JP7133917B2 - Al-containing ferritic stainless steel sheet with excellent surface properties and sulfidation corrosion resistance, and method for producing the same - Google Patents

Description

TECHNICAL FIELD The present invention relates to an Al-containing ferritic stainless steel sheet and a method for producing the same, and more particularly to an Al-containing ferritic stainless steel sheet having excellent surface properties and sulfidation corrosion resistance and a method for producing the same.

In recent years, photovoltaic power generation is developing into one of the main energies to replace fossil fuels, and the technological development of solar cells is accelerating. Conventionally, ceramics or glass has been widely used for solar cell substrates.

However, since glass is fragile and heavy, it is not easy to mass-produce solar cell substrates having a light absorption layer formed on the glass surface. Therefore, the application of a stainless steel plate, which has excellent heat resistance, as a substrate material has been studied. For example, an insulating material is used in which the smooth surface of a general-purpose ferritic stainless steel plate SUS430 (17Cr steel) is coated with an alumina, silicon oxide or silicon nitride film.

Here, among solar cells, compound solar cells such as CIS thin films are expected to spread in the future as solar cells having both low cost and high efficiency. A compound thin-film solar cell, for example, forms an insulating layer on a substrate, forms a first electrode layer comprising a Mo layer on the insulating layer, and forms a chalcopyrite-type compound layer thereon as a light absorbing layer. After forming a film, a second electrode layer is further formed. The chalcopyrite type compound is a quinary system alloy typified by a Cu--In--Ga--Se--S system (hereinafter referred to as a CIS system).

For example, stainless steel foils such as SUS430, SUS444 (18Cr-2Mo), and SUS447J1 (30Cr-2Mo) having a thickness of 20 to 200 μm are used as substrates for CIS thin-film solar cells. An insulating coating is formed on the surface of the stainless steel foil, and a back electrode made of Mo layer is formed thereon, followed by heat treatment for coating formation, so that the back electrode is made of Cu ₍ In1 _{- xGax} )Se2. A light absorption layer is formed.

Here, in Patent Documents 1 and 2, in the film-forming process of a compound-based thin-film solar cell, Cu, In, and Ga, which are precursors of a light-absorbing layer, are formed by sputtering on a substrate, and then converted into a CIS-based compound thin film. discloses a heat treatment process (sulfidation process) in which it is exposed to a highly corrosive gas atmosphere such as hydrogen sulfide (H ₂ S). In Patent Document 1, a method is used in which, for example, a silica film is formed by a sputtering method to form a corrosion-resistant layer on the back surface of a metal substrate exposed to a highly corrosive gas atmosphere.

In addition, as a method that does not rely on coating, Patent Documents 1 and 2 disclose a method of forming an alumina layer on the back surface of a metal substrate by applying a ferritic stainless steel substrate with a high Al content. . Specifically, in Patent Document 2, JFE18-3USR: Application of a ferritic stainless steel substrate containing 3.4% Al is studied.

Furthermore, in Patent Documents 3 and 4, it is possible to form an insulating surface that maintains the conversion efficiency of the solar cell at a high level without coating etc., and a solar cell substrate with a small thermal expansion coefficient and suitable for it A stainless steel plate is disclosed. The composition of the steel sheet is, in mass %, Cr: 9 to 25%, C: 0.03% or less, Mn: 2% or less, P: 0.05% or less, S: 0.01% or less, N: 0. 03% or less, Al: 0.005 to 5.0%, Si: 0.05 to 4.0%, the balance being Fe and unavoidable impurities, Al: 0.5% or more, and/or Si: 0.4% or more and a composition satisfying Cr+10Si+Mn+Al>24.5, or satisfying the above composition and containing 50% or more Al ₂ O ₃ on the surface, or Al ₂ O ₃ and SiO An oxide film containing 50% or more of the total of ₂ is formed.

In Patent Document 5, a stainless steel surface with good film-forming properties has a ten-point average roughness Rz of 0.3 μm or less and a characteristic average parameter Rsk in the height direction of less than 0.7. is disclosed. The material contains C: 0.01%, Si: 0.52%, Mn: 0.2%, Cr: 18.2%, Nb: 0.41%, and Cu: 0.48% by mass. A ferritic stainless steel sheet containing Fe and unavoidable impurities, C: 0.04%, Si: 0.48%, Mn: 0.2%, Cr: 18.4%, Ni: 8. An austenitic stainless steel sheet containing 2% nitrogen, 0.02% nitrogen, and the balance being Fe and unavoidable impurities is used.

Patent Document 6 discloses a compound-based thin-film solar cell that uses a generally available stainless substrate and has good device characteristics. Here, the stainless steel surface used as the substrate has an average roughness Ra of 32 nm or more and 62 nm or less, and a ferritic stainless steel plate such as SUS430 is used, for example.

Patent document 7 discloses a ferritic stainless steel sheet excellent in washability and a manufacturing method thereof. In mass%, C: 0.1% by mass or less, Si: 0.1 to 2.0% by mass, Cr: 10 to 32% by mass, and the balance is Fe and unavoidable impurities. A stainless steel sheet having a surface with an arithmetic mean roughness Ra of 0.03 μm or less in a direction perpendicular to the rolling direction of the steel sheet surface, and micropits having a depth of 0.5 μm or more and an opening area of 10 μm ² or more on the surface of the steel sheet. The pits are characterized by having an existence density of 10.0 or less per 0.01 mm ² and having an opening area ratio of 1.0% or less of the pits.

Patent Document 8 discloses a cold-rolled stainless steel sheet having dull patterns on the steel sheet surface as a cold-rolled stainless steel sheet excellent in cleanability and antiglare properties. The arithmetic mean roughness Ra in the direction perpendicular to the rolling direction on the steel plate surface is 0.2 to 1.2 μm, and the transfer rate, which is the area ratio of the dull pattern on the steel plate surface, is 15 to 70%. Micropits having a depth of 0.5 μm or more and an opening area of 10 μm ² or more have a density of 10.0 or less per 0.01 mm ² on the steel plate surface, and an opening area ratio of 1.0 on the steel plate surface. % or less.

JP 2014-203936 A JP 2014-127712 A JP 2014-218727 A Japanese Unexamined Patent Application Publication No. 2014-218728 Japanese Patent No. 5837284 JP 2014-107510 A JP 2013-208638 A Japanese Patent No. 5918127

In order to expand the spread of CIS-based compound thin-film solar cells as a major photovoltaic power generation in the future, it is an important issue to improve power generation efficiency and productivity. To this end, the stainless steel plate that serves as the substrate should (1) be improved in surface properties to prevent damage to the film layer itself and short circuits that occur during the film formation process, and (2) be improved in sulfidation corrosion resistance. A problem is to omit the complicated surface treatment for forming the corrosion-resistant layer.

The technique disclosed in Patent Document 1 includes a step of forming a corrosion-resistant layer against sulfidation corrosion on the back surface of the substrate, and has a problem in terms of productivity itself.
Further, in Patent Documents 1 and 2, the use of a ferritic stainless steel sheet containing Al as a substrate makes it possible to omit the step of forming a corrosion-resistant layer. However, the ferritic stainless steel sheet containing 3.4% by weight or 5.5% by weight of Al disclosed in the relevant art does not disclose any surface properties such as surface roughness. Since the Al content is high, coarse and hard MgO.Al ₂ O ₃ inclusions are likely to be formed, and it is thought that the problem of surface properties remains.

The techniques disclosed in Patent Documents 3 and 4 are characterized by the formation of an oxide film composed of Al ₂ O ₃ and SiO ₂ in an Al-containing ferritic stainless steel sheet characterized by Si addition. is not mentioned at all. Also, the surface properties are not disclosed.

Although Patent Documents 5 to 8 disclose surface properties, they are all aimed at Al-free stainless steel sheets, and when used as substrates for CIS solar cells, there remains a problem with sulfuric corrosion resistance.
As described above, as a stainless steel substrate for a CIS solar cell, there is no disclosure of a stainless steel plate that satisfies the two problems of (1) surface properties and (2) resistance to sulfidation corrosion.

An object of the present invention is to provide an Al-containing ferritic stainless steel sheet that has excellent surface properties and is resistant to sulfidation corrosion without surface treatment of a corrosion-resistant layer such as coating or plating, and a method for producing the same. .

In order to solve the above-described problems, the present inventors conducted intensive experiments and studies on the relationship between the surface properties and power generation efficiency, and between the surface coating and sulfidation corrosion resistance in Al-containing ferritic stainless steel sheets. I came to obtain the knowledge of

(a) In general, ferritic stainless steel sheets containing Al are finished by rough surface polishing because it is difficult to remove the oxide scale by pickling. Al-containing ferritic stainless steel sheets tend to form coarse and hard MgO·Al ₂ O ₃ inclusions even when they are finished by either long-term pickling or bright annealing instead of polishing. Cold rolling causes the formation of fine cracks and unevenness on the surface of the steel sheet, and the surface quality tends to deteriorate.

(b) In the above stainless steel sheet, by adding a small amount of Mg and Ga singly or in combination, hard MgO.Al ₂ O ₃ inclusions are finely dispersed, and cracks caused by inclusions pressed during cold rolling are prevented. and unevenness can be reduced, and the surface properties can be improved.

(c) It has been found that the index of the surface properties is not the arithmetic mean roughness measured by a roughness meter, but the area ratio of irregularities of less than 0.1 μm obtained by microscopic observation of the surface. The target surface texture of the present invention is to increase the area ratio of surface irregularities of less than 0.1 μm to 40% or more. It was found that damage to the membrane layer itself can be suppressed, and as a result, it is effective in improving power generation efficiency. In the prior art, the existence rate of micropits is defined by limiting the roughness parameter and the opening area of the micropits. For this reason, the opening area is out of specification, deep pits that exist locally are not taken into account, and the surface properties (form of unevenness on the surface) do not necessarily correspond to the damage to the film layer itself.

(d) When high-purity ferritic stainless steel sheets, represented by SUS430LX and SUS430J1L, are exposed to an atmosphere containing several percent hydrogen sulfide gas at a high temperature of 400 to 700°C for about 0.5 to 2 hours, the steel sheet base becomes Sulfidation corrosion progresses until it falls off. Such sulfidation corrosion proceeds when Fe and Cr, which are constituent elements of stainless steel, react with S in the atmosphere gas to form FeS ₂ and FeCr ₂ S ₄ , corroding the steel.

(e) In the case of a ferritic stainless steel sheet containing 3.5% or more of Al, a continuous film of Al ₂ O ₃ is formed in a high-temperature environment, so the above-mentioned sulfidation corrosion is improved. On the other hand, in the case of ferritic stainless steel sheets with an Al content of less than 3.5%, the sulfidation corrosion resistance is affected by the composition of the surface coating. Generally, a thin passive film of Fe and Cr having a thickness of several nanometers is formed on the surface after pickling or polishing. Sulfidation corrosion is likely to occur when a passivation film mainly composed of Fe and Cr is formed on the surface.

(f) Addition of Mg and Ga as trace elements in addition to Al is effective for reducing the Fe and Cr concentrations in the surface coating, and sulfidation corrosion can be suppressed even with a low Al content. It was also found that SiO ₂ impairs the protective properties of the continuous Al ₂ O ₃ film, and rather reduces the sulfidation corrosion resistance.

(g) Although the reason for the above is not necessarily clear, by adding Mg alone, or more effectively by adding Ga together with Mg, the production of SiO ₂ in addition to Fe or Cr is suppressed, and Al is reduced. We have acquired the knowledge that it works effectively for film formation. This effect makes it possible to reduce the Al content while maintaining the sulfidation corrosion resistance.

(h) We have found that heat treatment (bright annealing) in a temperature range of 700 to 1100° C. in an atmosphere containing hydrogen gas is effective for forming the surface unevenness and surface film.

(i) The above-described steel sheet finished by bright annealing is further subjected to electrolysis in a nitric acid solution, so that the corners of the overhang caused by the pressing of polishing lines and inclusions can be removed, and the surface properties can be improved. .

The present invention has been made based on the above findings, and the gist thereof is the following Al-containing ferritic stainless steel sheet and method for producing the same.

(1) in % by mass,
C: 0.03% or less,
Si: 0.02 to 2.0%,
Mn: 0.03-2.0%,
Cr: 13.0 to 22.0%,
P: 0.05% or less,
S: 0.01% or less,
Al: 0.20-3.5%,
Ti: 0.5% or less,
N: 0.03% or less,
and further Mg: 0.0004 to 0.01%,
or Mg: 0.0003 to 0.01%, Ga: 0.1% or less,
The sum of the Mg content and 1/2 of the Ga content is more than 0.0004% and 0.06% or less,
The balance is Fe and unavoidable impurities,
An Al-containing ferritic stainless steel sheet characterized by having a surface coating containing Al, having a surface unevenness of less than 0.1 μm in an area of 40% or more, and having a thickness of 100 nm or less.

(2) The Al-containing ferritic stainless steel sheet according to (1) above, wherein the content of the element contained in the surface coating satisfies the following formulas (i) to (iv).
Al>60% (i)
Fe<20% (ii)
Cr<20% (iii)
S<5% (iv)
However, the content of each element in the above formula represents the content (atomic %) of each element in the components other than O, C and N contained in the surface coating.

(3) The Al-containing ferritic stainless steel sheet according to any one of (1) and (2) above, characterized in that the region with surface irregularities of less than 0.1 µm is 55% or more.

(4) Furthermore, in % by mass,
Ni: 1% or less,
Cu: 1% or less,
Mo: 2% or less,
Nb: 0.5% or less,
V: 0.5% or less,
Sn: 0.2% or less,
Sb: 0.2% or less,
W: 1% or less,
Zr: 0.5% or less,
Co: 0.5% or less,
B: 0.005% or less,
Ca: 0.005% or less,
La: 0.1% or less,
Y: 0.1% or less,
Hf: 0.1% or less,
REM: 0.1% or less,
The Al-containing ferritic stainless steel sheet according to any one of (1) to (3) above, containing one or more selected from.

(5) The Al-containing ferritic stainless steel sheet according to any one of (1) to (4), which is used as a solar cell substrate.

(6) A stainless steel plate having the chemical composition described in either (1) or (4) above is heat-treated at a temperature range of 700 to 1100° C. in an atmosphere containing hydrogen gas, so that (1) is formed on the surface. Alternatively, a method for producing an Al-containing ferritic stainless steel sheet, characterized by forming the surface irregularities and the surface film according to (2).

(7) Furthermore, the surface is subjected to electrolytic pickling in a 3 to 20% by mass aqueous solution of nitric acid to form the surface unevenness and the surface film described in either (1) or (3) on the surface. A method for producing an Al-containing ferritic stainless steel sheet according to (6).

According to the present invention, it is possible to obtain an Al-containing ferritic stainless steel sheet which has excellent surface properties and is resistant to sulfidation corrosion without surface treatment such as coating or plating.

It is a figure which shows the roughness curved surface measurement result of the surface by a three-dimensional roughness analyzer.

Each requirement of the present invention will be described in detail below.

1. Chemical Composition of Base Material The reasons for limiting each element are as follows. In addition, "%" about content in the following description means "mass %."

C: 0.03% or less C forms a solid solution in the ferrite phase or forms Cr carbides to lower the oxidation resistance and inhibit the formation of the surface film according to the present invention. For this reason, the lower the C content is, the better, and it is set to 0.03% or less. The C content is preferably 0.02% or less. However, since excessive reduction leads to an increase in refining cost, the C content is preferably 0.001% or more, more preferably 0.002% or more.

Si: 0.02-2.0%
Si is an important element for ensuring the anti-sulfidation corrosion property which is the object of the present invention. Si promotes the formation of an oxide film mainly composed of Al, suppresses penetration of hydrogen sulfide (H ₂ S) into the steel, and improves sulfidation corrosion resistance. Therefore, the Si content should be 0.02% or more. Preferably, the Si content is 0.1% or more, more preferably 0.5% or more. However, if Si is contained excessively, SiO ₂ is formed, which impairs the protective properties of the continuous Al ₂ O ₃ film and significantly reduces the sulfidation corrosion resistance. Moreover, it causes deterioration of toughness and workability of the steel sheet. Therefore, from the viewpoint of sulfidation corrosion resistance and basic properties, the Si content should be 2.0% or less. It is preferably 1.8% or less, more preferably 1.5% or less.

Mn: 0.03-2.0%
Mn has the effect of suppressing the surface oxidation of Fe and promoting the formation of a surface film that is mainly composed of Al and falls within the specified range of the present invention. Therefore, the Mn content should be 0.03% or more. It is preferably 0.1% or more, more preferably 0.2% or more. However, an excessive Mn content lowers the oxidation resistance and leads to deterioration of the sulfuric corrosion resistance, so the Mn content is made 2.0% or less. From the viewpoint of oxidation resistance and sulfuric corrosion resistance, the Mn content is preferably 1.0% or less.

Cr: 13.0-22.0%
In addition to corrosion resistance, Cr is a basic constituent element for forming the surface film according to the present invention to ensure sulfidation corrosion resistance. If the Cr content is less than 13.0%, the targeted sulfidation corrosion resistance cannot be sufficiently ensured, and the linear expansion coefficient increases. Therefore, the Cr content should be 13.0% or more. However, if Cr is contained excessively, the Cr concentration in the surface film is increased, and when exposed to a high-temperature hydrogen sulfide atmosphere, the sulfidation corrosion resistance is deteriorated, and the alloy cost is increased. Therefore, the Cr content is set to 22.0% or less in terms of sulfidation corrosion resistance and alloy cost. The Cr content is preferably 15.0% or more, more preferably 16.0% or more. Also, the Cr content is preferably 20.0% or less, more preferably 19.0% or less.

P: 0.05% or less P is an element that deteriorates manufacturability and weldability, and the lower the content, the better. Therefore, the P content is set to 0.05% or less. The P content is preferably 0.04% or less, more preferably 0.03% or less. However, since excessive reduction leads to an increase in refining cost, the P content is preferably 0.003% or more, more preferably 0.005% or more, and more preferably 0.01% or more. More preferred.

S: 0.01% or less S is an unavoidable impurity element contained in steel, and deteriorates sulfidation corrosion resistance. In particular, S present in the surface film, or Mn-based inclusions or solid-solution S present in the steel act as fracture starting points of the surface film when exposed to a high-temperature hydrogen sulfide atmosphere. Therefore, the lower the S content, the better, and it is set to 0.01% or less. The S content is preferably 0.002% or less, more preferably 0.001% or less. However, excessive reduction leads to an increase in raw material and refining costs, so it is preferably 0.0001% or more, more preferably 0.0002% or more.

Al: 0.20-3.5%
In addition to being an element having a deoxidizing action, Al is an essential element for achieving the sulfidation corrosion resistance targeted by the present invention by modifying the surface film. In the present invention, if the Al content is 0.20% or more, the targeted sulfidation corrosion resistance is exhibited. Therefore, the Al content is set to 0.20% or more. On the other hand, if the Al content exceeds 3.5%, the amount of hard Al-based inclusions increases, which not only deteriorates the surface properties, but also deteriorates the manufacturability and increases the alloy cost. Therefore, the Al content is set to 3.5% or less. The Al content is preferably 0.8% or more and preferably 3.0% or less.

Ti: 0.5% or less Ti improves the oxidation resistance of the steel through the action of a stabilizing element that fixes C and N, thereby improving the oxidation resistance of the steel. It has the effect of causing Therefore, when exhibiting the effect of Ti, the Ti content is made 0.03% or more. However, excessive Ti content leads to an increase in alloy cost and a decrease in manufacturability due to an increase in recrystallization temperature. Therefore, the Ti content is set to 0.5% or less. The Ti content is preferably 0.05% or more, more preferably 0.1% or more. Also, the Ti content is preferably 0.35% or less, more preferably 0.25% or less.

N: 0.03% or less N, like C, is an element that deteriorates sulfuric corrosion resistance. Therefore, the lower the N content, the better, and it is set to 0.03% or less. The N content is preferably 0.02% or less. However, since an excessive reduction leads to an increase in refining cost, the N content is preferably 0.002% or more, more preferably 0.005% or more.

Mg: 0.0003-0.01%
As described above, Mg concentrates on the surface to suppress the oxidation of Fe and Cr, promotes the formation of a film mainly composed of Al, and has the effect of finely dispersing hard MgO.Al ₂ O ₃ inclusions. It is an element that has Therefore, the Mg content is set to 0.0003% or more. However, an excessive Mg content not only increases the refining cost of steel and deteriorates manufacturability, but also produces a large amount of hard inclusions described above, deteriorates surface properties, and inhibits film formation. , leading to deterioration of sulfidation corrosion resistance. Therefore, the Mg content should be 0.01% or less. The Mg content is preferably 0.001% or more and preferably 0.008% or less.

Ga: 0.1% or less Like Mg, Ga is an element that concentrates on the surface to suppress the oxidation of Fe and Cr and acts on the formation of a film mainly composed of Al. Therefore, Ga can be contained. When the Ga content is 0.0003% or more, the effect is remarkable. However, an excessive Ga content not only increases the refining cost of the steel and deteriorates the manufacturability, but rather deteriorates the sulfidation corrosion resistance. Therefore, the Ga content should be 0.1% or less. The Ga content is preferably 0.02% or less.
In the present invention, one type of Mg or two types of Mg and Ga are contained.
The total content of Mg+0.5Ga is 0.0003% or more and 0.060% or less. Containing more than 0.001% is particularly effective for exhibiting the above effects. More preferably, the total content of Mg+0.5Ga exceeds 0.002%. The upper limit of the total content of Mg+0.5Ga is preferably 0.02% or less, more preferably 0.01% or less.

The Al-containing ferritic stainless steel sheet of the present invention has a chemical composition containing the above elements from C to Ga, with the balance being Fe and unavoidable impurities.

Here, "inevitable impurities" are components that are mixed in by various factors in raw materials such as ores, scraps, and manufacturing processes when steel is manufactured industrially, and are allowed within a range that does not adversely affect the present invention. means to be

In addition to the above elements, the Al-containing ferritic stainless steel sheet according to the present invention may optionally contain the following amounts of Ga, Ni, Cu, Mo, Nb, V, Sn, Sb, W, Co, B , Ca, Zr, Hf, REM and Ta.

Ni: 1.0% or less Cu: 1.0% or less Mo: 2.0% or less Nb: 0.5% or less V: 0.5% or less Sn: 0.2% or less Sb: 0.2% or less W Co: 1.0% or less Co: 0.5% or less The above elements are effective in increasing the high-temperature strength and corrosion resistance of the stainless steel sheet. Therefore, it may be contained as necessary. However, an excessive content leads to an increase in alloy cost and deterioration in manufacturability. Therefore, the contents of Ni, Cu and W are set to 1.0% or less. Further, since Mo is an element effective in suppressing high-temperature deformation due to a decrease in linear expansion coefficient, the Mo content is set to 2.0% or less. Furthermore, the contents of Nb, V and Co are set to 0.5% or less. The contents of Sn and Sb are set to 0.2% or less from the viewpoint of manufacturability. In order to obtain the above effect, it is preferable to contain at least one of the above elements in an amount of 0.05% or more.

B: 0.005% or less Ca: 0.005% or less B and Ca are elements that improve hot workability and secondary workability, and thus may be contained as necessary. However, since an excessive content leads to deterioration of productivity, the contents of B and Ca are each set to 0.005% or less. In order to obtain the above effect, it is preferable to contain at least one of the above elements in an amount of 0.0001% or more.

Zr: 0.5% or less La: 0.1% or less Y: 0.1% or less Hf: 0.1% or less REM: 0.1% or less It is an element effective for improving oxidation resistance. Therefore, it may be contained as necessary. However, the sulfuric corrosion resistance targeted by the present invention does not depend on the effect of adding these elements. Therefore, the upper limit of each element is defined as above. In order to obtain the above effect, Zr: 0.01% or more, La: 0.001% or more, Y: 0.001% or more, Hf: 0.001% or more, REM: 0.001% It is preferable to satisfy at least one of the above.

Here, REM refers to lanthanoid elements not described above, and the content of REM refers to the total content of these elements.

In addition to the elements described above, impurity elements are allowed to be mixed within a range that does not impair the effects of the present invention. It is preferable to reduce the above-mentioned P and S, which are general impurity elements, as well as Zn, Bi, Pb, Se, etc., as much as possible. On the other hand, the content ratio of these elements is controlled as long as the problems of the present invention are solved, and if necessary, Zn: 500 ppm or less, Bi: 100 ppm or less, Pb: 100 ppm or less, Se: 100 ppm or less More than one species may be contained.

2. Surface Texture A stainless steel plate used as a substrate for a CIS-based compound thin-film solar cell is required to have good surface texture. In order to suppress damage to the film formed on the substrate, it is preferable to smoothen the shape of the surface of the steel sheet. Specifically, on the plate surface of the described stainless steel plate, the region having an unevenness of less than 0.1 μm is 40% or more. More preferably, it is 55% or more.

3. Surface Coating The Al-containing ferritic stainless steel sheet according to the present invention is provided with a coating having a film thickness of 100 nm or less and a predetermined composition on the surface. Long-time BA annealing is required to achieve a film thickness exceeding 100 nm. In that case, the content of Fe and Cr in the coating increases, and the sulfidation corrosion resistance may decrease. Moreover, since the productivity deteriorates, the film thickness is preferably 50 nm or less, more preferably 30 nm or less. On the other hand, the lower limit of the film thickness is not particularly limited, but it is preferably 3 nm or more in order to exhibit the effect of sulfidation corrosion resistance in hydrogen sulfide.

Also, in order to improve the resistance to sulfidation corrosion during the sulfidation process using H ₂ S or the like, it is necessary to increase the Al content in the coating and decrease the Fe, Cr, and S contents. Specifically, the content of each element contained in the film satisfies the following formulas (i) to (v).
Al>60% (i)
Fe<20% (ii)
Cr<20% (ii)
S<5% (iv)
However, each symbol in the above formula represents the content (atomic %) of each element in the components other than O, C and N contained in the film.
Al is concentrated from the inner layer of the surface coating to the interface of the base iron, and significantly suppresses the penetration of the corrosive gas S into the steel. These effects are remarkably exhibited by setting the Al content in the surface coating to an amount exceeding 5 atomic %. The Al content is preferably 8 atomic % or more, more preferably 10 atomic % or more. The upper limit of the Al content is not particularly limited, but is preferably 95 atomic % or less in consideration of the efficiency of annealing and pickling.

Fe and Cr contained in the coating are preferentially sulfided and promote sulfidation corrosion. Therefore, the content of Fe or Cr in the surface film should be 20% or less, respectively, or the sum of Fe and Cr should be 30% or less.

In order to suppress corrosion starting points due to hydrogen sulfide at high temperatures, the S content in the film is preferably less than 5.0 atomic %. The S content is more preferably less than 4.0 atomic %, more preferably less than 3.0 atomic %. The lower limit of the S content is not particularly limited, but the lower the better, and the non-detection=zero is preferred.

There are no particular restrictions on the method of analyzing the coating composition, as long as quantitative analysis is possible. For example, the content of Fe, Cr, Al, and S in the film can be measured by Auger electron spectroscopy (AES).

4. Manufacturing Method Regarding the manufacturing method of the Al-containing ferritic stainless steel sheet according to the present invention, the manufacturing method described later is preferable in order to incorporate the above-described surface properties and surface coating, but the manufacturing method is not necessarily limited. It can be manufactured by subjecting a steel material having a chemical composition to the following treatments. When using a steel plate as the steel material, for example, a thin plate, a foil, or a medium-thick plate can be used. Here, the thin plate has a plate thickness of 0.2 mm or more, the foil has a plate thickness of 0.02 to 0.2 mm, and the thick plate has a plate thickness of 6 mm or more. . The surface finish may be BA, 2B, 2D, polishing, etc. conforming to JIS (G 0203).

In addition, although there are no particular restrictions on the type of steel plate used as the steel material, it is possible to use a cold-rolled steel plate obtained by subjecting the hot-rolled steel strip to annealing, descaling, and cold-rolling as necessary. can. In order to form the film specified in the present invention on the surface of the cold rolled sheet, it is effective to carry out bright annealing in a low dew point atmosphere containing hydrogen gas after cold working. The atmosphere gas for bright annealing contains 50% by volume or more of hydrogen gas and the balance is an inert gas in order to suppress the oxidation of Cr and Fe and selectively oxidize the above elements on the surface. The dew point of the atmospheric gas is preferably −40° C. or less, and the hydrogen gas is preferably 80% by volume or more, more preferably 90% by volume or more. The rest of the inert gas is preferably nitrogen gas, which is industrially inexpensive, but Ar gas or He gas may also be used. In addition, oxygen or other gases may be mixed in the ambient gas in an amount of less than 5% by volume, as long as the formation of the surface film targeted by the present invention is promoted or not hindered. In particular, in the above-described low dew point hydrogen gas atmosphere, by mixing 0.1% by volume or more and less than 3% by volume of oxygen gas, Al and Mg are selectively oxidized, which is the target of the present invention. It can promote the formation of a surface film suitable for sulfuric corrosion resistance.

If the heating temperature in the final annealing is less than 700°C, the softening and recrystallization of the steel sheet may be insufficient, and the desired material properties may not be obtained. Therefore, the heating temperature is preferably 700° C. or higher. In order to lower the dew point of the atmosphere gas, the heating temperature is more preferably 800° C. or higher, more preferably 850° C. or higher. On the other hand, if the heating temperature exceeds 1100° C., the crystal grains become coarse and the toughness and ductility of the steel sheet may deteriorate. Therefore, the heating temperature is preferably 1100° C. or lower, more preferably 1000° C. or lower. For the formation of the surface film targeted by the present invention, the heating time is preferably 5 seconds or longer, more preferably 10 seconds or longer. On the other hand, when the time is 3600 seconds or more, the content of Fe and Cr in the surface film increases, and the sulfidation corrosion resistance may deteriorate. Therefore, the heating time is preferably less than 3600 seconds. 300 seconds or less is preferable, and 60 seconds or less is more preferable from the viewpoint of productivity and sulfuric corrosion resistance.

Further, it is preferable to perform electrolytic pickling in a nitric acid aqueous solution on the BA, 2B, 2D, and polished surfaces. The nitric acid concentration is preferably in the range of 3 to 20% by weight in order to dissolve the surface to reduce surface flaws and microcracks and to remove S and Fe adhering to the surface by electrolysis. When the nitric acid concentration is less than 3%, it is difficult to obtain the effect of electrolytic pickling. On the other hand, when the concentration of nitric acid exceeds 20%, the concentration of Cr in the surface film progresses, leading to deterioration of the sulfidation corrosion resistance. Nitric acid electrolysis is preferably carried out in an industrial electrolytic pickling apparatus. For example, in a facility equipped with a pretreatment tank and a plurality of electrolytic pickling tanks, washing processes such as water washing and dilute sulfuric acid electrolytic pickling are performed. After that, nitric acid electrolysis may be performed. The current density and temperature for nitric acid electrolysis are not particularly limited as long as they do not harm the film composition.

EXAMPLES The present invention will be described in more detail with reference to examples below, but the present invention is not limited to these examples.

A ferritic stainless steel (Steel A to Z) having the chemical composition shown in Table 1 is melted, hot rolled, annealed, and cold rolled to form a foil or sheet having a thickness of 0.1 to 1.5 mm. After that, each steel plate was subjected to the following treatment (Test Nos. 1 to 30). The foil or sheet is either bright annealed or air annealed at 900 to 950°C to complete recrystallization, and after air annealing is pickled with 2.5% HF-5% HNO ₃ at 55°C or polished. rice field. Test no. The steel sheets Nos. 8, 11, 13 and 15 were further subjected to nitric acid electrolytic pickling in an 8% nitric acid aqueous solution. Note that Ga in Table 1 was analyzed by mass spectrometry GD-MS using glow discharge.

The composition of the surface film of the obtained steel sheet was measured by AES analysis. For the AES analysis, a scanning FE Auger electron spectrometer (manufactured by ULVAC-PHI) was used, the acceleration voltage of the electron gun was 10 kV, the acceleration voltage of the ion gun (Ar) was 3 kV, and the sputtering rate was 10 nm/min (SiO ₂ measured value) and converted to the film thickness in the plate thickness direction.

From the obtained Auger electron spectrum, the Auger peak heights (heights between peaks in the differential spectrum) of all detected elements such as Fe, Cr, Mn, Al, Si, Ti, and S are read to determine the relative concentrations. Calculated. The film thickness was obtained by measuring the profile of O and obtaining the half-value width at which the intensity of O is half the value. Table 2 shows the coating thickness and composition.

Also, using the above steel plate, the roughness curved surface was measured. After degreasing, 4 visual fields were measured in an area of 500 μm×500 μm. At this time, the pitch interval was set to 0.1 μm. Based on the numerical data of the roughness curved surface, the roughness parameter and the unevenness area of less than 0.1 μm were calculated. SURFPAC (high-precision roughness measuring machine-three-dimensional roughness analyzer) manufactured by Mitutoyo Corporation was used as an apparatus.

An example of the above measurements is shown in FIG. In FIG. 1, the portions expressed in light color are the roughness parameter and the region with unevenness of less than 0.1 μm.

Subsequently, using each of the steel sheets described above, the degree of damage to the film-forming layer that affects the power generation efficiency of the solar cell was evaluated as the "degradation degree of the film-forming layer." An insulating film of 1.0 μm thick SiO ₂ film is formed on the surface of the steel plate, and 20 Al electrodes of 10 mm × 10 mm × 0.2 μm thick are vapor-deposited thereon. did. If it exceeds 1 MΩ, it is judged to be insulated, and the rate of insulation achieved is 90% or more with "◎", 70% or more and less than 90% is "○", 50% or more and less than 70% is "△", less than 50% was set to "x". In the present invention, it was determined that the degree of deterioration of the film formation layer was small and the power generation efficiency was excellent when the above results corresponded to "⊚", "○", and "Δ".

Subsequently, evaluation of sulfidation corrosion resistance was performed using each of the above steel sheets. Specifically, for each steel plate, 700° C., 2% H ₂ S-bal. A heat treatment was carried out by holding in an atmosphere of N ₂ for 1 hour. If there is no peeling of corrosion products and the corrosion weight gain is less than 1.0 mg/cm ² , "⊚" is given, and if it is less than 1.5 mg/cm ² , "○" is given, and if it is 2.5 mg/cm ² or less. A sample with a corrosion weight increase of more than 2.5 mg/cm ² due to peeling was rated as “×”. In the present invention, when the above results correspond to "⊚", "◯", and "Δ", it was determined that the sulfidation corrosion resistance was excellent.

The above results are also shown in Table 2.

Test no. Nos. 1 to 16 are steel sheets according to examples of the present invention that satisfy the provisions of the present invention.
As can be seen from Table 2, the steel sheets of the examples of the present invention have a small degree of deterioration of the film formation layer involved in power generation and have excellent sulfidation corrosion resistance.

On the other hand, Test No. using a steel sheet whose chemical composition is outside the scope of the present invention. In Nos. 17 to 30, both or one of the degree of deterioration and resistance to sulfidation corrosion of the film-forming layer was inferior. Specifically, the steel sheet having a surface unevenness of less than 40% in a region of less than 0.1 μm had an insulation achievement rate of less than 70%, and the degree of deterioration of the film layer was “×”. In addition, the sulfidation corrosion resistance was evaluated as "x" when the chemical composition did not meet the requirements of the present invention and the chemical composition of the film did not meet the requirements of the present invention.

According to the present invention, it is possible to obtain an Al-containing ferritic stainless steel sheet which has excellent surface properties and is resistant to sulfidation corrosion without surface treatment such as coating or plating. Therefore, the Al-containing ferritic stainless steel sheet according to the present invention is used as a substrate for compound thin-film solar cells manufactured through a film-forming process including heat treatment in which it is exposed to a sulfurizing atmosphere such as hydrogen sulfide gas in an oil-fired or combustion environment. It can be used preferably.

Claims (8)

In % by mass,
C: 0.03% or less,
Si: 0.02 to 1.83%,
Mn: 0.03-2.0%,
Cr: 13.0 to 22.0%,
P: 0.05% or less,
S: 0.01% or less,
Al: 0.35-3.5%,
Ti: 0.03% or more and 0.5% or less,
N: 0.025% or less,
and further Mg: 0.0003 to 0.01%, Ga: 0.1% or less,
The sum of the Mg content and 1/2 of the Ga content is 0.0007% or more and 0.06% or less,
The balance is Fe and unavoidable impurities,
When measuring four fields of view with a pitch interval of 0.1 µm in a range of 500 µm × 500 µm per field of view on the surface of the steel plate,
1. An Al-containing ferritic stainless steel sheet characterized by having an area ratio of 40% or more of a region having surface irregularities of less than 0.1 μm, a film thickness of 8 nm or more and 100 nm or less, and a surface coating containing 9% or more of Al. 2. The Al-containing ferritic stainless steel sheet according to claim 1, wherein the contents of the elements contained in the surface coating satisfy the formulas (i) to (iv).
Al>60% (i)
Fe<20% (ii)
Cr<20% (iii)
S<5% (iv)
However, the content of each element in the above formula represents the content (atomic %) of each element in the components other than O, C and N contained in the surface coating. In % by mass,
C: 0.03% or less,
Si: 0.02 to 1.83%,
Mn: 0.03-2.0%,
Cr: 13.0 to 22.0%,
P: 0.05% or less,
S: 0.01% or less,
Al: 0.35-3.5%,
Ti: 0.03% or more and 0.5% or less,
N: 0.025% or less,
and further Mg: 0.0007 to 0.01%,
including
The balance is Fe and unavoidable impurities,
When measuring four fields of view with a pitch interval of 0.1 µm in a range of 500 µm × 500 µm per field of view on the surface of the steel plate,
The area ratio of the region with surface unevenness of less than 0.1 μm is 40% or more,
A surface film with a film thickness of 8 nm or more and 100 nm or less,
An Al-containing ferritic stainless steel sheet, wherein the content of elements contained in the surface coating satisfies the formulas (i) to (iv).
Al>60% (i)
Fe<20% (ii)
Cr<20% (iii)
S<5% (iv)
However, the content of each element in the above formula represents the content (atomic %) of each element in the components other than O, C and N contained in the surface coating. 4. The Al-containing ferritic stainless steel sheet according to any one of claims 1 to 3, further comprising 55% or more of the surface irregularities of less than 0.1 μm. Furthermore, in mass % ,
Mo : 1.5 % or less,
Nb: 0.13 % or less,
V: 0.15 % or less ,
Zr : 0.01 % or less ,
B : 0.0005 % or less,
Ca: 0.005% or less,
La: 0.01 % or less,
Y: 0.01 % or less
The Al-containing ferritic stainless steel sheet according to any one of claims 1 to 4, containing 1.665% or less in total amount of one or more selected from . The Al-containing ferritic stainless steel sheet according to any one of claims 1 to 5, which is used as a solar cell substrate. By heat-treating a stainless steel plate having the chemical composition according to any one of claims 1, 3 or 5 in a temperature range of 700 to 1100° C. in an atmosphere containing hydrogen gas, the surface according to any one of claims 1 to 4 is formed. A method for producing an Al-containing ferritic stainless steel sheet, comprising forming the surface irregularities and the surface film according to item 1. Further, by electrolytic pickling in a 3 to 20% by mass nitric acid aqueous solution, the surface unevenness and surface film according to any one of claims 1 to 4 are formed on the surface. A method for producing the Al-containing ferritic stainless steel sheet described above.

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