JP7772115B2 - Ferritic stainless steel for furnace walls - Google Patents

Description

The present invention relates to ferritic stainless steel for furnace walls, and more specifically to Cr-containing steel, a ferritic stainless steel with excellent steam oxidation resistance that is suitable for use in furnace walls exposed to high temperatures of 500°C or higher, such as in heating furnaces and incinerators.

The walls of various furnaces, such as heating furnaces and incinerators, are made of ceramic insulation or stainless steel plates to provide insulation and prevent heat from escaping from the furnace. Ceramics have excellent insulating 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 insulating properties to ceramic insulation, but their excellent conductivity has the advantage of saving energy needed to heat up.

In recent years, with growing awareness of global environmental issues, efforts toward a carbon-neutral society have been underway worldwide, and consideration is being given to supplying electricity using _H2 , which does not emit _CO2 when burned, or alcohol, which uses _CO2 as a raw material during production, as fuel (at least in part). Hereinafter, these fuels will also 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.

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 commonly used stainless steel. SUS310S (25 mass% Cr-19 mass% Ni) is a stainless steel specialized for higher temperature applications.

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 the large amount of Cr and Ni it contains. Furthermore, because both SUS304 and SUS310S are austenitic, they have low thermal conductivity compared to other stainless steels and a high coefficient of thermal expansion. Therefore, when temperatures are repeatedly increased and decreased, not only are oxide scales prone to peeling off, but the material itself also deforms significantly, making it susceptible to breakage.

Furthermore, when stainless steel plates are used for the furnace walls, they are attached by bolts or welding. When SUS304 or SUS310S is welded, chromium forms as carbides at the weld, reducing the amount of chromium in the steel and creating the problem of 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 severe oxidizing environment (water vapor oxidizing environment) than air (approximately 3 vol% water vapor and approximately 20 vol% oxygen).

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 when temperatures are repeatedly increased and decreased (peeling and deformation of oxide scale caused by temperature increases and decreases).

As examples of ferritic stainless steels with excellent oxidation resistance, Patent Document 1, for example, discloses a ferritic stainless steel with improved oxidation resistance due to the addition of Al. Patent Document 2 discloses a ferritic stainless steel for fuel cells that, in addition to Al, also contains one or both of Sn and B to improve creep resistance. Patent Document 3 discloses an Fe-Cr-Al ferritic stainless steel that suppresses high-temperature deformation due to the addition of Nb. Patent Document 4 discloses a ferritic stainless steel containing Al to improve oxidation resistance. Patent Document 5 discloses a ferritic stainless steel containing appropriate amounts of Si and Al to improve oxidation resistance. Patent Document 6 discloses a low-density 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.

Japanese Patent Application Publication No. 6-220587 Patent No. 6113359 Japanese Patent Application Publication No. 10-251810 JP 2012-102376 A JP 2012-107314 A Japanese Patent Application Laid-Open No. 2018-168457 Japanese Patent Application Laid-Open No. 2018-188687

According to the inventors' research, Patent Document 1 discloses a steel with improved oxidation resistance through the addition of Al, but does not take into consideration the oxidation resistance of welds. Furthermore, it does not take into consideration oxidation resistance in the severe oxidizing environment (steam oxidizing environment) inside a furnace.

Patent Document 2 also 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. Furthermore, it does not consider oxidation resistance in the severe oxidizing environment inside a furnace.

Patent Document 3 discloses an Fe-Cr-Al ferritic stainless steel that suppresses high-temperature deformation 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.

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

Patent Document 5 discloses a ferritic stainless steel having 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 condition 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 and 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.

Patent Document 6 discloses a low-density ferritic stainless steel containing 0.50 to 10.0% Al and with a well-balanced Cr and Si content, but this has only been evaluated in air, not in a water vapor atmosphere.

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

As such, while many ferritic stainless steels with improved oxidation resistance have been disclosed in the prior art, none have evaluated the oxidation resistance of welds in the harsh environment of steam, leaving room for further investigation into ferritic stainless steels suitable for use in furnace walls.

The present invention therefore aims to provide a ferritic stainless steel that is suitable for furnace wall applications and has excellent steam oxidation resistance, even at welded joints.

In this specification, "excellent steam oxidation resistance" means that in a steam oxidation resistance test in which the weld is held at 600 to 900°C for 100 hours in nitrogen containing 20 vol% steam, neither abnormal oxidation (oxidation weight increase ≧ 50 g/ ^m2 ) nor spalling of oxide scale occurs, and further, after the steam oxidation resistance test, the weld is heated to 600 to 900°C in nitrogen containing 20 vol% steam, held for 30 minutes, and then cooled to 200°C in air, and this temperature rise/fall cycle is repeated 200 times, resulting in no spalling of oxide scale.

The inventors conducted extensive research into the steam oxidation resistance of various elements in order to develop a ferritic stainless steel whose weld zones have steam oxidation resistance equal to or better than that of SUS310S.

As a result, it was discovered that a ferritic stainless steel with superior steam oxidation resistance, even at welds, can be obtained by including, 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.

Based on the above findings, the present invention was completed by incorporating appropriate amounts of Al, Nb, and Cr. If even one of the above elements is not incorporated in an appropriate amount, the superior steam oxidation resistance desired by the present invention will not be achieved.

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: 0.05-2.00%,
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,
Contains 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%,
Ni: 0.05-1.00%,
Cu: 0.01-2.00%,
Mo: 0.3 to 3.0%,
W: 0.01-3.00%,
Ti: 0.005-0.300%,
Zr: 0.005-0.300%,
V: 0.01-0.50%,
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 to 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:

The present invention provides a ferritic stainless steel that has excellent steam oxidation resistance, even at welded joints. Therefore, the ferritic stainless steel of the present invention can be suitably used for the furnace walls of various furnaces, such as heating furnaces and incinerators.

The present invention is described in detail below.

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. 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.

Next, we will explain the chemical composition of the ferritic stainless steel of the present invention. Hereinafter, unless otherwise specified, percentages indicating the content of steel components refer to mass percentages.

C: 0.020% or less C is an effective element for 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.

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

Mn: 0.05-1.30%
Mn has the effect of increasing 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. And, more preferably, the Mn content is set to 0.50% or less.

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.

S: 0.010% or less S is a harmful element that reduces the corrosion resistance, which is a basic property of stainless steel, so it is desirable to reduce the S content 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 leads to an increase in costs, the S content is preferably 0.0005% or more.

Al: 2.00-6.00%
Al is an important element in the present invention because it improves 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 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 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 more pronounced 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.

N: 0.020% or less N is an element that reduces the toughness and workability of steel. If the N content exceeds 0.020%, the toughness and formability are significantly reduced, and the precipitation of coarse AlN prevents the above-mentioned oxidation resistance improvement effect of containing 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%. The lower limit of the N content is not particularly limited. However, since excessive denitrification leads to increased costs, the N content is preferably 0.004% or more.

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 occurs 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 preferably 21.0% or less, more preferably 20.0% or less, and even more preferably 19.0% or less.

Nb: More than 0.30% and 0.80% or less Nb is an important element in the present invention, as it forms carbonitrides with C and N to fix C and N and improve the steam oxidation resistance of welds. If the Nb content is 0.30% or less, N in the atmosphere reacts with Al during welding and precipitates 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 bonds with N preferentially over Al and precipitates as NbN, thereby suppressing the precipitation of AlN. Furthermore, since NbN is finer than AlN, it can also suppress grain coarsening in welds. If NbN precipitation is insufficient and the grains of the weld become coarse, the number of grain boundaries that serve as diffusion paths for Al is reduced when exposed to high temperatures, delaying the formation of highly protective Al ₂ O _{3 .} Even if the oxide film does not peel off during soaking in high-temperature steam, it will 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, an Nb content exceeding 0.80% hardens the steel and reduces its workability. Therefore, the Nb content is set to more than 0.30% but not more than 0.80%. The Nb content is preferably 0.40% or more, more preferably 0.45% or more. The Nb content is preferably less than 0.70%, more preferably less than 0.60%.

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

In addition to the essential components listed above, 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 ranges listed below.

Ni: 0.05-1.00%
Ni is an element that improves the toughness and oxidation resistance of steel. To achieve 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%.

Cu: 0.01-2.00%
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.

Mo: 0.3-3.0%
Mo is an element that improves the corrosion resistance and high-temperature strength of steel. This effect is obtained 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.

W: 0.01~3.00%
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.

Ti: 0.005-0.300%
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 preferentially combines with N over 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, unlike NbN, is less effective in suppressing grain coarsening in welds. A Ti content exceeding 0.300% produces coarse TiN, reducing the toughness of the steel. Therefore, when Ti is included, the Ti content is limited to 0.005% to 0.300%. The Ti content is preferably less than 0.200%, and more preferably 0.050% or less.

Zr: 0.005-0.300%
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%.

V:0.01~0.50%
V is an element effective in improving the workability of steel and also in improving its oxidation resistance. Adding V prevents 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 is 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.

Co:0.01~0.50%
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 achieve these effects, the Co content is preferably 0.01% or more. On the other hand, an excessive Co content exceeding 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%.

Sb: 0.01~0.50%
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.

Sn: 0.01~0.50%
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.

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:

B: 0.0002-0.0050%
B is an element effective for improving the workability of steel, particularly its secondary workability. This 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, reducing 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. The B content is preferably 0.0020% or less, and more preferably 0.0010% or less.

Ca: 0.0002-0.0050%
Ca is an effective component for preventing nozzle clogging due to inclusion precipitation, which 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.

Mg: 0.0002-0.0050%
Mg is an element effective in improving the equiaxed crystal ratio of slabs and 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 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 deteriorate. 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, more preferably 0.0020% or less.

Note that if 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 will be considered to be included as an unavoidable impurity.

Next, we will explain the method for producing the ferritic stainless steel of the present invention.

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 described above. This steel is then formed into a slab by continuous casting or ingot casting and blooming. The steel can then be produced through processes such as 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 step in between. The cold rolling, finish annealing, and pickling steps may also 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.

Preferred manufacturing conditions for the above manufacturing method are described below.

In the steelmaking process, steel melted in a converter or electric furnace is preferably subjected to secondary refining using a method such as VOD to produce steel containing the essential elements listed above, plus optional additional elements. The molten steel can be processed into steel stock using known methods, but continuous casting is preferred from the perspective of productivity and quality. The steel stock is then heated to a temperature of preferably 1050-1250°C and hot-rolled to the desired thickness (3-6 mm). Of course, hot processing can also be used to produce products other than sheet metal. The hot-rolled sheet is then subjected to continuous annealing at a temperature of 900-1150°C as needed to produce a hot-rolled annealed sheet, which is then preferably descaled by pickling or other methods 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.

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 perspective of improving productivity and 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 % N ₂ -75 vol % H ₂ ), and the descaling step such as pickling may be omitted.

The hot-rolled or cold-rolled products obtained as described above can then be cut, bent, drilled, and otherwise processed for use in the walls of various furnaces, such as heating furnaces and incinerators. Fixing methods for these components include bolting and welding. There are no particular limitations on the method for welding these components. Typical welding methods include MIG (Metal Inert Gas), MAG (Metal Active Gas), and TIG (Tungsten Inert Gas). Resistance welding methods such as spot welding and seam welding, high-frequency resistance welding such as electric resistance welding, high-frequency induction welding, and laser welding may also be used. The ferritic stainless steel of the present invention has excellent steam oxidation resistance, even in welds, making it suitable for use in furnace walls, such as the inner walls of furnaces. The ferritic stainless steel of the present invention exhibits excellent steam oxidation resistance, even in severely oxidizing environments where temperature increases and decreases are repeated, such as in combustion furnaces and incinerators.

The present invention will now be described in more detail with reference to examples.

Steels with the chemical compositions Nos. 1 to 37 listed in Table 1 were melted in a vacuum melting furnace and cast into 50 kg 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. After polishing to remove surface oxide scale, bead-on-plate welding was performed using a TIG welding process. The electrode distance was 2 mm, the torch angle was 60°, and a backside gas shield of 100% Ar was used, with a welding speed of 60 cm/min. The current and voltage were adjusted appropriately while checking the bead shape for each steel. 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 an oxidation test after TIG welding. The annealing temperature was determined for each steel while checking the structure within the above temperature range.

<Steam oxidation resistance test 1>
From the welded material obtained as described above, 30 mm x 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. After degreasing, the test piece was suspended in a furnace in an 80 vol% N ₂ -20 vol% H ₂ O atmosphere (dew point +60°C, flow rate 0.5 L/min per piece) heated and held at 600°C, 700°C, 800°C, and 900°C. Tests were then conducted in which the test pieces were 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 calculate the oxidation weight gain (g/m ² ). The oxidation weight gain did not include spalled oxide scale, and the presence or absence of spalled oxide scale was confirmed visually. Steam oxidation resistance was evaluated as follows.

[Steam oxidation resistance evaluation criteria 1]
◯: No abnormal oxidation (oxidation weight gain ≧50 g/m ² ) occurred and no scale spalling occurred at any of the temperatures of 600°C, 700°C, 800°C, and 900°C. Δ: No abnormal oxidation occurred at any of the temperatures of 600°C, 700°C, 800°C, and 900°C, but scale spalling occurred at one of these temperatures. ×: Abnormal oxidation (oxidation weight gain ≧50 g/m ² ) occurred at any of the temperatures of 600°C, 700°C, 800°C, and 900°C. The results are shown in Table 1. ◯ was rated as pass (excellent steam oxidation resistance), and Δ and × were rated as fail.

<Steam oxidation resistance test 2>
For specimens that showed no abnormal oxidation or oxide scale spalling in the above steam oxidation resistance test 1 (those with a rating of ○), the test specimens were subsequently heated to 600°C, 700°C, 800°C, and 900°C, respectively, and held in a furnace containing an 80 vol% N ₂ -20 vol% H ₂ O atmosphere (dew point +60°C, flow rate 0.5 L/min per specimen) for 30 minutes, then removed into the air at room temperature and cooled to 200°C over 30 minutes. This temperature rise/fall cycle was repeated 200 times. After the test, the specimens were visually inspected for oxide scale spalling, and the steam oxidation resistance was evaluated as follows:

[Steam oxidation resistance evaluation criteria 2]
○: No spalling of oxide scale occurred at any of the temperatures of 600°C, 700°C, 800°C, and 900°C. △: Spalling of oxide scale occurred at any of the temperatures of 600°C, 700°C, 800°C, and 900°C. The results are shown in Table 1. ○ was rated as pass (excellent resistance to steam oxidation), and △ was rated as fail.

Items that received a rating of 〇 in both Steam Oxidation Resistance Test 1 and Steam Oxidation Resistance Test 2 were rated as having superior steam oxidation resistance, and were given an overall rating of "Pass."

Table 1 shows that inventive steels No. 1, 2, 4, 5, 7, 9, 11-14, 16-23, and 31-35, no abnormal oxidation or oxide scale spalling occurred in steam oxidation resistance tests 1 and 2 at 600-900°C, demonstrating superior steam oxidation resistance. The inventive steels exhibited superior steam oxidation resistance to SUS310S (Steel No. 24), which exhibited oxide scale spalling.

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 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 the steam oxidation resistance test 1, but spalling of the oxide scale occurred at 600°C in the steam oxidation resistance test 2.
Steel No. 8 had an Al content of less than 2.00% and an Nb content of 0.30% or less, and passed the steam oxidation resistance test 1, but spalling of the oxide scale occurred at 900°C in the steam oxidation resistance test 2.
Steel No. 10, which had an Al content of less than 2.00%, passed the steam oxidation resistance test 1, but spalling of the oxide scale occurred at 900°C in the steam oxidation resistance test 2.
Steel No. 15, which had an Al content of less than 2.00%, passed the steam oxidation resistance test 1, but spalling of the oxide scale occurred at 900°C in the steam oxidation resistance test 2.
Steel No. 25 had a Si content exceeding 2.00%, and failed the steam oxidation resistance test 1 because peeling of the oxide scale occurred 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 had an Al content of less than 0.25%, and abnormal oxidation occurred at 600°C and 900°C in the 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 had an Al content of less than 2.00%, passed the steam oxidation resistance test 1, but spalling of the oxide scale occurred at 900°C in the steam oxidation resistance test 2.
Steel No. 37, which had an Nb content of 0.30% or less, passed the steam oxidation resistance test 1, but spalling of the oxide scale occurred at 600°C in the steam oxidation resistance test 2.

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 (4)

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,
Contains 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. Furthermore, in mass% ,
Ti : 0.005 to 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 : 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 to 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 . Furthermore, 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: