Ferrite-based stainless steel

Abstract

The present invention addresses the problem of suppressing intergranular cracking (providing nitriding resistance) and suppressing occurrence of red scale (providing oxidation resistance) even in a gas atmosphere containing nitrogen and water vapor such as an ammonia combustion gas in a medium to high temperature range ranging from 500°C to 700°C. The purpose of the present invention is to provide ferrite-based stainless steel which solves the problem.　The present invention has a certain component composition with which the nitriding tendency index is 5 or less, wherein a dislocation (strain) is introduced into a steel material surface by polishing the steel material surface so as to harden the steel material surface, and a hard Cr-Si-based oxide coating film, which is hard in an atmosphere at 500°C or higher, is formed for a surface layer by setting the difference between the surface hardness and the hardness of a central part of the steel material to 25 Hv or more. Consequently, excellent oxidation resistance and nitriding resistance are provided. Nitriding tendency index: 10Al + 2Mo + 3Ti + 0.5Cu - 1.5Si

Description

Ferritic stainless steel

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

[0002] Global warming has become an international environmental issue, and technological development is being actively carried out to realize a carbon-free society, such as carbon zero and carbon neutral. In this context, ammonia is attracting attention as an alternative fuel to carbon fuels. The combustion reaction of ammonia is 4NH ３ +30 ２ →2N ２ +6H ２ Ammonia is a low-carbon fuel, producing water and nitrogen, with a low environmental impact, and is therefore expected to be a recyclable fuel. The combustion temperature of ammonia is 1750°C (adiabatic flame temperature), which is lower than that of hydrogen (2120°C), methane (1970°C), and gasoline (approximately 2000°C). The combustion temperature in actual engines and gas turbines is also lower than that of existing fuels. Therefore, when ammonia is used as a fuel, the exhaust gas temperature is also lower, at around 500-700°C, which is lower than that of existing fuels. This temperature range of 500-700°C is a temperature at which steel materials used in exhaust pipes, etc., are susceptible to oxidation, and is also a temperature range where so-called red scale is likely to form.

[0003] Patent Document 1 proposes a ferritic stainless steel for use in a reformer for a fuel cell, which has oxidation resistance and red scale resistance even at high temperatures of around 600°C.

[0004] Patent Document 2 proposes a ferritic stainless steel for use in the exhaust pipe of an internal combustion engine equipped with a pollution control system containing urea or ammonia for reducing nitrogen oxides.

[0005] JP 2003-160844 A JP 2015-532681 A

[0006] Development is underway to utilize ammonia as a fuel not only by burning it alone, but also by co-firing it with other fuels (heavy oil, light oil, hydrogen, etc.). However, even in co-firing, 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 large amounts of nitrogen, water vapor, and hydrogen.

[0007] Because the steel contains water vapor and hydrogen, red scale is likely to form due to steam oxidation. Furthermore, the combustion gas temperature is around 500 to 700°C, which is also a temperature range in which red scale is likely to form. For this reason, oxidation resistance (red scale resistance) is required for steel materials used in ammonia combustion gas systems, etc.

[0008] Furthermore, the large amount of nitrogen contained in ammonia combustion exhaust gas causes nitrogen to penetrate (nitride) into the surface layer of the steel, which causes embrittlement due to intergranular cracking. Therefore, steel for use in ammonia combustion gas systems is also required to be nitriding-resistant (intergranular cracking-resistant).

[0009] The stainless steel of 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 intergranular cracking caused by surface nitriding in gases containing a large amount of nitrogen, such as ammonia combustion gas.

[0010] The stainless steel of Patent Document 2 has the effect of suppressing surface grain boundary cracking caused by urea or ammonia, but it cannot be said to have sufficient nitriding resistance against gases containing large amounts of nitrogen, such as the exhaust gas from combustion of ammonia.

[0011] The present invention addresses the problem of providing a ferritic stainless steel that has red scale resistance (oxidation resistance) and intergranular cracking resistance (nitriding resistance) even when exposed to gases containing large amounts of nitrogen, water (water vapor), and hydrogen at temperatures of about 500 to 700°C, such as ammonia combustion exhaust gas, and aims to provide such a steel material (ferritic stainless steel material).

[0012] In order to achieve the above object, the present inventors have conducted extensive research and have obtained the following findings.

[0013] (a) Development was carried out with the aim of forming a coating with strong oxidation and nitridation resistance on the surface of a steel material. As a result, it was discovered that introducing dislocations (strain) into the surface of a steel material to form a surface-hardened layer promotes the diffusion of Cr and Si, allowing the formation of a strong Cr-Si oxide coating on the surface (hereinafter referred to as a Cr-Si oxide coating). It was found that a strong Cr-Si oxide coating can be formed on the surface of a steel material if the surface-hardened layer exists from the surface of the steel material and its thickness is 0.5 μm or more. The surface-hardened layer is a region where the difference in hardness between the steel material and the center (the region from the steel surface to 3 / 8 to 5 / 8 of the steel thickness in the thickness direction) is 25 Hv or more in Vickers hardness. It was also discovered that if the surface hardness of a steel material differs from the center of the steel material by 25 Hv or more, the steel material has a surface-hardened layer with a thickness of at least 0.5 μm.

[0014] (b) From the viewpoint of suppressing nitriding, it was considered that it would be good to optimize the contents of Cr, Mo, Ti, Al, and Cu, which are elements that promote nitriding. Furthermore, it was discovered that Si not only has an effect of suppressing red scale caused by steam oxidation, but also has an effect of suppressing nitriding, although the reason for this is not yet clear. By taking into account the nitriding suppression effect of Si as well as the contents of the nitriding promoting elements Cr, Mo, Ti, Al, and Cu, a nitriding tendency index, which indicates the nitriding tendency of steel, was derived, and it was discovered that it would be good to keep this nitriding tendency index at 5.0 or less. Nitriding tendency index = 10Al + 2Mo + 3Ti + 0.5Cu - 1.5Si ≦ 5.0 ... (Equation 1)

[0015] (c) Although there are no particular limitations on the method for introducing strain into the surface layer of a steel material, it has been found that polishing, shot blasting, shot peening, or the like is effective on the steel surface. From the viewpoint of forming a strong oxide film, it has been found that a strong oxide film can be formed by manufacturing a stainless steel using a conventional manufacturing process, removing the oxide film formed on the surface, introducing strain into the activated surface, and then thermally oxidizing it. Furthermore, the temperature during this thermal oxidation is approximately 500 to 700°C, which overlaps with the temperature of ammonia combustion gas. Therefore, it has been found that a strong oxide film can be formed on the steel surface by applying surface-hardened stainless steel to ammonia combustion equipment or exhaust parts and exposing them to combustion gas.

[0016] (d) Furthermore, it was found that the presence of precipitates in the surface layer of the stainless steel material restricts the movement of nitrogen and suppresses nitriding of the surface layer of the stainless steel material. Specifically, in the surface layer of the stainless steel material (base material) (within the range of 20 μm from the steel surface in the depth direction), precipitates with a particle size of 0.1 μm or more (carbonitrides of Nb, Ti, W, B, V and precipitates of ε-Cu) were found to be present at a density of 10 / 1000 μm. ２ It has been found that the presence of more than this amount inhibits nitriding.

[0017] (e) It was found that the presence of the above-mentioned precipitates at grain boundaries (boundaries between crystal grains, hereinafter simply referred to as grain boundaries) which are likely to serve as paths for nitrogen migration further restricts the migration of nitrogen and suppresses nitriding. Specifically, it was found that nitriding is effectively suppressed when the above-mentioned precipitates with a particle size of 0.1 μm or more (preferably 2.0 μm or less) are present at grain boundaries in the surface layer at a density of 0.17 particles / μm or more.

[0018] The present invention was made based on these findings, and the gist of the present invention is as follows.

[0019] [1] In mass%, C: 0 to 0.030%, Si: 0.03 to 3.00%, Mn: 0.03 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%, A ferritic stainless steel material comprising: Zr: 0-0.60%, Sb: 0-0.60%, Co: 0-1.50%, W: 0-2.00%, Ta: 0-1.00%, Ga: 0-0.50%, Mg: 0-0.0050%, and REM: 0-0.20%, and satisfying the following formula 1, with the balance being Fe and impurities, wherein the Vickers hardness (Hvs) of the surface of the steel material and the Vickers hardness (Hvc) of the center of the steel material satisfy the following formula 2: 10Al+2Mo+3Ti+0.5Cu-1.5Si≦5.0 Formula 1 Hvs-Hvc≧25Hv Formula 2 wherein the element symbols in formula 1 indicate the content (mass%) of the element, and 0 is substituted if the element is not contained. The central portion of the steel material refers to a region in the range of 3 / 8 to 5 / 8 of the thickness of the steel material in the thickness direction in the cross section of the steel material. [2] The ferritic stainless steel material according to [1], wherein in a cross section perpendicular to the surface of the steel material, there is a surface hardened layer having a Vickers hardness that is 25 Hv or more higher than the Vickers hardness of the central portion of the steel material, and the thickness of the surface hardened layer is 0.5 μm or more in the thickness direction from the surface of the steel material. [3] In a cross section perpendicular to the surface of the steel material, there are 10 precipitates per 1000 μm, each having a particle size of 0.1 μm or more (preferably 2.0 μm or less), in a surface layer portion at a depth of 20 μm from the surface of the steel material. ２[4] The ferritic stainless steel material according to any one of [1] to [3], wherein, in a cross section perpendicular to the surface of the steel material, precipitates having a particle size of 0.1 μm or more (preferably 2.0 μm or less) are present at 0.17 particles / μm or more at grain boundaries in a surface layer portion at a depth of 20 μm from the surface of the steel material. [5] The ferritic stainless steel material according to any one of [1] to [4], wherein, in a cross section perpendicular to the surface of the steel material, a 100 μm square area is taken as one field of view, and the total length of intergranular cracks in any three fields of view is 5 μm or less. [6] The ferritic stainless steel material according to any one of [1] to [4], wherein, after holding the steel material at 600°C for 10 minutes, a region at a distance of 500 nm (0.5 μm) or less from the surface of the steel material in the thickness direction is subjected to GD-OES analysis, and the Cr peak intensity I Ｃｒ , Si peak intensity I Ｓｉ , and the N (nitrogen) peak intensity I Ｎ The ferritic stainless steel material according to any one of the above [1] to [5], wherein the following formula 3 is satisfied: Ｃｒ +2×I Ｓｉ -0.5 x I Ｎ≧0.30 Formula 3 [7] The ferritic stainless steel material for an ammonia burner according to any one of [1] to [6]. [8] A method for producing the ferritic stainless steel material according to any one of [1] to [6], comprising the steps of: holding a steel material having the composition according to [1] in a temperature range of 900 to 1100°C, cooling to a temperature of 50°C or less, pickling the steel material by immersing it 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, and then polishing, shot blasting, or shot peening the surface of the steel material. [9] The method for producing the ferritic stainless steel material according to [8], wherein the holding time in the temperature range of 900 to 1100°C is 30 seconds or more and 5 minutes or less, and the residence time in the temperature range of 800 to 300°C during the cooling is 30 seconds or more and 120 seconds or less.

[10] The method for producing a ferritic stainless steel material according to [8] or [9], wherein the polishing is dry polishing, in which polishing is performed with #220 followed by #400 and / or #600 in that order.

[11] A part having at least a part made of the ferritic stainless steel material according to any one of [1] to [6].

[12] The part according to

[11] , which is a part for an ammonia burning appliance.

[0020] The ferritic stainless steel according to the present invention can provide a ferritic stainless steel with good oxidation resistance and nitriding resistance even when it comes into contact with a gas containing a large amount of nitrogen and water (water vapor) at a temperature of about 500 to 700°C, such as ammonia combustion exhaust gas.

[0021] FIG. 1 is a graph showing the relationship between the nitriding resistance index and the length of intergranular cracks.

[0022] An embodiment of the present invention (hereinafter simply referred to as the present invention) will be described below. Unless otherwise specified, "%" for components indicates mass % in the steel. When no lower limit is specified or when the lower limit is 0%, this also includes the case where the component is not contained (0%). Furthermore, unless otherwise specified, hardness indicates Vickers hardness. In this specification, unless otherwise specified, ferritic stainless steel materials are simply referred to as steel materials, and their form is not particularly limited, and includes steel plates, steel bars, wire rods, steel sections, steel pipes, etc.

[0023] <Steel Composition> C: 0 to 0.030% Since C is an element that reduces formability (r-value), the lower the content the better, with the upper limit set to 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.

[0024] Si: 0.03 to 3.00% Si is an element that is effective in suppressing oxidation, especially steam oxidation, and is also effective in suppressing nitriding. ２ From the viewpoint of forming an internal oxide layer, the content of Si is 0.03% or more. The lower limit of Si is preferably 0.05%, 0.10%, 0.15%, 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, a Si oxide film (or SiO ２ The upper limit of Si is set to 3.00%, because the area ratio of the internal oxide layer increases and the workability and weldability deteriorate. The upper limit of Si is preferably set to 2.95% or 2.90%.

[0025] Mn: 0.03 to 1.20% Mn, like Si, is an element effective in improving oxidation resistance, so it is recommended that it be contained in an amount of 0.03% or more. The lower limit of Mn is preferably 0.05%, 0.07%, 0.09%, 0.11%, 0.13%, or 0.15%. On the other hand, excessive Mn content adversely affects oxidation resistance and deteriorates workability, so it is recommended that it be contained in an amount of 1.20% or less. The upper limit of Mn is preferably 1.10%, 1.00%, 0.90%, or 0.80%.

[0026] P: 0.050% or less P is harmful to stainless steel, as it reduces toughness, hot workability, and corrosion resistance, so the less P the better, and it should be 0.050% or less, and preferably 0.040% or less. However, an excessive reduction in P would increase the load during refining or require the use of expensive raw materials, so in reality it may contain 0.001% or more.

[0027] S: 0.0050% or less S is harmful to stainless steel, as it reduces toughness, hot workability, and corrosion resistance, so the less S the better, and the upper limit should be 0.0050% or less, preferably 0.0030% or less. However, an excessive reduction increases the load during refining or requires the use of expensive raw materials, so in reality, a content of 0.0001% or more may be acceptable.

[0028] Ni: 0 to 1.00% or less The addition of Ni has the effect of further enhancing the high corrosion resistance of stainless steel. However, because Ni is an expensive element, adding a large amount does not provide an effect commensurate with the increase in alloy cost. Therefore, the Ni content should be 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 to ensure the effect, it is preferable to add 0.01% or more.

[0029] Cr: 12.00 to 31.00% Cr is an important element that provides corrosion resistance to stainless steel, and should be contained in an amount of 12.00% or more, preferably 12.50% or more, 13.00% or more, 14.00% or more, 15.00% or more, 18.00% or more, or 20.00% or more. On the other hand, since a large amount of Cr content leads to a decrease in workability, it should be contained in an amount of 31.00% or less, preferably 30.00% or less, 29.00% or less, 28.00% or less, 26.00% or less, or 24.00% or less.

[0030] N: 0 to 0.030% From the viewpoint of suppressing intergranular cracking on the surface, it is preferable that the amount of N originally contained in the steel material is small. Furthermore, since N reduces workability and reduces corrosion resistance by combining with Cr, the smaller the content, the better. It is good to set it to 0.030% or less, and preferably 0.025% or less, 0.020% or less, 0.015% or less, or 0.010% or less. On the other hand, since an excessive reduction places a large burden on the refining process, it may be contained in an amount of preferably 0.001% or more, 0.003% or more, or 0.005% or more.

[0031] Nb: 0 to 1.00% Nb has the effect of improving formability and corrosion resistance. On the other hand, if added in excess of 1.00%, recrystallization becomes difficult and the structure becomes coarse, so it is best to keep the content at 1.00% or less, and preferably at most 0.90%, 0.80%, or 0.70%. There is no particular lower limit for the Nb content, but to ensure the desired effect, it is preferable to add 0.01% or more.

[0032] 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 content, which leads to embrittlement and a decrease in corrosion resistance. Therefore, it is recommended that the content be 2.50% or less, and preferably 2.20% or less, or 2.00% or less. There is no particular lower limit for the Mo content, but to ensure the effect of corrosion resistance, it is preferable to contain 0.01% or more.

[0033] 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 result in performance improvements that justify the manufacturing costs, so it is best to keep it at 3.00% or less, and preferably 2.50% or less, 2.20% or less, or 1.90% or less. There is no particular lower limit for the Cu content, but to ensure the desired effect, it is preferable to include 0.01% or more.

[0034] Al: 0.002 to 0.500% Al is an element that bonds 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, because 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.

[0035] 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 adding too much Ti Ｎ Since significant formation of Ti leads to nozzle clogging during production and surface defects in products, the Ti content should be 0.600% or less, 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 to ensure the effect, it is preferable to contain 0.001% or more.

[0036] 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 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 desired effect, it is preferable to add 0.01% or more, or 0.05% or more.

[0037] B: 0 to 0.0100% B is an element that increases the strength of grain boundaries and contributes to improving workability. However, excessive addition of B can actually reduce elongation and workability, so the content should be 0.0100% or less, and 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 desired effect, it is preferable to include 0.0001% or more, or 0.0005% or more.

[0038] Ca: 0 to 0.0150% If the Ca content is high, TI Ｎ The concentration of Ca in the oxides that promote the formation of slag increases, causing the loss of this ability, so the content should be 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 in it is unavoidable. In addition, complete removal is difficult, and excessive reduction increases the load during refining, so in practical operation, the content may be 0.0001% or more, or 0.0002% or more.

[0039] Sn: 0 to 1.00% The addition of Sn has the effect of further enhancing the high corrosion resistance of stainless steel. However, excessive addition leads to a decrease in workability, so the content should be 1.00% or less, and 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 to ensure the desired effect, it is preferable to include 0.001% or more, or 0.002% or more.

[0040] In addition, the stainless steel may further contain, 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%, and REM: 0-0.200%. The addition of these elements enhances the corrosion resistance of stainless steel. However, because these elements are expensive, excessive inclusion of these elements does not provide benefits commensurate with the increased cost, so upper limits have been set. While there are no particular lower limits on the content of these elements, to ensure the effects 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.

[0041] The balance of the above steel components is Fe and impurities. Here, the term "impurities" refers to components that are mixed in during industrial steel production due to various factors in the production process, including raw materials such as ore and scrap, and are acceptable within a range that does not adversely affect the present invention.

[0042] <Nitriding Tendency Index> In optimizing the steel composition from the viewpoint of suppressing nitriding of steel, the relationship between the contents of elements that affect nitriding was considered. Cr, Mo, Ti, and Al are known as elements that promote nitriding, but a certain amount of these may be contained in order to ensure the corrosion resistance and other functions of stainless steel. 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 nitriding, although the reason for this is unclear. In addition, as will be described later, a Si oxide film (SiO ２ Therefore, it was found that the nitriding tendency index for ferritic stainless steel can be evaluated as 10Al + 2Mo + 3Ti + 0.5Cu - 1.5Si, which is an index indicating the nitriding tendency. From the viewpoint of nitriding suppression, it was found that it is desirable to set this nitriding tendency index to 5.0 or less.

[0043] Nitriding tendency index=10Al+2Mo+3Ti+0.5Cu−1.5Si≦5.0 (Equation 1) In Equation 1, the element symbol indicates the content (mass%) of the element, and 0 is substituted if the element is not contained.

[0044] 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.

[0045] <Hardened Surface Layer> When the surface layer of the steel material is hardened, a strong Cr-Si oxide film can be formed in a temperature range of about 500 to 700° C. This is thought to be due to the following reasons.

[0046] That is, hardening the surface layer of steel introduces dislocations (strain) into the surface layer, and it is believed that these dislocations become diffusion paths for Cr and Si inside the steel. Furthermore, when steel is heated to 500°C or higher, thermal energy and high-density strain act as driving forces, accelerating the diffusion of Cr and Si from the inside of the steel toward the surface layer, where they combine with oxygen in the external environment to form a strong oxide film on the surface of the steel.

[0047] Furthermore, the formation of this strong Cr-Si oxide film on the surface can inhibit oxygen and nitrogen from the external environment from penetrating and diffusing into the steel material, and can inhibit excess consumption of Cr and Si in the steel material, which is thought to inhibit a decrease in Cr concentration and maintain corrosion resistance.Similarly, the strong Cr-Si oxide film on the surface also inhibits the penetration and diffusion of nitrogen, which is thought to inhibit nitriding in the surface layer of the steel material (the portion immediately below the surface of the steel material) and prevent intergranular cracking due to nitriding.

[0048] In order to introduce dislocations (strain) into the steel surface, it is preferable that a surface-hardened layer be present in the surface layer (directly below the surface of the steel). The surface-hardened layer is a region having a Vickers hardness that is 25 Hv or more higher than that of the central portion of the steel. It has been confirmed that the hardness of the surface-hardened layer is preferably 25 Hv or more higher than the hardness of the interior of the steel (referring to the interior of the steel other than the surface-hardened layer). Here, the hardness of the interior of the steel is preferably represented by the hardness of the central portion of the steel. The central portion of the steel refers to a region in a cross section perpendicular to the surface of the steel (hereinafter simply referred to as the "steel cross section"), extending from the surface of the steel to a distance of 3 / 8 to 5 / 8 of the thickness (thickness refers to the thickness in the case of a plate-shaped steel and the diameter in the case of a rod-shaped steel) in the thickness direction (the thickness direction is the direction perpendicular to the surface of the steel toward the center of the steel (the central direction)). If the steel material is a steel plate, it refers to the vicinity of the center in the thickness direction of the plate; if the steel material is a rod-shaped material such as a steel bar or wire rod, it refers to the vicinity of the central axis of the steel material; and if the steel material is a steel pipe, it refers to the center of the steel pipe in the thickness direction.

[0049] The surface-hardened layer is present directly below the surface of the steel material and has a thickness from the surface of the steel material in the thickness direction. The lower limit of the thickness of the surface-hardened layer is not particularly limited, but it is preferably at least 0.5 μm. Preferably, it is 0.6 μm or more, 0.7 μm or more, 0.8 μm or more, 0.9 μm or more, or 1.0 μm or more.

[0050] The upper limit of the thickness of the surface-hardened layer is not particularly limited. However, since it is difficult to form a thick surface-hardened layer, the thickness of the surface-hardened layer is preferably 50.0 μm or less. Preferably, the thickness is 45.0 μm or less, 40.0 μm or less, 35.0 μm or less, 30.0 μm or less, 25.0 μm or less, or 20.0 μm or less.

[0051] For example, a thickness of T μm in the surface hardened layer refers to a region from the surface of the steel material up to T μm in the thickness direction. That is, it indicates a region ranging from 0 to T μm in the thickness direction from the surface of the steel material. Therefore, T may be 0.5 μm or more, 0.6 μm or more, 0.7 μm or more, 0.8 μm or more, 0.9 μm or more, or 1.0 μm or more, and may be 50.0 μm or less, 45.0 μm or less, 40.0 μm or less, 35.0 μm or less, 30.0 μm or less, 25.0 μm or less, or 20.0 μm or less.

[0052] <Surface Hardness> It was confirmed that if the hardness of the surface of a steel material is 25 Hv or more higher than the hardness of the interior of the steel material (referring to the interior of the steel material excluding the surface hardened layer), the thickness of the surface hardened layer is at least 0.5 μm. In other words, when the Vickers hardness of the surface of the steel material is Hvs and the Vickers hardness of the center part of the steel material is Hvc, if the following formula 2 is satisfied, the steel material has a surface hardened layer with a thickness of 0.5 μm or more.

[0053] Hvs-Hvc≧25Hv...Formula 2

[0054] The difference in hardness between the surface hardness (Hvs) of the steel material and the center hardness (Hvc) of the steel material is preferably 26 Hv or more, 28 Hv or more, 30 Hv or more, 32 Hv or more, 34 Hv or more, 36 Hv or more, 38 Hv or more, or 40 Hv or more.

[0055] The hardness (Hvc) of the central part of the steel material is measured at any five points in the central part of the steel material cross section, and the arithmetic mean value of the measured hardness is defined as the hardness (Hvc) of the central part of the steel material. The hardness (Hvs) of the surface of the steel material is measured at any five points in the central part of the steel material cross section, and the arithmetic mean value of the measured hardness is defined as the hardness (Hvs) of the surface of the steel material.

[0056] <Method of Measuring Vickers Hardness> Vickers hardness is measured at five points with an indentation load of 200 g, and the arithmetic mean value of these is taken as the Vickers hardness of that portion.

[0057] The hardness of the surface of the steel material can be measured by measuring five points at any desired location on the surface of the steel material (for example, an arbitrarily selected location of 5 mm square), and the arithmetic mean value of the measurements can be used as the surface hardness.

[0058] The Vickers hardness of the interior of the steel material and the surface hardened layer is measured on the cross section of the steel material. The Vickers hardness of the central part of the steel material is measured at five points on the cross section of the steel material within a square area that is 1 / 4 (25%) of the plate thickness and has its center on the center line of the plate thickness in the case of a steel plate or steel pipe, or within a circular area (a concentric circle of the cross section of the steel bar or wire rod) that has its center on the central axis and has a diameter of 1 / 4 (25%) of the steel diameter, and the arithmetic average value of the measurements is used as the hardness of the central part of the steel material.

[0059] The Vickers hardness of the surface-hardened layer can be determined by measuring the Vickers hardness in the cross section of the steel material from the surface of the steel material in the thickness direction of the steel material, and identifying the area (surface-hardened layer) having a hardness difference of a predetermined hardness (25 Hv) or more compared to the hardness of the central part of the steel material. For example, it is advisable to measure the Vickers hardness in the thickness direction from the surface to a thickness of 1.0 μm at intervals of 0.1 μm, and at intervals of 1.0 μm from the thickness of 1.0 μm onwards.

[0060] <Precipitate density in surface layer> Furthermore, development was carried out from the perspective of suppressing the penetration of nitrogen from outside the steel material. As a result, it was found that the presence of precipitates in the surface layer of the stainless steel material restricts the movement of nitrogen and suppresses nitriding of the surface layer of the stainless steel material. Here, the surface layer of the stainless steel material (base metal) refers to the range from the surface of the steel material to 20 μm in the depth direction in the cross section of the steel material.

[0061] In the cross section of the steel material, the number density of precipitates with a particle size of 0.1 μm or more in the surface layer of the steel material (surface layer precipitate density) is 10 / 1000 μm ２ It has been found that the presence of more than 11 precipitates per 1000 μm inhibits nitriding. ２ Above, 12 pieces / 1000μm ２ Above, 15 pieces / 1000μm ２ Above, 18 pieces / 1000μm ２ or more, or 20 pieces / 1000 μm ２ There is no particular upper limit to the density of precipitates in the surface layer, but if the density of precipitates in the surface layer is too high, it will affect properties other than nitriding (for example, toughness), so it is preferably 200 precipitates / 1000 μm ２ Below, 150 pieces / 1000μm ２ Below, 100 pieces / 1000μm ２ Below, 70 pieces / 1000μm ２ or less, or 50 pieces / 1000 μm ２ It would be better to do the following:

[0062] Although there is no particular upper limit to the particle size of the precipitates to be measured, it is preferable to prevent the formation of large precipitates because if the precipitate particle size is too large, it will affect the strength and corrosion resistance of the steel material. From this perspective, it is preferable to keep the particle size of the precipitates to 2.0 μm or less, and therefore the particle size of the precipitates to be measured may also preferably be 2.0 μm or less.

[0063] The type of precipitate is not particularly limited, but may be, for example, one or more of Nb(C,N), Ti(C,N), W(C,N), B(C,N), V(C,N), and ε-Cu, where M(C,N) represents a carbonitride of element M (a composite compound of either or both of carbide and nitride).

[0064] <Precipitate Density at Grain Boundaries in the Surface Layer> Research into the restriction of nitrogen migration by precipitates has revealed that the presence of precipitates at grain boundaries, which are likely to serve as nitrogen migration paths, can block nitrogen migration and effectively restrict it. Tests have been conducted to investigate the relationship between nitriding and precipitates precipitated at grain boundaries in the surface layer. It has been found that nitriding is effectively suppressed when the number of precipitates with a grain size of 0.1 μm or more per grain boundary length (grain boundary precipitate density) is 0.17 / μm or more at the grain boundaries in the surface layer. The grain boundary precipitate density is preferably 0.20 / μm or more, 0.25 / μm or more, 0.30 / μm or more, 0.35 / μm or more, or 0.40 / μm or more. There is no particular upper limit to the grain boundary precipitate density, but if the grain boundary precipitate density is too high, it will affect properties other than nitriding (for example, toughness and corrosion resistance), so the density is preferably 10.00 particles / μm or less, 7.00 particles / μm or less, 5.00 particles / μm or less, 4.00 particles / μm or less, or 3.00 particles / μm or less.

[0065] The upper limit of the particle size of the precipitates to be measured is not particularly limited, as with the surface layer precipitates, but may preferably be 2.0 μm or less.

[0066] The type of precipitates is not particularly limited, as with the surface layer precipitates, but may be, for example, one or more of Nb(C,N), Ti(C,N), W(C,N), B(C,N), V(C,N), and ε-Cu.

[0067] <Method of measuring surface layer precipitate density and grain boundary precipitate density> A method of observing precipitates in the surface layer will be described. In the cross section of the steel material sample to be observed, a depth range of 20 μm from the surface is observed. For example, it is advisable to observe 20 μm from the surface in a direction parallel to the surface and a depth of 50 μm. In this case, the observation field area is 1000 μm. ２ The observation field of the sample is analyzed using an FE-EPMA (for example, JXA-8100, JXA-8350F) and the obtained observation image is analyzed. For example, the accelerating voltage is 15 kV, the current is 1 × 10 －７ The observation image obtained under condition A can be analyzed, and the above-mentioned precipitates can be observed. The observed precipitates can be mapped using the software provided with the EPMA. The particle size of the mapped precipitates is measured, and precipitates with particle sizes of 0.1 to 2 μm or more are extracted. The surface layer precipitate density can be calculated by dividing the number of extracted precipitates by the area of ​​the observation field.

[0068] The particle size of the precipitate is defined as the average value of the major axis and the minor axis, where the maximum width of the observed precipitate is defined as the major axis and the maximum width in the direction perpendicular to the major axis, respectively ((major axis + minor axis) / 2).

[0069] Similarly, grain boundaries can be identified from the obtained observation image, precipitates present on the grain boundaries can be extracted, and the grain boundary precipitate density (number / μm), which is the linear density of the precipitates, can be calculated from the grain boundary length and the number of precipitates extracted on the grain boundaries by image processing.

[0070] Measurements are made in at least three fields of view, and the surface layer precipitate density and grain boundary precipitate densities obtained are arithmetically averaged to determine the surface layer precipitate density and grain boundary precipitate density of the steel material.

[0071] <Grain boundary crack length> By forming the steel surface layer as described above, nitriding of the steel surface is suppressed, thereby suppressing grain boundary cracking due to the penetration of nitrogen (N). The length of grain boundary cracking can be measured by observing the crystal grain boundaries. Three 100 μm square areas are selected at any one time in the surface layer of the steel cross section (a portion including at least the nitrided portion), and the total length of the grain boundary cracks at these points is preferably 5.0 μm or less. If the total length of the grain boundary cracks at the three observation points is 5 μm or less, embrittlement of the steel surface can be suppressed, and the strength of the steel can be ensured in the temperature range of 500 to 700 °C. The shorter the total length of the grain boundary cracks, the more preferable it is, and it is more preferably 4.5 μm or less, 4.0 μm or less, 3.5 μm or less, 3.0 μm or less, 2.5 μm or less, or 2.0 μm or less.

[0072] The length of intergranular cracks in the surface layer of a steel material can be measured as follows. The cross section of the steel material to be used as a sample is observed under an optical microscope with a square observation field of 100 μm on each side, and the length of the intergranular cracks is measured. At this time, since the area closer to the steel material surface is more susceptible to the influence of nitrogen, it is advisable to set the observation field to the area directly below the steel material surface. It is preferable to perform the measurement by image processing. For example, the intergranular crack portion can be marked on the measurement image, and its length can be measured by image processing.

[0073] <Nitriding Layer Thickness> The shallower the nitriding depth, the better, from the viewpoint of suppressing nitriding cracks on the surface. The steel material according to the present invention has its components adjusted to suppress nitriding and has a Cr-Si oxide film on the surface, which enhances the nitriding suppression effect and results in a thinner nitrided layer. In particular, the smaller the nitriding tendency index (including negative values) is, the thinner the nitrided layer tends to be. While the nitrided layer thickness varies slightly depending on the nitrogen (N) content of the contacting gas, it has been confirmed that a thickness of approximately 45 μm or less suppresses surface embrittlement. The nitrided layer thickness is preferably 40 μm or less, 35 μm or less, 30 μm or less, 20 μm or less, or 10 μm or less. The nitrided layer is defined as a range showing a nitrogen concentration of 2 mass% or more.

[0074] <Form of Steel Material> The form of the steel material is not particularly limited, and may be, for example, a steel plate, a steel bar, a wire rod, a steel pipe, a steel section, etc. The steel material may also be a part obtained by processing a steel plate, etc. The processing method is not particularly limited, and may be, for example, a press process, a wire drawing process, a cutting process, etc.

[0075] <Evaluation of Nitridation Resistance Using Oxide Film> Because composite oxides increase in the temperature range of 500 to 700°C, Cr-Si oxide films, which are primarily composed of Cr or Si, are composed of strong oxides and can suppress the oxidation and nitridation of steel materials. Because Cr-Si oxide films have a particularly high nitridation suppression effect, we attempted to evaluate the nitridation resistance of Cr-Si oxide films. As a result, we found an index of nitridation suppression effect using the peak intensity of the emission intensity of each element measured by glow discharge optical emission spectrometry (GD-OES). That is, after holding a steel material at 600°C for 10 minutes, we performed GD-OES analysis on a region of the steel material at a distance of 500 nm (0.5 μm) or less in the thickness direction, and measured the emission intensity. The detected peak intensities of Cr, Si, and N were identified (qualitatively analyzed). The peak intensities of Cr, Si, and N were respectively calculated as I Ｃｒ , I Ｓｉ , I Ｎ It has been found that when the above formula is satisfied, the nitridation resistance index expressed by the following formula 3 is preferably 0.30 or more.

[0076] Nitriding resistance index = I Ｃｒ +2×I Ｓｉ -0.5 x I Ｎ ≧0.30 Formula 3 The lower limit of the nitridation resistance index is preferably 0.32, 0.34, 0.35, 0.36, 0.38, 0.40, 0.42, 0.43, 0.44, 0.45, 0.46, 0.47, 0.48, 0.49 or 0.50.

[0077] <Manufacturing method> Next, a manufacturing method will be described. The manufacturing method described below is one embodiment for obtaining the steel material according to the present invention, and the manufacturing method is not limited to this. As long as the steel material according to the present invention can be obtained, the manufacturing method is not limited.

[0078] In one embodiment of the method for producing a steel material according to the present invention, a steel material is produced by a conventional method, followed by acid washing, and then the surface is polished, shot blasted, shot peened, or the like to introduce strain, thereby obtaining a steel material according to the present invention in which the surface hardness is higher than that of the central portion. Hereinafter, an example will be described in which the steel material is a steel plate and polishing is used as the method for introducing strain into the surface.

[0079] The steel sheet before final annealing may be produced by a conventional production method, for example, steelmaking-hot rolling, steelmaking-hot rolling-annealing, or steelmaking-hot rolling-pickling-cold rolling.

[0080] However, in steelmaking, a suitable method is to melt steel containing the components adjusted to the above-described composition in a converter or electric furnace, followed by secondary refining. The molten steel thus adjusted to the desired composition is formed into a slab by a known casting method (e.g., continuous casting). The slab is heated to a predetermined temperature and hot-rolled to a desired thickness. After hot rolling, the steel may be cold-rolled (cold-rolled) as needed. Cold rolling may also be performed by a conventional method.

[0081] The conditions in the manufacturing process may be selected as appropriate. For example, the slab thickness, hot-rolled sheet thickness, etc. may be set as appropriate. After coiling, the hot-rolled sheet 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 also no particular restrictions on the pickling solution after hot rolling, and existing conditions, such as sulfuric acid or nitric hydrofluoric acid, may be used. Furthermore, the coil surface may be subsequently ground.

[0082] The hot-rolled steel sheet, hot-rolled annealed steel sheet, and cold-rolled steel sheet thus obtained are annealed to recrystallize. The annealing atmosphere is not particularly limited, and air may be used. After holding the steel in a temperature range of 900 to 1100°C, the steel is cooled to a temperature of 50°C or less. From the viewpoint of precipitating precipitates in the surface layer, if the annealing temperature is too high, the steel remains in a solid solution state and precipitation does not proceed. Therefore, the annealing temperature is preferably 1100°C or less, or 1050°C or less. On the other hand, if the annealing temperature is too low, the number of precipitates decreases. Therefore, the annealing temperature is preferably 900°C or more, 930°C or more, or 950°C or more. Although the mechanism behind this has not been elucidated, it is presumed that if the annealing temperature is low, precipitation nuclei of precipitates are not formed. The holding time in the temperature range of 900 to 1100°C is not particularly limited, but if it is too short, the precipitation time may be insufficient. Therefore, the holding time is preferably 30 seconds or more or 60 seconds or more. On the other hand, if the holding time is too long, the precipitates may become coarse and affect the toughness and corrosion resistance, so the holding time is preferably 5 minutes or less, 3 minutes or less, or 2 minutes or less. The annealing may be carried out after the processing of the steel material is completed, for example, after hot rolling, after hot rolling annealing, or after final cold rolling.

[0083] After annealing, the steel sheet is cooled. The cooling conditions are not particularly limited, but the residence time of the steel sheet in the temperature range of 800°C to 300°C is preferably 30 seconds or more and 120 seconds or less in order to grow the precipitates to a predetermined particle size. The residence time is more preferably 40 seconds or more, 50 seconds or more, or 60 seconds or more, and is preferably 110 seconds or less, 100 seconds or less, or 90 seconds or less.

[0084] The annealed steel sheet is cooled to 60°C or less and then pickled to etch away the upper Cr oxide and Fe oxide layers. To this end, the pickling solution is adjusted to contain 2.0% or less hydrofluoric acid (HF) and 6 to 15% nitric acid. The annealed steel sheet may be pickled by immersing it in a pickling solution at a temperature of 50 to 60°C for 40 to 60 seconds. This allows the steel sheet surface to be free of the upper Fe oxide, Cr oxide, and Si oxide layers formed during the manufacturing process.

[0085] The surface of the steel sheet after pickling is polished to impart dislocations (strain) to the surface and harden it. The thickness of the surface-hardened layer can be adjusted by adjusting the polishing conditions (e.g., polishing time and pressure). The polishing conditions are not particularly limited, but dry polishing or wet polishing can be used. For example, in the case of dry polishing, the steel sheet surface can be polished using a #220 abrasive, followed by a #400 to #600 abrasive. For example, the steel sheet surface can be polished using a #220 abrasive followed by a #400 and / or #600 abrasive. Polishing in order from coarse to fine abrasives reduces the surface roughness of the steel sheet and eliminates defects. The combination of abrasives is not particularly limited. The polishing amount is also not particularly limited, but is preferably approximately 10 μm to 50 μm in actual production. In practice, conditions vary depending on the steel material, so the amount can be appropriately determined by polishing in advance to check the surface hardness and surface defects.

[0086] In the case of wet polishing, oil mixed with abrasives is applied to the surface of the steel sheet, and the steel sheet surface is polished while rotating the polishing cloth. In this case, too, it is recommended to polish the steel sheet surface with abrasives in order from coarse to fine.

[0087] After polishing, the surface may be washed with water or with an alkali to remove any abrasive remaining on the surface of the steel sheet.

[0088] When shot blasting or shot peening is employed as a method for imparting dislocations (strain) to the surface, the conditions for shot blasting or shot peening are not particularly limited, and may be appropriately determined by checking the surface hardness and surface defects through a test in advance.

[0089] <Applications> Even when used in a gas environment with a high nitrogen (N) content, the steel material according to the present invention exhibits excellent nitriding resistance, resulting in little nitrogen penetration into the steel material surface layer and suppression of intergranular cracking. Furthermore, it also has oxidation resistance, and is effective in preventing red scale formation, a problem with conventional stainless steels, particularly in the medium- to high-temperature range of about 500 to 700°C. For this reason, it can be used, for example, in ammonia-burning equipment with a high nitrogen content and gas temperatures in the medium- to high-temperature range of 500 to 700°C. The entire equipment or component may be manufactured using the steel material according to the present invention, or the steel material according to the present invention may be used for at least a portion of the equipment or component. For example, the steel material according to the present invention may be used in ammonia-burning equipment components at least partially composed of the steel material according to the present invention.

[0090] Of course, due to its nitriding resistance and oxidation resistance, even when used in containers or piping parts that come into direct contact with ammonia or urea, the penetration of nitrogen ions in the solution and nitrogen in the evaporated gas from ammonia or urea into the steel surface layer is suppressed, thereby suppressing intergranular cracking. In addition, when the steel material according to the present invention is applied to parts that require nitriding resistance and oxidation resistance, the effects thereof can be enjoyed.

[0091] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples.

[0092] Steel having the chemical 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 obtained hot-rolled steel 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 (final annealing) by holding it at a temperature of 900 to 1100 ° C for 90 seconds, and then cooled to a temperature of 800 to 300 ° C so that the residence time was 90 seconds, and then cooled to 50 ° C or less. Then, the steel was 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.

[0093] For Sample 7, the holding time for final annealing was 30 seconds, and the subsequent cooling was performed so that the residence time between 800°C and 300°C was 90 seconds. For Sample 12, the holding time for final annealing was 30 seconds, and the subsequent cooling was performed so that the residence time between 800°C and 300°C was also 30 seconds.

[0094] Five test pieces measuring 20 mm x 25 mm were cut out from the obtained test material and the surfaces were polished. Polishing was performed by dry polishing, in which abrasive-coated abrasive paper was wrapped around a resin holder, and the holder was pressed against the surface of the test piece, and the holder was moved back and forth over a length of 300 mm at a constant speed (10 m / min) with the polishing pressure set as shown in Table 2. A #220 abrasive was used for the first pass, and a #400 abrasive was used for the second pass.

[0095] One of the specimens was cut to obtain a cross section perpendicular to the surface, and the surface Vickers hardness and central Vickers hardness were measured under a 50 g load. In addition to measuring the surface hardness, measurements were also taken from the surface to the thickness direction at 0.1 μm intervals up to a thickness of 1.0 μm, and then at 1 μm intervals from 1.0 μm onward to determine the thickness of the surface-hardened layer. Furthermore, the same steel cross section was observed at a depth of 20 μm from the surface, and image analysis of the precipitates within the observation field was performed using FE-EPMA. For precipitates with a particle size of 0.1 to 2.0 μm, the precipitate density in the surface layer (surface layer precipitate density) and the number of precipitates present at the grain boundaries between crystal grains were calculated by dividing the number of precipitates per μm of grain boundary (grain boundary precipitate density) by the total grain boundary length in the observation field.

[0096] Of the remaining three test pieces, nitriding and oxidation treatment was performed to simulate ammonia combustion gas. The nitriding and oxidation treatment was performed by introducing a gas atmosphere of 10 vol% ammonia, 10 vol% water vapor, and the balance nitrogen (N) into an annealing furnace, placing the three test pieces in the furnace, heating them to a temperature of 600°C, and holding them there for 50 hours. Thereafter, they were cooled and removed, and the length of intergranular cracks and the thickness of the nitrided layer were measured.

[0097] The length of intergranular cracking 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 100 μm × 100 μm area just below the steel sheet surface as one field of view, and three randomly selected locations on the sample cross section were observed to measure the length of intergranular cracking. If the total length of intergranular cracking at the three observation points was 5 μm or less, it could be determined that the nitriding resistance was good.

[0098] The thickness of the nitrided layer was determined by cutting the test piece after nitriding and oxidation treatment, measuring the nitrogen concentration distribution from the surface to the thickness direction by EPMA analysis of the cross section, and determining the range showing a nitrogen concentration of 2 mass % or more as the nitrided layer.

[0099] The red scale (oxidation resistance) was visually confirmed, and samples where no red scale was found were rated as pass (◯), and samples where 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 sheets according to the present invention did not develop intergranular cracks. A nitride layer thickness of 45 μm or less was rated as pass, and the steel sheets according to the present invention had a nitride layer thickness of 1.0 μm or less, confirming that almost no nitride layer was formed.

[0100] One of the remaining test pieces was heated to 600°C in a heat treatment furnace and held there for 10 minutes to form an oxide film on the surface. The emission intensity of the surface layer (a region 0.5 μm or less from the surface in the thickness direction) was measured by GD-OES analysis, and the peak intensity of Cr (I Ｃｒ ), Si peak intensity (I Ｓｉ ), N (nitrogen) peak intensity (I Ｎ ) was obtained. The GD-OES measurement conditions were as follows: GD-OES device: Rigaku Glow Discharge Optical Emission Spectrometer GDA750; Pulse frequency: 650 V, 2.8 hPa, 200 Hz; Sputtering rate: 0.5 μm / min; Anode diameter: 4.0 mm; Carrier gas: Ar (argon). The obtained peak values ​​of Cr, Si, and N were inserted into Equation 3 to evaluate the nitridation resistance of the oxide film. The evaluation results are shown in Table 2. Figure 1 shows a plot of the intergranular crack length and the nitridation resistance index (left side of Equation 3). From Figure 1, it can be seen that if the nitridation resistance index (left side of Equation 3) is 0.30 or higher, the intergranular cracking is suppressed to a level where it does not occur. In other words, it was confirmed that nitridation is suppressed by the Cr-Si oxide film.

[0101]

[0102]

[0103] The present invention can be used in a variety of industries, including the automobile industry and general machinery industry.

Claims

1. The components, by mass%, are: C: 0-0.030%, Si: 0.03-3.00%, Mn: 0.03-1.20%, P: 0.050% or less, S: 0.005% or less, 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%, A ferritic stainless steel material comprising Zr: 0-0.60%, Sb: 0-0.60%, Co: 0-1.50%, W: 0-2.00%, Ta: 0-1.00%, Ga: 0-0.50%, Mg: 0-0.0050%, and REM: 0-0.20%, and satisfying the following formula 1 with the balance being Fe and impurities, wherein the Vickers hardness (Hvs) of the surface of the steel material and the Vickers hardness (Hvc) of the center of the steel material satisfy the following formula 2: 10Al+2Mo+3Ti+0.5Cu-1.5Si≦5.0 Formula 1 Hvs-Hvc≧25Hv Formula 2 wherein the element symbols in formula 1 indicate the content (mass%) of the element, and 0 is substituted if the element is not contained. The central portion of the steel material refers to a region in the cross section of the steel material that is in the range of 3 / 8 to 5 / 8 of the thickness in the thickness direction of the steel material.

2. A ferritic stainless steel material as described in claim 1, which has a surface hardened layer having a Vickers hardness that is 25 Hv or more higher than the Vickers hardness of the central part of the steel material in a cross section perpendicular to the surface of the steel material, and the thickness of the surface hardened layer is 0.5 μm or more in the thickness direction from the surface of the steel material.

3. In a cross section perpendicular to the surface of the steel material, there are 10 precipitates with a grain size of 0.1 μm or more per 1000 μm in the surface layer at a depth of 20 μm from the surface of the steel material. ２ The ferritic stainless steel material according to claim 1 or 2, wherein the above-mentioned properties are present.

4. A ferritic stainless steel material according to any one of claims 1 to 3, in which, in a cross section perpendicular to the surface of the steel material, there are 0.17 precipitates / μm or more having a grain size of 0.1 μm or more at grain boundaries in a surface layer at a depth of 20 μm from the surface of the steel material.

5. A ferritic stainless steel material according to any one of claims 1 to 4, in which the total length of intergranular cracks in any three fields of view, each of which is 100 μm square in a cross section perpendicular to the surface of the steel material, is 5 μm or less.

6. After holding the steel material at 600°C for 10 minutes, the Cr peak intensity I obtained by GD-OES analysis of a region of the steel material that is 0.5 μm or less from the surface in the thickness direction is Ｃｒ , Si peak intensity I Ｓｉ , and N (nitrogen) peak intensity I Ｎ The ferritic stainless steel material according to claim 1 or 2, wherein the following formula 3 is satisfied. Ｃｒ +2×I Ｓｉ -0.5 x I Ｎ ≧0.30 ...Formula 3 7. A ferritic stainless steel material for an ammonia burning appliance according to any one of claims 1 to 6.

8. A method for producing a ferritic stainless steel material as defined in claim 1 or 2, comprising the steps of: holding a steel material having the components as defined in claim 1 in a temperature range of 900-1100°C, cooling it to a temperature of 50°C or less, immersing it in a pickling solution containing 2.0% or less hydrofluoric acid and 6-15% nitric acid at a temperature of 50-60°C for 40-60 seconds to pickle it, and then polishing, shot blasting or shot peening the surface of the steel material.

9. A method for producing ferritic stainless steel material as described in claim 8, wherein the holding time in the temperature range of 900 to 1100°C is 30 seconds or more and 5 minutes or less, and the residence time in the temperature range of 800°C to 300°C during the cooling is 30 seconds or more and 120 seconds or less.

10. A method for producing a ferritic stainless steel material according to claim 8 or 9, wherein the polishing is a dry polishing, in which #220 is followed by #400 and / or #600 in that order.

11. A part having at least a part made of the ferritic stainless steel material according to any one of claims 1 to 6.

12. The part according to claim 11, which is a part for an ammonia-burning appliance.

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