JP7526091B2 - Ferritic Stainless Steel - Google Patents

Description

The present invention relates to a ferritic stainless steel material with excellent corrosion resistance and a black oxide film on the surface.

Stainless steel is not only a material with excellent corrosion resistance, but it is also used in interior and exterior building materials, taking advantage of its shiny silver-white surface. Furthermore, in order to enhance the design of stainless steel, it is often given colors, typically black, using methods such as chemical coloring, painting, and oxidation treatment.

The method of forming a black oxide film on the surface of stainless steel by oxidation treatment methods such as those described in Patent Documents 1 and 2 allows for continuous processing using continuous annealing equipment, which is a general-purpose manufacturing process for stainless steel, and has higher productivity than other processes such as chemical coloring methods. Furthermore, the black oxide film formed by the oxidation treatment method has high corrosion resistance because a layer consisting mainly of protective Cr oxide is formed.

JP 2019-178392 A JP 2018-135591 A

However, while the oxide film formed by the oxidation treatment methods described in Patent Documents 1 and 2 has high corrosion resistance because it is mainly composed of a Cr oxide layer, there are cases where partial, point-like abnormal oxidation areas are inevitably formed.

The abnormal oxidation portion will be described in more detail below with reference to FIG.
FIG. 1 is a schematic diagram for explaining an abnormal oxidation part, in which (a) is a plan view showing a stainless steel material on which an oxide film having an abnormal oxidation part is formed, (b) is a cross-sectional view, and (c) is an enlarged view of the rectangular frame in (b). The stainless steel material shown in FIG. 1(a) has a black oxide film 4 formed on the surface by an oxidation treatment method. This black oxide film 4 is mainly composed of Cr oxide (Cr ₂ O ₃ ), and a composite oxide of Mn and Cr and a composite oxide of Ti and Cr are formed in addition to Cr ₂ O ₃ on the surface side, giving it a black color tone. However, when forming an oxide film by an oxidation treatment method, the black oxide film 4 mainly composed of Cr may not be formed, and Fe-based oxides may grow locally, forming an abnormal oxidation part 9 composed of the oxide film 4 mainly composed of Fe. In this specification, the entire oxide film formed on the surface of the stainless steel material, including the black oxide film 4 and the abnormal oxidation part 9, may be simply referred to as an "oxide film".

1(b) and (c), inclusions such as Al oxide (Al ₂ O ₃ ) and Ti oxide (TiO ₂ ) are present in the substrate surface layer 5 near the interface between the black oxide film 4 and the substrate 3, and furthermore, a Cr-deficient layer 51 is present on the surface side of the substrate surface layer 5, in which Cr is lost to the oxide film 4 side and the Cr concentration is relatively lower than that of the surrounding substrate 3. The Cr-based oxide film 4 has a dense and uniform structure, so it functions as a protective film that protects the substrate 3 underneath. On the other hand, the abnormally oxidized portion 9 made of an oxide film mainly made of Fe has a structure with more gaps than the Cr-based black oxide film 4, so it does not function sufficiently as a protective film. Since the abnormally oxidized portion 9 may be formed in a stainless steel material having an oxide film formed on its surface by an oxidation treatment method, there is a risk that the abnormally oxidized portion 9 may cause early rusting, and further improvement has been required.

The present invention has been made to solve the above problems, and aims to provide a ferritic stainless steel material with excellent corrosion resistance and a black oxide film on the surface.

As a result of intensive research conducted by the inventors to solve the above-mentioned problems, they have discovered that in order to improve the corrosion resistance of ferritic stainless steel materials having an oxide film formed by an oxidation treatment method, it is effective to remove the abnormally oxidized parts and the inclusions and Cr-depleted layers present therebelow, and further to form a highly corrosion-resistant passive film, i.e., a passive film with a high Cr concentration (high Cr fraction), on the exposed base material after removing the abnormally oxidized parts, and that electrolytic treatment in an aqueous solution of nitric acid, which is an oxidizing acid, is effective for this purpose.
In other words, the inventors have discovered that by subjecting a stainless steel material having an oxide film on its surface, in which the composition of the ferritic stainless steel and the color tone of the oxide film are controlled, to electrolytic treatment under specific conditions in an aqueous nitric acid solution, it is possible to dissolve the abnormally oxidized portion and the surface layer of the base material immediately below the abnormally oxidized portion without dissolving the normal black oxide film, and that a passive film with a high Cr fraction can be formed on the surface of the base material exposed after dissolution, and that as a result, it is possible to improve corrosion resistance while maintaining the black color tone, which has led to the present invention.

The gist of the present invention, which has been completed based on the above findings, is as follows.
(1) A ferritic stainless steel material according to one embodiment of the present invention has, in mass%, C: 0.050% or less, Si: 1.00% or less, Mn: 0.04 to 1.00%, P: 0.050% or less, S: 0.030% or less, Cr: 16.00 to 25.00%, Ni: 1.00% or less, Cu: 0.60% or less, Mo: 2.00% or less, N: 0.030% or less, Al: 0.500% or less, Ti: 0.080 to 0.500%, Nb: 0.500% or less, and the total content of Nb and Ti is 6 (C + N) or more (C and N represent the contents of C and N, respectively), with the balance being Fe and impurities. The ferritic stainless steel material has a composition of ^≦ 45 in brightness index and ≦ ^-5.0 in chromathetics index. A black oxide film satisfying a*≦5.0, -5.0≦b ^* ≦5.0 is formed on the surface, there is an exposed substrate portion on which the black oxide film is not formed, the area ratio of the exposed substrate portion is 0.01 to 3.0%, the maximum peeled area of the exposed substrate portion is 100 μm ² to 10,000 μm ² , and the passive film formed on the surface of the exposed substrate portion satisfies a Cr fraction of ≧40%.

(2) The ferritic stainless steel material described in (1) above may further contain, in mass %, one or more elements selected from the group consisting of Zr: 1.00% or less, Co: 1.00% or less, V: 1.00% or less, and W: 1.00% or less in the composition of the ferritic stainless steel.

(3) The ferritic stainless steel material described in (1) or (2) above may further contain, in mass %, one or more elements selected from the group consisting of REM: 0.100% or less and Ca: 0.100% or less in the composition of the ferritic stainless steel.

(4) The ferritic stainless steel material described in any one of (1) to (3) above may further contain, in mass%, one or more elements selected from the group consisting of Sn: 0.100% or less and B: 0.010% or less in the composition of the ferritic stainless steel.

According to the above aspect of the present invention, a ferritic stainless steel material having a black oxide film and excellent corrosion resistance can be provided.

Schematic diagrams for explaining an abnormally oxidized portion, in which (a) is a plan view showing a stainless steel material on which an oxide film having an abnormally oxidized portion is formed, (b) is a cross-sectional view, and (c) is an enlarged view of the rectangular frame in (b). FIG. 1 is a schematic diagram for explaining the process from removal of abnormally oxidized parts to formation of a passive film on exposed substrate parts in the electrolytic treatment step, in which (a) is a cross-sectional view showing the state in which an oxide film having abnormally oxidized parts has been formed, and (b) is a cross-sectional view showing the state after completion of nitric acid electrolysis. 1A and 1B are diagrams of the adhesive used in the corrosion resistance test, where (a) is a plan view and (b) is a side view.

The following is a detailed description of the embodiments of the present invention. The present invention is not limited to the following embodiments, and it should be understood that modifications and improvements to the following embodiments, as appropriate, based on the ordinary knowledge of those skilled in the art, fall within the scope of the present invention, provided they do not deviate from the spirit of the present invention.

(1. Ferritic stainless steel material according to the present embodiment)
1.1 Chemical Composition
The base material of the ferritic stainless steel material according to this embodiment (hereinafter, sometimes simply referred to as "stainless steel material") has a composition, in mass%, of C: 0.050% or less, Si: 1.00% or less, Mn: 0.04 to 1.00%, P: 0.050% or less, S: 0.030% or less, Cr: 16.00 to 25.00%, Ni: 1.00% or less, Cu: 0.60% or less, Mo: 2.00% or less, N: 0.030% or less, Al: 0.500% or less, Ti: 0.080 to 0.500%, Nb: 0.500% or less, and the total content of Nb and Ti is 6 (C + N) or more (C and N represent the contents of C and N, respectively), with the balance being Fe and impurities. That is, the base material of the stainless steel material according to this embodiment has a chemical composition in which the metal structure at room temperature is mainly ferrite phase.
In this specification, the term "impurities" refers to components that are mixed in during industrial production of the base ferritic stainless steel due to various factors in the raw materials such as ores and scraps, and in the manufacturing process, and are acceptable within a range that does not adversely affect the present invention. For example, impurities also include unavoidable impurities.

In addition, the base material of the ferritic stainless steel material according to this embodiment may further contain, by mass%, one or more elements selected from the group consisting of Zr: 1.00% or less, Co: 1.00% or less, V: 1.00% or less, and W: 1.00% or less.
In addition, the base material of the ferritic stainless steel material according to this embodiment may further contain, by mass%, one or more elements selected from the group consisting of REM: 0.100% or less and Ca: 0.100% or less.
Furthermore, the base material of the ferritic stainless steel material according to an embodiment of the present invention may further contain, by mass%, one or more elements selected from the group consisting of Sn: 0.100% or less and B: 0.0100% or less.
The reasons for limiting the content of each element will be explained below.

(C: 0.050% by mass or less)
C is an element that affects the intergranular corrosion resistance (sensitization suppression effect) and workability of stainless steel materials. If the C content is too high, the workability and intergranular corrosion resistance of stainless steel materials will be deteriorated. Therefore, the upper limit of the C content is 0.050 mass%, preferably 0.045 mass%, and more preferably 0.040 mass%. Although the lower limit is not particularly limited, a reduction in the C content leads to an increase in refining costs. Therefore, the lower limit of the C content is preferably 0.0005 mass%, more preferably 0.001 mass%. %.

(Si: 1.00% by mass or less)
Silicon is an element that improves the oxidation film peeling resistance and high-temperature oxidation resistance of stainless steel materials. If the silicon content is too high, the workability and toughness decrease. Therefore, the upper limit of the silicon content is On the other hand, the lower limit of the Si content is not particularly limited, but it is preferable that the Si content is 1.00 mass%, preferably 0.90 mass%, and more preferably 0.80 mass%. From the viewpoint of suppressing deterioration of surface quality due to oxidation, the content is preferably 0.05% by mass, more preferably 0.10% by mass, and even more preferably 0.15% by mass.

(Mn: 0.04-1.0% by mass)
Mn is a useful deoxidizing element and is also effective in improving oxidation film peeling resistance and high-temperature oxidation resistance. Mn also forms a complex oxide with Cr, which gives the steel a black color. If the Mn content is too high, MnS, which is the starting point of corrosion, is easily generated and the ferrite phase is destabilized. Therefore, the upper limit of the Mn content is set to 1.00 % by mass, preferably 0.95% by mass, and more preferably 0.90% by mass. On the other hand, if the Mn content is too low, the above effects may not be sufficiently obtained. The lower limit of the content is 0.04 mass %, preferably 0.05 mass %.

(P: 0.050% by mass or less)
P is an element that affects the properties of stainless steel, such as weldability and workability. If the P content is too high, the above properties may be deteriorated. Therefore, the upper limit of the P content is set at 100%. is 0.050 mass%, preferably 0.045 mass%, and more preferably 0.040 mass%. On the other hand, the lower limit of the P content is not particularly limited, but the P content is reduced. This leads to an increase in refining costs. Therefore, the lower limit of the P content is preferably 0.001 mass%, more preferably 0.010 mass%.

(S: 0.030% by mass or less)
S is an element that generates MnS, which is the starting point of corrosion, and affects the toughness and other properties of stainless steel materials. If the S content is too high, the above properties may be reduced. The upper limit of the S content is 0.030% by mass, preferably 0.025% by mass, and more preferably 0.020% by mass. On the other hand, the lower limit of the S content is not particularly limited, but A lower S content leads to an increase in refining costs, so the lower limit of the S content is preferably 0.0001 mass % or more, and more preferably 0.0005 mass % or more.

(Cr: 16.00 to 25.00% by mass)
Cr is an element that is effective in improving the corrosion resistance and oxidation resistance of stainless steel materials. If the Cr content is too high, the toughness of the stainless steel material decreases, and the growth of the oxide film is inhibited, resulting in a black color. Therefore, the upper limit of the Cr content is 25.00 mass%, preferably 24.50 mass%, and more preferably 24.00 mass%. If the amount is too small, the above-mentioned effects may not be sufficiently obtained, so the lower limit of the Cr content is 16.00 mass%, preferably 16.50 mass%.

(Ni: 1.00% by mass or less)
Ni is an element effective in improving the corrosion resistance and toughness of stainless steel materials. If the Ni content is too high, the ferrite phase becomes unstable and the manufacturing cost increases. Therefore, the Ni content is The upper limit of the Ni content is 1.00 mass%, preferably 0.90 mass%, and more preferably 0.80 mass%. On the other hand, the lower limit of the Ni content is not particularly limited, but if the above effect is not obtained, the lower limit is set to 0.80 mass%. From the viewpoint of obtaining the desired result, the content is preferably 0.01% by mass, and more preferably 0.05% by mass.

(Cu: 0.60% by mass or less)
Cu is an element effective in improving the corrosion resistance of stainless steel materials. If the Cu content is too high, the ferrite phase becomes unstable and the manufacturing cost increases. Therefore, the upper limit of the Cu content is set to 100%. The lower limit of the Cu content is not particularly limited, but is preferably 0.001 mass %, more preferably 0.50 mass %, and most preferably 0.60 mass %, still more preferably 0.55 mass %, still more preferably 0.50 mass %. %, preferably 0.01% by mass.

(Mo: 2.00% by mass or less)
Mo is an element effective in improving the corrosion resistance and oxidation resistance of stainless steel materials. If the Mo content is too high, the workability of the stainless steel material decreases, the manufacturing cost increases, and the oxidation resistance decreases. Therefore, the upper limit of the Mo content is 2.00 mass%, preferably 1.90 mass%. The lower limit is not particularly limited, but is preferably 0.001 mass %, and more preferably 0.01 mass %.

(N: 0.030% by mass or less)
N is an element that affects properties such as intergranular corrosion resistance (sensitization suppression effect) and workability. If the N content is too high, the intergranular corrosion resistance and workability of stainless steel materials will decrease. As described later, Ti is necessary for the formation of a black oxide film. However, if the N content is high, TiN precipitates, reducing the amount of Ti dissolved in the steel, and the black oxide film is not formed. The formation of a coating is inhibited. In addition, the formed nitrides are likely to become the starting points of corrosion, and reduce the corrosion resistance, especially the pitting corrosion resistance. Therefore, the upper limit of the N content is set to 0.030 mass%. The lower limit of the N content is not particularly limited, but reducing the N content increases the refining cost. Therefore, the lower limit of the N content is preferably 0.0005 mass %, and more preferably 0.001 mass %.

(Al: 0.500% by mass or less)
Al is an element that effectively prevents the oxide film from peeling off by forming Al oxide on the surface of the material when the black oxide film is formed. On the other hand, if the Al content is too high, the stainless steel The toughness of the steel material is reduced. Therefore, the upper limit of the Al content is 0.500 mass%, preferably 0.450 mass%, and more preferably 0.400 mass%. The lower limit is not particularly limited, but is preferably 0.001 mass %, and more preferably 0.010 mass %.

(Nb: 0.500 mass% or less, Ti: 0.080 to 0.500 mass%, total content of Nb and Ti: 6(C+N) or more)
Nb and Ti are elements that affect properties such as intergranular corrosion resistance (sensitization suppression effect), etc. Furthermore, Ti forms a complex oxide with Cr, thereby imparting a black appearance to the oxide film.
If the Nb content is too high, the workability and toughness of the stainless steel material are reduced, so the upper limit of the Nb content is 0.500 mass%, preferably 0.480 mass%, and more preferably 0.450 mass%.
Moreover, if the Ti content is too high, the workability and surface quality of the stainless steel material are deteriorated, so the upper limit of the Ti content is 0.500 mass%, preferably 0.480 mass%, and more preferably 0.450 mass%.
On the other hand, the lower limit of the content of Nb and Ti is controlled based on the relationship with the content of C and N, which reduce the intergranular corrosion resistance. Specifically, the lower limit of the total content of Nb and Ti is 6 (C+N), preferably 7 (C+N), and more preferably 8 (C+N). Here, C and N represent the contents of C and N, respectively. Also, if the content of Ti is too small, the above-mentioned effect may not be sufficiently obtained. Therefore, the lower limit of the content of Ti is 0.080 mass%, preferably 0.090 mass% or more, and more preferably 0.100 mass%.

(Zr: 1.00% by mass or less, Co: 1.00% by mass or less, V: 1.00% by mass or less, W: 1.00% by mass or less)
Zr, Co, V and W are elements effective for improving the oxidation resistance of stainless steel materials. If the content of Zr, Co, V and W is too high, the workability and toughness of the stainless steel material will decrease. Therefore, the upper limit of each of the contents of Zr, Co, V and W is set to 1.0 mass %, preferably 0.8 mass %, and more preferably 0.5 mass %. On the other hand, the lower limit of each of the Zr, Co, V and W contents is not particularly limited, but is preferably 0.001 mass %, more preferably 0.01 mass %.

(REM: 0.100% by mass or less, Ca: 0.100% by mass or less)
REM and Ca are elements effective in improving the oxidation resistance of ferritic stainless steel materials. If the content of REM and Ca is too high, it leads to an increase in the production cost of ferritic stainless steel. The upper limit of the content of REM and Ca is 0.100 mass%, preferably 0.080 mass%, and more preferably 0.050 mass%. On the other hand, the lower limit of the content of REM and Ca is Although not limited, the content is preferably 0.0001 mass %, and more preferably 0.003 mass %. Note that REM is a collective term for Sc, Y, and lanthanoids, a total of 17 elements, and means rare earth metals. The rare earth elements include La, Ce, Nd, etc., and one of these may be contained alone or two or more of them may be contained in combination. The content means the total content of these rare earth elements. As a method of addition, for example, misch metal (MM), which is a mixture of rare earth elements, may be used so that the REM content falls within the above range.

(Sn: 0.100% by mass or less)
Sn is an element effective for improving the corrosion resistance of stainless steel materials. If the Sn content is too high, Sn segregates and manufacturability decreases. Therefore, the upper limit of the Sn content is set to 0. 100% by mass, preferably 0.080% by mass, and more preferably 0.050% by mass. On the other hand, the lower limit of the Sn content is not particularly limited, but is preferably 0.001% by mass, and more preferably 0.010% by mass. is 0.005 mass%.

(B: 0.0100% by mass or less)
B is an element effective for improving the secondary workability of stainless steel materials. If the B content is too high, the fatigue strength of the stainless steel decreases. Therefore, the upper limit of the B content is The lower limit of the B content is not particularly limited, but is preferably 0.0001 mass%, more preferably 0.0050 mass%. The content is preferably 0.0005% by mass.

(1.2 Structure of stainless steel material)
As described later, Fig. 2(b) is a cross-sectional view in the plate thickness direction that typically illustrates the vicinity of the surface of the stainless steel material 1 according to this embodiment. Referring to Fig. 2(b), the stainless steel material 1 according to this embodiment has a base material 3 made of ferritic stainless steel and a black oxide film 4 formed on the surface of the base material 3.
(Oxide film)
The substrate 3 side of the black oxide film 4 in the stainless steel material 1 according to this embodiment is a Cr oxide region 42 in which Cr ₂ O ₃ is mainly formed. Such an oxide film mainly composed of Cr has a dense and uniform structure and functions as a protective film that protects the substrate 3, so that the stainless steel material 1 according to this embodiment has excellent corrosion resistance. In addition, the surface side of the black oxide film 4 in the stainless steel material 1 according to this embodiment is a Ti/Mn concentrated region 41 in which the ratio of Ti and Cr composite oxides and Mn and Cr composite oxides is higher than that of the Cr oxide region 42, and the presence of these composite oxides gives the oxide film a black color tone.

(Oxide film color tone)
The stainless steel material 1 according to this embodiment has a lightness index L ^* and chromanetics indices a ^* and b ^* in specific ranges with respect to the color tone of the surface (surface other than the exposed base portion) on which the black oxide film 4 is formed. These values are values obtained by performing color tone measurement in accordance with JIS Z 8722:2009 at any five points, averaging the values, and expressing the lightness index L ^* and chromanetics indices a ^* and b ^* in accordance with CIELAB (L ^* a ^* b ^* color system) in accordance with JIS Z 8781-4:2013. The surface of the black oxide film 4 of the stainless steel material 1 according to this embodiment has ranges of L ^* ≦45.0, -5.0≦a ^* ≦5.0, and -5.0≦b ^* ≦5.0.

(Area ratio and maximum area of exposed substrate)
In the stainless steel material 1 according to the present embodiment, the abnormally oxidized portion 9 formed locally in the oxide film and the substrate surface layer 5 immediately below the abnormally oxidized portion are dissolved by the electrolytic treatment process described later, so that the substrate exposed portion 7 is present in a specific range. That is, in the stainless steel material 1 according to the present embodiment, the area ratio of the substrate exposed portion 7 is 0.01 to 3.0%, and the maximum area of the substrate exposed portion 7 is 100 μm ² to 10,000 μm ^2. Although the removal of the abnormally oxidized portion 9 is effective for improving corrosion resistance, an increase in the area ratio and maximum area of the substrate exposed portion 7 leads to a deterioration in appearance. Therefore, when the total area of the substrate exposed portion 7 in the entire observation area on the surface of the stainless steel material 1 according to the present embodiment is defined as the area ratio of the substrate exposed portion 7, the area ratio of the substrate exposed portion 7 is 0.01 to 3.0%, more preferably 2.0% or less, and even more preferably 1.0% or less. The area of each exposed base portion 7 is 100 μm ² to 10,000 μm ² , preferably 9,000 μm ² or less, and more preferably 8,000 μm ² or less.

In this specification, the term "exposed substrate portion" refers to a portion where the black oxide film 4 has been removed and no black oxide film 4 has been formed, that is, a portion where the abnormally oxidized portion 9 in the oxide film and the substrate surface layer 5 directly below the abnormally oxidized portion have been dissolved by the electrolytic treatment process described below, thereby removing the abnormally oxidized portion 9 and the Cr-deficient layer 51 and inclusions 6 (hereinafter sometimes referred to as "abnormally oxidized portion, etc.") that exist below it. The portion where the abnormally oxidized portion, etc. has been removed does not mean that the surface of the substrate 3 is exposed, but rather that the passive film 8 formed on the surface of the substrate 3 by natural oxidation is exposed. However, the passive film 8 is extremely thin, about a few nm thick, and when the surface is viewed from above, the silver-white color of the substrate 3 is almost intact. Therefore, in this specification, the portion where the abnormally oxidized portion, etc. has been removed and the passive film 8 has been formed is referred to as the exposed substrate portion 7.

(Method of measuring area ratio and maximum area)
These values were obtained by observing an area of 1 mm ² (1 mm×1 mm) at any five points on the surface of the steel material excluding the edges, using an optical microscope.
The area of the exposed substrate 7 can be determined by image analysis using a photograph taken with an optical microscope at a magnification of 200 times. The area where the black oxide film 4 is present is observed to be black, the area where the abnormally oxidized portion 9 remains is black or brown, and the exposed substrate 7 is observed to be the silver-white color of the stainless steel substrate 3, so the area of the exposed substrate 7 can be determined by measuring the area of the white portion through a binarization process. When determining the area ratio of the exposed substrate 7, the entire measured area was measured and the arithmetic average of the values obtained from five locations was calculated. Moreover, when determining the maximum area of the exposed substrate 7, the measurement was performed at an enlarged portion near the largest exposed substrate 7, and the maximum value of the values obtained from the five locations was used.

(Composition of the passive film on exposed substrate)
The passive film 8 formed on the surface of the exposed substrate portion 7 of the stainless steel material 1 according to this embodiment satisfies the requirement of a Cr fraction of ≧40%. In the electrolytic treatment step described below, when removing the abnormally oxidized portion and the like, by using a nitric acid aqueous solution as the electrolytic treatment solution, the Cr fraction of the passive film 8 formed on the exposed substrate portion 7 after the abnormally oxidized portion and the like have been removed can be made to be 40% or more.
The composition of the passive film 8 on the surface of the exposed substrate 7 can be analyzed by glow discharge optical emission spectrometry (GDS), and is measured by glow discharge optical emission spectrometry (GD-OES) in accordance with JIS K 0144:2018. In this case, the analysis range is limited so that it includes only the exposed substrate. The Cr fraction (%) of the passive film 8 on the surface of the exposed substrate 7 is determined as the Cr concentration/(Fe concentration+Cr concentration+Mn concentration+Ti concentration)×100 at the point where O (oxygen) is three-quarters of the maximum value in the obtained surface profile.

In the stainless steel material 1 according to this embodiment, the Cr-deficient layer 51 and inclusions 6 present in the abnormally oxidized portion 8 and the underlying substrate surface layer 5 have been removed, and furthermore, a passive film 8 with a high Cr concentration is formed in the area where these have been removed (the surface of the substrate exposed portion 7), and this passive film composition satisfies a Cr fraction of 40% or more, so that it has excellent corrosion resistance. Therefore, the Cr fraction of the passive film on the surface of the substrate exposed portion 7 is preferably 43% or more, more preferably 45% or more. This makes it possible to provide a stainless steel material with excellent corrosion resistance while maintaining a black color tone.

The ferritic stainless steel material 1 according to this embodiment has excellent corrosion resistance while having the desired black color tone, since the chemical composition of the substrate, the color tone of the oxide film, the area ratio in the exposed portion of the substrate, the maximum area, and the passive film composition are all within the specified ranges. Therefore, the stainless steel material 1 according to this embodiment is suitable for use as a design component used in environments where high corrosion resistance is required. Design components include, but are not limited to, various components such as building materials, piping, and automotive exhaust system components such as mufflers that require design.

(2. Manufacturing method of ferritic stainless steel material according to the present embodiment)
The stainless steel material of this embodiment can be manufactured, for example, by the following method. A ferritic stainless steel slab having the above-mentioned chemical composition is hot-rolled, and the obtained hot-rolled coil is annealed and pickled, and then cold-rolled at a cold rolling rate of 50% or more. The surface is then finish-polished with a polishing belt of #180 or more. The obtained cold-rolled sheet (base material) is annealed in an oxidizing atmosphere at 1050 to 1080 ° C. for 2 to 3 minutes, thereby forming a black oxide film on the base material surface. The thickness of the black oxide film is 200 to 1000 nm, and preferably 250 to 900 nm. The thickness of the black oxide film can be adjusted by the annealing temperature and annealing time.

(Method for removing abnormally oxidized parts, Cr-depleted layers and inclusions)
Fig. 2 is a schematic diagram for explaining the process from removal of the abnormally oxidized portion to formation of a passive film on the exposed substrate in the electrolytic treatment step. Fig. 2(a) is a cross-sectional view showing the state in which an oxide film having an abnormally oxidized portion has been formed, and Fig. 2(b) is a cross-sectional view showing the state after completion of nitric acid electrolysis.
As shown in FIG. 2( a ), when an oxide film is formed on the surface of a base material 3 by an oxidation treatment method, a black oxide film 4 mainly composed of Cr is not formed, and Fe-based oxides grow locally, resulting in the formation of abnormally oxidized areas 9 mainly composed of Fe.
The abnormal oxides 9 that can be unavoidably formed on the stainless steel surface have a structure with many gaps compared to the Cr-based black oxide film 4, which has a dense and uniform structure, and therefore do not function adequately as a protective film, and the presence of the abnormally oxidized parts 9 may lead to early rusting. Therefore, in order to further improve corrosion resistance, it is effective to remove the abnormally oxidized parts 9.

1(c), in the substrate surface layer 5 near the interface between the black oxide film 4 and the substrate 3, there are inclusions 6 such as Al ₂ O ₃ and TiO ₂ , and furthermore, on the surface side of the substrate surface layer 5, there is a Cr-deficient layer 51 in which Cr has escaped to the black oxide film 4 side and has a relatively lower Cr concentration than the surrounding substrate. Therefore, in the area where the abnormally oxidized portion 9 is removed and the surface of the substrate 5 is exposed, the Cr-deficient layer 51 and inclusions 6 are present in the substrate surface layer 5, so sufficient corrosion resistance cannot be obtained by simply removing the abnormally oxidized portion 9. Therefore, in order to further improve corrosion resistance, it is effective to not only remove the abnormally oxidized portion 9, but also remove the Cr-deficient layer 51 and inclusions 6 present immediately below the abnormally oxidized portion 9.
These are removed by electrolysis using an aqueous nitric acid solution as the electrolysis solution.

Referring to FIG. 2, in this electrolytic treatment process, the abnormally oxidized portion 9 in the oxide film is removed by dissolving the Fe-based oxide in the nitric acid aqueous solution, which is an oxidizing acid. As a result, the surface of the substrate 3 is exposed in the area where the abnormally oxidized portion 9 has been removed. Furthermore, in the area where the substrate 3 is exposed after the abnormally oxidized portion 9 has been removed, the substrate surface layer 5 is dissolved in the nitric acid aqueous solution, removing the Cr-deficient layer and inclusions. In addition, since nitric acid electrolysis not only dissolves metals but also promotes metal oxidation, the surface of the substrate 3 (exposed substrate portion 7) after the Cr-deficient layer 51 and inclusions 6 have been removed is immersed in the nitric acid aqueous solution, forming a passive film 8 with a high Cr fraction.

As described above, in the stainless steel material 1 according to this embodiment, the abnormally oxidized portion 9 and the underlying Cr-deficient layer 51 and inclusions 6 have been removed, so that the surface vicinity of the stainless steel material 1 has a cross section as shown diagrammatically in FIG. 2(b). In other words, FIG. 2(b) is a cross-sectional view diagrammatically showing the surface vicinity of the stainless steel material 1 according to this embodiment in a cross section cut in the plate thickness direction.

(Electrolytic Treatment Solution)
The electrolytic treatment is carried out in an aqueous solution of nitric acid, which is an oxidizing acid. The concentration of the aqueous solution of nitric acid is 70 to 150 g/L, and the temperature of the solution is 45 to 60° C. If the nitric acid concentration or the solution temperature is too low, the Fe-based oxides do not dissolve sufficiently, making it difficult to remove the abnormally oxidized portion 9, or to remove the Cr-deficient layer 51 and the inclusions 6. If the nitric acid concentration or the solution temperature is too high, the black oxide film 4 is dissolved, which causes a deterioration in color tone. Furthermore, excessive dissolution of the base material 3 may cause the black oxide film 4 to peel off significantly, which may cause a deterioration in the appearance.

The amount of Fe ions contained in the nitric acid aqueous solution is 50 g/L or less. With use, Fe ions dissolved from precipitates such as the removed abnormally oxidized parts 9 and Cr-deficient layers 6 may become mixed into the nitric acid aqueous solution. However, if there are a large amount of Fe ions in the nitric acid aqueous solution, the dissolution of Fe-based oxides is not promoted, making it difficult to remove the abnormally oxidized parts 9. Furthermore, if there are a large number of Fe ions, the Fe ions are preferentially oxidized, which does not promote the oxidation of Cr contained in the base material 3 and prevents the regeneration of the passive film 8.

(Electrolytic Treatment)
The amount of electricity during electrolysis is 150 to 900 C/ ^m2 , and the current density at this time is 90 A/ ^m2 or less. If the amount of electricity is small, it becomes difficult to remove the abnormally oxidized parts 9, the Cr-deficient layer 51, and the inclusions 6. If the amount of electricity is large, it leads to dissolution of the black oxide film 4, which causes a deterioration in color tone. Furthermore, excessive dissolution of the base material 3 may cause the black oxide film to peel off in large areas, which may also cause a deterioration in appearance.

Under such electrolytic treatment conditions, the normally formed black oxide film 4 is hardly dissolved and remains in its almost untouched state, and only the locally formed abnormally oxidized portion 9 and the Cr-depleted layer 51 and inclusions 6 present in the substrate surface layer 5 underneath are removed.
Here, Fe-based oxides are more soluble than Cr oxides (Cr ₂ O ₃ ), and the stainless steel substrate 3 itself is more soluble than the corrosion-resistant Cr-based oxide film 4, so they dissolve when immersed in a nitric acid solution and a small current is passed through them. Therefore, by controlling the amount of electricity to be small, only the Fe-based oxides and the substrate surface layer 5 underneath can be preferentially dissolved, and the abnormally oxidized portion 9 and the Cr-deficient layer 51 and inclusions 6 present underneath can be removed.

Through the above steps, the ferritic stainless steel material 1 having excellent corrosion resistance and a black oxide film 4 on the surface according to this embodiment is manufactured.
According to the above-mentioned process, it is possible to improve the corrosion resistance while maintaining the desired appearance, so that the stainless steel material 1 according to the present embodiment has a black color tone and is excellent in corrosion resistance. Therefore, the stainless steel material 1 according to the present embodiment is suitable for use as a design component used in an environment where high corrosion resistance is required. The design component is not particularly limited, but various components such as building materials, piping, and automotive exhaust system components such as mufflers that require designability can be mentioned.

The present invention will be described in detail below with reference to examples, but the present invention should not be construed as being limited to these.

A ferritic stainless steel having the chemical composition shown in Table 1 was hot-rolled to produce a hot-rolled sheet having a thickness of 3.0 mm. The hot-rolled sheet was annealed at 1050°C for 3 minutes, and then the oxide scale on the surface was removed by dry honing. The sheet was then cold-rolled to a thickness of 1.0 mm, and finished annealed at 1000°C for 1 minute. The sheet was then hand-polished using No. 120 and No. 240 dry abrasive papers in sequence to remove the oxide scale on the steel sheet surface. The chemical composition shown in Table 1 is shown in mass%, with the remainder being Fe and impurities. The underlines indicate that the composition is outside the scope of the present invention. In the table, REM means rare earth elements, and here, misch metal (MM) containing La, Ce, and Nd was used.

After that, in order to form a black oxide film on the surface of the above-mentioned stainless steel plate obtained, a heat treatment was performed in an air atmosphere at 1050°C for 3 minutes, and then a test piece with a width of 70 mm and a length of 120 mm was cut out. A coated conductor was attached to the end by spot welding, and the end face width of 10 mm square and the back surface were covered with Teflon (registered trademark) sealing tape so that the conductor adhesive part was hidden, and used as a test material for electrolytic treatment. The type, concentration, Fe ion concentration, temperature, current density, and amount of electricity of the electrolytic treatment solution are shown in Table 2. A glass beaker containing 400 mL of electrolytic treatment solution was immersed in the test material and a platinum counter electrode while adjusting the solution temperature in a thermostatic bath, and electrolytic treatment was performed by passing a specified current from a power source. The electrolysis method in the solution was an indirect current method.

After electrolytic treatment, the color tone of the oxide film, the maximum area of the exposed substrate, the area ratio, and the Cr content of the passive film, as well as the corrosion resistance of the test materials were measured and evaluated. The measurement and evaluation methods are explained below, and the measurement and evaluation results are shown in Table 3.

(Method of measuring color tone)
Color measurements were performed in accordance with JIS Z 8722:2009 using a spectrophotometer with a measuring diameter of 3 mmφ at five points on the black oxide coating portion of the surface of the test material, excluding the edges, and the average values were expressed as the lightness index L ^* and chromanetics indices a ^* and b ^* , which are CIELAB (L ^* a ^* b ^* color system) in accordance with JIS Z 8781-4:2013.

The conditions for measuring the color tone are as follows.
Equipment: Konica Minolta Spectrophotometer CM-700d
Light source: Pulsed xenon lamp Light receiving element: Dual 36-element silicon photodiode array Target mask: Φ3 mm
Measurement: 10° viewing angle auxiliary Illuminant: D65 daylight, color temperature 6504K
Regular reflection processing mode: SCI

(Method of measuring the area ratio and maximum area of exposed substrate)
Furthermore, five locations of the black oxide film on the surface of the test specimen, excluding the edge portion, were observed at a magnification of 200 times within an area of 1 mm x 1 mm using a digital microscope. The area of the white area after binarization was measured at each location, and the area ratio and maximum area of the exposed base material were derived by image analysis. The area ratio was the average of the five locations, and the maximum area was the maximum value of the five locations.

(Method of measuring passive film composition)
The exposed substrate portion of the test material surface was subjected to GDS analysis by glow discharge optical emission spectroscopy (GD-OES) in accordance with JIS K 0144: 2018 to measure the Cr fraction (%) of the passive film on the exposed substrate portion. Here, the Cr fraction (%) of the passive film on the exposed substrate portion was determined as the Cr concentration/(Fe concentration+Cr concentration+Mn concentration+Ti concentration)×100 at the point where the O peak intensity was 3/4 of the maximum value.

The GDS measurement conditions are as follows.
Equipment: Rigaku Corporation GDA750
Analysis diameter (anode diameter): Φ4mm
Voltage: 650V
Ar pressure: 2.8 hPa

(Method of evaluating corrosion resistance)
A test piece 12 for measurement, 50 mm wide and 100 mm long, was cut out from the test material subjected to the electrolytic treatment. The test piece for measurement was used to prepare the bonded body 11 shown in FIG. 3 as follows. FIG. 3(a) is a plan view of the bonded body, and FIG. 3(b) is a side view. First, the cut end faces of three sides of the four cut end faces of the test piece for measurement 12, excluding one short side, were covered with rubber 13 (one-liquid condensation type RTV rubber KE44 manufactured by Shin-Etsu Silicone Co., Ltd.). Next, two polyethylene tubes 15 of 20 mmφ×10 mm were placed and bonded on a Bakelite plate 14 of 70 mm×150 mm, and the test piece for measurement 12 covered with rubber 13 was bonded thereon. The bonded body 11 thus obtained was used to evaluate corrosion resistance in a salt-dry-wet combined cycle test. Specifically, the bonded body 11 was placed in a combined cycle tester so that the test specimen 12 was at an angle of 75 degrees with respect to the horizontal plane and the uncoated short side was facing downward, and then five cycles were performed, each cycle consisting of 5% salt spray (35°C, 2 hours), drying (60°C, 25% RH, 4 hours), and wetting (50°C, 95% RH, 2 hours). After the combined cycle test, the bonded body 11 was washed with water and dried, and the rust rate of the surface of the bonded body 11 was evaluated in accordance with JIS G 0595:2004. When the rating number (RN) of the exposed base portion 7 was 9.5 or more (rust area rate ≦0.05%), it was evaluated as ○ (excellent corrosion resistance), and when the rating number (RN) was less than 9.5 (rust area rate >0.05%), it was evaluated as × (insufficient corrosion resistance).

The results of each of the above evaluations are shown in Table 3.

As shown in Table 3, the stainless steel sheets of Examples 1 to 7 within the scope of the present invention met the standards for the chemical composition of the substrate, the color tone of the oxide film, and the maximum area, area ratio, and Cr fraction of the passive film in the exposed substrate, and also showed good results in the corrosion resistance test.

In contrast, the stainless steel sheets of Comparative Examples 1 to 12 were inferior to the Examples in terms of the chemical composition of the substrate, the color tone of the oxide film, the maximum area of the exposed substrate, the area ratio, the composition of the passive film, or the corrosion resistance. In Comparative Examples 1 and 2, the solution concentration of the nitric acid electrolysis was high or the solution temperature was high, so the maximum peeled area of the exposed substrate did not meet the standard. In Comparative Example 3, the amount of electricity in the nitric acid electrolysis was large, so the area ratio and maximum peeled area of the exposed substrate did not meet the standard. In Comparative Example 4, the amount of electricity was very large, so most of the black oxide film peeled off and the color tone standard of the oxide film was not met. In Comparative Example 5, the solution concentration of the nitric acid electrolysis was low, in Comparative Example 6, the Fe ion concentration in the nitric acid electrolysis solution was high, in Comparative Example 7, the solution temperature of the nitric acid electrolysis was low, and in Comparative Example 8, the amount of electricity was low, so no silver-white exposed substrate was observed. In other words, it is considered that the abnormally oxidized parts were not sufficiently removed and remained, and the evaluation criteria for corrosion resistance were not met. In Comparative Example 9, the Cr content of the passive film on the exposed substrate was low due to neutral salt electrolysis using sodium sulfate, and the corrosion resistance evaluation criteria were not met.

In Comparative Examples 10 and 11, the S and C contents were higher than the standards, respectively, so the area ratio of exposed substrate did not meet the standards. It is presumed that when sulfides or carbides are exposed on the substrate surface, abnormally oxidized areas are likely to form in the surrounding area, resulting in the formation of many abnormal areas. In Comparative Example 12, the Cr content was lower than the standards, so the corrosion resistance of the substrate was low and the area ratio and maximum area of exposed substrate exceeded the standards, so the corrosion resistance evaluation was not met.

As can be seen from the above results, the present invention can provide a ferritic stainless steel material with excellent corrosion resistance and a black oxide film.

Reference Signs List 1 Stainless steel material 3 Base material 4 Black oxide film 41 Ti/Mn concentrated region 42 Cr oxide region 5 Base material surface layer 51 Cr-depleted layer 6 Inclusions
7 Exposed base material 8 Passive film 9 Abnormally oxidized part 11 Adhesive 12 Test piece for measurement 13 Rubber 14 Bakelite plate 15 Polyethylene tube

Claims (4)

a ferritic stainless steel substrate having a composition, in mass%, of C: 0.050% or less, Si: 1.00% or less, Mn: 0.04 to 1.00%, P: 0.050% or less, S: 0.030% or less, Cr: 16.00 to 25.00%, Ni: 1.00% or less, Cu: 0.60% or less, Mo: 2.00% or less, N: 0.030% or less, Al: 0.500% or less, Ti: 0.080 to 0.500%, Nb: 0.500% or less, and a total content of Nb and Ti of 6(C+N) or more (C and N represent the contents of C and N, respectively), with the balance being Fe and impurities;
A black oxide film that satisfies the following conditions is formed on the surface: lightness index L ^* ≦45, chromanetics index −5.0≦a ^* ≦5.0, −5.0≦b ^* ≦5.0;
The black oxide coating is not formed on the exposed base material.
A ferritic stainless steel material, wherein the area ratio of the exposed substrate portion is 0.01 to 3.0%, the maximum area of the exposed substrate portion is 100 μm ² to 10,000 μm ² , and the passive film formed on the surface of the exposed substrate portion satisfies a Cr fraction of ≧40%. The ferritic stainless steel material according to claim 1, further comprising, by mass%, one or more elements selected from the group consisting of Zr: 1.00% or less, Co: 1.00% or less, V: 1.00% or less, and W: 1.0% or less. The ferritic stainless steel material according to claim 1 or 2, further comprising, by mass%, one or more elements selected from the group consisting of REM: 0.100% or less and Ca: 0.100% or less. The ferritic stainless steel material according to any one of claims 1 to 3, further comprising, by mass%, one or more elements selected from the group consisting of Sn: 0.100% or less and B: 0.0100% or less.

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