JP5018257B2 - Ferritic stainless steel sheet excellent in abrasiveness and corrosion resistance and method for producing the same - Google Patents

Description

  The present invention relates to a ferritic stainless steel sheet, and more particularly to a ferritic stainless steel sheet excellent in abrasiveness and corrosion resistance, which is suitable for use as a material for parts used in a corrosive environment after polishing, and a method for producing the same.

  Since stainless steel plates are often used without being subjected to rust prevention treatment such as painting, excellent corrosion resistance is required. In particular, when used in kitchen equipment such as exterior materials and indoor sinks, etc., it is often required to have design characteristics based on a specific metallic luster in addition to corrosion resistance. In addition, when rust is generated in such application parts, it is common to scrape the rust off with a grinder or remove the rust with an acid. In this case, the surface of the part after processing loses metallic luster, and the aesthetics are significantly impaired.

  One commonly used ferritic stainless steel is SUS430 (18% Cr). This SUS430 has low rust resistance, and it is difficult to say that it has sufficient corrosion resistance for use in a corrosive environment where a large amount of acid, halogen, etc. are present, or in an air environment where there is a large amount of incoming salt. Then, as what improved rust resistance, there exists SUS436 (18% Cr-1-1.5% Mo) which added Mo to SUS430. This steel can suppress rusting even in an environment where there are a lot of acids, halogens, etc. or in an atmospheric environment where there is a lot of incoming salt.

  By the way, a stainless steel plate is usually annealed after cold rolling to ensure mechanical properties. This annealing may be performed in a reducing atmosphere called a so-called BA atmosphere such as ammonia decomposition gas or hydrogen gas, or in an oxidizing atmosphere such as a gas atmosphere in which coke oven gas is burned. . In the former annealing, an oxide film is not formed, and the surface of the steel sheet is glossy and has good polishing properties even after annealing, but the manufacturing cost increases because of the BA atmosphere. On the other hand, in the latter annealing, an oxide film is formed on the surface of the steel sheet after annealing, and in particular, SUS436 steel to which Mo is added has a dense oxide film due to Mo. It is common to require a process for removal.

The above-mentioned dense oxide film caused by Mo cannot be completely removed by neutral salt electrolysis in a solution such as Na ₂ SO ₄ which is usually performed to remove the oxide film, and then further contains an acid solution containing halogen. It is necessary to dissolve and remove the base material. Therefore, the surface of this steel plate does not have a metallic luster. Therefore, in order to give a metallic luster to the surface of the steel sheet, it is necessary to perform a heavy surface polishing, which is a so-called “poor polishing ability”.

  Further, since Mo-added steel rusts once and dense rust is formed, in order to remove the once formed rust with a grinder or acid, the ground must be greatly ground or melted. Therefore, in Mo-added steel, it is necessary to remove surface defects and scratches that are likely to cause rusting as much as possible, and when used for a long period of time, there was a problem in securing gloss with simple maintenance. .

  On the other hand, stainless steel that does not contain Mo and has excellent rust resistance has also been proposed. For example, Patent Document 1 discloses a component composition of Cr: 9 to 30%, Cu: 0.1 to 0.6%, Ti: 5 × C to 15 × C%, and Sb: 0.02 to 0.2%. Stainless steel for pitting corrosion resistance processing is disclosed. However, this steel improves the pitting corrosion resistance by suppressing the harmful effect of MnS, which is a main factor of pitting corrosion generation, by adding Ti, Cu and Sb in combination. There is a problem that the acid resistance is lowered due to the formation of an oxide of Sb.

  Patent Document 2 contains Cr: 11 to 23 wt%, V: 0.05 to 2.0 wt%, Cu: 0.5 to 2.0 wt%, and among Ti, Nb, Zr and Ta Ferritic stainless steels having a component composition containing at least one of the above in a range of 0.01 to 1.0 wt% and excellent in corrosion resistance are disclosed. However, in this steel, the corrosion resistance is improved by the combined addition of V and Cu, but the addition of V causes a decrease in hot workability, so that surface defects are likely to occur during hot rolling, Since the surface defect remains to the end, it has a problem of impairing the polishability.

Patent Document 3 contains Cr: 5 to 60 wt%, Ti: 4 × (C + N) to 0.5 wt%, Nb: 0.003 to 0.020 wt%, and further Cu to 0.15 to A chromium steel sheet having a component composition that can be contained in a range of 3.0 wt% and excellent in formability and weather resistance is disclosed. In this steel sheet, the deep drawability is improved by adding Nb, and the weather resistance is improved by adding Cu. However, this steel sheet has a problem that since fine precipitates are formed by the addition of Nb, the strength increases, the elongation decreases, and the workability deteriorates.
Japanese Patent Publication No. 50-6167 Japanese Patent Publication No. 64-4576 Japanese Patent No. 3420371

  As described above, there is no actual ferritic stainless steel sheet that has both polishing properties and corrosion resistance regardless of whether or not Mo is added.

  Accordingly, an object of the present invention is to propose a ferritic stainless steel sheet excellent in both abrasiveness and corrosion resistance and a method for producing the same.

  The inventors have solved the above-mentioned problems of the prior art and developed a ferritic stainless steel sheet that is excellent in abrasiveness and excellent in corrosion resistance, and the structure of the oxide film formed during annealing, as well as abrasiveness and corrosion resistance. We studied earnestly about the relationship. As a result, in Mo-added steel, the density of the oxide film generated during annealing differs depending on the Mo content, and when the Mo content exceeds a certain value, the oxide film generated during annealing becomes dense, and neutral salt electrolysis In the descaling process called + pickling, the surface was not dissolved uniformly, and it was found that the remaining oxide film and the base material part are likely to be the starting point of rust. In addition, when the Mo content exceeds a certain value, the Cr content in the oxide film increases and the Cr concentration at the grain boundaries at the iron-iron interface decreases, resulting in large grain boundary erosion holes due to halogen-containing acids. As a result, it was found that the polishing ability is greatly inhibited. Therefore, in order to ensure polishing properties, it is necessary to suppress the generation of grain boundary erosion holes in an acid containing halogen as much as possible, that is, to suppress the dissolution amount as much as possible.

However, if the amount of dissolution is suppressed, a portion where a thin oxide film remains on the surface is generated. Therefore, as a result of further examination, even in the case of Mo-added steel, the Cr content in the steel is increased to a certain value or more, and an appropriate amount of Cu and Ni is added. When a certain amount or more of an oxide film having a high SiO ₂ content is left on the surface, the oxide film part (that is, the temper color generating part) exhibits excellent corrosion resistance among acids containing halogen and is also abrasive. It was also found to be excellent. Furthermore, the present invention has found that a ferritic stainless steel sheet having excellent weldability, workability of welded parts, and corrosion resistance can be obtained by reducing impurity elements of the steel sheet and adding Ti as a stabilizing element. It came to complete.

  That is, the present invention includes C: 0.030 mass% or less, Si: 0.20 to 1.0 mass%, Mn: 0.3 mass% or less, P: 0.04 mass% or less, S: 0.02 mass% or less, Cr : 20.0 to 25.0 mass%, Mo: 0.01 to 0.15 mass%, Ti: 0.10 to 0.5 mass%, N: less than 0.025 mass%, Ni: more than 0.05 mass%, 0.5 mass % Or less, Cu: 0.2 to 1.0 mass%, the remainder is made of Fe and inevitable impurities, and the thickness of the oxide film remaining on the steel sheet surface layer is a ferritic stainless steel sheet having a thickness of 30 to 100 nm.

  Further, the present invention includes C: 0.030 mass% or less, Si: 0.20 to 1.0 mass%, Mn: 0.3 mass% or less, P: 0.04 mass% or less, S: 0.02 mass% or less, Cr : 20.0 to 25.0 mass%, Mo: 0.01 to 0.15 mass%, Ti: 0.10 to 0.5 mass%, N: less than 0.025 mass%, Ni: more than 0.05 mass%, 0.5 mass %, Cu: 0.2 to 1.0 mass%, and the rest of the cold-rolled sheet consisting of Fe and inevitable impurities is finally annealed to form an oxide film, and then immersed in a nitric hydrofluoric acid solution. A method for producing a ferritic stainless steel sheet is proposed in which an oxide film having a film thickness of 30 to 100 nm is left.

  According to the present invention, it is possible to obtain a ferritic stainless steel sheet having both abrasiveness and corrosion resistance. Moreover, according to this invention, the ferritic stainless steel plate which is excellent also in weldability, weld part workability, and weld part corrosion resistance can be obtained.

The component composition of the ferritic stainless steel sheet according to the present invention will be described.
C: 0.030 mass% or less C forms carbides with Cr, reduces the effective Cr amount, and causes rust, so the smaller the content, the better. In the present invention, 0.030 mass% or less And Preferably, it is 0.020 mass% or less. In addition, since the excessive reduction of C lengthens the decarburization time at the time of steelmaking, and raises manufacturing cost, it is preferable that a minimum shall be about 0.005 mass%.

N: Less than 0.025 mass% N has an effect of improving corrosion resistance when dissolved in steel. However, since N forms nitrides with Cr, excessive addition reduces the amount of effective Cr and also causes rusting. Therefore, N is preferably as small as possible. In the present invention, it is less than 0.025 mass%.
In addition, in order to suppress the production | generation of Cr carbonitride at a grain boundary, it is preferable that it is C + N <= 0.025mass%.

Si: 0.20 to 1.0 mass%
Si is an element added as a deoxidizer. Further, according to the knowledge of the inventors, when Si is contained in the steel in an amount of 0.20 mass% or more, an oxide film of SiO ₂ is formed on the outermost surface layer of the base iron during heat treatment. In particular, it has been clarified that the SiO ₂ oxide is concentrated at the grain boundary of the surface layer, and has the effect of suppressing the grain boundary erosion caused by the acid. In an acid containing halogen or a solution having a low pH, the above effect is great, and it is possible to prevent the grain boundary from being dug deeply and reducing the polishing property. Incidentally, the oxide film of SiO ₂ does not have to be continuous, the effect of moderately also improve the corrosion resistance by remaining on the surface. The above effect is manifested when the Si content is 0.20 mass% or more. However, when the Si content exceeds 1.0 mass%, Si is excessively concentrated at the interface between the oxide film and the ground iron when the oxide film is formed. An excessive amount of Si oxide is generated at the grain boundary part, and on the contrary, the abrasiveness is lowered. Therefore, in this invention, Si addition amount shall be the range of 0.20-1.0 mass%. Preferably, it is the range of 0.3-0.6 mass%.

Mn: 0.3 mass% or less Mn is added as a deoxidizing element. However, since excessive addition of Mn impairs workability by the solid solution strengthening action, it is desirable that the addition amount be low. Also, if Mn is added in an amount exceeding 0.3 mass%, the Mn concentrates in the spinel oxide present in the outer layer of the oxide film produced during heat treatment, and the corrosion resistance when the temper color oxide film remains is deteriorated. Let Therefore, in the present invention, the Mn content is set to 0.3 mass% or less. Preferably, it is 0.2 mass% or less.

P: 0.04 mass% or less P is an element that degrades corrosion resistance and hot workability, so the lower the content, the more desirable. In the present invention, P is 0.04 mass% or less.

S: 0.02 mass% or less S combines with Mn to form MnS, which is the starting point for initial rusting. Further, it is a harmful element that segregates at the grain boundaries and causes grain boundary embrittlement. In particular, if it exceeds 0.02 mass%, the adverse effect becomes significant, so S is made 0.02 mass% or less. Preferably, it is 0.015 mass% or less.

Cr: 20.0-25.0 mass%
Cr is an element necessary for ensuring corrosion resistance even when an oxide film (temper color) is attached to the surface, and for improving rust resistance in cooperation with Cu and Ni. When the Cr content is less than 20.0 mass%, when a temper color is generated on the surface, the Cr concentration at the interface between the temper color (scale) and the ground iron decreases, and when this part is locally exposed, Although it rusts in a very short time, if the Cr content is 20.0 mass% or more, even if an oxide film of the present invention is present, the Cr concentration of the base iron does not decrease so much, so that the corrosion resistance can be prevented from decreasing. . Furthermore, if Cr in the steel is 20.0 mass% or more, the passive film is stabilized by the presence of the Cu film described later, and exhibits particularly good rust resistance.

  On the other hand, if the Cr content exceeds 25.0 mass%, the Cr concentration in the oxide film increases, and the formation of the Si oxide at the grain boundary is suppressed, so that the acid containing halogen or the solution having a low pH is contained. The selective intergranular corrosion cannot be prevented, and even if the oxide film of the present invention is left, the intergranular erosion occurs, so that the abrasiveness is lowered. Moreover, when Cr exceeds 25.0 mass%, the elongation of a steel plate will fall and workability will fall. Therefore, the content of Cr is set to 20.0 to 25.0 mass%. The preferred range is 20.5 to 23.5 mass%.

Cu: 0.2-1.0 mass%
Cu is a component that improves rust resistance in cooperation with Cr. In particular, when the stainless steel base metal is melted, a film is formed on the surface thereof to reduce the dissolution of the base metal due to the anodic reaction and to prevent local dissolution. This effect is obtained by addition of 0.2 mass% or more of Cu. Particularly, the effect of a steel containing 20.0 mass% or more of Cr in a low pH solution containing halogen is large. Although this mechanism is not clear, it is presumed that Cu dissolved in a low-pH solution containing halogen is reattached to the base iron to improve dissolution resistance. Cu is also effective in improving rust resistance after polishing, and is a particularly useful component for improving crevice corrosion resistance. This effect is also manifested at 0.2 mass% or more. However, when Cu is added in excess of 1.0 mass%, dissolution of Cu after polishing is promoted, and the crevice corrosion resistance is reduced. Therefore, the addition amount of Cu is set to 0.2 to 1.0 mass%. Preferably it is the range of 0.3-0.7 mass%.

Ni: 0.05 mass% to 0.5 mass% or less Ni has the same effect as Cu, forms a film on the surface of stainless steel, reduces the dissolution of the base iron by the anode reaction, and prevents local dissolution effective. This effect can be obtained by addition exceeding 0.05 mass%, and particularly in a steel containing 20.0 mass% or more of Cr, it is large in a low pH solution containing halogen. On the other hand, when Ni is added in excess of 0.5 mass%, dissolution of Ni after polishing is promoted, and conversely, crevice corrosion resistance decreases. Therefore, the Ni content is more than 0.05 mass% and 0.5 mass% or less. Preferably it is the range of 0.15-0.3 mass%.

Mo: 0.01-0.15 mass%
Mo is an element that is extremely effective in improving rust resistance. Mo in stainless steel strengthens the passive film as the exposure period in the atmospheric environment becomes longer, so it is preferable to add it positively in consideration of the corrosion resistance after polishing. However, the inventors have studied in detail the oxide film formed during the heat treatment of the Mo-added steel. As a result, when the Mo content exceeds a certain level, the oxide film becomes a Cr-rich Cr ₂ O _3- based film. It has been found that oxidation at the grain boundaries of elements such as Si and Si that effectively act on the corrosion resistance is suppressed.

  The mechanism is not always clear, but if a large amount of Mo is present in the steel, the passive film at room temperature becomes thick, stable, and Cr-rich. Diffusion is suppressed, and a film having a high Cr concentration is generated. As a result, it is considered that the places where the Cr concentration is low increase in the vicinity of the grain boundary, promote grain boundary erosion, and reduce the polishing property. Therefore, it has been clarified that it is preferable to limit the Mo content to some extent in order to retain some of the oxide film and ensure the polishability as in the present invention.

  The inventors have Cr: 20.0 to 25.0 mass%, Si: 0.20 to 1.0 mass%, Cu: 0.2 to 1.0 mass%, Ni: more than 0.05 mass%, and 0.5 mass% or less. The relationship between the form of the oxide film and the Mo content was investigated using steel containing. As a result, when Mo exceeds 0.15 mass%, the effect of suppressing the formation of Si oxide is increased, and the polishing property is deteriorated. On the other hand, when the influence of the Mo content on the corrosion resistance after polishing, particularly the corrosion resistance in the gap portion was investigated, if the Mo content is less than 0.01 mass%, the effects of Cu, Ni, Cr, etc. described above are sufficient. There was an example that could not secure a good corrosion resistance. Therefore, in this invention, Mo content in steel is made into the range of 0.01-0.15 mass%. Preferably, the Mo content is 0.015 to 0.05 mass%.

Ti: 0.10 to 0.5 mass%
Ti has the effect of fixing S and preventing the generation of rust starting from MnS, and fixing C and N and preventing sensitization due to Cr carbonitride formation. If the addition amount of Ti is less than 0.1 mass%, it is not sufficient for fixing elements that deteriorate the corrosion resistance. On the other hand, if it exceeds 0.5 mass%, the toughness of the hot-rolled sheet decreases, and TiN cluster inclusions. It causes the generation of surface flaws and significantly impairs the polishing properties. Therefore, Ti is set to a range of 0.10 to 0.5 mass%. Preferably it is the range of 0.15-0.35 mass%.

Next, the most important oxide film on the steel sheet surface layer in the steel sheet of the present invention will be described.
The steel sheet of the present invention cannot achieve both excellent corrosion resistance and abrasiveness only by satisfying the above-described component composition, and by selectively leaving an oxide film on the surface layer of the steel sheet that does not deteriorate the corrosion resistance. For the first time, polishing and corrosion resistance can be ensured. In other words, in the present invention, from the viewpoint of ensuring corrosion resistance, the oxidation film, which is normally hated, is intentionally left on the surface of the steel sheet, thereby suppressing the dissolution of the base iron, particularly the erosion at the grain boundaries. In addition, the polishing property is improved, and sufficient corrosion resistance is secured when used after polishing.

  The inventors have Cr: 20.0 to 25.0 mass%, Si: 0.20 to 1.0 mass%, Cu: 0.2 to 1.0 mass%, Ni: more than 0.05 mass%, and 0.5 mass% or less. , Mo: A cold-rolled steel sheet containing 0.005 to 0.15 mass% is subjected to final annealing at 800 to 1050 ° C. for 10 to 600 seconds to form an oxide film on the steel sheet surface. The oxide film was left on the surface of the steel sheet by dissolving at different levels in a hydrofluoric acid solution, and the corrosion resistance, polishability and post-polishing corrosion resistance of the remaining oxide film were investigated. As a result, it has been newly found that as the oxide film remaining on the surface layer, a very good polishing property can be obtained when the Si concentration is high and the film thickness is 30 to 100 nm.

  However, in the steel having the component composition suitable for the present invention, when the remaining film thickness of the oxide film exceeds 100 nm on average, the spinel oxide containing a large amount of Mn and Fe existing in the outer layer of the oxide film becomes the starting point of rust. Rusts in a very short time. On the other hand, if the oxide film exceeds 100 nm, the polishing property deteriorates in order to remove the oxide film itself. Conversely, if the surface layer is dissolved until the average thickness of the oxide film is 30 nm or less, part of the grain boundary of the surface layer is deeply dug, the surface appearance becomes whitish, the metallic luster is lost, and polishing Deteriorates. Therefore, in the present invention, the thickness of the oxide film remaining on the steel sheet surface is set in the range of 30 to 100 nm. Preferably, it is the range of 40-80 nm.

Next, the manufacturing method of the stainless steel plate which concerns on this invention is demonstrated.
The steel having the proper composition described above is melted by a known method, and is made into a steel slab by a known method such as continuous casting. The slab is reheated to 1100 to 1200 ° C, and the finish rolling finish temperature is 700 to 900 ° C. The hot rolling is performed to obtain a hot rolled coil. Subsequently, this hot-rolled coil is annealed at a temperature of 800 to 1100 ° C., pickled, and cold-rolled to obtain a cold-rolled plate having a thickness of 3.0 mm or less. This is annealed in a coke oven combustion gas atmosphere, for example, at 1000 ° C. for about 60 seconds to form an oxide film having a thickness of 150 μm, and then a nitric hydrofluoric acid solution (nitric acid 50 g / l, hydrofluoric acid 30 g). / L, 60 ° C.) for about 20 seconds. At this time, the immersion time is adjusted to leave an oxide film having a thickness of 30 to 100 nm on the steel sheet surface layer. The acid concentration and the immersion time are examples, and may be appropriately adjusted so that an oxide film of 30 to 100 nm remains depending on changes in the Cr concentration in the steel and the like. It should be noted that there is no problem in performing the neutral salt electrolysis treatment that is normally performed in the pre-process of the nitric hydrofluoric acid solution immersion.

  Finally, in the present invention, it is preferable to carry out positive electrolysis and negative electrolysis at least once each in a nitric acid solution after being immersed in the nitric hydrofluoric acid solution to control the film thickness of the oxide film within a predetermined range. In electrolysis in nitric acid solution, it is easy to remove only the spinel oxide containing a large amount of Mn and Fe in the outer layer due to the difference in solubility. By applying positive electrolysis and negative electrolysis in nitric acid, This is because the Si concentration in the oxide can be increased and the corrosion resistance of the oxide film can be improved.

Ferritic stainless steels of steel symbols A to V having the composition shown in Table 1 are melted in a vacuum melting furnace and cast to obtain a 30 kg steel ingot, which is heated to 1200 ° C. and 1200 to 900 Hot rolling was performed in a temperature range of ° C. to obtain a hot-rolled sheet having a thickness of 3 mm. Subsequently, these hot-rolled sheets were subjected to hot-rolled sheet annealing at 1000 ° C., and then cold rolling and annealing were repeated to obtain cold-rolled sheets having a sheet thickness of 1.0 mm. The cold-rolled sheet thus obtained was annealed at 1000 ° C. for 60 seconds, and for some steel types, the current value (+30 C) in a neutral salt solution (Na ₂ SO ₄ : 200 g / l, 80 ° C.). / Dm ² , −30 C / dm ² ). Next, the thickness of the oxide film on the steel sheet surface layer is adjusted as shown in Table 2 by dipping in a nitric hydrofluoric acid solution (nitric acid: 50 g / l, hydrofluoric acid 20 g / l, temperature 55 ° C.) and changing the dipping time. did. Then, in a nitric acid solution of ^{100g / l, 45 ℃, +} 30C / dm 2, subjected twice each electrolytic treatment of -30C / dm ^2, and a cold-rolled stainless steel sheet for evaluation test. However, the electrolytic treatment was not performed on some cold-rolled plates. For reference, a stainless cold-rolled steel sheet for evaluation tests was produced in the same manner for a steel plate equivalent to SUS436 (steel symbols: U and V).

  Auger spectroscopic analysis and GDS analysis were performed on the surfaces of various stainless cold-rolled steel sheets obtained as described above, and the thickness of the remaining oxide film was measured.

Moreover, abrasiveness and corrosion resistance were evaluated in the following manner.
<Abrasiveness>
Evaluation was performed by performing wet polishing with organic resin buffol for 5 minutes using # 600 alumina abrasive grains and measuring the gloss after polishing.
<Corrosion resistance test>
Corrosion resistance was evaluated by a wet and dry repeated test using samples before and after the polishing test. The conditions of the dry / wet repeat test are 1 spray (NaCl: 5 mass% solution, temperature 35 ° C., 0.5 hour) → dry (temperature 60 ° C., 1 hour) → wet (temperature 40 ° C., 95% humidity, 1 hour). The cycle test was repeated and the time until rusting was measured.

  The result of the said test was shown to (No.1-28) of Table 2. From the results in Table 2, it can be seen that all of the ferritic stainless steel sheets having the component composition and oxide film satisfying the conditions of the present invention are excellent in polishing properties and corrosion resistance.

  The ferritic stainless steel sheet of the present invention is suitable for use in parts that require design and corrosion resistance, such as marine transport containers, articles, kitchen equipment, and joinery.

Claims (2)

C: 0.030 mass% or less, Si: 0.20 to 1.0 mass%, Mn: 0.3 mass% or less, P: 0.04 mass% or less, S: 0.02 mass% or less, Cr: 20.0 to 25 0.0 mass%, Mo: 0.01 to 0.15 mass%, Ti: 0.10 to 0.5 mass%, N: less than 0.025 mass%, Ni: more than 0.05 mass% and less than 0.5 mass%, Cu: 0 A ferritic stainless steel sheet containing .2 to 1.0 mass%, the balance being Fe and inevitable impurities, and the thickness of the oxide film remaining on the steel sheet surface layer being 30 to 100 nm. C: 0.030 mass% or less, Si: 0.20 to 1.0 mass%, Mn: 0.3 mass% or less, P: 0.04 mass% or less, S: 0.02 mass% or less, Cr: 20.0 to 25 0.0 mass%, Mo: 0.01 to 0.15 mass%, Ti: 0.10 to 0.5 mass%, N: less than 0.025 mass%, Ni: more than 0.05 mass% and less than 0.5 mass%, Cu: 0 A cold-rolled sheet containing 0.2 to 1.0 mass%, the balance being Fe and unavoidable impurities is finally annealed to form an oxide film, and then immersed in a nitric hydrofluoric acid solution to have a film thickness of 30 to 100 nm A method for producing a ferritic stainless steel sheet, characterized by leaving an oxide film of