JP7564707B2 - Ferritic Stainless Steel - Google Patents

Description

The present invention relates to a ferritic stainless steel material with excellent design and corrosion resistance, which has a black oxide film on the surface and a pattern formed by removing part of it to expose the stainless steel base.

Stainless steel is not only a material with excellent corrosion resistance, but it is also used for interior and exterior building materials, taking advantage of its lustrous silver-white surface. In recent years, in order to enhance the design of stainless steel, it has become common to impart colors, typically black, to stainless steel using methods such as chemical coloring, painting, and oxidation treatment. Furthermore, colored stainless steel may be polished or etched to partially expose the stainless steel base material and create a pattern.

Patent Document 1 describes a pattern forming method in which a stainless steel surface, which has been given a color tone using a chemical coloring method, is mechanically polished to roughen the surface into a specified pattern. In addition, Patent Document 2 describes a method for manufacturing a plated steel sheet in which a pattern is imparted to the surface of a Zn-plated steel sheet on which a black oxide film has been formed by an oxidation treatment method by mechanically polishing the surface.

Patent Document 3 and Patent Document 4 describe black stainless steel plates in which a black oxide film is formed on the stainless steel surface by an oxidation treatment method.

Japanese Unexamined Patent Publication No. 61-213380 JP 2017-218647 A JP 2019-178392 A JP 2018-135591 A

However, when a black oxide film is formed on the surface of stainless steel as a base material, and then a portion of the base material is polished to expose it to form a pattern, the design is attractive, but there is a problem in that the stainless steel base material exposed by polishing has lower corrosion resistance than the surrounding area covered with the black oxide film, and often rusts early.

The present invention has been made to solve the above problems, and aims to provide a ferritic stainless steel material with excellent design and corrosion resistance, which has a black oxide film on the surface and is patterned by removing part of the black oxide film to expose the stainless steel base material.

As a result of intensive research by the inventors to solve the above problems, they have discovered that the cause of the decrease in corrosion resistance that occurs when a stainless steel material having a black oxide film on its surface is polished to form a pattern is that part of the oxide film is removed by polishing, exposing a layer (inner oxide layer) containing a large number of inner oxides such as Ti oxide (TiO ₂ ) and Al oxide (Al ₂ O ₃ ) immediately below the oxide film, and that the composition of the passive film formed on the exposed base surface due to exposure to high temperatures during polishing has a low Cr concentration and a high Fe concentration.

In other words, it was discovered that the deterioration of corrosion resistance of the exposed substrate can be suppressed by controlling the composition of the ferritic stainless steel, removing the oxide film and inner oxide layer when exposing the substrate by polishing, and increasing the Cr fraction in the passive film formed on the exposed substrate. Furthermore, it was discovered that the removal state of the oxide film and inner oxide layer can be confirmed by checking the difference in brightness and gloss between the oxide film and the exposed substrate within a specific range, which led to the present invention. The gist of the present invention, which was 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 surface of the substrate has a lightness index L ^* ≦ 45.0, a chromathetics index -5.0 ≦ a The oxide film portion is formed with a black oxide film that satisfies the following conditions: ^b* ≦5.0, -5.0≦b ^* ≦5.0, and gloss GS(60°)≦40.0, and an exposed substrate portion on which the oxide film is not formed, and a pattern is formed by the oxide film portion and the exposed substrate portion, the difference in lightness index between the oxide film portion and the exposed substrate portion being ΔL ^* ≧30.0 and the difference in gloss ΔGS(60°)≧130.0, and the passive film formed on the surface of the exposed substrate portion satisfies a Cr fraction ≧30%.

(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: 0.0100% or less in the composition of the stainless steel.

According to the above aspect of the present invention, it is possible to provide a ferritic stainless steel material with excellent design and corrosion resistance, which has a black oxide film on the surface and is patterned by removing part of the black oxide film to expose the base material.

FIG. 1 is a schematic diagram for explaining the process until a pattern is formed, in which (a) is a cross-sectional view showing the vicinity of the surface layer of a substrate before a black oxide film is formed, (b) is a cross-sectional view showing the vicinity of the surface layer of a substrate after a black oxide film has been formed, and (c) is a cross-sectional view showing the vicinity of the surface layer of a stainless steel material after a pattern has been formed by polishing. FIG. 2 shows a top view and a side view of the bonded structure used in the corrosion resistance test.

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 this 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 consisting of, 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. 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, "impurities" refer to components that are mixed in during the industrial production of the base ferritic stainless steel due to various factors in the manufacturing process, such as raw materials such as ores and scraps, and are acceptable within the scope of not adversely affecting the present invention. For example, impurities also include unavoidable impurities.

The base material of the stainless steel material according to the present 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. The base material of the stainless steel material according to the present 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. The base material of the stainless steel material according to the present 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 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 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 materials, such as weldability and workability. If the P content is too high, the above properties may be reduced. 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 value is not particularly limited, but is preferably 0.001 mass %, 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 will be described later, Ti is required to form 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 formed. The formation of an oxide film of N 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. % by mass, preferably 0.028% by mass, and more preferably 0.025% by mass. On the other hand, the lower limit of the N content is not particularly limited, but reducing the N content leads to a reduction in refining costs. 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 stainless steel material when a black oxide film is formed. On the other hand, if the Al content is too high, The toughness of the stainless 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 of 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 will decrease. Therefore, the upper limit of the Nb content is 0.500 mass%, preferably 0.480 mass%, and more preferably 0.450 mass%. Also, if the Ti content is too high, the workability and surface quality of the stainless steel material will decrease. Therefore, 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 Nb and Ti content is controlled based on the relationship with the C and N contents, which reduce intergranular corrosion resistance. Specifically, the lower limit of the total Nb and Ti content is 6 (C+N), preferably 7 (C+N), and more preferably 8 (C+N). Here, C and N represent the C and N contents, respectively. Also, if the Ti content is too low, the above-mentioned effect may not be sufficiently obtained. Therefore, the lower limit of the Ti content 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 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 each of the amounts is 0.100% by mass, preferably 0.080% by mass, and more preferably 0.05% by mass. On the other hand, the lower limit of each of the 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., among which 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 is The total content of these rare earth elements is meant. 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. 1(c) is a cross-sectional view in the plate thickness direction that typically shows the vicinity of the surface layer of the stainless steel material according to this embodiment. Referring to FIG. 1(c), the stainless steel material 1 according to this embodiment has an oxide film 4 having a black color tone on the surface, and the black oxide film 4 is partially removed by polishing to have a design consisting of a pattern intentionally formed for decorative or other purposes. The stainless steel material according to this embodiment has a ferritic stainless steel base material 3 (hereinafter simply referred to as "base material") and an oxide film 4 having a black color tone formed on the surface of the base material 3. The stainless steel material 1 according to this embodiment also has a base material exposed portion 7 in which a part of the black oxide film 4 and the internal oxide layer 5 immediately below it have been removed to partially expose the base material. A passive film 8 is formed on the base material exposed portion 7. On the surface of the stainless steel material 1 according to this embodiment, a pattern is formed by an oxide film portion 41 on which the black oxide film 4 is formed, and a base material exposed portion 7 in which the black oxide film 4 has been removed to expose the silver-white base material 3. The pattern may be, for example, a stripe or wave pattern with a width of 0.5 to 20 mm in the exposed base portion 7, a checkered pattern (lattice pattern) of similar width, or a polka dot pattern with a diameter of 0.5 to 20 mm in the exposed base portion, or a combination of these.

(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 according to JIS Z 8722:2009 at any five points, averaging the values, and expressing the lightness index L ^* and chromanetics indices a ^* and b ^* in CIELAB (L ^* a ^* b ^* color system) according to JIS Z 8781-4:2013. The oxide film 4 (oxide film portion 41) of the stainless steel material 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.

(Glossiness of oxide film)
Furthermore, the stainless steel material 1 according to this embodiment has a glossiness GS (60°) of the surface (surface other than the exposed substrate portion) on which the black oxide film 4 is formed, which is within a specific range. This value is an average value obtained by performing glossiness measurements at any five points in accordance with JIS Z8741:1997. The glossiness GS (60°) indicates that the measurement was performed by irradiating light at an incident angle (60°). The oxide film 4 (oxide film portion 41) of the stainless steel material according to this embodiment has a surface with a GS (60°) of ≦40.0. This allows the black color to be obtained while suppressing the generation of excessive reflected light.

(Brightness difference and gloss difference)
The stainless steel material 1 according to this embodiment is polished so as to remove the oxide film 4 and the inner oxide layer 5 in the exposed substrate portion 7. As a result, the lightness difference ΔL ^* and the gloss difference ΔGS (60°) between the oxide film portion 41 and the exposed substrate portion 7 fall within a specific range. The method for measuring the lightness and gloss of the exposed substrate portion 7 is the same as the method for measuring the color tone and gloss of the oxide film described above.

The difference in lightness index (hereinafter, sometimes simply referred to as "lightness difference") ΔL ^* is the difference between the lightness index L ^* 1 of the oxide coating 4 (oxide coating portion 41) and the lightness index L ^* 2 of the exposed substrate portion 7. In this embodiment, L ^* 2 is usually a larger value than L ^* 1, and therefore ΔL ^* =L ^* 2-L ^* 1≧30.0 is satisfied. Also, preferably ΔL ^* ≧32.0, and more preferably ΔL ^* ≧35.0. The color tone difference ΔGS(60°) is the difference between the glossiness GS(60°)1 of the oxide coating 4 (oxide coating portion 41) and the glossiness GS(60°)2 of the exposed substrate portion 7. In this embodiment, GS(60°)2 is usually a larger value than GS(60°)1, and therefore ΔGS(60°)=GS(60°)2-GS(60°)1≧130.0 is satisfied. Preferably, ΔGS(60°)≧135.0, and more preferably ΔGS(60°)≧140.0 are satisfied. In other words, if the lightness difference ΔL ^* and the gloss difference ΔGS(60°) are within the above-mentioned ranges, the oxide film 4 and the inner oxide layer 5 are sufficiently removed in the exposed substrate portion 7. The closer the lightness difference ΔL ^* and the gloss difference ΔGS(60°) are to the above-mentioned preferred ranges, or the more preferred ranges, the more reliably the inner oxide layer 5 is removed.

(Composition of the passive film on exposed substrate)
On the surface of the substrate exposed portion 7 of the stainless steel material 1 according to this embodiment, the oxide film 4 and the internal oxide layer 5 immediately below the oxide film 4 are removed in a polishing process described later, and then the passive film 8 is formed by natural oxidation. Therefore, in the area where the oxide film 4 and the internal oxide layer 5 are removed and the oxide film 4 is not formed, the surface of the substrate 3 is not exposed, but the passive film 8 formed on the surface of the substrate 3 is exposed. However, the passive film 8 is extremely thin, having a thickness of about several nm, and when the surface is viewed from above, the silver-white color of the substrate 3 is seen almost as it is. Therefore, in this specification, the area where the oxide film 4 and the internal oxide layer 5 are removed by polishing and the passive film 8 is formed is referred to as the substrate exposed portion 7.

The passive film 8 formed on the surface of the exposed substrate 7 has a Cr fraction of 30% or more. When removing the oxide film 4 and the internal oxide layer 5 by polishing, the composition of the passive film 8 on the surface of the exposed substrate 7 can be made to have a Cr fraction of 30% or more by polishing so that the temperature of the stainless steel surface (temperature of the substrate surface) does not exceed 400°C. 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 K0144:2018. In this case, the analysis range is limited so that it includes only the exposed substrate 7. The Cr fraction (%) of the passive film 8 on the surface of the exposed substrate 7 is calculated by dividing the Cr concentration at the point where O (oxygen) is three-quarters of the maximum value in the obtained surface profile by (Fe concentration + Cr concentration + Mn concentration + Ti concentration) x 100. The stainless steel material 1 according to this embodiment has sufficient protective properties (corrosion resistance) because the Cr fraction of the passive film 8 on the surface of the exposed substrate 7 is 30% or more. Therefore, it is possible to prevent the design from being impaired by corrosion, oxidation, etc. The Cr fraction of the passive film 8 on the surface of the exposed substrate 7 is preferably 35% or more, more preferably 40% or more. This makes it possible to provide a stainless steel material 1 with superior design and corrosion resistance.

The stainless steel material 1 according to this embodiment has a high designability while suppressing a decrease in corrosion resistance because the chemical composition of the substrate, the color tone of the oxide film, the difference in brightness and glossiness between the oxide film portion and the exposed substrate portion, and the passive film composition of the exposed substrate portion are all within a specified range. Therefore, this stainless steel material 1 is suitable for use as a design component. Design components include, but are not limited to, building materials (especially interior components such as decorative panels) and various components that require designability, such as piping.

<2. Manufacturing method of ferritic stainless steel material>
The stainless steel material 1 according to the present embodiment described above can be manufactured, for example, by the method described below.
A preferred method for producing the stainless steel material 1 having the black oxide film 4 according to this embodiment is as follows:
(1) an oxidation treatment step of heating a ferritic stainless steel material (base material 3) having the above-mentioned chemical composition in an oxidizing atmosphere at a temperature of 1000° C. to 1100° C. (e.g., 1050 to 1080° C.) for 1 minute to 5 minutes (e.g., 2 to 3 minutes) to form an oxide film 4;
(2) A polishing process for removing the oxide film 4 and the inner oxide layer 5 formed immediately below the oxide film 4 (on the substrate 3 side near the interface with the oxide film 4) by polishing the surface of the stainless steel material on which the oxide film 4 has been formed while maintaining the surface temperature at 400° C. or less.
1 is a schematic diagram for explaining the flow up to the formation of a pattern. Each step will be explained below with reference to FIG.

(Preparation of stainless steel material)
FIG. 1(a) is a cross-sectional view showing the vicinity of the surface layer of a substrate before a black oxide film is formed.
First, prior to each of the above-mentioned steps, a ferritic stainless steel material is prepared. The material may have a composition containing the above-mentioned elements and may be manufactured by a known method.
For example, a cold-rolled sheet is produced by hot-rolling a ferritic stainless steel slab having the above-mentioned chemical composition, annealing and pickling the resulting hot-rolled coil, and then cold-rolling it at a cold rolling rate of 50% or more. Next, the surface of the resulting cold-rolled sheet is finish-polished with an abrasive belt of #180 or more to obtain the base material 3 shown in Fig. 1(a). By using the base material 3 with the surface 2 polished in this manner, the appearance unevenness of the black oxide film 4 formed on the base material surface 2 in the subsequent process is suppressed.

(Oxidation treatment process)
FIG. 1(b) is a cross-sectional view showing the vicinity of the surface layer of the substrate after the black oxide film has been formed.
1(b), this step is a step of forming a black oxide film 4 on the surface 2 of a base material 3 by heat treatment. The base material 3 is subjected to an oxidation treatment (annealing) in which the base material 3 is heated in an oxidizing atmosphere at a temperature of 1050 to 1080°C for 2 to 3 minutes, thereby forming a black oxide film 4 on the base material surface 2. The thickness of this oxide film 4 is 200 to 1000 nm, and preferably 250 to 900 nm. The thickness of the oxide film 4 can be adjusted by the annealing temperature and annealing time.
An inner oxide layer 5 is formed on the inner surface side near the surface layer of the base material 3. The inner oxide layer 5 is a region in which a large number of inner oxides 6 such as _TiO2 and _Al2O3 exist. The thickness of the inner oxide layer 5 is about 400 to 1000 _nm .

(Polishing process)
Fig. 1(c) is a cross-sectional view showing the vicinity of the surface layer of the stainless steel material after a pattern has been formed by polishing. In other words, Fig. 1(c) is a cross-sectional view showing a schematic view of the vicinity of the surface layer in a cross section of the stainless steel material according to this embodiment cut in the plate thickness direction.
1(c), this step is a step of removing an arbitrary portion of the black oxide film 4 and the internal oxide layer 5 immediately below the oxide film 4 (on the substrate 3 side near the interface with the oxide film 4) by polishing. By removing the black oxide film 4 and the internal oxide layer 5 immediately below the black oxide film 4 at an arbitrary portion on the substrate surface 2 to expose the silver-white substrate 3, a pattern is formed on the surface of the stainless steel material 1 by forming a black oxide film portion 41 and a silver-white substrate exposed portion 7, and a design is imparted. The oxide film 4 and the internal oxide layer 5 can be removed by polishing. The surface 2 of the substrate 3 from which the oxide film 4 and the internal oxide layer 5 have been removed by polishing is exposed to the atmosphere after polishing is completed, and a thin passive film 8 of about several nm is formed by natural oxidation.

In the stainless steel material 1 according to this embodiment, the oxide film 4 having the desired black color tone and the inner oxide layer 5 are removed from the exposed substrate 7, so that the brightness difference and gloss difference between the oxide film 41 and the exposed substrate 7 are within a predetermined range, and the stainless steel material 1 has excellent design. Furthermore, the Cr fraction of the passive film 8 formed on the surface of the exposed substrate 7 is 30% or more, so that the stainless steel material 1 has excellent corrosion resistance. In order to make the composition of the passive film 8 have a Cr fraction of 30% or more, it is necessary to polish the steel sheet so that the surface temperature of the steel sheet (temperature of the substrate surface 2) during polishing is kept at 400°C or less. For example, by polishing the steel sheet while cooling (water cooling, oil cooling, air cooling, etc.) and not applying too much load, the surface temperature of the steel sheet during polishing can be kept at 400°C or less. If the surface temperature of the steel sheet during polishing exceeds 400°C, a passive film 8 with a low Cr fraction and a high Fe fraction is formed, and the corrosion resistance of the exposed substrate 7 may deteriorate.

By the above process, the stainless steel material 1 according to this embodiment can be manufactured, which has a pattern on the surface and has excellent design and corrosion resistance. Furthermore, the stainless steel material 1 according to this embodiment can be used not only for decorative panels such as wall materials, but also for bending, press forming, and welding, and can therefore be widely applied to steel pipes, processed products, etc.

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. This hot-rolled sheet was annealed at 1050°C for 3 minutes, and then the oxide scale on the surface was removed using dry honing. It was then cold-rolled to a thickness of 1.0 mm, and finished annealed at 1000°C for 1 minute. It 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 in Table 1 is shown in mass%, with the remainder being Fe and impurities. In addition, REM in the table means rare earth elements, and here, misch metal (MM) containing La, Ce, and Nd was used.

Then, in order to blacken the surface of the above obtained stainless steel plate (to form a black oxide film on the surface of the stainless steel plate), heat treatment was performed in an air atmosphere at the temperature and time shown in Table 2. Then, a test piece 50 mm wide and 100 mm long was cut out, and the central 10 mm wide section was mechanically polished while cooling to remove the black oxide film and form an exposed base material portion, which was used as the test piece. In Comparative Example 3, mechanical polishing was performed without cooling. Comparative Example 2 had a shorter polishing time than Example 1, and Comparative Example 1 had an even shorter polishing time than Comparative Example 2.

(Color and gloss)
For five points of the oxide film part on the surface of the test piece where the black oxide film is formed, a color measurement was performed in accordance with JIS Z 8722: 2009 using a spectrophotometer with a measurement diameter of 3 mmφ, and the average value was expressed as the lightness index L ^* and chromanetics indexes a ^* and b ^* , which are CIELAB (L ^* a ^* b ^* color system) in accordance with JIS Z 8781-4: 2013. Similarly, for five arbitrary points of the oxide film part on the surface of the test piece where the black oxide film is formed, a gloss measurement was performed in accordance with JIS Z 8741 using a gloss meter, and the average value was expressed as GS (60 °). Similarly, color measurement and gloss measurement were performed for five arbitrary points of the exposed base material, and the lightness difference ΔL ^* and gloss difference ΔGS (60 °) from the oxide film (oxide film part) were derived. When the lightness difference ΔL ^* was 30 or more, it was evaluated as pass (excellent lightness difference), and when it was 30 or less, it was evaluated as fail (insufficient lightness difference). In addition, when the gloss difference ΔGS(60°) was 130 or more, it was evaluated as pass (excellent gloss difference), and when it was 130 or less, it was evaluated as fail (insufficient gloss difference).

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

The conditions for measuring the gloss level are as follows.
Equipment: Nippon Denshoku Co., Ltd. Handy gloss meter PG1M
Light source: tungsten lamp Detector: photodiode Incident angle: 60°

(Passive Film Composition)
Using the above test pieces, the composition of the passive film on the exposed substrate was analyzed 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. In the GDS analysis (GD-OES), the Cr fraction (%) of the passive film on the exposed substrate 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

(Corrosion resistance)
The above test piece was used to prepare the bonded body 11 shown in FIG. 2 as follows. FIG. 2(a) is a top view of the bonded body, and FIG. 2(b) is a side view. First, the cut end faces of three sides of the four cut end faces of the test piece 12 except for 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 12 covered with rubber 13 was bonded on the two polyethylene tubes. The thus obtained bonded body 11 was used to evaluate the corrosion resistance in accordance with the salt spray test specified in JIS Z2371:2015. 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 5% salt spray was performed for 48 hours. After the salt spray 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 G0595:2004. When the rating number (RN) of the exposed base material was 8 or more (rust area rate ≦ 0.062%), the corrosion resistance was evaluated as excellent (○), and when the rating number (RN) was less than 8 (rust area rate > 0.062%), the corrosion resistance was evaluated as insufficient (×).

The results of each of the above evaluations are shown in Table 2.

As shown in Table 2, the stainless steel sheets of Examples 1 to 7 within the scope of the present invention met the chemical composition of the substrate and the color tone standard for the oxide film portion. In addition, the lightness difference ΔL ^* and gloss difference ΔGS (60°) between the oxide film portion and the exposed substrate portion met the standards (i.e., the oxide film and the inner oxide layer were sufficiently removed), and the Cr fraction of the passive film in the exposed substrate portion was 30% or more. Therefore, no rust was observed in the corrosion resistance test and good corrosion resistance was demonstrated.

In contrast, the stainless steel sheets of Comparative Examples 1 to 7 were inferior to the Examples in either color tone or the corrosion resistance of the passive film at the exposed base. Comparative Example 1 met the color tone standard of the oxide film, but polishing was insufficient, the oxide film was not sufficiently removed, and the lightness difference ΔL ^* and gloss difference ΔGS (60°) did not meet the standard. Comparative Example 2 met the color tone of the oxide film, but polishing was insufficient, and the inner oxide layer was not sufficiently removed, so the gloss difference ΔGS (60°) did not meet the standard, and the corrosion resistance was also insufficient. The fact that the inner oxide layer was not sufficiently removed and remained was confirmed by the appearance of a Ti peak in the GDS analysis. Comparative Example 3 was polished without cooling, so the surface temperature of the base during polishing exceeded 400° C., and therefore the Cr fraction of the passive film at the exposed base was low, and the corrosion resistance was insufficient. In Comparative Example 4, the Cr content was low, and therefore the lightness difference ΔL ^* , gloss difference ΔGS (60°), and Cr fraction of the passive film met the standards, but the corrosion resistance was insufficient. Comparative Example 5 had a high Cr content, Comparative Example 6 had a low Ti content, and Comparative Example 7 had a low Mn content, so the oxide film color tone standards were not met.

As can be seen from the above results, the present invention can provide a ferritic stainless steel material with excellent design and corrosion resistance, which has a black oxide film on the surface and is patterned by removing part of the oxide film to expose the base material.

REFERENCE SIGNS LIST 1 Ferritic stainless steel material 2 Substrate surface 3 Substrate 4 Oxide film 41 Oxide film portion 5 Inner oxide layer 6 Inner oxide 7 Exposed substrate portion 8 Passive film 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 surface of the substrate has an oxide film portion on which a black oxide film satisfying lightness index L ^* ≦45.0, chromanetics indexes −5.0≦a ^* ≦5.0, −5.0≦b ^* ≦5.0, and gloss GS(60°)≦40.0 is formed, and a substrate exposed portion on which the oxide film is not formed, and a pattern is formed by the oxide film portion and the substrate exposed portion,
a difference in lightness index ΔL ^* between the oxide film portion and the exposed substrate portion satisfies ΔL≧30.0 and a difference in gloss ΔGS(60°) ≧130.0, and the passive film formed on the surface of the exposed substrate portion satisfies a Cr fraction ≧30%.
Here, the Cr fraction (%) is expressed as the Cr concentration at the point where O (oxygen) is three-quarters of the maximum value in a surface profile obtained by measurement using glow discharge optical emission spectrometry (GD-OES) in accordance with JIS K0144:2018/(Fe concentration+Cr concentration+Mn concentration+Ti concentration)×100, which is a ferritic stainless steel material. 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.

Images