to 1.00%, and preferably 0.90% or less, 0.70% or less, or 0.50% or less.
  • B 0 to 0.0100% B is an element that enhances the strength of grain boundaries and contributes to improving workability.
  • the content should be 0.0100% or less, preferably 0.0090% or less, 0.0070% or less, or 0.0050% or less.
  • the B content it is preferable to contain 0.0001% or more, or 0.0005% or more.
  • Ca 0 to 0.0150% If Ca is contained in a large amount, the concentration in the oxide for promoting TiN generation increases and the ability to do so is lost, so it is good to contain 0.0150% or less, and preferably 0.0120% or less, 0.0090% or less, 0.0070% or less, or 0.0050% or less. There is no particular lower limit, but Ca is the main component of slag, and some inclusion is unavoidable. In addition, it is difficult to completely remove it, and excessive reduction increases the load during refining, so it may be contained in an amount of 0.0001% or more, or 0.0002% or more in practical operation.
  • Sn 0 to 1.00%
  • the addition of Sn has the effect of further enhancing the high corrosion resistance of stainless steel.
  • the content should be 1.00% or less, preferably 0.70% or less, 0.50% or less, or 0.30% or less.
  • impurities refer to components that are mixed in during the industrial production of steel due to various factors in the manufacturing process, including raw materials such as ores and scraps, and are acceptable within the scope of not adversely affecting the present invention.
  • ⁇ Nitriding tendency index> In optimizing the steel components from the viewpoint of suppressing nitridation of steel, the relationship of the content of elements that affect nitridation was considered. Cr, Mo, Ti, and Al are known as elements that promote nitridation, but a certain amount of them may be contained in order to ensure the functions of stainless steel, such as corrosion resistance. Furthermore, the inventors have found that Si, an important element contained in the steel according to the present invention, not only has an effect of suppressing red scale caused by steam oxidation, but also has an effect of suppressing nitridation, although the reason is unclear. In addition, as will be described later, it is also effective to form a Si oxide film (SiO 2 film) on the steel surface.
  • SiO 2 film Si oxide film
  • Nitriding tendency index 10Al + 2Mo + 3Ti + 0.5Cu - 1.5Si ⁇ 5.0 ....
  • Equation 1 In the formula 1, the symbol of an element indicates the content (mass%) of the element, and 0 is substituted when the element is not contained.
  • the nitriding tendency index is, in short, an index of the ease of nitriding, and a smaller value is preferable. Therefore, the upper limit of the nitriding tendency index is preferably 4.8, 4.6, 4.4, 4.2, 4.0, 3.9, 3.8, 3.7, 3.6, or 3.5.
  • SiO2 film ⁇ Area ratio of surface Si oxide film ( SiO2 film)> It is preferable that a Si oxide film (SiO 2 film) is present on the steel sheet surface. This is because the portion where the Si oxide film is present does not penetrate into the steel even if nitrogen (N) comes into contact with the portion, and nitridation of the portion is suppressed. Therefore, it is preferable that the Si oxide film is present at an area ratio of 5.0% or more on the steel sheet surface. Preferably, it is 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 upper limit to the area ratio of the Si oxide film.
  • the Si oxide film inhibits the luster and design of stainless steel, and also deteriorates workability and weldability. Therefore, it is preferable that the Si oxide film is present at an area ratio of 50.0% or less on the steel sheet surface. Preferably, it may be 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 Si oxide that is internally oxidized during the manufacturing process and is exposed on the steel sheet surface by removing the Fe-based and Cr-based oxides on the surface. Therefore, it is different from the Si oxide with an amorphous structure in the FeCr-based oxide layer (passive film).
  • the area ratio of the Si oxide film on the steel plate surface can be measured as follows.
  • the observation field is a 30 ⁇ m square on the stainless steel plate surface to be measured, and the observation surface is analyzed by EPMA.
  • the area of the oxide formed on the surface that has an Si content of 5 wt% or more is measured as the Si oxide film, and the area ratio in the observation field is calculated.
  • three or more observation fields can be arbitrarily selected, and the area ratio of the Si oxide film obtained in each can be calculated as an arithmetic average.
  • photo editing software e.g., ImageJ
  • Si oxide (SiO 2 ) is also present in the surface layer of the steel sheet.
  • the surface layer of the steel sheet refers to a region from the surface of the steel sheet to 10 ⁇ m in the sheet thickness direction.
  • the presence of Si-based oxides in the surface layer of the steel sheet prevents the intrusion of nitrogen (N) into the steel and suppresses nitriding. Therefore, it is preferable that Si oxides having a grain size of 1 ⁇ m or more are present in an area ratio of 3.0% or more in an observation surface of 30 ⁇ m in width in the surface layer of the steel sheet.
  • the Si oxide in the steel sheet surface layer is an Si oxide that is internally oxidized during the manufacturing process. Therefore, it is different from the Si oxide with an amorphous structure in the FeCr-based oxide layer (passive film).
  • the area ratio of the Si oxide on the surface of the steel plate can be measured as follows. In the vertical cross section of the steel plate surface of the stainless steel plate to be measured, a rectangular observation surface 30 ⁇ m wide and 10 ⁇ m from the steel plate surface in the plate thickness direction is arbitrarily selected, and the observation surface is analyzed by EPMA. Among the oxides observed, the parts with a Si content of 5 wt% or more are regarded as Si oxides, and their shapes (particularly the major and minor axes) are measured, and Si oxides with an average particle size of 1 ⁇ m or more are identified, and the area ratio on the observation surface is calculated.
  • the average particle size is the diameter of a circle equivalent to the area (diameter of a circle equivalent to the area).
  • Intergranular crack length As a result of suppressing nitridation of the steel sheet surface, intergranular cracking due to the intrusion of nitrogen (N) is suppressed. Intergranular cracking can be measured by observing the crystal grain boundaries, and three arbitrary 100 ⁇ m square ranges are selected from the surface layer portion (at least the portion including the nitrided portion) of the steel sheet cross section, and the total intergranular crack length in these areas is preferably 20 ⁇ m or less. If the total intergranular crack length in the three observation surfaces is 20 ⁇ m or less, embrittlement of the steel sheet surface can be suppressed, and the steel sheet strength in the temperature range of 500 to 700 ° C. can be ensured. The shorter the total intergranular crack length, the more preferable it is, and it is more preferable that it is 18 ⁇ m or less, 16 ⁇ m or less, 14 ⁇ m or less, 12 ⁇ m or less, or 10 ⁇ m or less.
  • the intergranular crack length in the surface layer of the steel plate can be measured as follows.
  • the cross section of the sample steel plate is observed under an optical microscope with a square observation field of 100 ⁇ m on each side, and the intergranular crack length is measured.
  • the observation field is set to the area directly below the steel plate surface.
  • the intergranular crack area can be marked on the measurement image, and its length can be measured using image processing.
  • the steel plate according to the present invention has a shallow nitriding depth on average because the components are adjusted to suppress nitriding and the surface has a silicon oxide film.
  • the nitriding depth varies slightly depending on the nitrogen (N) content of the contacting gas, but it has been confirmed that surface embrittlement is suppressed if the nitriding depth is generally 220 ⁇ m or less.
  • the nitriding depth is preferably 210 ⁇ m or less, 200 ⁇ m or less, 190 ⁇ m or less, or 180 ⁇ m or less.
  • the manufacturing method described below is one embodiment for obtaining the steel plate according to the present invention, and the manufacturing method is not limited to this. As long as the steel plate according to the present invention can be obtained, the manufacturing method is not limited.
  • a steel plate is manufactured by a conventional method, and then a Si oxide layer is formed under the surface of the steel plate by internal oxidation during final annealing, and a Cr oxide layer and an Fe oxide layer are formed on top of that (on the surface side of the steel plate). After final annealing, the Cr oxide layer and the Fe oxide layer are etched away by acid washing, whereby the Si oxide layer appears on the surface and becomes a Si oxide film, thereby obtaining the steel plate according to the present invention.
  • Steel sheets before final annealing can be manufactured using standard manufacturing methods. For example, they can be manufactured using the process of steelmaking-hot rolling, steelmaking-hot rolling-annealing, or steelmaking-hot rolling-pickling-cold rolling.
  • a suitable method is to melt steel containing the components adjusted to the composition described above in a converter or electric furnace, followed by secondary refining.
  • the molten steel thus adjusted to the desired composition is made into slabs using a known casting method (e.g., continuous casting).
  • the slabs are heated to a desired temperature and hot rolled to a desired thickness.
  • the steel may be cold rolled (cold rolling) if necessary. Cold rolling may also be performed using conventional methods.
  • the slab thickness and hot-rolled sheet thickness may be set as appropriate. After coiling the hot-rolled sheet, it may be immersed in a water-cooled pool.
  • the mechanical descaling method such as shot blasting, bending, or brushing, may be selected as appropriate.
  • the pickling solution after hot rolling either, so existing conditions such as sulfuric acid, nitric hydrofluoric acid, etc. may be used.
  • the coil surface may be ground after this.
  • the hot-rolled steel sheet, hot-rolled annealed steel sheet, and cold-rolled steel sheet thus obtained are then subjected to final annealing.
  • the annealing atmosphere is not particularly limited, and may be an air atmosphere. Annealing is preferably performed in a temperature range of 900 to 1100°C.
  • the holding time is not particularly limited, but is preferably 30 seconds to 5 minutes.
  • a silicon oxide layer is formed under the surface of the steel sheet (internal oxidation).
  • a Cr oxide layer is formed above the silicon oxide layer (on the surface side of the steel sheet) due to the diffusion of Cr in the steel sheet, and an Fe oxide layer is formed due to the Fe in the steel sheet.
  • the steel sheet after final annealing is cooled to 60°C or less and pickled to etch away the upper Cr oxide layer and Fe oxide layer.
  • the pickling solution should contain 2.0% or less hydrofluoric acid (HF) and 6-15% nitric acid, and should be adjusted to a temperature of 50-60°C and an immersion time of 40-60 seconds. This removes the upper Fe oxide and Cr oxide layers, and the internally oxidized Si oxide layer appears on the surface, forming a Si oxide film with an appropriate area ratio. Hydrofluoric acid is not necessary, but if excessive Si oxide remains, it will not only deteriorate the design due to surface coloring, but also deteriorate workability and weldability.
  • hydrofluoric acid in the pickling solution at a concentration of preferably 0.1% or more, preferably 0.2% or more, 0.3% or more, 0.4% or more, or 0.5% or more to dissolve part of the Si oxide and leave an appropriate Si oxide film. If there is too much HF, excessive silicon oxide will be removed, so it should be 2.0% or less, preferably 1.5% or less, or 1.0% or less.
  • a brushing step of brushing the steel sheet surface may be added after pickling.
  • the Fe oxide and Cr oxide in the upper layer can be reliably removed, and the amount of removal can be adjusted, so that the Si oxide film can be exposed on the steel sheet surface so as to have a desired area ratio.
  • Brushing is sufficient if at least a part of the steel sheet surface is brushed, and the entire steel sheet surface may also be brushed.
  • brushing may be performed on either the front or back surface of the steel sheet, or both.
  • the type of brush for brushing is not particularly limited. It is advisable to select a brush based on the difference in hardness between the Fe oxide and Cr oxide to be removed and the Si oxide to be left behind. This is because it is possible to selectively remove the Fe oxide and Cr oxide without removing the surface Si oxide. For example, it is advisable to use an abrasive brush with abrasive grains having adjusted roughness.
  • the steel sheet according to the present invention is used in a gas environment with a high nitrogen (N) content, the good nitriding resistance reduces nitrogen penetration into the surface layer of the steel sheet, suppressing grain boundary cracking. Furthermore, the steel sheet according to the present invention has oxidation resistance, and is effective against the generation of red scale, which is a problem with conventional stainless steels, particularly in the medium to high temperature range of about 500 to 700°C. For this reason, the steel sheet according to the present invention can be used, for example, for ammonia combustion equipment with a high nitrogen content and a medium to high gas temperature range of 500 to 700°C. In particular, the steel sheet can be used for exhaust parts of ammonia combustion equipment.
  • the nitrogen ions in the solution and the nitrogen in the evaporated gases of ammonia and urea are prevented from penetrating into the steel sheet surface, thereby suppressing grain boundary cracking.
  • the steel sheet according to the present invention can be used to obtain the same effects when applied to parts that require resistance to nitridation and oxidation.
  • the liquid temperature conditions and immersion time of the final pickling are shown in Table 2.
  • the surface was finished by brushing using a SiC abrasive brush at a load current of 80 to 120 A, a rotation speed of 1000 rpm, and a reduction of 0.5 to 1.0 mm.
  • test pieces measuring 20 mm x 25 mm were cut from the obtained test material, one of which was used to measure the area ratio of the surface silicon oxide film, and the remaining three were subjected to nitriding and oxidation treatment to simulate ammonia combustion gas.
  • the area ratio of the Si oxide film on the steel sheet surface was measured using an EPMA under the following conditions: acceleration voltage: 15 kV, irradiation current: 2.0 ⁇ 10 ⁇ 7 A, analysis area: 30 ⁇ m ⁇ 30 ⁇ m, measurement time: 50 ms.
  • the Si oxide film (SiO 2 ) was identified from the obtained image, and the image was binarized using photo editing software (ImageJ), and the area ratio relative to the observation field area was calculated using image processing software.
  • the nitriding and oxidation treatment involved introducing 10 vol% ammonia, 10 vol% water vapor, and the balance nitrogen (N) into the atmospheric gas of an annealing furnace, placing the remaining test pieces in the furnace, heating them to a temperature of 600°C and holding them there for 50 hours, then cooling and removing them, and measuring the intergranular crack length and nitriding depth.
  • the intergranular crack length was measured by cutting the test piece after nitriding and oxidation treatment so that the cross section in the thickness direction could be observed, and observing the cross section of the test piece using an optical microscope. The observation was performed with a field of view of 100 ⁇ m x 100 ⁇ m just below the steel plate surface, and three randomly selected points on the sample cross section were observed to measure the intergranular crack length. A satisfactory result was obtained if the total intergranular crack length on the three observation surfaces was 20 ⁇ m or less.
  • the nitriding depth was measured by cutting the test piece after the nitriding and oxidation treatment, electrolytically etching it with a 10% oxalic acid aqueous solution at a voltage of 6 V for 5 seconds, and observing it with an optical microscope. The nitriding depth was measured using a photograph. The red scale was visually confirmed, and samples in which no red scale was found were rated as pass ( ⁇ ), and samples in which even a small amount of red scale was found were rated as fail ( ⁇ ). The results of these measurements are shown in Table 2. The data in Table 2 show that the steel plate according to the present invention has a reduced intergranular crack length.
  • This invention can be used in all industries, including the automotive and general machinery industries.

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Abstract

The present invention provides a ferritic stainless steel sheet in which intergranular cracking is inhibited (nitrification resistance), and a red scale is inhibited from occurring (oxidation resistance), in a moderate- to high-temperature region of 500-700°C, even in a gas atmosphere containing nitrogen and water vapor, such as ammonia combustion gas.　The ferritic stainless steel sheet according to the present invention has a predetermined component composition having a nitrification tendency factor, described below, of at most five. In addition, a Si oxide film is provided on a surface of the steel sheet at an area ratio of at least 5%. The steel sheet can be obtained by subjecting a steel sheet to finishing-annealing to form an internal oxide layer (Si oxide layer), subsequently performing acid cleaning to remove a Cr oxide layer and an Fe oxide layer from a surface layer, and forming a Si oxide layer in the surface layer.　Nitrification tendency factor = 10Al + 2Mo + 3Ti + 0.5Cu - 1.5Si

Description

フェライト系ステンレス鋼板、その製造方法、および部品Ferritic stainless steel sheet, its manufacturing method, and parts
　本発明は、フェライト系ステンレス鋼とその製造方法および当該フェライト系ステンレス鋼板を用いた部品に関する。 The present invention relates to ferritic stainless steel, its manufacturing method, and parts made from said ferritic stainless steel sheet.
　地球温暖化が国際的な環境問題となっており、カーボンゼロやカーボンニュートラルなどの脱炭素化社会の実現への技術開発が活発に行われている。そのような流れの中、炭素燃料に代わる燃料としてアンモニアが注目されている。アンモニアの燃焼反応式は４ＮＨ３＋３Ｏ２→２Ｎ２＋６Ｈ２Ｏであり、水と窒素が生成され環境負荷が小さく、循環可能な燃料として期待されている。アンモニアの燃焼温度は、断熱火炎温度で１７５０℃であり、水素の２１２０℃、メタンの１９７０℃、ガソリンの約２０００℃と比較しても低く、実際のエンジンやガスタービン中での燃焼温度も、これら既存の燃料と比べて低くなる。そのため、アンモニアを燃料として使用した場合、その排ガス温度も既存燃料の場合より低温である５００～７００℃程度になる。この５００～７００℃に温度域は排気管などに用いられる鋼材が酸化し易い温度であり、いわゆる赤スケールが発生し易い温度域である。 Global warming has become an international environmental issue, and technological development is being actively carried out to realize a decarbonized society, such as carbon zero and carbon neutral. In this trend, ammonia is attracting attention as a fuel to replace carbon fuels. The combustion reaction of ammonia is 4NH 3 + 3O 2 → 2N 2 + 6H 2 O, and it is expected to be a circulable fuel because it produces water and nitrogen and has a small environmental impact. The combustion temperature of ammonia is 1750°C in adiabatic flame temperature, which is lower than the 2120°C of hydrogen, the 1970°C of methane, and the approximately 2000°C of gasoline, and the combustion temperature in actual engines and gas turbines is also lower than these existing fuels. Therefore, when ammonia is used as fuel, the exhaust gas temperature is about 500 to 700°C, which is lower than that of existing fuels. This temperature range of 500 to 700°C is a temperature range at which steel materials used in exhaust pipes, etc. are easily oxidized, and is a temperature range at which so-called red scale is easily generated.
　特許文献１には、燃料電池用改質器用鋼材であって、６００℃程度の高温であっても耐酸化性、耐赤スケール性を有するフェライト系ステンレス鋼が提案されている。 Patent Document 1 proposes a ferritic stainless steel for fuel cell reformers that is resistant to oxidation and red scale even at high temperatures of around 600°C.
　特許文献２には、窒素酸化物を還元処理するため尿素やアンモニアを有する汚染制御システムを備えた内燃機関の排気管用フェライト系ステンレス鋼が提案されている。 Patent Document 2 proposes a ferritic stainless steel for use in the exhaust pipes of internal combustion engines equipped with a pollution control system that contains urea or ammonia to reduce nitrogen oxides.
特開２００３－１６０８４４号公報JP 2003-160844 A 特開２０１５－５３２６８１号公報JP 2015-532681 A
　アンモニアの燃料としての活用は、単独燃焼だけでなく他の燃料（重油、軽油、水素等）との混焼での開発も行われている。しかし、混焼とはいえ、既存燃料よりも燃焼温度の低いアンモニアを加えて燃焼させるため、燃焼排ガス温度は既存燃料より低く５００～７００℃程度となる。さらにアンモニアの燃焼ガス中には多量の窒素と水蒸気が含有されている。
　水蒸気の含有により水蒸気酸化や赤スケールが発生し易くなる。さらに、燃焼ガス温度が５００～７００℃程度と赤スケールが発生し易い温度域でもある。このため、アンモニア燃焼ガス系などに用いられる鋼材には耐酸化性（耐赤スケール性）が要求される。 Development is being carried out not only for the use of ammonia as a fuel, but also for its co-combustion with other fuels (heavy oil, light oil, hydrogen, etc.). However, even in the case of co-combustion, ammonia, which has a lower combustion temperature than existing fuels, is added and burned, so the combustion exhaust gas temperature is lower than that of existing fuels, at around 500 to 700°C. Furthermore, ammonia combustion gas contains a large amount of nitrogen and water vapor.
The inclusion of water vapor makes it easy for steam oxidation and red scale to occur. Furthermore, the combustion gas temperature is about 500 to 700°C, which is also a temperature range in which red scale is likely to occur. For this reason, oxidation resistance (red scale resistance) is required for steel materials used in ammonia combustion gas systems, etc.
　さらに、アンモニア燃焼排ガス中に含まれる多量の窒素により、鋼材表層に窒素が浸入（窒化）し粒界割れに起因する脆化が顕在化してくる。そのため、アンモニア燃焼ガス系用鋼材には耐窒化性（耐粒界割れ性）も要求される。 Furthermore, the large amount of nitrogen contained in ammonia combustion exhaust gas causes nitrogen to penetrate (nitridation) into the surface layer of the steel, resulting in embrittlement caused by grain boundary cracking. For this reason, steel for ammonia combustion gas systems is also required to be nitridation-resistant (grain boundary cracking-resistant).
　特許文献１のステンレス鋼は、排ガス系部品用鋼材として耐高温酸化性および耐赤スケール性を確保したものであるが、アンモニアの燃焼ガスのように窒素を多量に含んだガスに対する表層窒化による粒界割れへの対策は考慮されていない。 The stainless steel in Patent Document 1 has high-temperature oxidation resistance and red scale resistance as a steel material for exhaust gas system parts, but does not take into consideration measures against grain boundary cracking caused by surface nitriding in gases that contain large amounts of nitrogen, such as ammonia combustion gas.
　特許文献２のステンレス鋼は、一応、尿素やアンモニアによる表層粒界割れの抑制効果はあるが、アンモニアの燃焼排ガスのように多量の窒素含有ガスに対しては、耐窒化性が十分とは言えない。 The stainless steel of Patent Document 2 does have the effect of suppressing surface grain boundary cracking caused by urea and ammonia, but it cannot be said to have sufficient nitriding resistance against gases containing large amounts of nitrogen, such as the exhaust gas from ammonia combustion.
　本発明は、フェライト系ステンレス鋼であって、アンモニア燃焼排ガスのような５００～７００℃程度で多量の窒素と水（水蒸気）を含有したガスに対しても、耐赤スケール性（耐酸化性）と耐粒界割れ性（耐窒化性）を有することを課題とし、そのような鋼材（フェライト系ステンレス鋼）を提供することを目的とする。 The present invention aims to provide a ferritic stainless steel that has red scale resistance (oxidation resistance) and intergranular cracking resistance (nitridation resistance) even when exposed to gases containing large amounts of nitrogen and water (water vapor), such as ammonia combustion exhaust gas, at temperatures of about 500 to 700°C.
　上記課題を達成するため、本発明者らは鋭意検討し、以下の知見を得た。 In order to achieve the above objective, the inventors conducted extensive research and obtained the following findings.
（ａ）窒化を抑制する観点から、窒化を助長する元素であるＭｏ、Ｔｉ、Ａｌ、Ｃｕの含有量を最適化するとよいことを考えた。 (a) From the perspective of suppressing nitridation, we considered it beneficial to optimize the content of Mo, Ti, Al, and Cu, which are elements that promote nitridation.
（ｂ）さらに、鋼材表層に耐赤スケール性、耐窒化性皮膜を形成することを想起して、鋼材表面にＳｉ酸化物皮膜（ＳｉＯ２皮膜。Ｓｉ酸化物層と言う場合もある。）を有することにより、耐赤スケール性、耐窒化性に有効であることを見出した。耐赤スケール性、耐窒化性を確保するには、鋼板表面にＳｉ酸化物皮膜を５％以上生成することがよいことを見出した。 (b) Furthermore, by considering the formation of a red-scale resistant and nitridation resistant film on the surface layer of a steel material, it was found that having a Si oxide film ( SiO2 film, also called a Si oxide layer) on the surface of the steel material is effective for red-scale resistance and nitridation resistance. It was found that in order to ensure red-scale resistance and nitridation resistance, it is good to generate a Si oxide film of 5% or more on the surface of the steel sheet.
（ｃ）さらにＳｉは、水蒸気酸化による赤スケールに対し抑制効果があるだけでなく、理由は定かではないものの窒化を抑制する効果があることも見出した。窒化を助長する元素Ｍｏ、Ｔｉ、Ａｌ、Ｃｕの含有量とともに、窒化を抑制するＳｉの含有量とから、鋼材の窒化傾向を示す窒化傾向指数を導出し、この窒化傾向指数が５．０以下にするとよいことを見出した。

窒化傾向指数＝１０Ａｌ＋２Ｍｏ＋３Ｔｉ＋０．５Ｃｕ－１．５Ｓｉ≦５．０
・・・・・（式１） (c) Furthermore, it was found that Si not only has an inhibitory effect against red scale caused by steam oxidation, but also has an inhibitory effect against nitridation, although the reason is unclear. A nitridation tendency index, which indicates the nitridation tendency of steel material, was derived from the contents of the elements Mo, Ti, Al, and Cu that promote nitridation, and the content of Si that inhibits nitridation, and it was found that it is desirable for this nitridation tendency index to be 5.0 or less.

Nitriding tendency index = 10Al + 2Mo + 3Ti + 0.5Cu - 1.5Si ≦ 5.0
.... (Equation 1)
（ｄ）鋼材表面に適度にＳｉＯ２皮膜を生成する方法についても検討した。その結果、所定の成分を有するフェライト系ステンレス鋼の表面下にＳｉＯ２の内部酸化層を生成し、その後ＳｉＯ２の内部酸化層上にＣｒ酸化物層、酸化鉄層を生成する。次にこのような鋼板を酸洗浄することによりＣｒ酸化物層と酸化鉄層をエッチング除去することにより、表層に適度にＳｉ酸化物皮膜を残存させることができることを見出した。 (d) A method for forming an appropriate SiO2 film on the steel surface was also investigated. As a result, an internal oxide layer of SiO2 is formed under the surface of a ferritic stainless steel having a specified composition, and then a Cr oxide layer and an iron oxide layer are formed on the internal oxide layer of SiO2 . Next, the steel sheet is acid-washed to etch away the Cr oxide layer and the iron oxide layer, and it was found that an appropriate Si oxide film can be left on the surface.
　本発明は、これら知見を基になしたものであり、その要旨は以下のとおりである。 The present invention is based on these findings and is summarized as follows:
［１］
　　質量％で、
Ｃ　：０～０．０３０％、
Ｓｉ：０．０５～３．００％、
Ｍｎ：０．０５～１．２０％、
Ｐ　：０．０５０％以下、
Ｓ　：０．００５％以下、
Ｎｉ：０～１．００％、
Ｃｒ：１２．０～３１．０％、
Ｎ　：０～０．０３０％、
Ｎｂ：０～１．００％
Ｍｏ：０～２．５０％
Ｃｕ：０～３．００％
Ａｌ：０．００２～０．５００％、
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％、
ＲＥＭ：０～０．２０％、を含み、
式１を満足し、
残部Ｆｅおよび不純物からなり、
以下の式１を満足し、
鋼板表面を垂直上方から見たときに表面上にＳｉ酸化物皮膜が面積％で５．０％以上存在することを特徴とするフェライト系ステンレス鋼板。
１０Ａｌ＋２Ｍｏ＋３Ｔｉ＋０．５Ｃｕ－１．５Ｓｉ≦５．０
・・・・・（式１）
ただし、式１中の元素記号は、当該元素の含有量（質量％）を示し、含有しない場合は０を代入する。
［２］
　前記鋼板表面に垂直な断面において、幅３０μｍで、前記鋼板表面から鋼板厚さ方向に１０μｍまでの領域内に、粒子径１μｍ以上のＳｉ系酸化物が面積率で３．０％以上存在する上記［１］に記載のフェライト系ステンレス鋼板。
［３］
　前記鋼板の板厚方向断面で、１００μｍ四方の範囲を１視野として、任意の３視野の粒界割れ長さの合計が２０μｍ以下である、［１］または［２］に記載のフェライト系ステンレス鋼板。
［４］
　前記Ｓｉ酸化物皮膜が面積％で５０％以下存在する、［１］～［３］のいずれか１つ記載のフェライト系ステンレス鋼板。
［５］
　アンモニア燃焼機器用である前記［１］～［４］のいずれか１つに記載のフェライト系ステンレス鋼板。
［６］
　上記［１］～［４］のいずれか１つに記載のフェライト系ステンレス鋼板の製造方法であって、前［１］に記載の成分を有する鋼板を、最終冷延後に、９００～１１００℃に加熱保持した後、５０℃以下の温度まで冷却し、フッ酸２．０％以下、硝酸６～１５％を含有し、温度５０～６０℃の酸洗液中に４０～６０秒間浸漬する酸洗工程を有することを特徴とするフェライト系ステンレス鋼板の製造方法。
［７］
　前記酸性工程の後に、前記鋼板表面の少なくとも一部をブラッシングする上記［６］に記載のフェライト系ステンレス鋼板の製造方法。
［８］
　前記［１］～［４］のいずれか１つに記載のフェライト系ステンレス鋼板を少なくとも一部に有する、部品。
［９］
　アンモニア燃焼機器用部品である前記［８］に記載の部品。 [1]
In mass percent,
C: 0 to 0.030%,
Si: 0.05 to 3.00%,
Mn: 0.05 to 1.20%,
P: 0.050% or less,
S: 0.005% or less,
Ni: 0 to 1.00%,
Cr: 12.0 to 31.0%,
N: 0 to 0.030%,
Nb: 0 to 1.00%
Mo: 0 to 2.50%
Cu: 0 to 3.00%
Al: 0.002 to 0.500%,
Ti: 0 to 0.600%
V: 0 to 1.00%,
B: 0 to 0.0100%,
Ca: 0 to 0.0150%
Sn: 0 to 1.00%,
Hf: 0 to 0.60%,
Zr: 0 to 0.60%,
Sb: 0 to 0.60%,
Co: 0 to 1.50%,
W: 0 to 2.00%,
Ta: 0 to 1.00%,
Ga: 0 to 0.50%,
Mg: 0 to 0.0050%,
REM: 0 to 0.20%;
Satisfying Equation 1,
The balance is Fe and impurities,
The following formula 1 is satisfied:
A ferritic stainless steel sheet characterized in that a silicon oxide film is present on the surface of the steel sheet in an area percentage of 5.0% or more when the surface is viewed vertically from above.
10Al+2Mo+3Ti+0.5Cu-1.5Si≦5.0
.... (Equation 1)
In the formula 1, the symbol of an element indicates the content (mass%) of the element, and 0 is substituted when the element is not contained.
[2]
The ferritic stainless steel sheet according to the above-mentioned [1], wherein in a cross section perpendicular to the steel sheet surface, within a region having a width of 30 μm and extending from the steel sheet surface to 10 μm in the thickness direction of the steel sheet, Si-based oxides having particle sizes of 1 μm or more are present at an area ratio of 3.0% or more.
[3]
The ferritic stainless steel sheet according to [1] or [2], wherein the total length of intergranular cracks in any three visual fields, each of which is a 100 μm square area in a cross section in the thickness direction of the steel sheet, is 20 μm or less.
[4]
The ferritic stainless steel sheet according to any one of [1] to [3], wherein the Si oxide film is present in an area percentage of 50% or less.
[5]
The ferritic stainless steel sheet according to any one of [1] to [4] above, which is for an ammonia burning appliance.
[6]
A method for producing a ferritic stainless steel sheet according to any one of the above [1] to [4], comprising a pickling step of heating and holding a steel sheet having the composition according to the above [1] at 900 to 1100°C after final cold rolling, and then cooling to a temperature of 50°C or less, and immersing the steel sheet in a pickling solution containing 2.0% or less hydrofluoric acid and 6 to 15% nitric acid at a temperature of 50 to 60°C for 40 to 60 seconds.
[7]
The method for producing a ferritic stainless steel sheet according to the above [6], wherein after the acid step, at least a part of the surface of the steel sheet is brushed.
[8]
A part having at least a part made of the ferritic stainless steel sheet according to any one of [1] to [4] above.
[9]
The part according to [8] above, which is an ammonia burning appliance part.
効果effect
　本発明に係るフェライト系ステンレス鋼により、アンモニア燃焼排ガスのような温度５００～７００℃程度で窒素や水（水蒸気）を多量に含有するガスが接触しても、耐食性、耐摩耗性の良好なステンレス鋼を得ることができる。 The ferritic stainless steel of the present invention makes it possible to obtain stainless steel with good corrosion resistance and wear resistance, even when it comes into contact with gases containing large amounts of nitrogen and water (water vapor) at temperatures of about 500 to 700°C, such as ammonia combustion exhaust gas.
　以下、本発明の実施態様（以下、単に本発明という。）について説明する。特に断りのない限り、成分に関する「％」は鋼中の質量％を示す。特に下限を規定していない場合や下限が０％となっているものは、含有しない場合（０％）も含む。 The following describes an embodiment of the present invention (hereinafter, simply referred to as the present invention). Unless otherwise specified, "%" for components indicates mass% in the steel. When no lower limit is specified or the lower limit is 0%, this also includes cases where the component is not contained (0%).
　＜鋼成分について＞
　Ｃ：０～０．０３０％
　Ｃは、成形性（ｒ値）を低下させる元素であるため少ない方が好ましく、上限を０．０３０％とする。成形性の観点から０．０２０％以下、または０．０１０％以下が好ましい。下限は特に限定しないが、過度な低減は精錬コストの上昇を招くため０．００１％以上が好ましく、さらに好ましくは０．００２％以上である。 <Steel composition>
C: 0 to 0.030%
C is an element that reduces formability (r value), so the less C the better, with the upper limit set at 0.030%. From the viewpoint of formability, 0.020% or less, or 0.010% or less is preferred. There is no particular lower limit, but an excessive reduction leads to an increase in refining costs, so 0.001% or more is preferred, and 0.002% or more is even more preferred.
　Ｓｉ：０．０５～３．００％
　Ｓｉは酸化、特に水蒸気酸化の抑制に対し有効であるとともに、窒化の抑制にも有効な元素である。さらに、鋼板表面直下にＳｉＯ２内部酸化層を生成する観点から０．０５％以上含有する。Ｓｉの下限は、好ましくは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％、または２．６０％であるとよい。一方、Ｓｉ含有量を多くするとＳｉ酸化物皮膜（もしくはＳｉＯ２内部酸化層）の面積率が増加し加工性や溶接性を劣化させるため、３．００％を上限とする。Ｓｉの上限は、好ましくは２．９５％、または２．９０％とするとよい。 Si: 0.05 to 3.00%
Si is an element that is effective in suppressing oxidation, particularly steam oxidation, and is also effective in suppressing nitridation. Furthermore, from the viewpoint of generating an SiO 2 internal oxidation layer just below the steel sheet surface, the content is 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 increased, the area ratio of the Si oxide film (or the SiO 2 internal oxidation layer) increases, deteriorating the workability and weldability, so the upper limit is set to 3.00%. The upper limit of Si is preferably 2.95% or 2.90%.
　Ｍｎ：０．０５～１．２０％
　Ｍｎは、Ｓｉと同様で耐酸化性に有効な元素であることから０．０５％以上含有するとよい。Ｍｎの下限は、好ましくは０．０７％、０．０９％、０．１１％、０．１３％、または０．１５％であるとよい。一方、Ｍｎを多く含有すると加工性を劣化させるので、１．２０％以下含有するとよい。Ｍｎの上限は、好ましくは１．１０％、１．００％、０．９０、または０．８０％であるとよい。 Mn: 0.05 to 1.20%
Mn, like Si, is an element effective for oxidation resistance, so it is preferable to contain 0.05% or more. The lower limit of Mn is preferably 0.07%, 0.09%, 0.11%, 0.13%, or 0.15%. On the other hand, since a large amount of Mn deteriorates workability, it is preferable to contain 1.20% or less. The upper limit of Mn is preferably 1.10%, 1.00%, 0.90%, or 0.80%.
　Ｐ：０．０５０％以下
　Ｐは靱性や熱間加工性、耐食性を低下させる等、ステンレス鋼にとって有害であるため、少ないほど良く、０．０５０％以下にするとよく、好ましくは０．０４０％以下にするとよい。ただし、過剰な低下は精錬時の負荷を高くするか、または高価格の原料を用いる必要があるため、現実的には０．００１％以上含有してもよい。 P: 0.050% or less P is harmful to stainless steels, as it reduces toughness, hot workability, and corrosion resistance, so the less the better, and it is better to keep it at 0.050% or less, and preferably at 0.040% or less. However, an excessive decrease in P content increases the load during refining or requires the use of expensive raw materials, so in reality it may contain 0.001% or more.
　Ｓ：０．００５％以下
　Ｓは靱性や熱間加工性、耐食性を低下させる等、ステンレス鋼にとって有害であるため、少ないほど良く、上限を０．００５％以下にするとよく、好ましくは０．００３％以下にするとよい。ただし、過剰な低下は精錬時の負荷が高くするか、または高価格の原料を用いる必要があるため、現実的には０．０００１％以上含有してもよい。 S: 0.005% or less S is harmful to stainless steels, as it reduces toughness, hot workability, and corrosion resistance, so the less the better, and the upper limit should be 0.005% or less, preferably 0.003% or less. However, an excessive reduction in S content increases the load during refining or requires the use of expensive raw materials, so in reality, a content of 0.0001% or more is acceptable.
　Ｎｉ：０～１．００％以下
　Ｎｉは添加することでステンレス鋼の高い耐食性をさらに高める作用がある。一方、高価な元素であるため多量に含有しても合金コストの増大に見合う効果が得られないため、１．００％以下にするとよく、好ましくは０．８０％以下、０．６０％以下、または０．５０％以下にするとよい。Ｎｉ含有量の下限は特に限定しないが、効果を確実に得るため好ましくは０．０１％以上含有するとよい。 Ni: 0 to 1.00% or less Ni has the effect of further enhancing the high corrosion resistance of stainless steel by adding it. On the other hand, since Ni is an expensive element, even if it is contained in a large amount, the effect is not commensurate with the increase in alloy cost, so it is good to make it 1.00% or less, and preferably 0.80% or less, 0.60% or less, or 0.50% or less. There is no particular lower limit for the Ni content, but it is preferable to contain 0.01% or more to ensure the effect.
　Ｃｒ：１２．０～３１．０％
　Ｃｒはステンレス鋼に耐食性をもたらす重要な元素であり、１２．０％以上含有するとよく、好ましくは１２．５％以上、１３．０％以上、１４．０％以上、１５．０％以上、１８．０％以上、または２０．０％以上にするとよい。その一方で多量の含有は加工性の低下を招くため、３１．０％以下にするとよく、好ましくは３０．０％以下、２９．０％以下、２８．０％以下、２６．０％以下、または２４．０％以下にするとよい。 Cr: 12.0 to 31.0%
Cr is an important element that provides corrosion resistance to stainless steel, and should be contained at 12.0% or more, preferably 12.5% or more, 13.0% or more, 14.0% or more, 15.0% or more, 18.0% or more, or 20.0% or more. On the other hand, since a large content of Cr leads to a decrease in workability, it should be contained at 31.0% or less, preferably 30.0% or less, 29.0% or less, 28.0% or less, 26.0% or less, or 24.0% or less.
　Ｎ：０～０．０３０％
　表面のＮによる粒界割れを抑制する観点から、鋼材に元々含有するＮは少ない方が好ましい。さらに、Ｎは加工性を低下させ、Ｃｒと結合して耐食性を低下させるため、少ない方が好ましく、０．０３０％以下にするとよく、好ましくは０．０２５％以下、０．０２０％以下、０．０１５％以下、または０．０１０％以下にするとよい。一方、過剰な低減は精錬工程上の負荷が大きいため、０．００１％以上含有してもよい。 N: 0 to 0.030%
From the viewpoint of suppressing grain boundary cracking due to surface N, it is preferable that the N content in the steel material is small. Furthermore, since N reduces workability and reduces corrosion resistance by combining with Cr, it is preferable that the content is small, and it is good to set it to 0.030% or less, preferably 0.025% or less, 0.020% or less, 0.015% or less, or 0.010% or less. On the other hand, since excessive reduction places a large load on the refining process, it may be contained at 0.001% or more.
　Ｎｂ：０～１．００％
　Ｎｂは成形性や耐食性を高める作用がある。一方、１．００％を超えて添加すると再結晶しにくくなって組織が粗くなるため、１．００％以下にするよく、好ましくは０．９０％以下、０．８０％以下、または０．７０％以下にするとよい。Ｎｂ含有量の下限は特に限定しないが、効果を確実に得るため好ましくは０．０１％以上含有するとよい。 Nb: 0 to 1.00%
Nb has the effect of improving formability and corrosion resistance. On the other hand, if it is added in excess of 1.00%, recrystallization becomes difficult and the structure becomes coarse, so it is preferable to set the content at 1.00% or less, and preferably at 0.90% or less, 0.80% or less, or 0.70% or less. There is no particular lower limit for the Nb content, but it is preferable to include 0.01% or more in order to ensure the effect.
　Ｍｏ：０～２．５０％
　Ｍｏは添加することでステンレス鋼の高い耐食性をさらに高める作用がある。一方、窒化を助長する元素であるばかりか、高Ｃｒで脆いシグマ相を形成して脆化と耐食性の低下を招くため、２．５０％以下にするとよく、好ましくは２．２０％以下、または２．００％以下にするとよい。Ｍｏ含有量の下限は特に限定しないが、耐食性の効果を確実に得るため好ましくは０．０１％以上含有するとよい。 Mo: 0 to 2.50%
The addition of Mo has the effect of further enhancing the high corrosion resistance of stainless steel. On the other hand, Mo is not only an element that promotes nitriding, but also forms a brittle sigma phase with high Cr, which leads to embrittlement and a decrease in corrosion resistance, so it is recommended that the content be 2.50% or less, preferably 2.20% or less, or 2.00% or less. There is no particular lower limit for the Mo content, but it is preferable to contain 0.01% or more in order to reliably obtain the effect of corrosion resistance.
　Ｃｕ：０～３．００％
　Ｃｕは添加することでステンレス鋼の高い耐食性をさらに高める作用がある。一方、過剰な添加は製造上のコストに見合う性能向上がなされないため、３．００％以下にするとよく、好ましくは２．５０％以下、２．２０％以下、または１．９０％以下にするとよい。Ｃｕ含有量の下限は特に限定しないが、効果を確実に得るため好ましくは０．０１％以上含有するとよい。 Cu: 0 to 3.00%
The addition of Cu has the effect of further enhancing the high corrosion resistance of stainless steel. On the other hand, excessive addition does not improve performance to justify the manufacturing cost, so it is preferable to set it to 3.00% or less, and preferably to 2.50% or less, 2.20% or less, or 1.90% or less. There is no particular lower limit for the Cu content, but it is preferable to contain 0.01% or more to ensure the effect.
　Ａｌ：０．００２～０．５００％
　ＡｌはＮと結びつきＡｌＮを生成し、窒化を助長する元素であり、さらに過剰な添加は加工性を低下させるため、Ａｌ含有量を０．５００％以下にするとよく、好ましくは０．４５０％以下、０．４００％以下、０．３５０％以下、０．３００％以下、０．２５０％以下、または０．２００％以下にするとよい。一方、脱硫して耐食性を向上する効果があることから、Ａｌ含有量を０．００２％以上にするとよく、好ましくは０．００４％以上、０．００７％以上、または０．０１０％以上にするとよい。 Al: 0.002 to 0.500%
Al is an element that combines with N to form AlN and promotes nitriding, and since excessive addition reduces workability, the Al content should be 0.500% or less, and preferably 0.450% or less, 0.400% or less, 0.350% or less, 0.300% or less, 0.250% or less, or 0.200% or less. On the other hand, since Al has the effect of desulfurizing and improving corrosion resistance, the Al content should be 0.002% or more, and preferably 0.004% or more, 0.007% or more, or 0.010% or more.
　Ｔｉ：０～０．６００％
　ＴｉはＣやＮの安定化作用により耐食性を担保する。一方、Ｔｉは窒化を助長する元素であり、過剰に添加するとＴｉＮが著しく生成して製造時のノズル閉塞や製品の表面欠陥を招くため、０．６００％以下にするとよく、好ましくは０．５００％以下、０．４００％以下、または０．３００％以下にするとよい。Ｔｉ含有量の下限は特に限定しないが、効果を確実に得るため好ましくは０．００１％以上含有するとよい。 Ti: 0 to 0.600%
Ti ensures corrosion resistance by stabilizing C and N. On the other hand, Ti is an element that promotes nitriding, and if added in excess, TiN is significantly generated, causing nozzle blockage during production and surface defects in the product, so it is preferable to make the content 0.600% or less, and preferably 0.500% or less, 0.400% or less, or 0.300% or less. There is no particular lower limit for the Ti content, but it is preferable to contain 0.001% or more to ensure the effect.
　Ｖ：０～１．００％
　Ｖは添加することでステンレス鋼の高い耐食性をさらに高める作用がある。一方、高濃度に含有すると靱性の低下を招くため、その上限を１．００％とするとよく、好ましくは０．９０％以下、０．７０％以下、または０．５０％以下にするとよい。Ｖ含有量の下限は特に限定しないが、効果を確実に得るため好ましくは０．０１％以上、または０．０５％以上含有するとよい。 V: 0 to 1.00%
The addition of V has the effect of further enhancing the high corrosion resistance of stainless steel. On the other hand, since a high concentration of V leads to a decrease in toughness, the upper limit of V should be set to 1.00%, and preferably 0.90% or less, 0.70% or less, or 0.50% or less. There is no particular lower limit for the V content, but to ensure the effect, it is preferable to include 0.01% or more, or 0.05% or more.
　Ｂ：０～０．０１００％
　Ｂは粒界の強度を高める元素であり、加工性の向上に寄与する。一方、過剰な添加は却って延びの低下による加工性低下を招くため、含有量を０．０１００％以下にするとよく、好ましくは０．００９０％以下、０．００７０％以下、または０．００５０％以下にするとよい。Ｂ含有量の下限は特に限定しないが、効果を確実に得るため好ましくは０．０００１％以上、または０．０００５％以上含有するとよい。 B: 0 to 0.0100%
B is an element that enhances the strength of grain boundaries and contributes to improving workability. On the other hand, excessive addition of B leads to a decrease in elongation and hence a decrease in workability, so the content should be 0.0100% or less, preferably 0.0090% or less, 0.0070% or less, or 0.0050% or less. There is no particular lower limit for the B content, but to ensure the effect, it is preferable to contain 0.0001% or more, or 0.0005% or more.
　Ｃａ：０～０．０１５０％
　Ｃａは多く含有すると、ＴｉＮ生成を促進するための酸化物中の濃度が上昇し、その能力を失わせるため０．０１５０％以下含有するとよく、好ましくは０．０１２０％以下、０．００９０％以下、０．００７０％以下、または０．００５０％以下にするとよい。下限は特に限定しないが、Ｃａはスラグの主成分であり、多少の巻き込みは避けられない。また、完全に除去することは難しく、過剰な低下は精錬時の負荷が高くなるため、実操業としては０．０００１％以上、または０．０００２％以上含有してもよい。 Ca: 0 to 0.0150%
If Ca is contained in a large amount, the concentration in the oxide for promoting TiN generation increases and the ability to do so is lost, so it is good to contain 0.0150% or less, and preferably 0.0120% or less, 0.0090% or less, 0.0070% or less, or 0.0050% or less. There is no particular lower limit, but Ca is the main component of slag, and some inclusion is unavoidable. In addition, it is difficult to completely remove it, and excessive reduction increases the load during refining, so it may be contained in an amount of 0.0001% or more, or 0.0002% or more in practical operation.
　Ｓｎ：０～１．００％
　Ｓｎは添加することでステンレス鋼の高い耐食性をさらに高める効果がある。一方で過剰な添加は加工性の低下につながるため、１．００％以下にするとよく、好ましくは０．７０％以下、０．５０％以下、または０．３０％以下にするとよい。Ｓｎ含有量の下限は特に限定しないが、効果を確実に得るため好ましくは０．００１％以上、または０．００２％以上含有するとよい。 Sn: 0 to 1.00%
The addition of Sn has the effect of further enhancing the high corrosion resistance of stainless steel. On the other hand, excessive addition leads to a decrease in workability, so the content should be 1.00% or less, preferably 0.70% or less, 0.50% or less, or 0.30% or less. There is no particular lower limit for the Sn content, but in order to ensure the effect, it is preferable to contain 0.001% or more, or 0.002% or more.
　その他、さらに質量％で、Ｈｆ：０～０．６００％、Ｚｒ：０～０．６００％、Ｓｂ：０～０．６００％、Ｃｏ：０～１．５００％、Ｗ：０～２．０００％、Ｔａ：０～１．０００％、Ｇａ：０～０．５００％、Ｍｇ：０～０．００５０％、ＲＥＭ：０～０．２００％を含んでも良い。これら元素は添加することでステンレス鋼の耐食性を高める作用がある。一方で、高価な元素であるため、過剰に含有してもコストの増大に見合う効果が得られないため、上限を設けた。これら元素の含有量の下限は特に限定しないが、含有した効果を確実に得るため好ましくは、Ｍｇは０．０００１％以上、Ｍｇ以外の元素はそれぞれ０．００１％以上含有するとよい。 Other elements may be included in mass %: Hf: 0-0.600%, Zr: 0-0.600%, Sb: 0-0.600%, Co: 0-1.500%, W: 0-2.000%, Ta: 0-1.000%, Ga: 0-0.500%, Mg: 0-0.0050%, REM: 0-0.200%. Adding these elements has the effect of increasing the corrosion resistance of stainless steel. On the other hand, since these elements are expensive, even if they are included in excess, the effect is not worth the increased cost, so an upper limit has been set. There is no particular lower limit for the content of these elements, but to ensure the effect of their inclusion, it is preferable to include 0.0001% or more of Mg and 0.001% or more of each of the elements other than Mg.
　上記鋼成分の残部はＦｅおよび不純物である。ここで不純物とは、鋼を工業的に製造する際に、鉱石やスクラップ等のような原料をはじめとして、製造工程の種々の要因によって混入する成分であって、本発明に悪影響を与えない範囲で許容されるものを意味する。 The balance of the above steel components is Fe and impurities. Here, impurities refer to components that are mixed in during the industrial production of steel due to various factors in the manufacturing process, including raw materials such as ores and scraps, and are acceptable within the scope of not adversely affecting the present invention.
　＜窒化傾向指数＞
　鋼の窒化を抑制する観点から鋼成分を最適化するに当たり、窒化に影響する元素の含有量の関係を考えた。窒化を助長する元素としてＣｒ、Ｍｏ、Ｔｉ、Ａｌが知られているが、ステンレス鋼としての耐食性などの機能を確保する上では一定量含有してもよい。さらに、本発明に係る鋼で重要な含有元素であるＳｉには、水蒸気酸化による赤スケールに対し抑制効果があるだけでなく、理由は定かではないものの窒化を抑制する効果も有することを、本発明者らは見出した。また、後述するが、鋼表面にＳｉ酸化物皮膜（ＳｉＯ２皮膜）を形成させることも有効である。そこで、これら窒化を助長する元素であるＭｏ、Ｔｉ、Ａｌ、Ｃｕと、窒化抑制に効果のあるＳｉとをバランスよく組み合わせることを想起し、フェライト系ステンレス鋼の場合は窒化傾向を示す指数として１０Ａｌ＋２Ｍｏ＋３Ｔｉ＋０．５Ｃｕ－１．５Ｓｉで評価することができることを見出した。窒化抑制の観点から、この窒化傾向指数が５．０以下にするとよいことを見出した。

窒化傾向指数＝１０Ａｌ＋２Ｍｏ＋３Ｔｉ＋０．５Ｃｕ－１．５Ｓｉ≦５．０
・・・・・（式１）
　ただし、式１中の元素記号は、当該元素の含有量（質量％）を示し、含有しない場合は０を代入する。

　窒化傾向指数は一言でいうと窒化し易さの指標であり、値が小さい方が好ましい。そのため、窒化傾向指数の上限値は、好ましくは４．８、４．６、４．４、４．２、４．０、３．９、３．８、３．７、３．６、または３．５であるとよい。 <Nitriding tendency index>
In optimizing the steel components from the viewpoint of suppressing nitridation of steel, the relationship of the content of elements that affect nitridation was considered. Cr, Mo, Ti, and Al are known as elements that promote nitridation, but a certain amount of them may be contained in order to ensure the functions of stainless steel, such as corrosion resistance. Furthermore, the inventors have found that Si, an important element contained in the steel according to the present invention, not only has an effect of suppressing red scale caused by steam oxidation, but also has an effect of suppressing nitridation, although the reason is unclear. In addition, as will be described later, it is also effective to form a Si oxide film (SiO 2 film) on the steel surface. Therefore, it was envisioned to combine Mo, Ti, Al, and Cu, which are elements that promote nitridation, with Si, which is effective in suppressing nitridation, in a well-balanced manner, and it was found that in the case of ferritic stainless steel, an index indicating the nitridation tendency can be evaluated as 10Al + 2Mo + 3Ti + 0.5Cu - 1.5Si. From the viewpoint of suppressing nitridation, it was found that it is good to make this nitridation tendency index 5.0 or less.

Nitriding tendency index = 10Al + 2Mo + 3Ti + 0.5Cu - 1.5Si ≦ 5.0
.... (Equation 1)
In the formula 1, the symbol of an element indicates the content (mass%) of the element, and 0 is substituted when the element is not contained.

The nitriding tendency index is, in short, an index of the ease of nitriding, and a smaller value is preferable. Therefore, the upper limit of the nitriding tendency index is preferably 4.8, 4.6, 4.4, 4.2, 4.0, 3.9, 3.8, 3.7, 3.6, or 3.5.
　＜表面Ｓｉ酸化物皮膜（ＳｉＯ２皮膜）の面積率＞
　鋼板表面にはＳｉ酸化物皮膜（ＳｉＯ２皮膜）が存在するとよい。Ｓｉ酸化物皮膜が存在する部分は、窒素（Ｎ）を接触しても鋼中に侵入することがなくその部分の窒化が抑制されるからである。そのため、鋼板表面において、Ｓｉ酸化物皮膜が面積率で５．０％以上存在するとよい。好ましくは、６．０％以上、７．０％以上、８．０％以上、９．０％以上、１０．０％以上、１１．０％以上、１２．０％以上、１３．０％以上、１４．０％以上、または１５．０％以上であるとよい。Ｓｉ酸化物皮膜が面積率の上限は特に限定しない。しかし、Ｓｉ酸化膜はステンレス鋼の有する光沢や意匠性を阻害し、また加工性および溶接性を悪化させる。そのため、鋼板表面においてＳｉ酸化物皮膜が面積率で５０．０％以下であるとよい。好ましくは、４５．０％以下、４０．０％以下、３５．０％以下、３０．０％以下、２５．０％以下、または２０．０％以下であってもよい。本発明のＳｉ酸化物皮膜は製造過程で内部酸化したＳｉ酸化物が、表面のＦｅ系およびＣｒ系酸化物の除去により、鋼板表面に露出したものである。従って、ＦｅＣｒ系酸化物層（不働態皮膜）中のアモルファス構造のＳｉ酸化物とは異なる。 <Area ratio of surface Si oxide film ( SiO2 film)>
It is preferable that a Si oxide film (SiO 2 film) is present on the steel sheet surface. This is because the portion where the Si oxide film is present does not penetrate into the steel even if nitrogen (N) comes into contact with the portion, and nitridation of the portion is suppressed. Therefore, it is preferable that the Si oxide film is present at an area ratio of 5.0% or more on the steel sheet surface. Preferably, it is 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 upper limit to the area ratio of the Si oxide film. However, the Si oxide film inhibits the luster and design of stainless steel, and also deteriorates workability and weldability. Therefore, it is preferable that the Si oxide film is present at an area ratio of 50.0% or less on the steel sheet surface. Preferably, it may be 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 Si oxide that is internally oxidized during the manufacturing process and is exposed on the steel sheet surface by removing the Fe-based and Cr-based oxides on the surface. Therefore, it is different from the Si oxide with an amorphous structure in the FeCr-based oxide layer (passive film).
　鋼板表面のＳｉ酸化物皮膜の面積率は次のように測定することができる。測定対象となるステンレス鋼板表面において、観察視野を３０μｍ四方の正方形として、当該観察面をＥＰＭＡ分析する。そこで表面に生成された酸化物のうち、Ｓｉ含有量が５ｗｔ％以上の部分をＳｉ酸化物皮膜として、その面積を測定し、観察視野中の面積率を算出することで得られる。同一のステンレス鋼板において、観察視野を任意に３箇所以上選定し、それぞれで得られたＳｉ酸化物皮膜の面積率を算術平均して求めるとよい。面積の測定方法は特に限定されないが、ＥＰＭＡで得られた写真を写真編集ソフト（例えば、ＩｍａｇｅＪ）に取込み、写真を２値化して画像処理ソフトにより算出するとよい。 The area ratio of the Si oxide film on the steel plate surface can be measured as follows. The observation field is a 30 μm square on the stainless steel plate surface to be measured, and the observation surface is analyzed by EPMA. The area of the oxide formed on the surface that has an Si content of 5 wt% or more is measured as the Si oxide film, and the area ratio in the observation field is calculated. On the same stainless steel plate, three or more observation fields can be arbitrarily selected, and the area ratio of the Si oxide film obtained in each can be calculated as an arithmetic average. There are no particular restrictions on the method of measuring the area, but it is advisable to import the photo obtained by EPMA into photo editing software (e.g., ImageJ), binarize the photo, and calculate the area using image processing software.
　＜表層のＳｉ酸化物の面積率＞
　鋼板表層にもＳｉ酸化物（ＳｉＯ２）が存在するとよい。鋼板表層とは、鋼板表面から板厚方向に１０μｍまでの領域を指す。鋼板表層にＳｉ系酸化物が存在することにより、窒素（Ｎ）の鋼中への侵入を防ぎ窒化が抑制されるからである。そのため、鋼板表層において、幅３０μｍの観察面内に粒径１μｍ以上のＳｉ酸化物が面積率で３．０％以上存在するとよい。好ましくは、４．０％以上、５．０％以上、６．０％以上、７．０％以上、８．０％以上、９．０％以上、または１０．０％以上であるとよい。幅３０μｍの観察面におけるＳｉ酸化物が個数の上限は特に限定しない。しかし、鋼板表層のＳｉ酸化物は加工性および溶接性を悪化させる。そのため、鋼板表層において、幅３０μｍの観察面内に粒径１μｍ以上のＳｉ酸化物は面積率で２０．０％以下であるとよい。好ましくは、１９．０％以下、１８．０％以下、１７．０％以下、１６．０％以下、または１５．０％以下であるとよい。鋼板表層のＳｉ酸化物は製造過程で内部酸化したＳｉ酸化物である。従って、ＦｅＣｒ系酸化物層（不働態皮膜）中のアモルファス構造のＳｉ酸化物とは異なる。 <Area ratio of silicon oxide in surface layer>
It is preferable that Si oxide (SiO 2 ) is also present in the surface layer of the steel sheet. The surface layer of the steel sheet refers to a region from the surface of the steel sheet to 10 μm in the sheet thickness direction. The presence of Si-based oxides in the surface layer of the steel sheet prevents the intrusion of nitrogen (N) into the steel and suppresses nitriding. Therefore, it is preferable that Si oxides having a grain size of 1 μm or more are present in an area ratio of 3.0% or more in an observation surface of 30 μm in width in the surface layer of the steel sheet. Preferably, it is 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 limit to the upper limit of the number of Si oxides in the observation surface of 30 μm in width. However, Si oxides in the surface layer of the steel sheet deteriorate workability and weldability. Therefore, it is preferable that Si oxides having a grain size of 1 μm or more are present in an area ratio of 20.0% or less in an observation surface of 30 μm in width in the surface layer of the steel sheet. Preferably, the content is 19.0% or less, 18.0% or less, 17.0% or less, 16.0% or less, or 15.0% or less. The Si oxide in the steel sheet surface layer is an Si oxide that is internally oxidized during the manufacturing process. Therefore, it is different from the Si oxide with an amorphous structure in the FeCr-based oxide layer (passive film).
　鋼板表層のＳｉ酸化物の面積率は次のように測定することができる。測定対象となるステンレス鋼板の鋼板表面に垂直断面において、幅３０μｍで鋼板表面から板厚方向に１０μｍの長方形の観察面を任意に選択して、当該観察面をＥＰＭＡ分析する。そこで観察された酸化物のうち、Ｓｉ含有量が５ｗｔ％以上の部分をＳｉ酸化物として、その形状（特に長径と短径）を測定し、平均粒径１μｍ以上のＳｉ酸化物を特定して、観察面中の面積率を算出することで得られる。ここで平均粒径は、面積円相当径（面積等価円直径）である。同一のステンレス鋼板において、観察面を任意に３箇所以上選定し、それぞれで得られたＳｉ酸化物の面積率を算術平均して求めるとよい。個数の測定方法は特に限定されないが、ＥＰＭＡで得られた写真を写真編集ソフト（例えば、ＩｍａｇｅＪ）に取込み、写真を２値化して画像処理ソフトにより測定するとよい。 The area ratio of the Si oxide on the surface of the steel plate can be measured as follows. In the vertical cross section of the steel plate surface of the stainless steel plate to be measured, a rectangular observation surface 30 μm wide and 10 μm from the steel plate surface in the plate thickness direction is arbitrarily selected, and the observation surface is analyzed by EPMA. Among the oxides observed, the parts with a Si content of 5 wt% or more are regarded as Si oxides, and their shapes (particularly the major and minor axes) are measured, and Si oxides with an average particle size of 1 μm or more are identified, and the area ratio on the observation surface is calculated. Here, the average particle size is the diameter of a circle equivalent to the area (diameter of a circle equivalent to the area). It is advisable to arbitrarily select three or more observation surfaces on the same stainless steel plate, and calculate the area ratio of the Si oxide obtained at each of them by arithmetic averaging. There is no particular limitation on the method of measuring the number, but it is advisable to import the photo obtained by EPMA into photo editing software (e.g., ImageJ), binarize the photo, and measure it using image processing software.
　＜粒界割れ長さ＞
　鋼板表面の窒化が抑制された結果、窒素（Ｎ）の侵入による粒界割れが抑制される。粒界割れは、結晶粒界を観察して長さを測定することができ、鋼板断面の表層部（少なくとも窒化部分を含む部分）の任意の１００μｍ四方の範囲を３箇所選定し、それらにおいて粒界割れ長さの合計が２０μｍ以下であるとよい。３箇所の観察面での粒界割れ長さの合計が２０μｍ以下であれば、鋼板表面の脆化を抑制することができ、５００～７００℃の温度域での鋼板強度を確保することができる。粒界割れ長さの合計は短いほど好ましく、１８μｍ以下、１６μｍ以下、１４μｍ以下、１２μｍ以下、または１０μｍ以下であるとより好ましい。 <Intergranular crack length>
As a result of suppressing nitridation of the steel sheet surface, intergranular cracking due to the intrusion of nitrogen (N) is suppressed. Intergranular cracking can be measured by observing the crystal grain boundaries, and three arbitrary 100 μm square ranges are selected from the surface layer portion (at least the portion including the nitrided portion) of the steel sheet cross section, and the total intergranular crack length in these areas is preferably 20 μm or less. If the total intergranular crack length in the three observation surfaces is 20 μm or less, embrittlement of the steel sheet surface can be suppressed, and the steel sheet strength in the temperature range of 500 to 700 ° C. can be ensured. The shorter the total intergranular crack length, the more preferable it is, and it is more preferable that it is 18 μm or less, 16 μm or less, 14 μm or less, 12 μm or less, or 10 μm or less.
　鋼板表層部の粒界割れ長さの測定は、次のように測定することができる。試料となる鋼板断面を光学顕微鏡下で観察視野を１００μｍ四方の正方形として観察し、粒界割れ長さを測定する。この時、鋼板表面に近いほど窒素の影響を受けやすいので、鋼板表面の直下に相当する部分を観察視野にするとよい。測定に際し画像処理により測定することが好ましい。例えば、測定画像上に粒界割れ部分をマーキングし、画像処理によりその長さを測定することができる。 The intergranular crack length in the surface layer of the steel plate can be measured as follows. The cross section of the sample steel plate is observed under an optical microscope with a square observation field of 100 μm on each side, and the intergranular crack length is measured. At this time, since the area closer to the steel plate surface is more susceptible to the effects of nitrogen, it is advisable to set the observation field to the area directly below the steel plate surface. It is preferable to perform the measurement using image processing. For example, the intergranular crack area can be marked on the measurement image, and its length can be measured using image processing.
　＜窒化深さ＞
　本発明に係る鋼板は、窒化が抑制されるよう成分を調整されていることと、表面にＳｉ酸化物皮膜を有していることから、窒化深さが平均すると浅くなる。特に、明らかに窒化傾向指数の値が小さい（負の値も含めて）ほど、窒化深さも浅くなる傾向がある。窒化深さは接触するガスの窒素（Ｎ）含有量でも若干異なるが、概ね２２０μｍ以下であれば表面脆化を抑制されることを確認した。窒化深さは、好ましくは２１０μｍ以下、２００μｍ以下、１９０μｍ以下、または１８０μｍ以下であるとよい。 <Nitriding depth>
The steel plate according to the present invention has a shallow nitriding depth on average because the components are adjusted to suppress nitriding and the surface has a silicon oxide film. In particular, the smaller the nitriding tendency index value (including negative values), the shallower the nitriding depth tends to be. The nitriding depth varies slightly depending on the nitrogen (N) content of the contacting gas, but it has been confirmed that surface embrittlement is suppressed if the nitriding depth is generally 220 μm or less. The nitriding depth is preferably 210 μm or less, 200 μm or less, 190 μm or less, or 180 μm or less.
　＜製造方法＞
　次に製造方法について説明する。以下に説明する製造方法は、本発明に係る鋼板を得るための一実施形態であり、この製造方法に限定されるものではない。本発明に係る鋼板が得られるのであれば、製造方法は限定されない。 <Production Method>
Next, a manufacturing method will be described. The manufacturing method described below is one embodiment for obtaining the steel plate according to the present invention, and the manufacturing method is not limited to this. As long as the steel plate according to the present invention can be obtained, the manufacturing method is not limited.
　本発明に係る鋼板の製造方法の一実施形態は、常法により鋼板を製造したのち、最終焼鈍にて鋼板表層下に内部酸化によりＳｉ酸化物層を形成し、その上部（鋼板表面側）にＣｒ酸化物層、Ｆｅ酸化物層を形成し、最終焼鈍後に酸洗浄によりＣｒ酸化物層、Ｆｅ酸化物層をエッチング除去するものであり、これにより、Ｓｉ酸化物層が表面に現れＳｉ酸化物皮膜とすることにより本発明に係る鋼板が得ることができる。 In one embodiment of the method for manufacturing steel plate according to the present invention, a steel plate is manufactured by a conventional method, and then a Si oxide layer is formed under the surface of the steel plate by internal oxidation during final annealing, and a Cr oxide layer and an Fe oxide layer are formed on top of that (on the surface side of the steel plate). After final annealing, the Cr oxide layer and the Fe oxide layer are etched away by acid washing, whereby the Si oxide layer appears on the surface and becomes a Si oxide film, thereby obtaining the steel plate according to the present invention.
　最終焼鈍前の鋼板は、常法の製造方法によって製造すればよい。例えば、製鋼－熱間圧延、製鋼－熱間圧延－焼鈍あるいは製鋼－熱間圧延－酸洗－冷延の工程で製造することができる。 Steel sheets before final annealing can be manufactured using standard manufacturing methods. For example, they can be manufactured using the process of steelmaking-hot rolling, steelmaking-hot rolling-annealing, or steelmaking-hot rolling-pickling-cold rolling.
　ただし、製鋼においては、前記説明した成分になるよう調整した成分を含有する鋼を転炉や電気炉にて溶製し、続いて２次精錬を行う方法が好適である。こうして所定の成分に調整した溶鋼は、公知の鋳造方法（例えば連続鋳造法）に従ってスラブにする。スラブは、所定の温度に加熱され、所定の板厚に熱間圧延される。熱延後、必要に応じて冷間圧延（冷延）してもよい。冷延も、常法にて行えばよい。 However, in steelmaking, a suitable method is to melt steel containing the components adjusted to the composition described above in a converter or electric furnace, followed by secondary refining. The molten steel thus adjusted to the desired composition is made into slabs using a known casting method (e.g., continuous casting). The slabs are heated to a desired temperature and hot rolled to a desired thickness. After hot rolling, the steel may be cold rolled (cold rolling) if necessary. Cold rolling may also be performed using conventional methods.
　製造工程における諸条件は適宜選択すれば良い。例えば、スラブ厚さ、熱間圧延板厚などは適宜設定すれば良い。熱延板の巻取後に水冷プールに浸漬しても構わない。熱延後あるいは熱延焼鈍後の酸洗工程については特に限定せず、ショットブラスト、ベンディング、ブラシ等のメカニカルデスケール方法については適宜選択すれば良い。熱延後の酸洗液についても特に限定しないので、例えば硫酸、硝弗酸等の既設条件で構わない。さらに、この後にコイル研削を表面に施しても構わない。 The various conditions in the manufacturing process may be selected as appropriate. For example, the slab thickness and hot-rolled sheet thickness may be set as appropriate. After coiling the hot-rolled sheet, it may be immersed in a water-cooled pool. There are no particular restrictions on the pickling process after hot rolling or after hot-rolling annealing, and the mechanical descaling method, such as shot blasting, bending, or brushing, may be selected as appropriate. There are no particular restrictions on the pickling solution after hot rolling, either, so existing conditions such as sulfuric acid, nitric hydrofluoric acid, etc. may be used. Furthermore, the coil surface may be ground after this.
　こうして得られた熱延鋼板、熱延焼鈍鋼板、冷延鋼板を最終焼鈍する。焼鈍雰囲気は特に限定されず、大気雰囲気でもよい。温度９００～１１００℃の温度域にて焼鈍するとよい。保定時間は特に限定しないが、好ましくは３０秒～５分であるとよい。この温度域で焼鈍することにより、Ｓｉ酸化物層が鋼板表面下に形成される（内部酸化）。さらにＳｉ酸化物層の上層（鋼板表面側）には鋼板中のＣｒが拡散することによりＣｒ酸化物層と、鋼板中のＦｅによるＦｅ酸化物層が形成される。 The hot-rolled steel sheet, hot-rolled annealed steel sheet, and cold-rolled steel sheet thus obtained are then subjected to final annealing. The annealing atmosphere is not particularly limited, and may be an air atmosphere. Annealing is preferably performed in a temperature range of 900 to 1100°C. The holding time is not particularly limited, but is preferably 30 seconds to 5 minutes. By annealing in this temperature range, a silicon oxide layer is formed under the surface of the steel sheet (internal oxidation). Furthermore, a Cr oxide layer is formed above the silicon oxide layer (on the surface side of the steel sheet) due to the diffusion of Cr in the steel sheet, and an Fe oxide layer is formed due to the Fe in the steel sheet.
　最終焼鈍後の鋼板を６０℃以下まで冷却し、酸洗して、上層のＣｒ酸化物層とＦｅ酸化物層をエッチング除去する。そのため酸洗液はフッ酸（ＨＦ）２．０％以下、硝酸６～１５％を含む酸洗溶液で、温度５０～６０℃で浸漬時間４０～６０秒になるように調整するとよい。これにより、上層のＦｅ酸化物、Ｃｒ酸化物が除去され、内部酸化されたＳｉ酸化物層が表層に現れ、適度な面積率となるＳｉ酸化物皮膜を形成する。フッ酸は含まなくてもよいが、過剰にＳｉ酸化物が残存すると表面着色による意匠性悪化のみならず、加工性や溶接性を悪化させる。そのため、酸洗液中にフッ酸を好ましくは０．１％以上、好ましくは０．２％以上、０．３％以上、０．４％以上、または０．５％以上含有させてＳｉ酸化物の一部を溶解し、適度にＳｉ酸化物皮膜を残存させるとよい。ＨＦが多過ぎると、過剰にＳｉ酸化物が除去されるため、２．０％以下、好ましくは１．５％以下、または１．０％以下とするとよい。 The steel sheet after final annealing is cooled to 60°C or less and pickled to etch away the upper Cr oxide layer and Fe oxide layer. The pickling solution should contain 2.0% or less hydrofluoric acid (HF) and 6-15% nitric acid, and should be adjusted to a temperature of 50-60°C and an immersion time of 40-60 seconds. This removes the upper Fe oxide and Cr oxide layers, and the internally oxidized Si oxide layer appears on the surface, forming a Si oxide film with an appropriate area ratio. Hydrofluoric acid is not necessary, but if excessive Si oxide remains, it will not only deteriorate the design due to surface coloring, but also deteriorate workability and weldability. Therefore, it is recommended to include hydrofluoric acid in the pickling solution at a concentration of preferably 0.1% or more, preferably 0.2% or more, 0.3% or more, 0.4% or more, or 0.5% or more to dissolve part of the Si oxide and leave an appropriate Si oxide film. If there is too much HF, excessive silicon oxide will be removed, so it should be 2.0% or less, preferably 1.5% or less, or 1.0% or less.
　さらに、酸洗後に鋼板表面をブラッシングするブラッシング工程を追加してもよい。鋼板表面をブラッシングすることにより、上層のＦｅ酸化物、Ｃｒ酸化物を確実に除去し、その除去量も調整することができるので、所望の面積率となるようにＳｉ酸化膜皮膜を鋼板表面に露出することができる。ブラッシングは鋼板表面の少なくとも一部をブラッシングすればよく、鋼板表面の全面を行ってもよい。また、鋼板の表裏面のどちらか一方でもよく、両方でもよい。
　ブラッシング用ブラシの種類は特に限定されない。除去するＦｅ酸化物、Ｃｒ酸化物と、残存させるＳｉ酸化物の硬度差からブラシを選定するとよい。こうすることで、表面Ｓｉ酸化物を除去することなく、Ｆｅ酸化物、Ｃｒ酸化物を選択的に除去することができるからである。例えば、粗さを調整した砥粒を付けた砥粒ブラシなどを適用するとよい。 Furthermore, a brushing step of brushing the steel sheet surface may be added after pickling. By brushing the steel sheet surface, the Fe oxide and Cr oxide in the upper layer can be reliably removed, and the amount of removal can be adjusted, so that the Si oxide film can be exposed on the steel sheet surface so as to have a desired area ratio. Brushing is sufficient if at least a part of the steel sheet surface is brushed, and the entire steel sheet surface may also be brushed. In addition, brushing may be performed on either the front or back surface of the steel sheet, or both.
The type of brush for brushing is not particularly limited. It is advisable to select a brush based on the difference in hardness between the Fe oxide and Cr oxide to be removed and the Si oxide to be left behind. This is because it is possible to selectively remove the Fe oxide and Cr oxide without removing the surface Si oxide. For example, it is advisable to use an abrasive brush with abrasive grains having adjusted roughness.
＜用途＞
　本発明に係る鋼板は、窒素（Ｎ）含有量の多いガス環境において使用しても、良好な耐窒化性から鋼板表層の窒素浸入が少なく、粒界割れが抑制される。さらに、耐酸化性も備え、特に５００～７００℃程度の中高温域において、従来のステンレス鋼で問題となる赤スケール発生に対しても有効である。このことから、例えば窒素含有量が高く、ガス温度が５００～７００℃の中高温域であるアンモニアの燃焼機器用に用いることができる。特にアンモニア燃焼機器の排気部品などに使用することができる。
　もちろん、耐窒化性、耐酸化性の性質から、例えばアンモニアや尿素などに直接接する容器や配管部品などに使用しても、溶液中の窒素イオンやアンモニアや尿素が蒸発ガス中の窒素が鋼板表層へ侵入することが抑えられ、粒界割れが抑制される。
　その他、耐窒化性と耐酸化性を要求する部品に本発明に係る鋼板を適用すると、その効果を得ることができる。 <Applications>
Even when the steel sheet according to the present invention is used in a gas environment with a high nitrogen (N) content, the good nitriding resistance reduces nitrogen penetration into the surface layer of the steel sheet, suppressing grain boundary cracking. Furthermore, the steel sheet according to the present invention has oxidation resistance, and is effective against the generation of red scale, which is a problem with conventional stainless steels, particularly in the medium to high temperature range of about 500 to 700°C. For this reason, the steel sheet according to the present invention can be used, for example, for ammonia combustion equipment with a high nitrogen content and a medium to high gas temperature range of 500 to 700°C. In particular, the steel sheet can be used for exhaust parts of ammonia combustion equipment.
Of course, due to their nitridation resistance and oxidation resistance, even if they are used in containers or piping parts that come into direct contact with ammonia, urea, etc., the nitrogen ions in the solution and the nitrogen in the evaporated gases of ammonia and urea are prevented from penetrating into the steel sheet surface, thereby suppressing grain boundary cracking.
In addition, the steel sheet according to the present invention can be used to obtain the same effects when applied to parts that require resistance to nitridation and oxidation.
　以下、実施例によって本発明をより具体的に説明するが、本発明はこれらの実施例に限定されるものではない。 The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples.
　表１に示す成分組成の鋼を溶製しスラブに鋳造し、スラブを熱間圧延して板厚４ｍｍの熱延鋼板を得た。その後熱延版の焼鈍を９００～１１００℃の温度で行い、酸洗を行い冷間圧延して、板厚１．５ｍｍの冷延鋼板を得た。得られた冷延鋼板を９００～１１００℃の温度で焼鈍（最終焼鈍）し、その後４０～６０℃の酸洗液（２％フッ酸＋１０％硝酸＋水）中に４０～９０秒浸漬し（最終酸洗）、水洗して試験材を得た。最終酸洗の液温度条件、浸漬時間を表２に示す。
　最終酸洗の後、表面をブラッシングして仕上げた。ブラッシングはＳｉＣ砥粒付きブラシを用いて、負荷電流８０～１２０Ａ、回転数１０００ｒｐｍ、圧下量は０．５～１．０ｍｍで行った。 Steel having the composition shown in Table 1 was melted and cast into a slab, and the slab was hot-rolled to obtain a hot-rolled steel sheet having a thickness of 4 mm. The hot-rolled sheet was then annealed at a temperature of 900 to 1100 ° C, pickled, and cold-rolled to obtain a cold-rolled steel sheet having a thickness of 1.5 mm. The obtained cold-rolled steel sheet was annealed at a temperature of 900 to 1100 ° C (final annealing), and then immersed in a pickling solution (2% hydrofluoric acid + 10% nitric acid + water) at 40 to 60 ° C for 40 to 90 seconds (final pickling), and washed with water to obtain a test material. The liquid temperature conditions and immersion time of the final pickling are shown in Table 2.
After the final pickling, the surface was finished by brushing using a SiC abrasive brush at a load current of 80 to 120 A, a rotation speed of 1000 rpm, and a reduction of 0.5 to 1.0 mm.
　得られた試験材から、２０ｍｍ×２５ｍｍの試験片を４本切り出して、1本を表面Ｓｉ酸化物皮膜の面積率測定に、残り３本はアンモニア燃焼ガスを想定して窒化および酸化処理をした。 Four test pieces measuring 20 mm x 25 mm were cut from the obtained test material, one of which was used to measure the area ratio of the surface silicon oxide film, and the remaining three were subjected to nitriding and oxidation treatment to simulate ammonia combustion gas.
　鋼板表面のＳｉ酸化物皮膜の面積率測定はＥＰＭＡにて行った。加速電圧：１５ｋＶ，照射電流：２．０×１０－７Ａ、分析面積：３０μｍ×３０μｍ、測定時間：５０ｍｓの条件で測定した。得られた画像からＳｉ酸化物皮膜（ＳｉＯ２）を判別し、写真編集ソフト（ＩｍａｇｅＪ）にて画像を２値化し、画像処理ソフトを用いて観察視野面積に対する面積率を求めた。 The area ratio of the Si oxide film on the steel sheet surface was measured using an EPMA under the following conditions: acceleration voltage: 15 kV, irradiation current: 2.0×10 −7 A, analysis area: 30 μm×30 μm, measurement time: 50 ms. The Si oxide film (SiO 2 ) was identified from the obtained image, and the image was binarized using photo editing software (ImageJ), and the area ratio relative to the observation field area was calculated using image processing software.
　窒化・酸化処理は、焼鈍炉に雰囲気ガスにアンモニア１０ｖｏｌ％、水蒸気１０ｖｏｌ％、残部窒素（Ｎ）のガスを導入し、残りの試験片を炉内に載置し、温度６００℃に加熱後５０時間保持し、その後冷却して取り出し、粒界割れ長さ、窒化深さを測定した。 The nitriding and oxidation treatment involved introducing 10 vol% ammonia, 10 vol% water vapor, and the balance nitrogen (N) into the atmospheric gas of an annealing furnace, placing the remaining test pieces in the furnace, heating them to a temperature of 600°C and holding them there for 50 hours, then cooling and removing them, and measuring the intergranular crack length and nitriding depth.
　粒界割れ長さは、窒化・酸化処理後の試験片を板厚方向の断面が観察できるように切断し、光学顕微鏡を用いて試験片の断面を観察した。観察は、鋼板表面直下の１００μm×１００μmの範囲を１視野とし、試料断面中のランダムに選択した３カ所を観察し、粒界割れ発生長さを測定した。３か所の観察面での粒界割れ長さの合計が２０μm以下であれば良好である。 The intergranular crack length was measured by cutting the test piece after nitriding and oxidation treatment so that the cross section in the thickness direction could be observed, and observing the cross section of the test piece using an optical microscope. The observation was performed with a field of view of 100 μm x 100 μm just below the steel plate surface, and three randomly selected points on the sample cross section were observed to measure the intergranular crack length. A satisfactory result was obtained if the total intergranular crack length on the three observation surfaces was 20 μm or less.
　窒化深さは、窒化・酸化処理後の試験片を切断し、１０％シュウ酸水溶液で、電圧６Ｖで５秒間電解エッチングを行い、光学顕微鏡を用いて観察した。窒化深さの測定は、写真を用いて行った。
　赤スケール性は目視にて確認し、赤スケールが発見できなかったものを合格（〇）、赤スケールが少しでも発見されたものを不合格（×）とした。
　これら測定結果を表２に示す。表２のデータから、本発明に係る鋼板は粒界割れ長さが低減していることが分かる。 The nitriding depth was measured by cutting the test piece after the nitriding and oxidation treatment, electrolytically etching it with a 10% oxalic acid aqueous solution at a voltage of 6 V for 5 seconds, and observing it with an optical microscope. The nitriding depth was measured using a photograph.
The red scale was visually confirmed, and samples in which no red scale was found were rated as pass (◯), and samples in which even a small amount of red scale was found were rated as fail (×).
The results of these measurements are shown in Table 2. The data in Table 2 show that the steel plate according to the present invention has a reduced intergranular crack length.
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000002
　本発明は、自動車工業、一般機械産業などあらゆる産業において利用することができる。 This invention can be used in all industries, including the automotive and general machinery industries.

Claims (9)

  1. 　　質量％で、
    Ｃ　：０～０．０３０％、
    Ｓｉ：０．０５～３．００％、
    Ｍｎ：０．０５～１．２０％、
    Ｐ　：０．０５０％以下、
    Ｓ　：０．００５％以下、
    Ｎｉ：０～１．００％、
    Ｃｒ：１２．０～３１．０％、
    Ｎ　：０～０．０３０％、
    Ｎｂ：０～１．００％
    Ｍｏ：０～２．５０％
    Ｃｕ：０～３．００％
    Ａｌ：０．００２～０．５００％、
    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％、
    ＲＥＭ：０～０．２０％、を含み、
    式１を満足し、
    残部Ｆｅおよび不純物からなり、
    以下の式１を満足し、
    鋼板表面を垂直上方から見たときに表面上にＳｉ酸化物皮膜が面積％で５．０％以上存在することを特徴とするフェライト系ステンレス鋼板。
    １０Ａｌ＋２Ｍｏ＋３Ｔｉ＋０．５Ｃｕ－１．５Ｓｉ≦５．０
    ・・・・・（式１）
    ただし、式１中の元素記号は、当該元素の含有量（質量％）を示し、含有しない場合は０を代入する。     In mass percent,
    C: 0 to 0.030%,
    Si: 0.05 to 3.00%,
    Mn: 0.05 to 1.20%,
    P: 0.050% or less,
    S: 0.005% or less,
    Ni: 0 to 1.00%,
    Cr: 12.0 to 31.0%,
    N: 0 to 0.030%,
    Nb: 0 to 1.00%
    Mo: 0 to 2.50%
    Cu: 0 to 3.00%
    Al: 0.002 to 0.500%,
    Ti: 0 to 0.600%
    V: 0 to 1.00%,
    B: 0 to 0.0100%,
    Ca: 0 to 0.0150%
    Sn: 0 to 1.00%,
    Hf: 0 to 0.60%,
    Zr: 0 to 0.60%,
    Sb: 0 to 0.60%,
    Co: 0 to 1.50%,
    W: 0 to 2.00%,
    Ta: 0 to 1.00%,
    Ga: 0 to 0.50%,
    Mg: 0 to 0.0050%,
    REM: 0 to 0.20%;
    Satisfying Equation 1,
    The balance is Fe and impurities,
    The following formula 1 is satisfied:
    A ferritic stainless steel sheet characterized in that a silicon oxide film is present on the surface of the steel sheet in an area percentage of 5.0% or more when the surface is viewed vertically from above.
    10Al+2Mo+3Ti+0.5Cu-1.5Si≦5.0
    .... (Equation 1)
    In the formula 1, the symbol of an element indicates the content (mass%) of the element, and 0 is substituted when the element is not contained.
  2. 　前記鋼板表面に垂直な断面において、幅３０μｍで、前記鋼板表面から鋼板厚さ方向に１０μｍまでの領域内に、粒子径１μｍ以上のＳｉ系酸化物が面積率で３．０％以上存在する請求項１に記載のフェライト系ステンレス鋼板。 The ferritic stainless steel sheet according to claim 1, wherein in a cross section perpendicular to the steel sheet surface, within a region 30 μm wide and 10 μm from the steel sheet surface in the thickness direction of the steel sheet, Si-based oxides having particle diameters of 1 μm or more are present at an area ratio of 3.0% or more.
  3. 　前記鋼板の板厚方向断面で、１００μｍ四方の範囲を１視野として、任意の３視野の粒界割れ長さの合計が２０μｍ以下である、請求項１または２に記載のフェライト系ステンレス鋼板。 The ferritic stainless steel plate according to claim 1 or 2, in which the total length of intergranular cracks in any three fields of view, each of which is a 100 μm square area in the thickness direction cross section of the steel plate, is 20 μm or less.
  4. 　前記Ｓｉ酸化物皮膜が面積％で５０％以下存在する、請求項１～３のいずれか１項に記載のフェライト系ステンレス鋼板。 The ferritic stainless steel sheet according to any one of claims 1 to 3, in which the silicon oxide film is present in an area percentage of 50% or less.
  5. 　アンモニア燃焼機器用である請求項１～４のいずれか１項に記載のフェライト系ステンレス鋼板。 The ferritic stainless steel sheet according to any one of claims 1 to 4, which is for use in ammonia burning equipment.
  6. 　請求項１～４のいずれか１項に記載のフェライト系ステンレス鋼板の製造方法であって、請求項１に記載の成分を有する鋼板を、最終冷延後に、９００～１１００℃に加熱保持した後、５０℃以下の温度まで冷却し、フッ酸２．０％以下、硝酸６～１５％を含有し、温度５０～６０℃の酸洗液中に４０～６０秒間浸漬する酸洗工程を有することを特徴とするフェライト系ステンレス鋼板の製造方法。 A method for producing a ferritic stainless steel sheet according to any one of claims 1 to 4, characterized in that the method comprises a pickling step in which a steel sheet having the composition according to claim 1 is heated and held at 900 to 1100°C after final cold rolling, cooled to a temperature of 50°C or less, and immersed in a pickling solution containing 2.0% or less hydrofluoric acid and 6 to 15% nitric acid at a temperature of 50 to 60°C for 40 to 60 seconds.
  7. 　前記酸性工程の後に、前記鋼板表面の少なくとも一部をブラッシングする請求項６に記載のフェライト系ステンレス鋼板の製造方法。 The method for producing a ferritic stainless steel sheet according to claim 6, wherein at least a portion of the steel sheet surface is brushed after the acid step.
  8. 　請求項１～４のいずれか１項に記載のフェライト系ステンレス鋼板を少なくとも一部に有する、部品。 A part having at least a portion of the ferritic stainless steel plate according to any one of claims 1 to 4.
  9. 　アンモニア燃焼機器用部品である請求項８に記載の部品。 The part according to claim 8, which is an ammonia combustion equipment part.
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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2018181257A1 (en) * 2017-03-30 2018-10-04 新日鐵住金ステンレス株式会社 Ferrite-based stainless steel sheet having low specific gravity and production method therefor
WO2022085788A1 (en) * 2020-10-23 2022-04-28 日鉄ステンレス株式会社 Ferrite-based stainless steel and component for exhaust gas

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2018181257A1 (en) * 2017-03-30 2018-10-04 新日鐵住金ステンレス株式会社 Ferrite-based stainless steel sheet having low specific gravity and production method therefor
WO2022085788A1 (en) * 2020-10-23 2022-04-28 日鉄ステンレス株式会社 Ferrite-based stainless steel and component for exhaust gas

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