JP7526090B2 - 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 is composed mainly of Cr oxides and has high corrosion resistance, it can inevitably form partial, point-like abnormally oxidized areas.

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 Ti and Cr and a composite oxide of Mn 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 is not formed, and Fe-based oxides grow locally, and an abnormal oxidation part 9 composed of an oxide film mainly composed of Fe may be formed. 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 black oxide film 4 side and the Cr concentration is relatively lower than that of the surrounding substrate. The Cr-based black 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, the abnormally oxidized portion 9 may cause early rusting, and further improvement was 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.

The inventors conducted extensive research to solve the above problems and found that removing the abnormally oxidized parts and the inclusions and Cr-depleted layers underneath them is effective in improving the corrosion resistance of ferritic stainless steel materials with oxide films produced by oxidation treatment, and that electrolytic treatment in an aqueous solution is effective for this purpose. In other words, by subjecting ferritic stainless steel materials with oxide films on the surface, in which the composition of the ferritic stainless steel and the color tone of the oxide film are controlled, it is possible to dissolve the abnormally oxidized parts and the surface layer of the base material directly underneath the abnormally oxidized parts without dissolving the normal black oxide film, thereby improving corrosion resistance while maintaining the black color tone, which 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: 0.05% or less, Si: 1.0% or less, Mn: 0.04 to 1.00%, P: 0.05% or less, S: 0.03% or less, Cr: 16.00 to 25.00%, Ni: 1.0% or less, Cu: 0.6% or less, Mo: 2.0% or less, N: 0.03% or less, Al: 0.50% or less, Ti: 0.08 to 0.5%, Nb: 0.50% 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 as a base material, and has a lightness index L ^* ≦ 45.0, chromathetics index -5.0 ≦ a ^* ≦ 5.0, -5.0 ≦ b ^* A black oxide film satisfying the condition of ≦5.0 is formed on the surface, and the steel has exposed base material portions on which the black oxide film is not formed, the area ratio of the exposed base material portions being 0.1 to 5.0%, and the maximum area of the exposed base material portions being ^0.100 mm2 or less.

(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: 1.0% or less, Co: 1.0% or less, V: 1.0% or less, and W: 1.0% 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: 0.10% or less and Ca: 0.10% 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: 0.10% or less and B: 0.01% 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). 1A is a schematic diagram for explaining the process from removal of abnormally oxidized portions to formation of a passive film on exposed portions of the substrate during electrolytic treatment, in which (a) is a cross-sectional view showing the state in which an oxide film having an abnormally oxidized portion has been formed, (b) is a cross-sectional view showing the state in which the abnormally oxidized portion has been removed by neutral salt electrolysis, (c) is a cross-sectional view showing the state in which the Cr-deficient layer and inclusions on the substrate surface have been removed by sulfuric acid electrolysis, and (d) is a cross-sectional view showing the state in which a passive film has been formed on the substrate surface after the Cr-deficient layer and inclusions have been removed by natural oxidation after completion of electrolytic treatment. 1A and 1B are diagrams of an adhesive used in a corrosion resistance test, in which (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 this embodiment 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 The lower limit of the Si content is, but is not particularly limited to, the amount of the oxide film formed during the production of the stainless steel material. From the viewpoint of suppressing deterioration of surface quality due to peeling, the content is preferably 0.05% by mass, more preferably 0.10% by mass, and further preferably 0.15% by mass.

(Mn: 0.04 to 1.00% 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 mass%, preferably 0.025 mass%, and more preferably 0.020 mass%. On the other hand, the lower limit of the S content is not particularly limited, but A reduction in the 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 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, it will lead to a decrease in the workability of the stainless steel material, an increase in the manufacturing cost, and a decrease in the oxidation resistance. 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 are deteriorated. Therefore, the upper limit of each of the contents of Zr, Co, V and W is set to 1.00 mass%, preferably 0.80 mass%, and more preferably 0.50 mass%. On the other hand, the lower limit of each of the contents of Zr, Co, V and W is not particularly limited, but is preferably 0.001% by mass, and more preferably 0.01% by 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 stainless steel materials. If the content of REM and Ca is too high, it leads to an increase in the manufacturing cost of stainless steel. Therefore, the content of REM and Ca is The upper limit of the amount of REM and Ca is 0.100% by mass, preferably 0.080% by mass, and more preferably 0.050% by mass. On the other hand, the lower limit of REM and Ca is not particularly limited, but The content is preferably 0.0001 mass%, more preferably 0.003 mass%. REM is a collective term for 17 elements in total, including Sc, Y, and lanthanoids, and means rare earth metals. Specifically, La , Ce, Nd, etc., and among these, one kind may be contained alone or two or more kinds may be contained in combination. When two or more kinds of rare earth elements are contained, the above-mentioned REM 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 Stainless Steel Material Composition)
As described later, Fig. 2(d) 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(d), 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 a Cr-based black oxide film 4 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)
In the stainless steel material 1 according to this embodiment, the lightness index L ^* and chromanetics indices a ^* and b ^* are 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. In this specification, the numerical value indicating the color tone according to this embodiment is a value expressed as the lightness index L ^* and chromanetics indices a* and b ^* which are CIELAB (L ^* a ^* b ^* color system) according to JIS Z 8781-4:2013, which are average values obtained by performing color tone measurement according to JIS Z 8722:2009 at any ^five points. The black oxide film 4 of the stainless steel material 1 according to this embodiment has a surface in the 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 below, 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 0.1 to 5.0%, and the maximum area of the substrate exposed portion 7 is ^0.100 mm2 or less. Although the removal of the abnormally oxidized portion 9 and the like 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 0.1 to 5.0%, more preferably 0.2 to 4.0%, and even more preferably 0.3 to 3.0%. The area of each exposed base portion 7 is preferably 0.100 mm ² or less, preferably 0.090 mm ² or less, and more preferably 0.080 mm 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 9, 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 9, etc. has been removed and the passive film 8 has been formed is referred to as the exposed substrate portion 7.

These values were obtained by observing an area of 25 mm ² (5 mm x 5 mm) at any five points on the steel surface excluding the edge portion using an optical microscope. The area of the exposed substrate 7 can be obtained 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 exists is black, the area where the abnormally oxidized portion 9 remains is black or brown, and the exposed substrate 7 is observed as the silver-white color of the stainless steel substrate. Therefore, the area of the white portion can be measured by a binarization process to obtain the area of the exposed substrate 7. When obtaining the area ratio of the exposed substrate 7, the entire measured area was measured, and the arithmetic average of the values obtained from the five points was calculated. In addition, when obtaining the maximum area of the exposed substrate 7, the enlarged portion near the largest exposed substrate 7 was measured, and the maximum value of the values obtained from the five points was used.

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 base material, the color tone of the oxide film, and the area ratio and maximum area of the exposed base material are within a specified range. 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. or more 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 portion in the electrolytic treatment step.
Fig. 2(a) is a cross-sectional view showing a state in which an oxide film having an abnormally oxidized portion has been formed. 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 made of Cr is not formed, and Fe-based oxides grow locally, resulting in the formation of an abnormally oxidized portion 9 mainly made 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. This electrolysis consists of neutral salt electrolysis using a sodium sulfate aqueous solution and sulfuric acid electrolysis using a sulfuric acid aqueous solution. The electrolysis conditions are explained below.

(Neutral salt electrolysis)
Fig. 2(b) is a cross-sectional view showing the state in which the abnormally oxidized portion 9 has been removed by neutral salt electrolysis. As shown in Fig. 2(b), in neutral salt electrolysis, the abnormally oxidized portion 9 in the oxide film is removed by dissolving the abnormally oxidized portion 9 in the sodium sulfate aqueous solution. As a result, the surface of the base material 3 is exposed in the area where the abnormally oxidized portion 9 has been removed.
The concentration of the sodium sulfate aqueous solution used in the neutral salt electrolysis is 170 to 230 g/L, and the solution temperature is 70 to 85° C. If the solution concentration or temperature is too low, the Fe-based oxides do not dissolve sufficiently, making it difficult to remove the abnormally oxidized portion 9. If the solution concentration or temperature is too high, the black oxide film 4 dissolves, causing a deterioration in color tone.
The quantity of electricity during neutral salt electrolysis is 500 to 1000 C/ ^m2 . The current density at this time is 80 A/ ^m2 or less. If the quantity of electricity is small, the Fe-based oxides do not dissolve sufficiently, making it difficult to remove the abnormally oxidized parts 9. If the quantity of electricity is large, the black oxide film 4 dissolves, causing a deterioration in color tone.

In such neutral salt electrolysis, the normally formed black oxide film 4 is hardly dissolved and remains almost as it is, and only the locally formed abnormally oxidized parts 9 are removed. Here, Fe-based oxides are more soluble than Cr oxides (Cr ₂ O ₃ ), and are dissolved by immersing the material in a neutral salt solution and passing a small current through it. Therefore, by controlling the amount of electricity to be small, it is possible to preferentially dissolve the Fe-based oxides and remove only the abnormally oxidized parts 9.

(Sulfuric acid electrolysis)
Fig. 2(c) is a cross-sectional view showing the state in which the Cr-depleted layer and inclusions in the substrate surface layer have been removed by sulfuric acid electrolysis. As shown in Fig. 2(c), in the sulfuric acid electrolysis performed after the neutral salt electrolysis, the abnormally oxidized portion 9 is removed and the exposed substrate surface layer 5 is dissolved in the sulfuric acid aqueous solution, thereby removing the Cr-depleted layer 51 and inclusions 6.
The concentration of the sulfuric acid aqueous solution used in sulfuric acid electrolysis is 45 to 90 g/L, and the solution temperature is 30 to 50° C. If the solution concentration or temperature is too low, it becomes difficult to remove the Cr-deficient layer 51 and the inclusions 6. On the other hand, if the solution concentration or temperature is too high, excessive dissolution of the base material 3 may occur, causing large portions of the black oxide film 4 to peel off, which may cause a deterioration in the appearance.
The quantity of electricity during sulfuric acid electrolysis is 800 to 1500 C/ ^m2 , and the current density is 80 A/ ^m2 or less. If the quantity of electricity is small, it becomes difficult to remove the Cr-deficient layer 51 in the defective portion and the inclusions 6. If the quantity of electricity is large, excessive dissolution of the base material 3 may occur, causing the black oxide film 4 to peel off in large areas, which causes a deterioration in the appearance.

In this type of sulfuric acid electrolysis, the normally formed black oxide film 4 is hardly dissolved and remains almost as it is, while the surface layer of the base material 3 that is exposed after the removal of the abnormally oxidized portion 9 after neutral salt electrolysis dissolves, removing the Cr-deficient layer 51 and inclusions 6. Here, the base material 3 of the stainless steel material 1 itself is more soluble than the corrosion-resistant Cr-based black oxide film 4, and dissolves when immersed in a sulfuric acid aqueous solution and a small current is passed through it. Therefore, by controlling the amount of electricity to be small, it is possible to preferentially dissolve only the base material surface layer 5 that is exposed after the removal of the abnormally oxidized portion 9, thereby removing the Cr-deficient layer 51 and inclusions 6.

(After electrolysis)
Fig. 2(d) is a cross-sectional view showing a state in which a passive film is formed on the surface of the substrate after the Cr-depleted layer and inclusions are removed by natural oxidation after the electrolytic treatment. As shown in Fig. 2(d), the surface of the substrate 3 after the Cr-depleted layer and inclusions are removed by sulfuric acid electrolysis is exposed to the atmosphere after the electrolytic treatment, whereby a passive film 8 is formed by natural oxidation.
Through the above steps, the ferritic stainless steel material 1 according to this embodiment, which has excellent corrosion resistance and has a black oxide film 4 on its surface, is manufactured. As described above, the stainless steel material 1 according to this embodiment has the abnormally oxidized portion 9 and the Cr-depleted layer 51 and inclusions 6 present thereunder removed, so that the surface vicinity of the stainless steel material 1 has a cross section as shown diagrammatically in Fig. 2(d). In other words, Fig. 2(d) is a cross-sectional view diagrammatically showing the surface vicinity in a cross section of the stainless steel material 1 according to this embodiment cut in the plate thickness direction.

The above-mentioned process allows for improved corrosion resistance while maintaining the desired appearance, so the stainless steel material 1 according to this embodiment has a black color tone and excellent corrosion resistance. 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.

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 finish-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, 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 side 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 electrolytic treatment solution concentration was 200 g / L for neutral salt solution and 85 g / L for sulfuric acid solution. The electrolytic treatment was performed by immersing the test material and platinum counter electrode in a glass beaker containing 400 mL of electrolytic treatment solution while adjusting the solution temperature in a thermostatic bath, and passing a specified current from a power source. The solution temperature, current density, and quantity of electricity for electrolytic treatment are shown in Table 2. The electrolysis method in the solution was an indirect current method.

After electrolytic treatment, the test materials were evaluated for the color tone of the oxide film, the maximum area and area ratio of the exposed base material, and corrosion resistance. The evaluation methods are explained below, and the 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 edges, were observed at a magnification of 200 times within an area of 25 ^mm2 (5 mm x 5 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 value of the five locations, and the maximum area was the maximum value of the five locations.

(Method of measuring 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 thus obtained bonded body 11 was used to evaluate the 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 piece 12 was at an angle of 75 degrees to the horizontal plane and the uncoated short side was on the lower side, and then three 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 bonded body surface was evaluated in accordance with JIS G 0595:2004. When the rating number (RN) of the exposed base material 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). Note that the underlined parts indicate that they are outside the scope of the present invention.

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 and area ratio of the exposed substrate, and also showed good corrosion resistance in the corrosion resistance test.

In contrast, the stainless steel sheets of Comparative Examples 1 to 8 were inferior to the Examples in either the chemical composition of the substrate, the color tone of the oxide film, the maximum area and area ratio of the exposed substrate, or the corrosion resistance. Comparative Example 1 was not subjected to electrolytic treatment, so no silver-white exposed substrate was observed, and it did not meet the evaluation criteria for corrosion resistance. This is thought to be due to the presence of abnormally oxidized areas. Comparative Examples 2 and 3 did not meet the color tone requirements because the solution temperature or the amount of electricity used in the neutral salt electrolytic treatment was high, which caused the black oxide film to dissolve.

Comparative Examples 4 and 5 did not meet the evaluation criteria for corrosion resistance because the solution temperature in the neutral salt electrolysis treatment was low or the amount of electricity was small. This is presumably due to insufficient removal of abnormally oxidized areas. Comparative Examples 6 and 7 had a high solution temperature in the sulfuric acid electrolysis treatment or a large amount of electricity, which caused the dissolution of the base material to proceed more than necessary, so the area ratio or maximum area of exposed base material did not meet the criteria. Comparative Examples 8 and 9 did not meet the evaluation criteria for corrosion resistance because the solution temperature in the sulfuric acid electrolysis treatment was low or the amount of electricity was small. This is presumably due to insufficient removal of Cr-deficient layers and inclusions.

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. This is presumably because when sulfides or carbides are exposed on the substrate surface, abnormally oxidized areas are likely to form around them, resulting in the formation of many abnormal areas. In Comparative Example 12, the Cr content was lower than the standards, so the substrate had low corrosion resistance, and the area ratio and maximum area of exposed substrate exceeded the standards, so the corrosion resistance evaluation criteria were 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 Ferritic 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 ^* ≦45.0, chromanetics index −5.0≦a ^* ≦5.0, −5.0≦b ^* ≦5.0;
The black oxide coating is not formed on the exposed base material.
The ferritic stainless steel material has an area ratio of 0.1 to 5.0% and a maximum area of the exposed base material of 0.100 ^mm2 or less. 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: 1.00% 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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