Ferritic stainless steel for furnace walls

Abstract

To provide a ferritic stainless steel that is suitable for furnace walls and exhibits enhanced steam oxidation resistance even in the welded areas.SOLUTION: A ferritic stainless steel for furnace walls comprises, in mass%, C: 0.020% or less, Si: 0.05-2.00%, Mn: 0.05-1.30%, P: 0.050% or less, S: 0.010% or less, Al: 2.00-6.00%, N: 0.020% or less, Cr: 16.0-25.0%, and Nb: more than 0.30% to 0.80% or less, with the balance being Fe and inevitable impurities.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a ferritic stainless steel for furnace walls, and more particularly to a Cr-containing steel, which has excellent steam oxidation resistance and is suitable for use in furnace walls exposed to high temperatures of 500°C or higher, such as in heating furnaces and incinerators. [Background technology]

[0002] Ceramic insulation materials and stainless steel plates are used for the walls of various furnaces, such as heating furnaces and incinerators, to provide insulation and prevent heat from escaping from the furnace. Ceramics have excellent insulation properties, but they have issues such as requiring a lot of energy to heat up, being prone to cracking due to temperature differences, and being prone to fine peeling. On the other hand, stainless steel plates have slightly inferior insulation properties to ceramic insulation materials, but their excellent conductivity has the advantage of saving the energy needed to heat up.

[0003] In recent years, growing awareness of global environmental issues has led to efforts around the world toward a carbon-neutral society, and the use of fuels such as H2, which does not emit CO2 when burned, and alcohol, which uses CO2 as a raw material during production, as fuels (at least in part) to supply electricity, is being considered. These fuels will hereafter be referred to as low-carbon fuels. While H2 does not emit CO2 when burned, and alcohol fuels do emit CO2, using CO2 during production can reduce total CO2 emissions, but it is expected that the amount of H2O produced during combustion will increase.

[0004] Stainless steel plates used for the walls of various furnaces exposed to high temperatures of 500°C or higher include SUS304 and SUS310S, as specified in JIS G4305:2021. SUS304 (18 mass% Cr-8 mass% Ni) is the most general-purpose stainless steel. SUS310S (25 mass% Cr-19 mass% Ni) is a stainless steel specialized for higher temperature applications.

[0005] However, SUS304 does not have sufficient oxidation resistance when exposed to temperatures above 500°C, and there is a high risk of the large amounts of oxides that form peeling off. SUS310S, on the other hand, has excellent oxidation resistance, but is very expensive due to its high Cr and Ni content. Furthermore, because both SUS304 and SUS310S are austenitic, they have low thermal conductivity among stainless steels and a high thermal expansion coefficient. Therefore, when subjected to repeated temperature increases and decreases, not only are oxide scales prone to peeling off, but the material itself also deforms significantly, making it susceptible to breakage.

[0006] Furthermore, when stainless steel plates are used for the furnace walls, they are attached by bolts or welding. With SUS304 or SUS310S, welding causes chromium to form as carbides at the welded joint, reducing the amount of chromium in the steel and resulting in insufficient oxidation resistance. The atmosphere inside the furnace varies depending on the furnace. If the atmosphere inside the furnace contains a lot of water vapor (approximately 5 vol% or more) and little oxygen (approximately 2 vol% or less), it becomes a significantly more oxidizing environment (steam oxidation environment) than air (approximately 3 vol% water vapor and approximately 20 vol% oxygen).

[0007] On the other hand, ferritic stainless steel has a higher thermal conductivity and a lower thermal expansion coefficient than austenitic stainless steel, so it is less likely to suffer from the problems mentioned above that occur when the temperature is repeatedly increased and decreased (peeling and deformation of oxide scale caused by temperature increases and decreases).

[0008] As examples of ferritic stainless steels with excellent oxidation resistance, Patent Document 1 discloses a ferritic stainless steel in which oxidation resistance is improved by adding Al. Patent Document 2 discloses a ferritic stainless steel for fuel cells in which creep resistance is improved by adding one or both of Sn and B in addition to Al. Patent Document 3 discloses an Fe-Cr-Al ferritic stainless steel in which high-temperature deformation is suppressed by adding Nb. Patent Document 4 discloses a ferritic stainless steel containing Al and having improved oxidation resistance. Patent Document 5 discloses a ferritic stainless steel containing appropriate amounts of Si and Al and having improved oxidation resistance. Patent Document 6 discloses a low-gravity ferritic stainless steel containing Al and with a balance between the Cr and Si contents. Patent Document 7 discloses a heat-resistant ferritic stainless steel containing Al. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Publication No. 6-220587 [Patent Document 2] Patent No. 6113359 [Patent Document 3] Japanese Patent Application Publication No. 10-251810 [Patent Document 4] Japanese Patent Application Laid-Open No. 2012-102376 [Patent Document 5] Japanese Patent Application Laid-Open No. 2012-107314 [Patent Document 6] Japanese Patent Application Laid-Open No. 2018-168457 [Patent Document 7] Japanese Patent Application Publication No. 2018-188687 Summary of the Invention [Problem to be solved by the invention]

[0010] According to the research of the present inventors, Patent Document 1 discloses a steel with improved oxidation resistance due to the addition of Al, but does not take into consideration the oxidation resistance of welds, nor the oxidation resistance in the severe oxidation environment (steam oxidation environment) inside a furnace.

[0011] Patent Document 2 discloses a steel that improves creep resistance by adding B and Sn in addition to Al, but does not take into consideration the oxidation resistance of welds or the oxidation resistance in the severe oxidizing environment inside a furnace.

[0012] Patent Document 3 discloses an Fe-Cr-Al ferritic stainless steel in which high-temperature deformation is suppressed by adding more than 0.1% but not more than 0.3% Nb. However, this was evaluated only in air, and the properties in an environment containing water vapor were not evaluated.

[0013] Patent Document 4 discloses a ferritic stainless steel containing 0.20 to 1.00% Al to improve oxidation resistance, but this has also only been evaluated in air, not in a water vapor atmosphere.

[0014] Patent Document 5 discloses a ferritic stainless steel with improved oxidation resistance, which contains 0.4 to 1.0% Si and more than 0.30% but not more than 1.0% Al so as to satisfy the relationship Si≧Al. The oxidation resistance in water vapor is also evaluated, but the test atmosphere contains 5% O2, which is an environment in which a protective oxide film of Al or Cr is likely to form, and there is room for testing in an even more severe environment for evaluation in a water vapor oxidation environment.

[0015] Patent Document 6 discloses a low-density ferritic stainless steel containing 0.50 to 10.0% Al and taking into consideration the balance between the Cr and Si contents, but this has also only been evaluated in air, not in a water vapor atmosphere.

[0016] Patent Document 7 discloses a heat-resistant ferritic stainless steel containing 0.0005 to 0.500% Al, but does not consider oxidation resistance.

[0017] As described above, in the prior art, many ferritic stainless steels with improved oxidation resistance have been disclosed, but none have evaluated the oxidation resistance of welds in the harsh environment of steam, and there is still room for further study on ferritic stainless steels suitable for use in furnace walls.

[0018] Therefore, an object of the present invention is to provide a ferritic stainless steel that is suitable for furnace wall applications and has excellent steam oxidation resistance even at welded joints.

[0019] In the present invention, "excellent steam oxidation resistance" means that the welded portion is not subject to abnormal oxidation (oxidation weight gain ≧ 50 g / m) in a steam oxidation resistance test in which the welded portion is kept in nitrogen containing 20 vol% of steam at 600 to 900°C for 100 hours. 2 In addition to the above, no peeling of oxide scale occurs, and in a steam oxidation resistance test in which, after the steam oxidation resistance test, the material is further heated to 600 to 900°C in nitrogen containing 20 vol% steam, held there for 30 minutes, and then cooled to 200°C in air, this temperature increase / decrease cycle is repeated 200 times, and no peeling of oxide scale is observed. [Means for solving the problem]

[0020] The present inventors have conducted extensive research into the steam oxidation resistance of various elements in order to develop a ferritic stainless steel having steam oxidation resistance at weld zones that is equal to or better than that of SUS310S.

[0021] As a result, it was discovered that a ferritic stainless steel with superior steam oxidation resistance can be obtained even in welds by containing, by mass%, 2.00 to 6.00% Al, more than 0.30% but not more than 0.80% Nb, and 16.0 to 25.0% Cr.

[0022] Based on the above findings, the present invention was completed by adding appropriate amounts of Al, Nb, and Cr. If even one of the above elements is not added in an appropriate amount, the superior steam oxidation resistance desired by the present invention cannot be obtained.

[0023] The present invention has been made based on the above findings and as a result of further investigation, and has the following gist. [1] In mass%, C: 0.020% or less, Si: 0.05 to 2.00%, Mn: 0.05 to 1.30% P: 0.050% or less, S: 0.010% or less, Al: 2.00~6.00%, N: 0.020% or less, Cr: 16.0~25.0%, and Nb: more than 0.30% and not more than 0.80%; A ferritic stainless steel for furnace walls, the balance of which consists of Fe and unavoidable impurities. [2]Furthermore, in mass %, Ni: 0.05 to 1.00%, Cu: 0.01 to 2.00%, Mo: 0.3 to 3.0% W: 0.01 to 3.00%, Ti: 0.005 to 0.300%, Zr: 0.005 to 0.300%, V: 0.01 to 0.50%, Co: 0.01 to 0.50%, Sb: 0.01 to 0.50%, and Sn: 0.01 to 0.50% The ferritic stainless steel for furnace walls according to [1], containing one or more selected from the following: [3] Furthermore, in mass %, B: 0.0002~0.0050%, Ca: 0.0002 to 0.0050%, and Mg: 0.0002 to 0.0050% The ferritic stainless steel for furnace walls according to [1] or [2], containing one or more selected from the following: [Effects of the Invention]

[0024] According to the present invention, a ferritic stainless steel having excellent steam oxidation resistance even at welded joints can be provided, and therefore the ferritic stainless steel of the present invention can be suitably used for the walls of various furnaces such as heating furnaces and incinerators. DETAILED DESCRIPTION OF THE INVENTION

[0025] The present invention will be described in detail below.

[0026] The ferritic stainless steel for furnace walls of the present invention contains, by mass%, C: 0.020% or less, Si: 0.05 to 2.00%, Mn: 0.05 to 1.30%, P: 0.050% or less, S: 0.010% or less, Al: 2.00 to 6.00%, N: 0.020% or less, Cr: 16.0 to 25.0%, Nb: more than 0.30% and 0.80% or less, with the balance being Fe and unavoidable impurities. Note that hereinafter, the ferritic stainless steel for furnace walls of the present invention will also be simply referred to as the ferritic stainless steel of the present invention.

[0027] Next, the chemical composition of the ferritic stainless steel of the present invention will be explained. Hereinafter, % indicating the content of steel components means % by mass unless otherwise specified.

[0028] C: 0.020% or less C is an element effective in increasing the strength of steel, but if the C content exceeds 0.020%, toughness and workability will be significantly reduced. Therefore, the C content is set to 0.020% or less. From the viewpoint of ensuring better workability, the C content is preferably set to 0.010% or less. From the viewpoint of ensuring strength, the C content is preferably set to 0.001% or more. More preferably, the C content is set to 0.003% or more. Furthermore, more preferably, the C content is set to 0.008% or less.

[0029] Si: 0.05 to 2.00% Silicon is an element effective in improving oxidation resistance. To achieve this effect, a silicon content of 0.05% or more is necessary. On the other hand, in steels containing a large amount of aluminum, such as those used in the present invention, an excess silicon content of more than 2.00% actually makes oxide scale more likely to peel off in the severe oxidation environment (steam oxidation environment) inside a furnace. Therefore, the upper limit of the silicon content is set to 2.00%. Preferably, the silicon content is set to 0.10% or more. Also, preferably, the silicon content is set to 1.00% or less. More preferably, the silicon content is set to 0.50% or less.

[0030] Mn: 0.05 to 1.30% Mn has the effect of improving the resistance to exfoliation of oxide scale. To achieve this effect, a Mn content of 0.05% or more is necessary. On the other hand, an excessive Mn content of more than 1.30% leads to abnormal growth of oxide scale in a steam oxidation environment, reducing steam oxidation resistance. Therefore, the Mn content is set to 0.05% or more and 1.30% or less. Preferably, the Mn content is set to 0.10% or more. Also, preferably, the Mn content is set to 1.00% or less. More preferably, the Mn content is set to 0.50% or less.

[0031] P:0.050% or less P is a harmful element that reduces the toughness of steel, and it is desirable to reduce its content as much as possible. Therefore, the P content is set to 0.050% or less. Preferably, the P content is 0.040% or less. More preferably, the P content is 0.030% or less. There is no particular lower limit for the P content. However, since excessive dephosphorization increases costs, the P content is preferably 0.005% or more.

[0032] S: 0.010% or less S is a harmful element that reduces the corrosion resistance, a basic property of stainless steel, so it is desirable to reduce it as much as possible. Therefore, in the present invention, the S content is set to 0.010% or less. Preferably, the S content is 0.005% or less. There is no particular lower limit for the S content. However, since excessive desulfurization increases costs, the S content is preferably 0.0005% or more.

[0033] Al: 2.00 to 6.00% Al is an important element in the present invention for improving the steam oxidation resistance of steel. At high temperatures, Al preferentially bonds with O over Fe and Cr to form a highly protective Al2O3 film, significantly enhancing steam oxidation resistance. Furthermore, since oxide films formed in steam are less protective and more prone to peeling than oxide films formed in air, Al also has the effect of suppressing this peeling. To achieve this effect, an Al content of 2.00% or more is required. With an Al content of less than 2.00%, even if the oxide film does not peel when soaked in high-temperature steam, peeling of the oxide film will occur when the temperature is repeatedly increased and decreased. On the other hand, Al also has the disadvantage of increasing the thermal expansion coefficient, and this effect becomes significant when the Al content exceeds 6.00%. For these reasons, the Al content is set to 2.00 to 6.00%. The Al content is preferably 2.50% or more. Furthermore, the Al content is preferably 3.00% or less.

[0034] N: 0.020% or less N is an element that reduces the toughness and workability of steel. If the content exceeds 0.020%, not only will the toughness and formability be significantly reduced, but the precipitation of coarse AlN will also prevent the aforementioned oxidation resistance improvement effect of Al from being obtained. Therefore, the N content is set to 0.020% or less. From the viewpoint of ensuring toughness and workability, it is preferable to reduce N as much as possible, and the N content is desirably less than 0.010%. There is no particular lower limit for the N content. However, excessive denitrification increases costs, so the N content is preferably 0.004% or more.

[0035] Cr: 16.0~25.0% Cr is an important element effective in improving the corrosion resistance and steam oxidation resistance that are characteristic of stainless steel. If the Cr content is less than 16.0%, even if an appropriate amount of Al is contained, a sufficiently protective oxide film cannot be formed in high-temperature steam. Even if peeling does not occur during soaking, peeling will occur when the temperature is repeatedly increased and decreased, resulting in insufficient steam oxidation resistance. On the other hand, if the Cr content exceeds 25.0%, a second phase (σ phase) mainly composed of Fe and Cr precipitates, resulting in a decrease in steam oxidation resistance. Therefore, the upper limit of the Cr content is set to 25.0%. The Cr content is preferably 17.0% or more, more preferably 18.0% or more. The Cr content is also preferably 21.0% or less, more preferably 20.0% or less, and even more preferably 19.0% or less.

[0036] Nb: More than 0.30% and less than 0.80% Nb is an important element in the present invention, as it forms carbonitrides with C and N, immobilizing them and improving the steam oxidation resistance of welds. If the Nb content is 0.30% or less, N reacts with Al in the atmosphere during welding, precipitating as AlN in the steel, reducing the Al's effect of improving steam oxidation resistance, particularly the protective properties of the oxide film in steam. On the other hand, if the Nb content exceeds 0.30%, Nb preferentially bonds with N over Al and precipitates as NbN, thereby suppressing AlN precipitation. Furthermore, since NbN is finer than AlN, it can also suppress grain coarsening in welds. If NbN precipitation is insufficient and grains in the welds become coarse, fewer grain boundaries serve as diffusion paths for Al when exposed to high temperatures. This delays the formation of highly protective Al2O3. Even if the oxide film does not peel off during soaking in high-temperature steam, it may peel off when the temperature is repeatedly increased and decreased. Therefore, in the present invention, the Nb content is set to more than 0.30%. However, a Nb content exceeding 0.80% hardens the steel and reduces workability. Therefore, the Nb content is set to more than 0.30% and not more than 0.80%. The Nb content is preferably 0.40% or more, and more preferably 0.45% or more. Furthermore, the Nb content is preferably less than 0.70%, and more preferably less than 0.60%.

[0037] In the ferritic stainless steel of the present invention, the balance consists of Fe and unavoidable impurities.

[0038] In addition to the above essential components, the ferritic stainless steel of the present invention may further contain one or more elements selected from Ni, Cu, Mo, W, Ti, Zr, V, Co, Sb, and Sn within the following ranges:

[0039] Ni: 0.05 to 1.00% Ni is an element that improves the toughness and oxidation resistance of steel. To obtain these effects, the Ni content is set to 0.05% or more. However, because Ni is a strong γ-phase forming element, an excessive Ni content of more than 1.00% generates γ-phase at high temperatures and reduces oxidation resistance. Therefore, when Ni is contained, the Ni content is set to 0.05 to 1.00%. The Ni content is preferably 0.10% or more. Furthermore, the Ni content is preferably less than 0.30%, more preferably less than 0.20%.

[0040] Cu: 0.01 to 2.00% Cu is an element that has the effect of improving the corrosion resistance of steel and can be contained as needed. This effect is obtained with a Cu content of 0.01% or more. On the other hand, if the Cu content exceeds 2.00%, oxide scale becomes more likely to peel off, and oxidation resistance decreases. Therefore, when Cu is contained, the Cu content is set to 0.01 to 2.00%. The Cu content is preferably 0.30% or more, and more preferably 1.00% or more. Furthermore, the Cu content is preferably 1.50% or less.

[0041] Mo: 0.3 to 3.0% Mo is an element that improves the corrosion resistance and high-temperature strength of steel. This effect can be achieved with a Mo content of 0.3% or more. On the other hand, an excessive Mo content of more than 3.0% hardens the steel and reduces its workability. Therefore, when Mo is contained, the Mo content is set to 0.3 to 3.0%. The Mo content is preferably 1.0% or more, and more preferably more than 2.0%. Furthermore, the Mo content is preferably 2.5% or less.

[0042] W: 0.01 to 3.00% Like Mo, W is an element that significantly improves high-temperature strength. This effect is achieved with a W content of 0.01% or more. On the other hand, an excessive W content of more than 3.00% not only significantly hardens the steel, but also generates strong scale during the annealing process during manufacturing, making descaling during pickling difficult. Therefore, when W is contained, the W content is set to 0.01 to 3.00%. The W content is preferably 0.30% or more, and more preferably 1.00% or more. Furthermore, the W content is preferably 2.00% or less, and more preferably 1.50% or less.

[0043] Ti: 0.005 to 0.300% Ti is an element that combines with C and N and prevents Al from combining with N to form AlN, thereby enhancing oxidation resistance. By including Ti, Ti combines with N more preferentially than Nb during welding, resulting in precipitation in the steel as TiN. This effect is achieved with a Ti content of 0.005% or more. However, TiN is coarser than NbN and is less effective at suppressing grain coarsening in welds than NbN. A Ti content exceeding 0.300% produces coarse TiN, reducing the toughness of the steel. Therefore, when Ti is included, the Ti content is set to 0.005 to 0.300%. The Ti content is preferably less than 0.200%, and more preferably 0.050% or less.

[0044] Zr: 0.005 to 0.300% Zr is an element that improves oxidation resistance and can be contained as needed in the present invention. To achieve this effect, the Zr content is preferably 0.005% or more. However, if the Zr content exceeds 0.300%, Zr intermetallic compounds precipitate, embrittling the steel. Therefore, if Zr is contained, the Zr content is set to 0.005 to 0.300%.

[0045] V: 0.01 to 0.50% V is an element effective in improving the workability of steel and also in improving its oxidation resistance. Addition of V inhibits V from combining with N in the steel and Al from combining with N to form coarse nitrides. If Al forms coarse AlN, the excellent steam oxidation resistance important in the present invention cannot be obtained. This effect becomes significant when the V content is 0.01% or more. However, excessive V content exceeding 0.50% leads to the precipitation of coarse V(C,N) and reduces toughness. Therefore, when V is contained, the V content is set to 0.01 to 0.50%. The V content is preferably 0.05% or more. Furthermore, the V content is preferably 0.10% or less.

[0046] Co: 0.01 to 0.50% Co is known as an element effective in improving the toughness of steel. Furthermore, in the present invention, it also has the effect of reducing the thermal expansion coefficient, which is increased by the inclusion of Al, and improving thermal fatigue properties. To obtain these effects, the Co content is preferably 0.01% or more. On the other hand, an excessive Co content of more than 0.50% not only reduces the toughness of the steel but also reduces its workability. Therefore, when Co is contained, the Co content is set to 0.01 to 0.50%. The Co content is preferably 0.03% or more. The Co content is preferably less than 0.30%, and more preferably less than 0.10%.

[0047] Sb: 0.01 to 0.50% Sb is an element that has the effect of improving the toughness of steel. When the alloying element content is high as in the present invention, the toughness of the steel decreases as the alloying element content increases, and cracks may occur during processing into parts, etc., so Sb is added as needed. This effect is obtained with an Sb content of 0.01% or more. On the other hand, an excessive Sb content of more than 0.50% actually reduces toughness, so the upper limit of the Sb content is set to 0.50%. Therefore, when Sb is added, the Sb content is set to 0.01 to 0.50%. The Sb content is preferably 0.03% or more. Furthermore, the Sb content is preferably 0.30% or less.

[0048] Sn: 0.01 to 0.50% Sn is an element that has the effect of improving the corrosion resistance and high-temperature strength of steel, and is contained as needed. This effect is obtained with an Sn content of 0.01% or more. On the other hand, an excessive Sn content of more than 0.50% reduces the workability of steel, so the upper limit of the Sn content is set to 0.50%. Therefore, when Sn is contained, the Sn content is set to 0.01 to 0.50%. The Sn content is preferably 0.03% or more. Furthermore, the Sn content is preferably 0.30% or less.

[0049] The ferritic stainless steel of the present invention may further contain one or more elements selected from B, Ca and Mg within the following ranges.

[0050] B: 0.0002 to 0.0050% B is an element effective for improving the workability of steel, particularly its secondary workability. Such an effect can be obtained with a B content of 0.0002% or more. On the other hand, an excessive B content of more than 0.0050% generates BN, which reduces workability. Therefore, when B is contained, the B content is set to 0.0002 to 0.0050%. The B content is preferably 0.0005% or more. Furthermore, the B content is preferably 0.0020% or less, and more preferably 0.0010% or less.

[0051] Ca: 0.0002 to 0.0050% Ca is an effective component for preventing nozzle clogging due to the precipitation of inclusions that is likely to occur during continuous casting. This effect can be achieved with a Ca content of 0.0002% or more. On the other hand, in order to obtain good surface quality without generating surface defects, the Ca content is preferably 0.0050% or less. Therefore, when Ca is contained, the Ca content is set to 0.0002 to 0.0050%. The Ca content is preferably 0.0005% or more. Furthermore, the Ca content is preferably 0.0030% or less, more preferably 0.0020% or less.

[0052] Mg: 0.0002 to 0.0050% Mg is an element that improves the equiaxed crystal ratio of a slab and is effective in improving workability and toughness. In Nb-containing steels such as those of the present invention, Mg also has the effect of suppressing the coarsening of Nb carbonitrides. This effect is achieved with a Mg content of 0.0002% or more. Coarsening of Nb carbonitrides reduces the amount of Nb in solid solution in the steel, leading to a deterioration in thermal fatigue properties. On the other hand, if the Mg content exceeds 0.0050%, the surface properties of the steel may be deteriorated. Therefore, when Mg is contained, the Mg content is set to 0.0002 to 0.0050%. The Mg content is preferably 0.0004% or more. The Mg content is preferably 0.0030% or less, and more preferably 0.0020% or less.

[0053] In addition, when the content of Ni, Cu, Mo, W, Ti, Zr, V, Co, Sb, Sn, B, Ca, or Mg described above as optional components is less than the lower limit, the component is considered to be contained as an unavoidable impurity.

[0054] Next, a method for producing the ferritic stainless steel of the present invention will be described.

[0055] The method for producing the ferritic stainless steel of the present invention can be basically any conventional method for producing ferritic stainless steel, and is not particularly limited. For example, steel is melted in a known melting furnace such as a converter or electric furnace, or further refined by ladle refining or vacuum refining to produce a steel having the composition of the present invention. This steel is then formed into a slab by continuous casting or ingot casting / blooming. The steel can then be produced by a manufacturing process that involves hot rolling, hot-rolled sheet annealing, pickling, cold rolling, finish annealing, and pickling to produce a cold-rolled annealed sheet. The cold rolling may be performed once or twice or more times with an intermediate annealing in between. The cold rolling, finish annealing, and pickling steps may be repeated. Furthermore, hot-rolled sheet annealing may be omitted. If adjustment of the surface gloss or roughness of the steel sheet is required, skin-pass rolling may be performed after cold rolling or finish annealing. Depending on the application, the hot-rolled annealed sheet may also be used as is.

[0056] Preferred production conditions for the above production method will be described below.

[0057] In the steelmaking process for producing smelted steel, steel melted in a converter or electric furnace is preferably subjected to secondary refining using a VOD method or the like to produce steel containing the above-mentioned essential components and optional additional components. The molten steel can be produced into a steel material by known methods, but continuous casting is preferred from the standpoints of productivity and quality. The steel material is then heated to a temperature of preferably 1050 to 1250°C and hot-rolled into a hot-rolled sheet of the desired thickness (3 mm to 6 mm). Of course, hot processing other than sheet material is also possible. The hot-rolled sheet is then subjected to continuous annealing at a temperature of 900 to 1150°C as needed to produce a hot-rolled annealed sheet, which is then descaled by pickling or the like to produce a hot-rolled product. The annealing step may be omitted. If necessary, scale may be removed by shot blasting or brush grinding before pickling.

[0058] Furthermore, the hot-rolled annealed sheet or hot-rolled sheet may be subjected to a process such as cold rolling to produce a cold-rolled product with a thickness of 0.3 mm to 3 mm. In this case, cold rolling may be performed once, or two or more cold rolling processes with intermediate annealing in between may be performed from the viewpoint of productivity and improving the required quality. The total reduction ratio of one or more cold rolling processes is preferably 40% or more, more preferably 50% or more, and even more preferably 60% or more. The cold-rolled steel sheet is then preferably subjected to continuous annealing (finish annealing) at a temperature of preferably 900 to 1150°C, more preferably 950 to 1100°C, followed by pickling to produce a cold-rolled product. Furthermore, depending on the application location, after finish annealing, skin-pass rolling or the like may be performed to adjust the shape, surface roughness, and material properties of the steel sheet. Brush grinding may be performed before the pickling. Furthermore, the finish annealing may be performed in a reducing atmosphere (for example, 25 vol% N2-75 vol% H2), and the descaling step such as pickling may be omitted.

[0059] The hot-rolled or cold-rolled product obtained as described above can then be cut, bent, drilled, or otherwise processed for use in furnace walls of various furnaces, such as heating furnaces and incinerators. Fixing methods for these components include bolting and welding. The welding method for these components is not particularly limited. Conventional arc welding, such as metal inert gas (MIG), metal active gas (MAG), and tungsten inert gas (TIG), can be used. Resistance welding, such as spot welding and seam welding, as well as high-frequency resistance welding, such as electric resistance welding, and high-frequency induction welding, may also be used. The ferritic stainless steel of the present invention has excellent steam oxidation resistance, even in welds, and is therefore suitable for use in furnace walls, such as furnace inner walls. The ferritic stainless steel of the present invention has excellent steam oxidation resistance, even in severely oxidizing environments where temperature increases and decreases are repeated, such as in combustion furnaces and incinerators. [Example]

[0060] The present invention will be described in detail below with reference to examples.

[0061] Steels having the chemical compositions Nos. 1 to 37 shown in Table 1 were melted in a vacuum melting furnace and cast into 50 kg steel ingots. These were then heated to 1170°C and hot-rolled into 35 mm thick sheet bars. 200 mm lengths were cut from these sheet bars, heated to 1150°C, and hot-rolled into 4.5 mm thick hot-rolled sheets. These were then annealed at temperatures ranging from 1000 to 1150°C and ground to produce hot-rolled annealed sheets. Subsequently, the sheets were cold-rolled with a reduction of 67% and finish-annealed at temperatures ranging from 1000 to 1150°C to produce 1.5 mm thick cold-rolled annealed sheets. Surface oxide scale was removed by polishing, and then bead-on-plate welding was performed using TIG welding. The electrode distance was 2 mm, the torch angle was 60°, and a back gas shield of 100% Ar was used. The welding speed was 60 cm / min. The current and voltage were adjusted appropriately for each steel while checking the bead shape. This welded material was subjected to an oxidation test. For reference, cold-rolled annealed sheets of SUS310S (No. 24) were also prepared in the same manner as above, and subjected to oxidation tests after TIG welding. The annealing temperature was determined for each steel while checking the structure within the above temperature range.

[0062] <Steam oxidation resistance test 1> From the welded material obtained as described above, 30 mm × 20 mm test pieces were cut out to include the weld. The weld was positioned at the center of the 30 mm direction of the test piece, extending in the 20 mm direction of the test piece. A 4 mm diameter hole was drilled at the top of the 30 mm direction of the test piece, and after degreasing, the test piece was suspended in a furnace heated to 600°C, 700°C, 800°C, and 900°C in an 80 vol% N2-20 vol% H2O atmosphere (dew point +60°C, flow rate 0.5 L / min / piece). Tests were then conducted, each held for 100 hours. After the test, the weight of the test piece was measured, and the difference from the weight of the test piece measured before the test was calculated to determine the oxidation weight gain (g / m2). 2 ) was calculated. The oxidation weight gain did not include the amount of spalled oxide scale, and the presence or absence of spalled oxide scale was confirmed visually. The steam oxidation resistance was evaluated as follows:

[0063] [Steam oxidation resistance criteria 1] ○: Abnormal oxidation (oxidation weight gain ≧ 50 g / m) at all temperatures of 600°C, 700°C, 800°C, and 900°C 2 ) did not occur, and no scale peeling occurred. △: Abnormal oxidation did not occur at any of the temperatures of 600℃, 700℃, 800℃, and 900℃, but scale spalling occurred at any of the above temperatures. ×: Abnormal oxidation (oxidation weight gain ≥ 50 g / m) at any of the temperatures 600°C, 700°C, 800°C, and 900°C 2 ) occurred The results are shown in Table 1. ◯ was judged to be acceptable (excellent resistance to steam oxidation), and △ and × were judged to be unacceptable.

[0064] <Steam oxidation resistance test 2> For specimens that showed no abnormal oxidation or spalling of oxide scale in the above Steam Oxidation Resistance Test 1 (those with a rating of ○), the specimens were subsequently heated to 600°C, 700°C, 800°C, and 900°C, respectively, and held in a furnace containing an 80 vol% N2-20 vol% H2O atmosphere (dew point +60°C, flow rate 0.5 L / min per specimen) for 30 minutes, then removed from the furnace and cooled to 200°C over 30 minutes. This heating and cooling cycle was repeated 200 times. After the test, the specimens were visually inspected for spalling of oxide scale, and their steam oxidation resistance was evaluated as follows:

[0065] [Steam oxidation resistance criteria 2] ○: No peeling of oxide scale occurred at any of the temperatures of 600℃, 700℃, 800℃, and 900℃. △: Oxide scale peeling occurred at any of the following temperatures: 600℃, 700℃, 800℃, 900℃ The results are shown in Table 1. ◯ was evaluated as passing (excellent resistance to steam oxidation), and △ was evaluated as failing.

[0066] A sample that was judged as excellent in both the steam oxidation resistance test 1 and the steam oxidation resistance test 2 was evaluated as having excellent steam oxidation resistance, and was given an overall pass rating.

[0067] [Table 1]

[0068] As can be seen from Table 1, inventive steels Nos. 1, 2, 4, 5, 7, 9, 11 to 14, 16 to 23, and 31 to 35, neither abnormal oxidation nor oxide scale spalling occurred in steam oxidation resistance tests 1 and 2 at 600 to 900°C, and they had excellent steam oxidation resistance. The inventive steels exhibited superior steam oxidation resistance to SUS310S (steel No. 24), which exhibited oxide scale spalling.

[0069] On the other hand, Steel No. 3, which had an Al content of less than 2.00%, passed Steam Oxidation Resistance Test 1, but spalling of the oxide scale occurred at 900°C in Steam Oxidation Resistance Test 2. Steel No. 6, which had a Cr content of less than 16.0%, passed Steam Oxidation Resistance Test 1, but spalling of the oxide scale occurred in Steam Oxidation Resistance Test 2 at 600°C. Steel No. 8 had an Al content of less than 2.00% and an Nb content of 0.30% or less, and passed steam oxidation resistance test 1, but spalling of the oxide scale occurred in steam oxidation resistance test 2 at 900°C. Steel No. 10, which has an Al content of less than 2.00%, passed Steam Oxidation Resistance Test 1, but spalling of the oxide scale occurred in Steam Oxidation Resistance Test 2 at 900°C. Steel No. 15, which has an Al content of less than 2.00%, passed Steam Oxidation Resistance Test 1, but spalling of the oxide scale occurred at 900°C in Steam Oxidation Resistance Test 2. Steel No. 25 had a Si content exceeding 2.00%, and failed the steam oxidation resistance test 1 because the oxide scale peeled off at 600°C. Steel No. 26 had a Mn content exceeding 1.30%, and abnormal oxidation occurred at 600°C in the steam oxidation resistance test 1, resulting in failure. Steel No. 27 contained less than 0.25% Al, and abnormal oxidation occurred at 600°C and 900°C in Steam Oxidation Resistance Test 1, resulting in failure. Steel No. 28 had a Cr content of less than 12.0%, and abnormal oxidation occurred at all temperatures from 600 to 900°C in Steam Oxidation Resistance Test 1, resulting in failure. Steel Nos. 29 and 30 had an Nb content of less than 0.20%, and therefore failed the steam oxidation resistance test 1 due to abnormal oxidation at 600°C. Steel No. 36, which has an Al content of less than 2.00%, passed Steam Oxidation Resistance Test 1, but spalling of the oxide scale occurred at 900°C in Steam Oxidation Resistance Test 2. Steel No. 37, which had a Nb content of 0.30% or less, passed Steam Oxidation Resistance Test 1, but spalling of the oxide scale occurred at 600°C in Steam Oxidation Resistance Test 2. [Industrial Applicability]

[0070] The ferritic stainless steel of the present invention is suitable for use as the furnace walls of various furnaces such as heating furnaces and incinerators, particularly as the inner walls of furnaces that are exposed to an atmosphere containing high-temperature steam.

Claims

1. In mass%, C: 0.020% or less, Si: 0.05-2.00%, Mn: 0.05-1.30%, P: 0.050% or less, S: 0.010% or less, Al: 2.00-2.85%, N: 0.020% or less, Cr: 16.0 to 25.0%, and Nb: more than 0.30% and not more than 0.80%; A ferritic stainless steel for furnace walls, the balance of which consists of Fe and unavoidable impurities.

2. Furthermore, in mass%, Ti: 0.005-0.300%, Zr: 0.005 to 0.300%, and V:0.01~0.50% 2. The ferritic stainless steel for furnace walls according to claim 1, comprising one or more selected from the following:

3. In mass %, C: 0.020% or less, Si: 0.05-2.00%, Mn: 0.05-1.30%, P: 0.050% or less, S: 0.010% or less, Al: 2.00-6.00%, N: 0.020% or less, Cr: 16.0 to 25.0%, and Nb: more than 0.30% and not more than 0.80%; Furthermore, in mass%, Ni: 0.05-1.00%, Cu: 0.01-2.00%, Mo: 0.3 to 3.0%, W: 0.01-3.00%, Co: 0.01 to 0.50%, Sb: 0.01 to 0.50%, Sn: 0.01~0.50% B: 0.0002 to 0.0050%, Ca: 0.0002 to 0.0050%, and Mg: 0.0002-0.0050% Contains one or more selected from A ferritic stainless steel for furnace walls, the balance of which consists of Fe and unavoidable impurities.

4. Further, in mass %, Ti: 0.005-0.300%, Zr: 0.005 to 0.300%, and V:0.01~0.50% 4. The ferritic stainless steel for furnace walls according to claim 3, which contains one or more selected from the following:

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