Ferritic stainless steel sheet, method for manufacturing the same, and parts

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

Technical Field

[0001] The present invention relates to a ferritic stainless steel, a method for producing the same, and a component using the ferritic stainless steel sheet.

Background Art

[0002] Global warming has become an international environmental issue, and active technological development is being carried out to achieve a decarbonized society such as carbon zero and carbon neutral. Among such trends, ammonia is attracting attention as a fuel to replace carbon fuels. The combustion reaction formula of ammonia is 4NH3 + 3O2 → 2N2 + 6H2O, and water and nitrogen are generated, resulting in a small environmental load and being expected as a recyclable fuel. The combustion temperature of ammonia is 1750°C at the adiabatic flame temperature, which is lower than that of hydrogen at 2120°C, methane at 1970°C, and gasoline at about 2000°C. The combustion temperature in an actual engine or gas turbine is also lower than that of these existing fuels. Therefore, when ammonia is used as a fuel, the 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 range in which the steel materials used for exhaust pipes and the like are easily oxidized, and a so-called red scale is easily generated.

[0003] In Patent Document 1, a ferritic stainless steel used for a reformer for a fuel cell, which has oxidation resistance and red scale resistance even at a high temperature of about 600°C, has been proposed.

[0004] In Patent Document 2, a ferritic stainless steel for an exhaust pipe of an internal combustion engine equipped with a pollution control system having urea or ammonia for reducing nitrogen oxides has been proposed.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

[0006] The use of ammonia as a fuel is being explored not only through single-fuel combustion but also through co-firing with other fuels (heavy oil, light oil, hydrogen, etc.). However, even with co-firing, since ammonia has a lower combustion temperature than existing fuels, the combustion exhaust gas temperature is lower than that of existing fuels, at around 500-700°C. Furthermore, ammonia combustion gases contain large amounts of nitrogen and water vapor. The presence of water vapor makes the material more susceptible to water vapor oxidation and red scale formation. Furthermore, the combustion gas temperature of around 500-700°C is also a temperature range where red scale formation is likely. For this reason, steel materials used in ammonia combustion gas systems and similar applications require oxidation resistance (red scale resistance).

[0007] Furthermore, the large amount of nitrogen contained in ammonia combustion exhaust gas causes nitrogen to penetrate (nitriding) into the surface of the steel, leading to embrittlement caused by intergranular cracking. Therefore, nitriding resistance (resistance to intergranular cracking) is also required for steel used in ammonia combustion gas systems.

[0008] The stainless steel described in Patent Document 1 ensures high-temperature oxidation resistance and red scale resistance as a steel material for exhaust gas system components, but it does not take into account measures against grain boundary cracking due to surface nitriding in gases containing a large amount of nitrogen, such as ammonia combustion gas.

[0009] While the stainless steel described in Patent Document 2 does have some effect in suppressing surface grain boundary cracking caused by urea and ammonia, its nitriding resistance to large amounts of nitrogen-containing gases, such as ammonia combustion exhaust gas, is not sufficient.

[0010] The present invention aims to provide a ferritic stainless steel that exhibits resistance to red scale (oxidation resistance) and intergranular cracking (nitriding resistance) even when exposed to gases containing large amounts of nitrogen and water (water vapor) at temperatures of around 500 to 700°C, such as ammonia combustion exhaust gas. [Means for solving the problem]

[0011] To achieve the above objectives, the inventors conducted diligent research and obtained the following findings.

[0012] (a) From the viewpoint of suppressing nitriding, we considered that it would be beneficial to optimize the content of elements that promote nitriding, such as Mo, Ti, Al, and Cu.

[0013] (b) Furthermore, considering the formation of a red scale-resistant and nitriding-resistant film on the surface of the steel material, we found that having a Si oxide film (SiO2 film; sometimes called a Si oxide layer) on the surface of the steel material is effective in improving red scale resistance and nitriding resistance. We found that to ensure red scale resistance and nitriding resistance, it is best to create a Si oxide film of 5% or more on the surface of the steel sheet.

[0014] (c) Furthermore, we 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 clear. We derived a nitriding tendency index that indicates the nitriding tendency of steel materials from the content of elements that promote nitriding, such as Mo, Ti, Al, and Cu, as well as the content of Si that suppresses nitriding, and found that it is good to have this nitriding tendency index of 5.0 or less. Nitriding tendency index = 10Al + 2Mo + 3Ti + 0.5Cu - 1.5Si ≤ 5.0 ...(Formula 1)

[0015] (d) A method for appropriately forming a SiO2 film on the surface of steel materials was also investigated. As a result, an internal oxide layer of SiO2 is formed beneath the surface of a ferritic stainless steel having a predetermined composition, and then a Cr oxide layer and an iron oxide layer are formed on the internal oxide layer of SiO2. Next, by pickling such a steel sheet to etch away the Cr oxide layer and the iron oxide layer, it was found that an appropriate Si oxide film can be left on the surface layer.

[0016] The present invention is based on these findings, and the gist thereof is as follows.

[0017] [1] In mass %, 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​​​​​​​​​​ The remainder consists of Fe and impurities. The following equation 1 is satisfied, A ferritic stainless steel sheet characterized by the presence of a Si oxide film covering 5.0% or more of the surface area when the steel sheet surface is viewed from vertically above. 10Al+2Mo+3Ti+0.5Cu-1.5Si≦5.0 ...(Formula 1) However, the element symbols in Equation 1 indicate the content (mass%) of the element, and 0 should be substituted if the element is not present. [2] The ferritic stainless steel sheet according to [1] above, wherein in a cross section perpendicular to the surface of the steel sheet, the area ratio of Si-based oxides with a particle size of 1 μm or more is 3.0% or more within a region with a width of 30 μm and extending from the surface of the steel sheet to 10 μm in the direction of the thickness of the steel sheet. [3] The ferritic stainless steel sheet according to [1] or [2], wherein, in the cross-section in the thickness direction of the steel sheet, the sum of the grain boundary crack lengths of any three fields of view, with a 100 μm square area being defined as one field of view, is 20 μm or less. [4] A 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] A ferritic stainless steel sheet according to any one of the above [1] to [4], for use in ammonia combustion equipment. [6] A method for manufacturing a ferritic stainless steel sheet as described in any one of [1] to [4] above, characterized in that, after the final cold rolling of a steel sheet having the components described in [1] above, it is heated and held at 900 to 1100°C, then cooled to a temperature of 50°C or lower, and immersed for 40 to 60 seconds in an acid pickling solution containing 2.0% or less hydrofluoric acid and 6 to 15% nitric acid at a temperature of 50 to 60°C. [7] The aforementioned acid wash A method for manufacturing a ferritic stainless steel sheet according to [6] above, wherein at least a portion of the surface of the steel sheet is brushed after the process. [8] A component having at least a portion of the ferritic stainless steel sheet described in any one of the above [1] to [4]. [9] The part described in [8] above, which is a component for ammonia combustion equipment. Effect

[0018] The ferritic stainless steel according to the present invention provides stainless steel with good corrosion resistance and wear resistance even when in contact with gases containing large amounts of nitrogen and water (water vapor) at temperatures of around 500 to 700°C, such as ammonia combustion exhaust gas. [Modes for carrying out the invention]

[0019] The embodiments of the present invention (hereinafter simply referred to as "the present invention") will be described below. Unless otherwise specified, "%" for components indicates mass % in steel. In cases where no lower limit is specified or where the lower limit is 0%, it also includes cases where the component is not contained (0%).

[0020] <About the steel composition> C: 0~0.030% Since carbon (C) is an element that reduces moldability (r value), it is preferable to have a low amount, with an upper limit of 0.030%. From the viewpoint of moldability, 0.020% or less, or 0.010% or less, is preferable. There is no particular lower limit, but excessive reduction will lead to an increase in refining costs, so 0.001% or more is preferable, and more preferably 0.002% or more.

[0021] Si: 0.05~3.00% Si is an element that is effective in suppressing oxidation, especially steam oxidation, and is also effective in suppressing nitriding. Furthermore, it should be contained in a quantity of 0.05% or more from the viewpoint of forming an internal SiO2 oxide layer directly beneath the surface of the steel sheet. 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, increasing the Si content increases the area ratio of the Si oxide film (or internal SiO2 oxide layer), which deteriorates workability and weldability, so the upper limit is set at 3.00%. The upper limit of Si is preferably 2.95% or 2.90%.

[0022] Mn: 0.05~1.20% Since Mn, like Si, is an element effective in improving oxidation resistance, it is desirable to include 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 high Mn content degrades processability, it is desirable to include 1.20% or less. The upper limit of Mn is preferably 1.10%, 1.00%, 0.90%, or 0.80%.

[0023] P:0.050% or less Since phosphorus (P) is harmful to stainless steel, reducing its toughness, hot workability, and corrosion resistance, the less P it contains, the better. It is desirable to keep it below 0.050%, and preferably below 0.040%. However, excessive reduction would increase the load during refining or necessitate the use of more expensive raw materials, so in practice, it may be present at 0.001% or more.

[0024] S: 0.005% or less Since sulfur (S) is harmful to stainless steel, reducing its toughness, hot workability, and corrosion resistance, the less S it contains, the better. Ideally, the upper limit should be 0.005% or less, and preferably 0.003% or less. However, excessive reduction increases the burden during refining or necessitates the use of expensive raw materials, so in practice, it may be present in amounts of 0.0001% or more.

[0025] Ni: 0~1.00% or less Adding nickel (Ni) further enhances the high corrosion resistance of stainless steel. However, because it is an expensive element, including large amounts does not justify the increased alloy cost. Therefore, it is best to keep the amount below 1.00%, preferably below 0.80%, 0.60%, or 0.50%. While there is no particular lower limit to the Ni content, it is preferable to include at least 0.01% to ensure the desired effect.

[0026] Cr: 12.0~31.0% Cr is an important element that provides corrosion resistance to stainless steel, and it is good to have a content of 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, a large amount of Cr leads to a decrease in workability, so it is good to have a content of 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.

[0027] N: 0~0.030% From the viewpoint of suppressing intergranular cracking caused by surface nitrogen, it is preferable for steel to have a low amount of nitrogen originally contained in it. Furthermore, since nitrogen reduces workability and reduces corrosion resistance by bonding with chromium, it is preferable to have a low amount, preferably 0.030% or less, and more preferably 0.025% or less, 0.020% or less, 0.015% or less, or 0.010% or less. On the other hand, excessive reduction places a heavy burden on the refining process, so it may contain 0.001% or more.

[0028] Nb: 0~1.00% Nb has the effect of improving moldability and corrosion resistance. On the other hand, if added in amounts exceeding 1.00%, recrystallization becomes difficult and the structure becomes coarser, so it is best to keep it below 1.00%, preferably below 0.90%, below 0.80%, or below 0.70%. There is no particular lower limit to the Nb content, but to ensure the effect is obtained, it is preferable to include 0.01% or more.

[0029] Mo: 0~2.50% Mo, when added, further enhances the high corrosion resistance of stainless steel. However, it is an element that promotes nitriding and forms a brittle sigma phase with high Cr content, leading to embrittlement and a decrease in corrosion resistance. Therefore, it is best to keep the amount below 2.50%, preferably below 2.20%, or below 2.00%. There is no particular lower limit to the Mo content, but to reliably obtain the effect of corrosion resistance, it is preferable to include 0.01% or more.

[0030] Cu: 0~3.00% Adding copper (Cu) further enhances the high corrosion resistance of stainless steel. However, excessive addition does not justify the performance improvement in terms of manufacturing costs, so it is best to keep the amount below 3.00%, preferably below 2.50%, below 2.20%, or below 1.90%. There is no particular lower limit to the Cu content, but to ensure the effect is obtained, it is preferable to include 0.01% or more.

[0031] Al: 0.002~0.500% Al combines with N to form AlN, an element that promotes nitriding, and excessive addition reduces processability. Therefore, the Al content should be 0.500% or less, 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 it has the effect of desulfurizing and improving corrosion resistance, the Al content should be 0.002% or more, preferably 0.004% or more, 0.007% or more, or 0.010% or more.

[0032] Ti: 0~0.600% Ti ensures corrosion resistance through the stabilizing effects of C and N. On the other hand, Ti is an element that promotes nitriding, and if added in excess, TiN is significantly generated, leading to nozzle clogging during manufacturing and surface defects in the product. Therefore, it is best to keep the amount below 0.600%, preferably below 0.500%, below 0.400%, or below 0.300%. There is no particular lower limit to the Ti content, but to reliably obtain the effect, it is preferable to include 0.001% or more.

[0033] V: 0~1.00% V, when added, further enhances the high corrosion resistance of stainless steel. However, high concentrations can lead to a decrease in toughness, so the upper limit should be set at 1.00%, preferably 0.90% or less, 0.70% or less, or 0.50% or less. There is no particular lower limit to the V content, but to ensure the effect is obtained, it is preferable to include 0.01% or more, or 0.05% or more.

[0034] B: 0~0.0100% B is an element that increases the strength of grain boundaries and contributes to improved workability. On the other hand, excessive addition can lead to a decrease in workability due to a reduction in elongation, so the content should be kept below 0.0100%, preferably below 0.0090%, below 0.0070%, or below 0.0050%. There is no particular lower limit to the B content, but to ensure the effect is obtained, it is preferable to include 0.0001% or more, or 0.0005% or more.

[0035] Ca: 0~0.0150% If Ca is present in large quantities, the concentration in the oxide that promotes TiN formation increases, causing it to lose its ability. Therefore, it is preferable to have Ca present at 0.0150% or less, 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. Furthermore, it is difficult to remove it completely, and excessive reduction increases the load during refining, so in actual operation, it may be present at 0.0001% or more, or 0.0002% or more.

[0036] Sn: 0~1.00% Adding sn (Sn) further enhances the high corrosion resistance of stainless steel. However, excessive addition can lead to a decrease in workability, so it is best to keep the amount below 1.00%, preferably below 0.70%, below 0.50%, or below 0.30%. There is no particular lower limit to the sn content, but to ensure the effect is obtained, it is preferable to include 0.001% or more, or 0.002% or more.

[0037] In addition, the following elements may be included in mass percent: 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%, and REM: 0-0.200%. Adding these elements enhances the corrosion resistance of stainless steel. However, since these are expensive elements, an upper limit has been set because excessive inclusion does not justify the increased cost. There is no particular lower limit for the content of these elements, but to ensure the desired effect is obtained, it is preferable to include at least 0.0001% Mg and at least 0.001% of each of the other elements.

[0038] The remainder of the above steel components consists of 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 ore and scrap, and are acceptable as long as they do not adversely affect the present invention.

[0039] <Nitriding tendency index> In optimizing the steel composition from the perspective of suppressing nitriding of steel, we considered the relationship between the content of elements that affect nitriding. Cr, Mo, Ti, and Al are known to promote nitriding, but a certain amount is acceptable to ensure the corrosion resistance and other functions of stainless steel. Furthermore, the inventors have found that Si, an important element in the steel according to the present invention, not only has an inhibitory effect on red scale caused by steam oxidation, but also has an effect of suppressing nitriding, although the reason is unclear. Also, as will be described later, forming a Si oxide film (SiO2 film) on the steel surface is effective. Therefore, we conceived of combining these nitriding-promoting elements Mo, Ti, Al, and Cu with Si, which is effective in suppressing nitriding, in a balanced manner, and found that in the case of ferritic stainless steel, the nitriding tendency index can be evaluated as 10Al + 2Mo + 3Ti + 0.5Cu - 1.5Si. From the perspective of suppressing nitriding, we found that it is desirable to have this nitriding tendency index of 5.0 or less. Nitriding tendency index = 10Al + 2Mo + 3Ti + 0.5Cu - 1.5Si ≤ 5.0 ...(Formula 1) However, the element symbols in Equation 1 indicate the content (mass%) of the element, and 0 should be substituted if the element is not present. In short, the nitriding tendency index is an indicator of how easily a material can be nitrided, 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.

[0040] <Area ratio of surface Si oxide film (SiO2 film)> It is desirable for a Si oxide film (SiO2 film) to be present on the surface of the steel sheet. This is because nitrogen (N) does not penetrate into the steel when it comes into contact with the areas where the Si oxide film is present, thus suppressing nitriding in those areas. For this reason, it is desirable for the Si oxide film to cover 5.0% or more of the surface area on the steel sheet. Preferably, it should be 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 surface area of ​​the Si oxide film. However, the Si oxide film impairs the luster and design properties of stainless steel, and also worsens workability and weldability. For this reason, it is desirable for the Si oxide film to cover 50.0% or less of the surface area on the steel sheet. Preferably, the content 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 formed when Si oxide that has been internally oxidized during the manufacturing process is exposed on the surface of the steel sheet by removing the Fe-based and Cr-based oxides on the surface. Therefore, it is different from the amorphous structure of Si oxide in the FeCr-based oxide layer (passivation film).

[0041] The area ratio of the Si oxide film on the surface of a steel plate can be measured as follows: On the surface of the stainless steel plate to be measured, the observation field is set to a 30 μm square, and the observation surface is analyzed using EPMA. Of the oxides formed on the surface, the portion with a Si content of 5 wt% or more is considered the Si oxide film, its area is measured, and the area ratio within the observation field is calculated. It is preferable to select three or more observation fields on the same stainless steel plate and calculate the area ratio by arithmetic mean of the Si oxide film area ratios obtained from each. The method of measuring the area is not particularly limited, but it is preferable to import the photograph obtained from EPMA into photo editing software (e.g., ImageJ), binarize the photograph, and calculate the area using image processing software.

[0042] <Area ratio of Si oxide on the surface> It is desirable for Si oxide (SiO2) to be present on the surface of the steel sheet. The surface of the steel sheet refers to the region from the surface of the steel sheet to 10 μm in the thickness direction. This is because the presence of Si oxide on the surface of the steel sheet prevents nitrogen (N) from penetrating into the steel and suppresses nitriding. Therefore, it is desirable for Si oxide particles with a particle size of 1 μm or larger to be present in an area ratio of 3.0% or more within a 30 μm wide observation area on the surface of the steel sheet. Preferably, it should be 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 upper limit on the number of Si oxide particles in the 30 μm wide observation area. However, Si oxide on the surface of the steel sheet deteriorates workability and weldability. Therefore, it is desirable for Si oxide particles with a particle size of 1 μm or larger to be present in an area ratio of 20.0% or less within a 30 μm wide observation area on the surface 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 on the surface of the steel sheet is Si oxide that has been internally oxidized during the manufacturing process. Therefore, it is different from the amorphous Si oxide in the FeCr-based oxide layer (passivation film).

[0043] The area ratio of Si oxides on the surface of a steel sheet can be measured as follows: A rectangular observation area, 30 μm wide and 10 μm long in the thickness direction from the surface of the stainless steel sheet to be measured, is arbitrarily selected in a cross-section perpendicular to the surface of the sheet. This observation area is then subjected to EPMA analysis. Of the observed oxides, those with a Si content of 5 wt% or more are considered Si oxides. Their shape (especially the major and minor axes) is measured, and Si oxides with an average particle size of 1 μm or more are identified. The area ratio within the observation area is then calculated. Here, the average particle size is the area equivalent circle diameter. It is recommended to select three or more observation areas on the same stainless steel sheet and calculate the arithmetic mean of the Si oxide area ratios obtained from each area. While there are no particular limitations on the method of measuring the number of particles, it is recommended to import the EPMA-obtained photograph into photo editing software (e.g., ImageJ), binarize the photograph, and measure it using image processing software.

[0044] <Granular boundary crack length> As a result of suppressing nitriding on the steel sheet surface, grain boundary cracking due to nitrogen (N) penetration is suppressed. Grain boundary cracks can be measured by observing the grain boundaries. Three arbitrary 100 μm square areas are selected on the surface layer (at least the part including the nitrided portion) of the steel sheet cross-section, and it is preferable that the sum of the grain boundary crack lengths in these areas is 20 μm or less. If the sum of the grain boundary crack lengths in the three observation areas is 20 μm or less, embrittlement of the steel sheet surface can be suppressed, and the strength of the steel sheet can be ensured in the temperature range of 500 to 700°C. The shorter the sum of the grain boundary crack lengths, the better, 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.

[0045] The length of grain boundary cracks in the surface layer of a steel sheet can be measured as follows: The cross-section of the steel sheet sample is observed under an optical microscope with a 100 μm square field of view, and the length of the grain boundary cracks is measured. At this time, since the area closer to the surface of the steel sheet is more susceptible to the effects of nitrogen, it is best to make the area just below the surface of the steel sheet the field of view. It is preferable to measure using image processing. For example, the grain boundary crack area can be marked on the measurement image, and its length can be measured by image processing.

[0046] <Nitriding depth> The steel sheet according to the present invention has its composition adjusted to suppress nitriding and has a Si oxide film on its surface, resulting in a shallower nitriding depth 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 also varies slightly depending on the nitrogen (N) content of the gas in contact, but it has been confirmed that surface embrittlement is suppressed when it is generally 220 μm or less. Preferably, the nitriding depth is 210 μm or less, 200 μm or less, 190 μm or less, or 180 μm or less.

[0047] <Manufacturing method> Next, the manufacturing method will be described. The manufacturing method described below is one embodiment for obtaining the steel sheet according to the present invention, and is not limited to this manufacturing method. The manufacturing method is not limited as long as the steel sheet according to the present invention can be obtained.

[0048] One embodiment of the steel sheet manufacturing method according to the present invention involves manufacturing a steel sheet by a conventional method, then forming a Si oxide layer beneath the surface layer of the steel sheet by internal oxidation during final annealing, forming a Cr oxide layer and an Fe oxide layer on top of it (on the surface side of the steel sheet), and after final annealing, etching off the Cr oxide layer and Fe oxide layer by acid cleaning. As a result, the Si oxide layer appears on the surface, forming a Si oxide film, thereby obtaining the steel sheet according to the present invention.

[0049] Steel sheets before final annealing can be manufactured using conventional methods. For example, they can be manufactured using processes such as steelmaking-hot rolling, steelmaking-hot rolling-annealing, or steelmaking-hot rolling-pickling-cold rolling.

[0050] However, in steelmaking, it is preferable 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 predetermined composition is formed into slabs according to a known casting method (e.g., continuous casting). The slabs are heated to a predetermined temperature and hot-rolled to a predetermined thickness. After hot-rolling, cold-rolling may be performed as needed. Cold-rolling may also be carried out by conventional methods.

[0051] The conditions in the manufacturing process can be selected as appropriate. For example, the slab thickness and hot-rolled sheet thickness can be set as appropriate. The hot-rolled sheet may be immersed in a water-cooling pool after coiling. There are no particular limitations on the pickling process after hot rolling or hot-rolled annealing, and mechanical descaling methods such as shot blasting, bending, and brushing can be selected as appropriate. There are no particular limitations on the pickling solution after hot rolling, so existing conditions such as sulfuric acid and nitrate-hydrofluoric acid can be used. Furthermore, coil grinding may be performed on the surface afterward.

[0052] The hot-rolled steel sheet, hot-rolled annealed steel sheet, and cold-rolled steel sheet obtained in this way are then subjected to final annealing. The annealing atmosphere is not particularly limited and may be an air atmosphere. Annealing is preferably carried out in the temperature range of 900 to 1100°C. The holding time is not particularly limited, but preferably 30 seconds to 5 minutes. By annealing in this temperature range, a Si oxide layer is formed beneath the surface of the steel sheet (internal oxidation). Furthermore, above the Si oxide layer (on the surface side of the steel sheet), a Cr oxide layer is formed by the diffusion of Cr in the steel sheet, and an Fe oxide layer is formed by Fe in the steel sheet.

[0053] After the final annealing, the steel sheet is cooled to below 60°C and pickled to etch away the upper Cr oxide and Fe oxide layers. The pickling solution should contain 2.0% or less hydrofluoric acid (HF) and 6-15% nitric acid, adjusted to a temperature of 50-60°C and an immersion time of 40-60 seconds. This removes the upper Fe and Cr oxide layers, exposing the internally oxidized Si oxide layer to the surface and forming a Si oxide film with an appropriate surface area. While hydrofluoric acid is not strictly necessary, excessive Si oxide residue can worsen not only the aesthetic appearance due to surface discoloration but also the workability and weldability. Therefore, it is preferable to include 0.1% or more, preferably 0.2%, 0.3%, 0.4%, or 0.5% or more of hydrofluoric acid in the pickling solution to dissolve some of the Si oxide and leave an appropriate amount of Si oxide film. If there is too much HF, excessive Si oxide will be removed, so it is best to keep it at 2.0% or less, preferably 1.5% or less, or 1.0% or less.

[0054] Furthermore, a brushing step may be added after pickling, in which the surface of the steel sheet is brushed. By brushing the surface of the steel sheet, the upper layers of Fe oxide and Cr oxide can be reliably removed, and the amount removed can be adjusted, so that the Si oxide film can be exposed on the surface of the steel sheet to the desired area ratio. Brushing only needs to be performed on at least a portion of the surface of the steel sheet, or it may be performed on the entire surface of the steel sheet. Also, it may be done on either the front or back surface of the steel sheet, or both. The type of brush used for brushing is not particularly limited. It is best 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 allows for the selective removal of Fe oxide and Cr oxide without removing the surface Si oxide. For example, an abrasive brush with abrasive grains of adjusted coarseness would be suitable.

[0055] <Application> The steel sheet according to the present invention exhibits excellent nitriding resistance, resulting in minimal nitrogen penetration into the surface layer and suppression of intergranular cracking, even when used in gas environments with high nitrogen (N) content. Furthermore, it possesses oxidation resistance and is particularly effective against red scale formation, a problem with conventional stainless steel, especially in the medium-to-high temperature range of 500-700°C. For this reason, it can be used, for example, in ammonia combustion equipment where the nitrogen content is high and the gas temperature is in the medium-to-high temperature range of 500-700°C. It can be used particularly in exhaust components of ammonia combustion equipment. Of course, due to its nitriding and oxidation resistance properties, even when used in containers or piping components that come into direct contact with substances such as ammonia or urea, the penetration of nitrogen ions, ammonia, urea, and nitrogen from the evaporated gas into the surface layer of the steel plate is suppressed, thereby inhibiting intergranular cracking. Furthermore, the steel sheet according to the present invention can be applied to parts that require nitriding resistance and oxidation resistance, thereby achieving these effects. [Examples]

[0056] The present invention will be described more specifically below with reference to examples, but the present invention is not limited to these examples.

[0057] Steel with the component composition shown in Table 1 was melted and cast into slabs, and the slabs were hot-rolled to obtain hot-rolled steel sheets with a thickness of 4 mm. Subsequently, the hot-rolled sheets were annealed at a temperature of 900 to 1100°C, pickled, and cold-rolled to obtain cold-rolled steel sheets with a thickness of 1.5 mm. The obtained cold-rolled steel sheets were annealed at a temperature of 900 to 1100°C (final annealing), 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 rinsed with water to obtain test specimens. The liquid temperature conditions and immersion time for the final pickling are shown in Table 2. After the final pickling, the surface was finished by brushing. Brushing was performed using a SiC abrasive brush with a load current of 80-120A, a rotation speed of 1000 rpm, and a reduction amount of 0.5-1.0 mm.

[0058] From the obtained test material, four 20mm x 25mm test pieces were cut out, and one of them had its surface Si For measuring the area ratio of the oxide film, the remaining three samples underwent nitriding and oxidation treatments, simulating ammonia combustion gas.

[0059] The area ratio of the Si oxide film on the steel plate surface was measured using EPMA. Acceleration voltage: 15kV, irradiation current: 2.0 × 10⁻¹⁴ -7 Measurements were taken under the following conditions: Analysis area: 30 μm × 30 μm, Measurement time: 50 ms. From the obtained images, the Si oxide film (SiO2) was identified, the images were binarized using photo editing software (ImageJ), and the area ratio relative to the observation field area was determined using image processing software.

[0060] For the nitriding and oxidation treatment, a gas atmosphere consisting of 10 vol% ammonia, 10 vol% water vapor, and the remainder nitrogen (N) was introduced into an annealing furnace. The remaining test specimens were placed in the furnace, heated to 600°C, held for 50 hours, then cooled and removed, and the grain boundary crack length and nitriding depth were measured.

[0061] The length of intergranular cracks was determined by cutting the specimen after nitriding and oxidation treatment so that the cross-section in the thickness direction could be observed, and then observing the cross-section of the specimen using an optical microscope. The observation was performed at a 100 μm × depth just below the surface of the steel plate. A 100 μm area was defined as one field of view, and three randomly selected locations within the sample cross-section were observed, focusing on the grain boundaries. The crack initiation length was measured. If the sum of the grain boundary crack lengths at three observation points is 20 μm or less, It is good.

[0062] The nitriding depth was determined by cutting the test specimen after nitriding and oxidation treatment, electrolytic etching with a 10% oxalic acid aqueous solution at a voltage of 6V for 5 seconds, and observation using an optical microscope. The nitriding depth was measured using photographs. The presence of red scale was visually checked. Samples that showed no red scale were deemed to pass (○), while those that showed even a slight red scale were deemed to fail (×). These measurement results are shown in Table 2. From the data in Table 2, it can be seen that the steel sheet according to the present invention has a reduced grain boundary crack length.

[0063] [Table 1]

[0064] [Table 2] [Industrial applicability]

[0065] This invention can be used in a wide range of industries, including the automotive industry and the general machinery industry.

Claims

1. In mass percent, C: 0 to 0.030%, Si: 0.05-3.00%, Mn: 0.05-1.20%, P: 0.050% or less, S: 0.005% or less, Ni: 0 to 1.00%, Cr: 12.0-31.0%, N: 0 to 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 to 1.00%, B: 0 to 0.0100%, Ca: 0-0.0150% Sn: 0-1.00%, Hf: 0-0.60%, Zr: 0 to 0.60%, Sb: 0 to 0.60%, Co: 0 to 1.50%, W: 0-2.00%, Ta: 0 to 1.00%, Ga: 0-0.50%, Mg: 0 to 0.0050%, REM: Contains 0-0.20%, The equation satisfies equation 1, The remainder consists of Fe and impurities. The following equation 1 is satisfied, A ferritic stainless steel sheet characterized by the presence of a Si oxide film covering 5.0% or more of the surface area when the steel sheet surface is viewed from vertically above. 10Al+2Mo+3Ti+0.5Cu-1.5Si≦5.0 ...(Formula 1) However, the element symbols in Equation 1 indicate the content (mass%) of the element, and 0 should be substituted if the element is not present.

2. The ferritic stainless steel sheet according to claim 1, wherein in the cross-section of the steel sheet in the thickness direction, the sum of the grain boundary crack lengths in any three fields of view, with a 100 μm square area being defined as one field of view, is 20 μm or less.

3. The ferritic stainless steel sheet according to claim 1 or 2, wherein the Si oxide film is present in an area percentage of 50% or less.

4. A ferritic stainless steel sheet according to claim 1 or 2, for use in ammonia combustion equipment.

5. A method for producing a ferritic stainless steel sheet according to claim 1 or 2, characterized in that, after the final cold rolling of a steel sheet having the components according to claim 1, it is heated and held at 900 to 1100°C, then cooled to a temperature of 50°C or lower, and immersed for 40 to 60 seconds in an acid pickling solution containing 2.0% or less hydrofluoric acid and 6 to 15% nitric acid at a temperature of 50 to 60°C.

6. The method for manufacturing a ferritic stainless steel sheet according to claim 5, wherein at least a portion of the surface of the steel sheet is brushed after the pickling step.

7. A component having at least a portion of the ferritic stainless steel sheet described in claim 1 or 2.

8. The part according to claim 7, which is a part for ammonia combustion equipment.

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