Stainless steel material for blackening heat treatment and method for manufacturing the same, and method for manufacturing black stainless steel material

Abstract

This invention provides a stainless steel material for blackening heat treatment that offers excellent workability, minimizes exposure of the base material during processing, and provides good corrosion resistance even if the base material is partially exposed during processing. [Solution] A stainless steel material for blackening heat treatment having a composition on a mass basis containing C: 0.100% or less, Si: 1.00% or less, Mn: 0.05~1.00%, P: 0.100% or less, S: 0.100% or less, Cr: 16.00~25.00%, Ni: 1.00% or less, Cu: 1.00% or less, Mo: 2.00% or less, N: 0.100% or less, Ti: 0.08~0.50%, with the remainder being Fe and impurities, having a surface Vickers hardness of 210 HV or more, and the difference between the Vickers hardness at a position 1 / 8 of the thickness from the surface and the Vickers hardness at a position 1 / 2 of the thickness from the surface is 40 HV or less.

Description

[Technical Field]

[0001] This invention relates to stainless steel material for blackening heat treatment, a method for manufacturing the same, and a method for manufacturing black stainless steel material. [Background technology]

[0002] Stainless steel is a material with excellent corrosion resistance, and its lustrous silvery-white surface makes it suitable for use in various parts such as interior and exterior building materials and exhaust system components. Furthermore, to enhance the aesthetic appeal of stainless steel, it is often given various colors, such as black, using methods such as chemical coloring, painting, and oxidation treatment. For example, Patent Document 1 describes a black stainless steel material in which a black film (oxide film) has been formed on the surface of the stainless steel material by an oxidation treatment method. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2019-178392 [Overview of the Initiative] [Problems that the invention aims to solve]

[0004] When manufacturing various products using black stainless steel, various processes such as bending, welding, and polishing are performed. Conventional black stainless steel materials, such as those described in Patent Document 1, do not have sufficient processability. Depending on the processing conditions (for example, bending with a large bending radius), the oxide film formed on the surface of the stainless steel material may crack or peel off, partially exposing the base material. In the parts of the black stainless steel material where the base material is exposed, the oxide film is not present on the surface, resulting in reduced corrosion resistance and causing corrosion.

[0005] The present invention has been made to solve the above problems, and an object thereof is to provide a method for manufacturing a black stainless steel material that is excellent in workability, difficult for the base material to be exposed during processing, and has good corrosion resistance even if the base material is partially exposed during processing. Another object of the present invention is to provide a stainless steel material for blackening heat treatment that can produce a black stainless steel material having the above characteristics by blackening heat treatment, and a method for manufacturing the same.

Means for Solving the Problems

[0006] As a result of intensive research on black stainless steel materials, the present inventors have found that the above problems can be solved by forming an oxide film having predetermined characteristics on the surface of a base material having a predetermined composition and average crystal grain size. Further, the present inventors have found that this black stainless steel material can be produced by subjecting a stainless steel material for blackening heat treatment having a predetermined composition and characteristics to blackening heat treatment under predetermined conditions. The present invention has been completed based on these backgrounds.

[0007] That is, the present invention contains, on a mass basis, C: 0.100% or less, Si: 1.00% or less, Mn: 0.05 to 1.00%, P: 0.100% or less, S: 0.100% or less, Cr: 16.00 to 25.00%, Ni: 1.00% or less, Cu: 1.00% or less, Mo: 2.00% or less, N: 0.100% or less, Ti: 0.08 to 0.50%, and the balance is Fe and impurities, the Vickers hardness of the surface is 210 HV or more, a stainless steel material for blackening heat treatment in which the difference between the Vickers hardness at a position 1 / 8 of the thickness from the surface and the Vickers hardness at a position 1 / 2 of the thickness from the surface is 40 HV or less.

[0008] Further, the present invention provides the stainless steel material for blackening heat treatment, wherein the O2 concentration is 1 to 10% by volume, the water vapor concentration is 5 to 20% by volume, and the following formula (1): 2 × O2 concentration + water vapor concentration ···(1) It is a method for manufacturing a black stainless steel material by performing a blackening heat treatment at a temperature of 900 to 1100 °C for 30 to 120 seconds in an atmosphere where the value represented by is 15 to 30.

[0009] Further, the present invention is a method for manufacturing a stainless steel material for blackening heat treatment, comprising: Based on mass, C: 0.100% or less, Si: 1.00% or less, Mn: 0.05 to 1.00%, P: 0.100% or less, S: 0.100% or less, Cr: 16.00 to 25.00%, Ni: 1.00% or less, Cu: 1.00% or less, Mo: 2.00% or less, N: 0.100% or less, Ti: 0.08 to 0.50%, and the balance being Fe and impurities. After hot rolling, annealing, and pickling the slab having such a composition, single cold rolling or two or more cold rollings sandwiching annealing and pickling are performed. It is a method for manufacturing a stainless steel material for blackening heat treatment, controlling the average crystal grain size before the single cold rolling or the average crystal grain size before the final cold rolling among the two or more cold rollings to 300 μm or less.

Effects of the Invention

[0010] According to the present invention, it is possible to provide a method for manufacturing a black stainless steel material that is excellent in workability, difficult for the base material to be exposed during processing, and has good corrosion resistance even when the base material is partially exposed during processing. Further, according to the present invention, it is possible to provide a stainless steel material for blackening heat treatment and a method for manufacturing the same that can manufacture a black stainless steel material having the above characteristics by blackening heat treatment.

Embodiments for Carrying Out the Invention

[0011] Hereinafter, embodiments of the present invention completed based on the above viewpoints will be specifically described. The present invention is not limited to the following embodiments, and it should be understood that modifications, improvements, etc. appropriately added to the following embodiments based on the ordinary knowledge of those skilled in the art also fall within the scope of the present invention without departing from the gist of the present invention. In this specification, the “%” display regarding components means “mass %” unless otherwise specified.

[0012] <Black stainless steel material> The black stainless steel material according to an embodiment of the present invention comprises a base material and an oxide film (black film) formed on the surface of the base material. In this specification, "stainless steel material" means a material formed from stainless steel, and its shape is not particularly limited. Examples of shapes include plates (including strips), rods, and tubes. Furthermore, it may be various types of shaped steel, such as T-shaped and I-shaped cross-sections.

[0013] The base material has a composition containing C: 0.100% or less, Si: 1.00% or less, Mn: 0.05-1.00%, P: 0.100% or less, S: 0.100% or less, Cr: 16.00-25.00%, Ni: 1.00% or less, Cu: 1.00% or less, Mo: 2.00% or less, N: 0.100% or less, and Ti: 0.08-0.50%, with the remainder being Fe and impurities. Herein, in this specification, "impurities" means components that are mixed in during the industrial production of stainless steel materials due to various factors in the raw materials such as ore and scrap, and the manufacturing process, and that are acceptable as long as they do not adversely affect the present invention. For example, impurities include unavoidable impurities. Examples of impurities include oxygen (O). Furthermore, regarding the content of each element, "xx% or less" means that it is xx% or less, but also contains an amount greater than 0% (especially above the impurity level).

[0014] Furthermore, the substrate may further include at least one selected from Nb: 1.00% or less, Al: 1.00% or less, Zr: 1.00% or less, Co: 1.00% or less, V: 1.00% or less, W: 1.00% or less, REM: 0.100% or less, Ca: 0.100% or less, Sn: 0.100% or less, and B: 0.0100% or less. The reasons for limiting the content of each element mentioned above will be explained.

[0015] (C: 0.100% or less) Carbon (C) is an element that affects properties such as intergranular corrosion resistance (sensitization suppression) and workability of black stainless steel. However, if the C content is too high, the workability and intergranular corrosion resistance of black stainless steel will decrease. Therefore, the upper limit of the C content is 0.100%, preferably 0.080%, and more preferably 0.060%. On the other hand, there is no particular lower limit for the C content, but excessively low C content leads to an increase in refining costs. Therefore, the lower limit of the C content is preferably 0.0003%, and more preferably 0.0005%.

[0016] (Si:1.00% or less) Si is an element that improves the oxidation resistance of black stainless steel. However, if the Si content is too high, the workability and toughness of the welded joint will decrease. Therefore, the upper limit of the Si content is 1.00%, preferably 0.90%, and more preferably 0.80%. On the other hand, the lower limit of the Si content is not particularly limited, but from the viewpoint of obtaining the effects of Si, it is preferably 0.005%, more preferably 0.01%, and even more preferably 0.015%.

[0017] (Mn: 0.05~1.00%) Mn is an effective element for ensuring the color tone of the oxide film (black film). In particular, Mn provides a black color tone by forming a complex oxide with Cr. However, if the Mn content is too high, it becomes easier to generate MnS, which is a corrosion initiation site, and also destabilizes the ferrite phase. Therefore, the upper limit of the Mn content is 1.00%, preferably 0.95%, and more preferably 0.90%. On the other hand, if the Mn content is too low, the above effects may not be sufficiently obtained. Therefore, the lower limit of the Mn content is 0.05%, preferably 0.055%, and more preferably 0.06%.

[0018] (P:0.100% or less) P is an element that affects properties such as weldability and workability of black stainless steel. If the P content is too high, the above properties may deteriorate. Therefore, the upper limit of the P content is 0.100%, preferably 0.080%, and more preferably 0.060%. On the other hand, there is no particular lower limit for the P content, but excessively low P content leads to an increase in refining costs. Therefore, the lower limit of the P content is preferably 0.001%, and more preferably 0.005%.

[0019] (S:0.100% or less) S is an element that generates MnS, which acts as a corrosion initiation site, and affects properties such as weld toughness of black stainless steel. If the S content is too high, the above properties may deteriorate. Therefore, the upper limit of the S content is 0.100%, preferably 0.080%, and more preferably 0.060%. On the other hand, there is no particular lower limit for the S content, but excessively low S content leads to an increase in refining costs. Therefore, the lower limit of the S content is preferably 0.0001%, and more preferably 0.0002%.

[0020] (Cr: 16.00~25.00%) Cr is an effective element for improving the corrosion resistance and oxidation resistance of black stainless steel materials. Furthermore, Cr is also an effective element for ensuring the color tone of the oxide film (black film). However, if the Cr content is too high, the toughness of the black stainless steel material decreases, and the growth of the oxide film is inhibited, preventing the formation of an oxide film with a black color tone. Therefore, the upper limit of the Cr content is 25.00%, preferably 24.50%, and more preferably 24.00%. On the other hand, if the Cr content is too low, the above effects cannot be fully obtained. Therefore, the lower limit of the Cr content is 16.00%, preferably 16.25%, and more preferably 16.50%.

[0021] (Ni: 1.00% or less) Ni is an effective element for improving the corrosion resistance and weld toughness of black stainless steel. However, if the Ni content is too high, the ferrite phase becomes unstable and the manufacturing cost increases. Therefore, the upper limit of the Ni content is 1.00%, preferably 0.90%, and more preferably 0.80%. On the other hand, the lower limit of the Ni content is not particularly limited, but from the viewpoint of obtaining the above effects, it is preferably 0.005%, and more preferably 0.01%.

[0022] (Cu:1.00% or less) Cu is an effective element for improving the corrosion resistance of black stainless steel. However, if the Cu content is too high, the ferrite phase becomes unstable and manufacturing costs increase. Therefore, the upper limit of the Cu content is 1.00%, preferably 0.90%, and more preferably 0.80%. On the other hand, the lower limit of the Cu content is not particularly limited, but is preferably 0.005%, and more preferably 0.01%.

[0023] (Mo: 2.00% or less) Mo is an effective element for improving the corrosion resistance and oxidation resistance of black stainless steel. However, if the Mo content is too high, it leads to a decrease in the workability of the black stainless steel and an increase in manufacturing costs. Therefore, the upper limit of the Mo content is 2.00%, preferably 1.95%, and more preferably 1.90%. On the other hand, the lower limit of the Mo content is not particularly limited, but is preferably 0.001%, and more preferably 0.005%.

[0024] (N:0.100% or less) Nitrogen (N) is an element that affects properties such as resistance to intergranular corrosion (sensitization inhibition) and workability. However, if the N content is too high, the intergranular corrosion resistance and workability of black stainless steel materials will decrease. In addition, if the N content is high, TiN will precipitate more easily, reducing the amount of solid-solution Ti in the steel, and inhibiting the formation of a black film after blackening heat treatment. Furthermore, the formed nitrides are prone to becoming corrosion initiation sites, reducing corrosion resistance. For this reason, the upper limit of the N content is 0.100%, preferably 0.095%, and more preferably 0.090%. On the other hand, there is no particular lower limit for the N content, but excessively low N content leads to increased refining costs. For this reason, the lower limit of the N content is preferably 0.001%, and more preferably 0.003%.

[0025] (Ti: 0.08~0.50%) Ti is an element that affects resistance to intergranular corrosion (sensitization suppression). Ti is also an effective element for ensuring the color tone of the oxide film (black film). In particular, Ti forms a complex oxide with Cr, giving it a black color, and also suppresses cracking and peeling of the oxide film by forming Ti oxide (TiO2) on the surface. However, if the Ti content is too high, the workability and surface quality of the black stainless steel material will decrease. Therefore, the upper limit of the Ti content is 0.50%, preferably 0.45%, and more preferably 0.40%. Conversely, if the Ti content is too low, the above effects cannot be fully obtained. Therefore, the lower limit of the Ti content is 0.08%, preferably 0.085%, and more preferably 0.09%.

[0026] (Nb:1.00% or less) Nb is an element that affects properties such as resistance to intergranular corrosion (sensitization suppression). However, if the Nb content is too high, the workability and surface quality of the black stainless steel material will deteriorate. Therefore, the upper limit of the Nb content is 1.00%, preferably 0.80%, and more preferably 0.50%. On the other hand, the lower limit of the Nb content is not particularly limited, but is preferably 0.005%, and more preferably 0.01%.

[0027] (Al: 1.00% or less, Zr: 1.00% or less, Co: 1.00% or less, V: 1.00% or less, W: 1.00% or less) Al, Zr, Co, V, and W are effective elements for improving the oxidation resistance of black stainless steel. However, if the content of Al, Zr, Co, V, and W is too high, the workability and toughness of the black stainless steel will decrease, and the manufacturing cost will increase. Therefore, the upper limit for the content of Al, Zr, Co, V, and W is 1.00%, preferably 0.95%, and more preferably 0.90%. On the other hand, the lower limit for the content of Al, Zr, Co, V, and W is not particularly limited, but is preferably 0.0001%, and more preferably 0.0005%.

[0028] (REM: 0.100% or less, Ca: 0.100% or less) REM and Ca are effective elements for improving the oxidation resistance of black stainless steel. However, if the content of REM and Ca is too high, it will lead to an increase in the manufacturing cost of black stainless steel. Therefore, the upper limit for the content of both REM and Ca is 0.100%, preferably 0.090%, and more preferably 0.080%. On the other hand, the lower limit for both REM and Ca is not particularly limited, but is preferably 0.0001%, and more preferably 0.0003%. REM is a collective term for 17 elements including Sc, Y, and lanthanides, and refers to rare earth metals. Specifically, examples include La, Ce, and Nd, and one of these can be included individually or in combination of two or more. If two or more rare earth elements are included, the above REM content refers to the total content of these rare earth elements.

[0029] (Sn:0.100% or less) Sn is an effective element for improving the oxidation resistance of black stainless steel materials. However, if the Sn content is too high, Sn segregates, reducing manufacturability. Therefore, the upper limit of the Sn content is 0.100%, preferably 0.090%, and more preferably 0.080%. On the other hand, the lower limit of the Sn content is not particularly limited, but is preferably 0.001%, and more preferably 0.002%.

[0030] (B:0.0100% or less) B is an effective element for improving the secondary workability of black stainless steel. However, if the B content is too high, the fatigue strength of the black stainless steel decreases. Therefore, the upper limit of the B content is 0.0100%, preferably 0.0090%, and more preferably 0.0080%. On the other hand, the lower limit of the B content is not particularly limited, but is preferably 0.0001%, and more preferably 0.0003%.

[0031] The substrate has an average crystal grain size of 100 μm or less. The reason for this limitation will be explained. The cause of cracking and peeling of the oxide film during processing of black stainless steel is the unevenness along the crystal grains that occurs in the processed area of ​​the black stainless steel. Therefore, by reducing the average crystal grain size of the base material, cracking and peeling of the oxide film during processing can be suppressed, and the base material can be less likely to be exposed. From the viewpoint of obtaining such effects, the average crystal grain size of the base material is set to 100 μm or less. From the viewpoint of stably obtaining the above effects, the average crystal grain size of the base material is preferably 95 μm or less, and more preferably 90 μm or less. On the other hand, the lower limit of the average crystal grain size of the base material is not particularly limited, but for example, it is 1 μm, preferably 5 μm, and more preferably 10 μm.

[0032] Here, in this specification, the average crystal grain size of the substrate can be determined by observing the metal structure of the substrate with an optical microscope in accordance with JIS G0551:2013. The observation by the optical microscope is carried out in accordance with JIS G0551:2013. A straight line is drawn at an arbitrary position on the optical microscope image, the number of intersections of the straight line and the crystal grain boundaries is measured, and the average section length is taken as the crystal grain size. The measurement of the crystal grain size is performed by drawing 20 or more straight lines in a plurality of fields of view and measuring them, and their average value is taken as the average crystal grain size.

[0033] Note that the metal structure of the substrate is ferritic. Here, in this specification, "ferritic" means that the metal structure is mainly a ferrite phase at room temperature.

[0034] The oxide film has a carrier density of 2.00×10 20 per cm 3 or less, L * a * b * in the lightness index L * in the colorimetric system is 50.0 or less, and the chromaticity indices a * and b * are within ±5.00. Also, it is preferable that the oxide film has a Cr2O3 inner layer thickness of 50 nm or more and / or an overall thickness of 300 to 1000 nm. The reasons for these limitations will be explained.

[0035] (Carrier density: 2.00×10 20 per cm 3 or less) The corrosion of metal proceeds by the paired occurrence of metal dissolution (anode reaction) and oxygen reduction (cathode reaction) around the metal dissolution part. This is because electrical neutrality is maintained by consuming the electrons generated in the anode reaction in the cathode reaction. Therefore, if the cathode reaction in the black stainless steel material can be suppressed, the anode reaction (metal dissolution) can also be suppressed. The cathode reaction occurs in the presence of water when electrons generated in the anode reaction move within the metal and react with dissolved oxygen in the water at the surface. Therefore, the cathode reaction can be suppressed by coating the metal surface with an insulating material that hinders electron movement. However, oxide films mainly composed of Cr2O3 do not provide sufficient insulation because structural defects (e.g., oxygen vacancies or metals with different valencies than Cr) act as carriers for electron movement, and thus cannot adequately suppress the cathode reaction. Consequently, controlling the carrier density of the oxide film is crucial to adequately suppress the cathode reaction in black stainless steel materials.

[0036] For the reasons mentioned above, the upper limit of the carrier density of the oxide film is 2.00 × 10⁻⁶. 20 pieces / cm 3 Preferably 1.00 × 10 20 pieces / cm 3 Furthermore, by setting the upper limit of the carrier density of the oxide film as described above, corrosion becomes less likely to occur even if the base material is partially exposed during processing. On the other hand, the lower limit of the carrier density of the oxide film is not particularly limited, as a smaller value increases the effect of suppressing the cathode reaction, but it is generally 1.00 × 10⁻⁶. 14 pieces / cm 3 Preferably 1.00 × 10 15 pieces / cm 3 That is the case. In this specification, the carrier density of the oxide film can be measured by electrochemical impedance measurement.

[0037] (Lightness index L * : Below 50.0, Chromanetics Index a * and b * (Within ±5.00) The oxide film is L * a * b * Lightness index L in a color system * If the index is 50.0 or less, the Chromanetics index a * and b * The value is within ±5.00. Brightness index L * , Chromanetics Index a* and b * If the value falls within the above range, it can be said that the desired shade of black has been obtained. Here, in this specification, "lightness index L * " and "Chromanetics Index a * and b * This can be measured in accordance with JIS Z8722:2009.

[0038] (Thickness of the Cr2O3 inner layer: 50 nm or more) The oxide film has a Cr2O3 inner layer with a thickness of 50 nm or more. The Cr2O3 inner layer is a layer formed on the substrate side of the oxide film and has the function of ensuring the barrier properties (substrate protection ability) of the oxide film. From the viewpoint of ensuring this function, the lower limit of the thickness of the Cr2O3 inner layer is 50 nm, preferably 60 nm, and more preferably 70 nm. On the other hand, the upper limit of the thickness of the Cr2O3 inner layer is not particularly limited, but is preferably 900 nm, more preferably 800 nm.

[0039] (Overall thickness: 300-1000nm) The overall thickness of the oxide film affects the color tone. From the viewpoint of imparting a desired black color tone, the lower limit of the overall thickness is 300 nm, preferably 310 nm, and more preferably 320 nm. On the other hand, if the overall thickness is large, cracking and peeling of the oxide film are more likely to occur during processing of the black stainless steel material. Therefore, the upper limit of the overall thickness is 1000 nm, preferably 950 nm, and more preferably 900 nm.

[0040] In this specification, the total thickness of the oxide film is defined as the depth from the surface to the point where the oxygen (O) concentration is 1 / 4 of its maximum value, as shown in the depth-direction component concentration profile obtained using glow discharge emission spectroscopy (GD-OES). The thickness of the Cr2O3 inner layer is defined as the portion of the oxide film where the Cr concentration / (Fe concentration + Cr concentration + Mn concentration + Ti concentration) × 100 is 70% or more. The concentration of each element can be determined by glow discharge emission spectroscopy (GD-OES).

[0041] The oxide film preferably has a composite oxide of Mn and Cr (Mn-Cr spinel oxide) on its surface. By providing such a composite oxide of Mn and Cr on the surface, it becomes easier to control the carrier density of the oxide film within the above range.

[0042] The black stainless steel material according to the embodiment of the present invention has a base material and oxide film having the above-described characteristics, and therefore exhibits excellent workability, is less likely to expose the base material during processing, and has good corrosion resistance even if the base material is partially exposed during processing. For this reason, the black stainless steel material according to the embodiment of the present invention is suitable for use in the manufacture of various processed products.

[0043] <Stainless steel material for blackening heat treatment> The stainless steel material for blackening heat treatment according to the embodiment of the present invention is used as a raw material for manufacturing the above-mentioned black stainless steel material. That is, the stainless steel material for blackening heat treatment according to the embodiment of the present invention can be used to manufacture the above-mentioned black stainless steel material by blackening heat treatment.

[0044] The stainless steel material for blackening heat treatment according to the embodiment of the present invention has a composition comprising C: 0.100% or less, Si: 1.00% or less, Mn: 0.05 to 1.00%, P: 0.100% or less, S: 0.100% or less, Cr: 16.00 to 25.00%, Ni: 1.00% or less, Cu: 1.00% or less, Mo: 2.00% or less, N: 0.100% or less, Ti: 0.08 to 0.50%, with the remainder being Fe and impurities. Furthermore, the stainless steel material for blackening heat treatment according to the embodiment of the present invention may further include at least one selected from Nb: 1.00% or less, Al: 1.00% or less, Zr: 1.00% or less, Co: 1.00% or less, V: 1.00% or less, W: 1.00% or less, REM: 0.100% or less, Ca: 0.100% or less, Sn: 0.100% or less, and B: 0.0100% or less. Since the composition of the stainless steel material for blackening heat treatment according to the embodiment of the present invention is basically the same as the composition of the black stainless steel material described above, its explanation will be omitted.

[0045] The stainless steel material for blackening heat treatment according to the embodiment of the present invention has a surface Vickers hardness of 210 HV or more, and the difference between the Vickers hardness at a position 1 / 8 of the thickness from the surface and the Vickers hardness at a position 1 / 2 of the thickness from the surface is 40 HV or less. The reasons for these limitations will be explained.

[0046] (Surface Vickers hardness: 210HV or higher) Because blackening heat treatment is often performed at high temperatures for extended periods, the crystal grains tend to coarseen, reducing workability and making the oxide film in the treated area prone to cracking and peeling. Therefore, materials subjected to blackening heat treatment are required to maintain fine crystal grains even after the treatment. Therefore, in the stainless steel material for blackening heat treatment according to the embodiment of the present invention, a predetermined amount of strain is applied from the viewpoint of maintaining fine crystal grains. In the stainless steel material for blackening heat treatment, the amount of strain applied (hereinafter referred to as "amount of strain") is related to the Vickers hardness. Specifically, a higher Vickers hardness results in a higher amount of strain, and conversely, a lower Vickers hardness results in a lower amount of strain.

[0047] To suppress cracking and peeling of the oxide film on the processed area, it is necessary to maintain the fine crystal grains on the surface by increasing the amount of strain on the surface of the stainless steel material for blackening heat treatment. In particular, in order to maintain the fine crystal grains on the surface even after blackening heat treatment, it is necessary to set the Vickers hardness of the surface of the stainless steel material for blackening heat treatment to 210 HV or higher. From the viewpoint of stably obtaining this effect, the Vickers hardness of the surface of the stainless steel material for blackening heat treatment is preferably 220 HV or higher, and more preferably 230 HV or higher. On the other hand, the upper limit of the Vickers hardness of the surface of the stainless steel material for blackening heat treatment is not particularly limited, but is preferably 500 HV, and more preferably 400 HV. Here, the Vickers hardness of the surface of stainless steel material for blackening heat treatment can be measured using a Vickers hardness tester. In measuring Vickers hardness, a load of 0.01 kg is applied, measurements are taken at five arbitrary points, and the average value is used as the result.

[0048] (Difference between Vickers hardness at 1 / 8 of the thickness from the surface and Vickers hardness at 1 / 2 of the thickness from the surface: 40 HV or less) As described above, in stainless steel materials for blackening heat treatment, the amount of strain is related to the Vickers hardness. In particular, in order to improve the workability of stainless steel materials for blackening heat treatment, it is necessary to maintain fine crystal grains in the thickness direction by reducing the difference in the amount of strain in the thickness direction of the stainless steel material for blackening heat treatment. To obtain this effect, it is necessary to keep the difference between the Vickers hardness at a position 1 / 8 of the thickness from the surface of the stainless steel material for blackening heat treatment and the Vickers hardness at a position 1 / 2 of the thickness from the surface of the stainless steel material for blackening heat treatment 40 HV or less. From the viewpoint of obtaining this effect stably, it is preferable that this difference is 35 HV or less, more preferably 30 HV or less, and even more preferably 25 HV or less. On the other hand, since the smaller the difference, the better, the lower limit is not particularly limited, but for example it is 1 HV. Here, the Vickers hardness at positions 1 / 8 and 1 / 2 of the thickness from the surface can be measured using the same method as described above, with a sample of stainless steel material for blackening heat treatment having its cross-section exposed.

[0049] In the embodiment of the present invention, the stainless steel material for blackening heat treatment is preferably a cold-rolled material or a cold-rolled material that has been polished after cold rolling, from the viewpoint of ensuring a predetermined amount of strain.

[0050] In the embodiment of the present invention, the stainless steel material for blackening heat treatment has refined crystal grains by controlling the Vickers hardness at predetermined locations, so that fine crystal grains are maintained even after blackening heat treatment. Therefore, it is possible to manufacture a black stainless steel material in which the base material is less likely to be exposed during processing, and even if the base material is partially exposed during processing, it has good corrosion resistance.

[0051] <Manufacturing method for black stainless steel material> The method for manufacturing the black stainless steel material according to the embodiment of the present invention is not particularly limited as long as it is a method capable of producing a black stainless steel material having the above-described characteristics. For example, the method for manufacturing the black stainless steel material according to the embodiment of the present invention is carried out by performing a blackening heat treatment on the above-described stainless steel material for blackening heat treatment at a temperature of 900 to 1100°C for 30 to 120 seconds in an atmosphere in which the O2 concentration is 1 to 10 volume%, the water vapor concentration is 5 to 20 volume%, and the value represented by the following formula (1) is 15 to 30. 2 × O2 concentration + water vapor concentration ... (1)

[0052] If the atmosphere during the blackening heat treatment is not under the above conditions, it is not possible to control the carrier density of the oxide film within the above range. For example, if the O2 concentration exceeds 10 volume%, the amount of Mn and Fe oxides in the oxide film increases, and the carrier density becomes high. On the other hand, if the O2 concentration is less than 1 volume%, it becomes difficult to form an oxide film of a predetermined thickness. From the viewpoint of stably controlling the carrier density of the oxide film within the above range, the O2 concentration is preferably 1.5 to 9.5 volume%, more preferably 2 to 9 volume%. If the water vapor concentration is outside the range of 5 to 20 volume%, lattice defects in the oxide film increase, and the carrier density becomes high. From the viewpoint of stably controlling the carrier density of the oxide film within the above range, the water vapor concentration is preferably 6 to 19 volume%, more preferably 7 to 18 volume%. If the value represented by equation (1) is outside the range of 15 to 30, the number of lattice defects in the oxide film increases, and the carrier density becomes higher. From the viewpoint of stably controlling the carrier density of the oxide film within the above range, the value represented by equation (1) is preferably 15.5 to 29, more preferably 16 to 28.

[0053] If the blackening heat treatment temperature is less than 900°C and the duration is less than 30 seconds, the oxide film will not grow sufficiently, and it will not be possible to form an oxide film of the desired thickness. On the other hand, if the blackening heat treatment temperature exceeds 1100°C and the duration exceeds 120 seconds, the oxide film will become too thick or the crystal grains will become coarse. The temperature for the blackening heat treatment is preferably 910 to 1100°C, more preferably 930 to 1100°C, from the viewpoint of stably forming an oxide film of the desired thickness. Furthermore, the heat treatment time is preferably 40 to 120 seconds, more preferably 50 to 120 seconds, from the viewpoint of stably forming an oxide film of the desired thickness.

[0054] <Manufacturing method for stainless steel material for blackening heat treatment> The method for manufacturing a stainless steel material for blackening heat treatment according to the embodiments of the present invention is not particularly limited as long as it is a method capable of manufacturing a stainless steel material for blackening heat treatment having the above-described characteristics. For example, the method for manufacturing a stainless steel material for blackening heat treatment according to the embodiments of the present invention is carried out by hot-rolling a slab having a composition containing C: 0.100% or less, Si: 1.00% or less, Mn: 0.05 to 1.00%, P: 0.100% or less, S: 0.100% or less, Cr: 16.00 to 25.00%, Ni: 1.00% or less, Cu: 1.00% or less, Mo: 2.00% or less, N: 0.100% or less, Ti: 0.08 to 0.50%, with the remainder being Fe and impurities, followed by annealing and pickling, and then performing one cold-rolling, or two or more cold-rolling with annealing and pickling in between. In addition, polishing may be performed after cold-rolling as needed. Furthermore, in order to maintain the specified amount of strain, annealing should not be performed after cold rolling or grinding. Here, two or more cold rolling processes with annealing and pickling in between means, for example, two cold rolling processes with annealing and pickling in between, which are carried out in the order of first cold rolling - annealing and pickling - second cold rolling. Also, in the case of three cold rolling processes with annealing and pickling in between, which are carried out in the order of first cold rolling - annealing and pickling - second cold rolling - annealing and pickling - third cold rolling.

[0055] The slab may further contain at least one selected from Nb: 1.00% or less, Al: 1.00% or less, Zr: 1.00% or less, Co: 1.00% or less, V: 1.00% or less, W: 1.00% or less, REM: 0.100% or less, Ca: 0.100% or less, Sn: 0.100% or less, and B: 0.0100% or less. The composition of the slab used in the method for manufacturing blackened heat-treated stainless steel according to the embodiment of the present invention is basically the same as the composition of the blackened stainless steel described above, so its explanation will be omitted.

[0056] In the above manufacturing method, the average grain size before one cold rolling or the average grain size before the final cold rolling among two or more cold rollings (hereinafter, these average grain sizes are referred to as "average grain size before final cold rolling") is controlled to 300 μm or less. By controlling the average grain size before final cold rolling to this range, it is possible to refine the grains during the blackening heat treatment (i.e., to make the average grain size 100 μm or less). From the viewpoint of stably obtaining this effect, the average grain size before final cold rolling is preferably 280 μm or less, more preferably 260 μm or less, and even more preferably 250 μm or less. The lower limit of the average grain size before final cold rolling is not particularly limited, but for example, it is 10 μm. Here, the average grain size before one cold rolling cycle refers to the average grain size of the hot-rolled, annealed material after hot rolling, annealing, and pickling. Furthermore, the average grain size before the final cold rolling cycle in a case of two or more cold rolling cycles refers to, for example, the average grain size of the cold-rolled, annealed material immediately before the second cold rolling cycle (material that has been annealed and pickled after the first cold rolling cycle) in the case of two cold rolling cycles with annealing and pickling in between. Similarly, in the case of three cold rolling cycles with annealing and pickling in between, it refers to the average grain size of the cold-rolled, annealed material immediately before the third cold rolling cycle (material that has been annealed and pickled after the second cold rolling cycle).

[0057] Here, the average grain size before final cold rolling can be determined by observation with an optical microscope, in accordance with JIS G0551:2013, similar to the average grain size of the base material described above. The observation method using an optical microscope is the same as described above, so the explanation is omitted.

[0058] The average grain size before the final cold rolling can be controlled by adjusting the rolling ratio during the rolling process prior to the final cold rolling (e.g., during hot rolling or intermediate cold rolling). Here, intermediate cold rolling refers to cold rolling other than the final cold rolling when two or more cold rolling processes are performed. For example, the rolling ratio in hot rolling is preferably 40% or more, and more preferably 50% or more. By adjusting the rolling ratio to this extent, it becomes easier to control the average grain size before final cold rolling within the above range. The upper limit of the rolling ratio in hot rolling is not particularly limited, but is typically 98%, for example, 99%.

[0059] Similarly, the rolling ratio of the intermediate cold rolling is preferably 40% or more, and more preferably 50% or more. By adjusting the rolling ratio to this extent, it becomes easier to control the average grain size before the final cold rolling within the above range. The upper limit of the rolling ratio of the intermediate cold rolling is not particularly limited, but is typically 80%, for example, 90%.

[0060] The rolling ratio of a single cold rolling or the rolling ratio of the final cold rolling in two or more cold rolling processes is not particularly limited, but is preferably 40% or more, and more preferably 45% or more. By adjusting to such a rolling ratio, it becomes easier to refine the average grain size to 100 μm or less during the blackening heat treatment. The upper limit of this rolling ratio is not particularly limited, but is, for example, 90%, and typically 80%.

[0061] <Processed products> The processed product according to the embodiment of the present invention is a processed product of the above-mentioned stainless steel material. In this specification, "stainless steel processed product" means a product obtained by processing stainless steel material by various known methods. The processing method is not particularly limited, but examples include pressing, polishing, and roll forming.

[0062] In the embodiment of the present invention, the exposed surface area ratio of the base material in the processed area is 30% or less. By controlling the exposed surface area ratio within this range, corrosion of the exposed surface area can be suppressed even if there is an exposed surface area in the processed area. In this specification, the exposed substrate area ratio refers to the ratio of the area of ​​the substrate that is exposed in the processed area after various processing steps have been performed. Furthermore, the size of the processed area is preferably within φ5 mm.

[0063] The processed products according to the embodiment of the present invention use black stainless steel as the base material, which is less likely to expose the base material during processing and has good corrosion resistance even if the base material is partially exposed during processing. Therefore, they have good corrosion resistance and can be used in various products where corrosion resistance is required. Examples of processed products are not particularly limited, but include exhaust system parts such as mufflers, and exterior building material panels that are joined to mounting jigs. [Examples]

[0064] The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples.

[0065] Stainless steel having the composition shown in Table 1 (the remainder being Fe and impurities) was melted, and a cold-rolled sheet with a thickness of 1 mm (stainless steel material for blackening heat treatment) was obtained by carrying out the manufacturing process shown in Table 2. In Test No. 2-1, in order to achieve a final thickness of 1 mm, the hot-rolled material was sliced ​​to a thickness of 5 mm after hot rolling, and then the thickness of the hot-rolled material was further adjusted to 3 mm by cutting. In Test No. 2-2, in order to achieve a final thickness of 1 mm, the thickness of the hot-rolled material was adjusted to 1.5 mm by cutting after hot rolling.

[0066] For the material before final cold rolling, 10 mm square test specimens were cut by machining, and then embedded in resin so that the cross-section in the rolling direction became the observation surface. Next, the resin-embedded test specimens were mirror-finished by wet polishing, and the metallographic structure revealed by etching with hydrofluoric acid was observed with an optical microscope. The optical microscope observation was performed in accordance with JIS G0551:2013, by drawing a straight line at an arbitrary position on the optical microscope image, measuring the number of intersections between the line and the grain boundary, and the average intercept length was defined as the grain size. The grain size was measured by drawing 20 or more straight lines in multiple fields of view, and the average value of these measurements was defined as the average grain size. The results are shown in Table 2 as "Average grain size before final cold rolling".

[0067] After cutting 10 mm square test pieces from the cold-rolled sheet obtained above, the Vickers hardness of the surface (rolled surface) was measured using a Vickers hardness tester. For the measurement of surface Vickers hardness, a load of 0.01 kg was applied, and measurements were taken at five arbitrary points, with the average value being used as the result. The results are shown in Table 2 as "Surface Hardness". Furthermore, after cutting 10 mm square test pieces from the cold-rolled sheet obtained above, resin embedding was performed so that the cross-section in the rolling direction became the observation surface. Next, the resin-embedded test pieces were mirror-finished by wet polishing, and the Vickers hardness was measured at a position 1 / 8 of the thickness from the surface and at a position 1 / 2 of the thickness from the surface using a Vickers hardness tester. For the measurement of Vickers hardness at each position, a load of 0.01 kg was set, and measurements were taken at five arbitrary points, with the average value being used as the result. The difference between the Vickers hardness at the position 1 / 8 of the thickness from the surface and the Vickers hardness at the position 1 / 2 of the thickness from the surface was then calculated. The results are shown in Table 2 as "Difference in Hardness".

[0068] [Table 1]

[0069] [Table 2]

[0070] Next, the cold-rolled sheet (stainless steel material for blackening heat treatment) obtained above was subjected to blackening heat treatment at a temperature of 1100°C for 120 seconds under the atmosphere shown in Table 3 to obtain a black stainless steel sheet (black stainless steel material). During the blackening heat treatment, the predetermined O2 concentration and water vapor concentration were controlled by adjusting the introduction ratio of O2 gas, N2 gas, and water vapor in the atmosphere furnace. A test piece measuring 300 mm (rolling direction) × 100 mm (width direction) was cut from the obtained black stainless steel sheet.

[0071] [Table 3]

[0072] The following evaluations were performed on the test specimens obtained above.

[0073] (Average crystal grain size of the substrate) After cutting a 10 mm square specimen from the test specimen by machining, it was embedded in resin so that the cross-section in the rolling direction would be the observation surface. Next, the resin-embedded specimen was polished to a mirror finish by wet polishing, and the exposed metal structure was etched with hydrofluoric acid and observed with an optical microscope. The optical microscope observation was performed in accordance with JIS G0551:2013, by drawing a straight line at an arbitrary position on the optical microscope image, measuring the number of intersections between the line and the grain boundary, and the average intercept length was defined as the grain size. The grain size was measured by drawing 20 or more straight lines in multiple fields of view, and the average value of these measurements was defined as the average grain size.

[0074] (Total thickness of the oxide film and the thickness of the Cr2O3 inner layer) A 50mm square test piece was cut from the test specimen, and its surface was degreased with acetone. Next, the pre-treated coating was analyzed using glow discharge emission spectroscopy (GD-OES) in accordance with JIS K0144:2018. In GD-OES, the thickness of the oxide film was defined as the depth from the surface to the point where the oxygen (O) concentration was 1 / 4 of its maximum value, based on the obtained depth-direction component concentration profile. Furthermore, the thickness of the Cr2O3 inner layer was defined as the portion of the oxide film where the Cr concentration / (Fe concentration + Cr concentration + Mn concentration + Ti concentration) × 100 was 70% or higher. The concentrations of each element were calculated from the depth-direction component concentration profile obtained by GD-OES.

[0075] (Carrier density of oxide film) A 20mm x 15mm test piece was cut from the specimen, a wire was spot-welded to one end, and the portion other than the 10mm x 10mm test surface was coated with silicone resin (one-component condensation type RTV rubber KE44 manufactured by Shin-Etsu Chemical Co., Ltd.). Next, an electrochemical impedance measurement was performed at 30°C under an Ar degassing atmosphere using a 0.1 mol / L Na2SO4 aqueous solution as the test solution. The electrochemical impedance measurement was performed using the HZ-7000 electrochemical measurement system manufactured by Hokuto Denko Co., Ltd., by measuring the impedance change by applying a sine wave at a predetermined potential, and the capacitance C of the oxide film was measured. The measurement potential was -0.2~0.5V vs. SSE (SSE indicates a saturated KCl silver-silver chloride electrode type reference electrode), the sine wave was 1Hz~100kHz, and the amplitude was 10mV. Based on the obtained C, a 1 / C plot called a Mott-Schottky plot was created. 2 -In the V-curve graph, the slope A of the graph from 0 to -0.4V was derived using the least squares method. The relationship between slope A and carrier density N q Since the relationship can be expressed by the following equation, the carrier density of the oxide film was derived from the slope A based on this equation.

[0076]

number

[0077] In the above formula, N q is the carrier density, A is the slope of the Mott-Schottky plot, and e is the elementary charge (1.62 × 10⁻¹⁰). -19 C) is the dielectric constant of the oxide film (12), and ε0 is the dielectric constant of vacuum (8.854 × 10⁻¹⁴).-12 It is F / m.

[0078] (color tone) For five arbitrary locations on the oxide film, color measurements were performed using a spectrophotometer with a measurement diameter of 3 mmφ in accordance with JIS Z8722:2009, and the average value was used to determine the CIELAB (L) color in accordance with JIS Z8781-4:2013. * a * b * Lightness index L (color system) * , Chromanetics Index a * , b * As shown.

[0079] The measurement conditions for the above color tones were as follows: Equipment: Konica Minolta CM-700d Spectrophotometer Light source: Pulsed xenon lamp Photodetector: Dual 36-element silicon photodiode array Target mask: φ3mm Measurement: 10° field of view Auxiliary illuminant: D65 Daylight, Color temperature 6504K Specular reflection processing mode: SCI

[0080] (Processability: Percentage of exposed substrate area) A 100mm (rolling direction) x 50mm (width direction) sample was cut from the test piece, and a thrust bend was performed perpendicular to the rolling direction at the center of the sample in the rolling direction. The thrust bend was performed with a bending radius of 5mmφ and a bending speed of 10cm / min. Next, three arbitrary locations within a 5mm x 5mm area centered on the top of the bent section were observed using a digital microscope, and the percentage of exposed substrate area at each of the three locations was determined, with the average value of these values ​​being used as the result. In this evaluation, if the percentage of exposed substrate area is 30% or less, it can be judged that the processability is good.

[0081] (Corrosion resistance: CCT test) The corrosion resistance of the sample was evaluated using a combined cycle test (CCT) using a sample prepared for processability evaluation. The sample was placed on a 70 mm x 150 mm bakelite plate with the top of the bent portion perpendicular to the horizontal plane. Ten cycles were performed, each 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 that, the sample was washed with water and dried, and the rust area ratio on the surface of the bent portion was evaluated (in accordance with JIS Z2371:2015). In this evaluation, a rating number (RN) of 8.0 or higher (corresponding to a rust area ratio of 0.25% or less) indicates good corrosion resistance, while an RN of less than 8.0 indicates poor corrosion resistance. The results are shown in Table 4.

[0082] [Table 4]

[0083] As shown in Tables 1-4, the black stainless steel plates No. 1-1 to 1-5 (examples of the present invention) have a predetermined composition and consist of a substrate with an average grain size of 100 μm or less and a carrier density of 2.00 × 10 20 pieces / cm 3 Below, L * a * b * Lightness index L in a color system * If the index is 50.0 or less, the Chromanetics index a * and b * Because it has an oxide film with a tolerance of ±5.00 or less, it has good processability (the base material is less likely to be exposed during processing), and the corrosion resistance of the exposed base material is also good.

[0084] In contrast, the black stainless steel sheet in Test No. 2-1 (comparative example) had an average grain size that was too large before the final cold rolling. As a result, the grain refinement was insufficient during the blackening heat treatment, and the average grain size of the base material became large. Consequently, the workability decreased (the exposed area of ​​the base material increased during processing), and the corrosion resistance of the exposed base material was also insufficient. In Test No. 2-2, the black stainless steel sheet (comparative example) had a Vickers hardness that was too low on the surface of the cold-rolled sheet (stainless steel material for blackening heat treatment) used for blackening heat treatment. Furthermore, the difference between the Vickers hardness at a position 1 / 8 of the thickness from the surface and the Vickers hardness at a position 1 / 2 of the thickness from the surface was also large. As a result, the grain refinement was not sufficient during the blackening heat treatment, and the average grain size of the base material became large. Consequently, the workability decreased (the exposed area of ​​the base material increased during processing), and the corrosion resistance of the exposed base material was also insufficient. In the black stainless steel sheet (comparative example) of Test No. 2-3, the Vickers hardness of the surface of the cold-rolled sheet (stainless steel material for blackening heat treatment) used for blackening heat treatment was too low. As a result, the grain refinement was not sufficient during the blackening heat treatment, and the average grain size of the base material became large. Consequently, the workability decreased (the exposed area of ​​the base material increased during processing), and the corrosion resistance of the exposed base material was also insufficient.

[0085] In tests No. 2-4 and 2-5, the black stainless steel plates (comparative examples) suffered from an inappropriate atmospheric condition during the blackening heat treatment, resulting in an excessively high carrier density in the oxide film. Consequently, the desired corrosion resistance could not be achieved. In Test No. 2-6, the black stainless steel sheet (comparative example) failed to achieve the desired black color because the Mn content of the substrate was too low. The black stainless steel sheet (comparative example) in test No. 2-7 did not achieve the desired black color because the base material did not contain Ti. In Test No. 2-8, the black stainless steel sheet (comparative example) had too little chromium content in the substrate, resulting in an excessively high carrier density. Consequently, the desired corrosion resistance could not be achieved.

[0086] As can be seen from the above results, the present invention provides a method for manufacturing a black stainless steel material that has excellent workability, is less likely to expose the base material during processing, and has good corrosion resistance even if the base material is partially exposed during processing. Furthermore, the present invention provides a stainless steel material for blackening heat treatment and a method for manufacturing the same, which can produce a black stainless steel material having the above-mentioned properties by blackening heat treatment.

Claims

1. The composition, by mass, contains C: 0.100% or less, Si: 1.00% or less, Mn: 0.05 to 1.00%, P: 0.100% or less, S: 0.100% or less, Cr: 16.00 to 25.00%, Ni: 1.00% or less, Cu: 1.00% or less, Mo: 2.00% or less, N: 0.100% or less, Ti: 0.08 to 0.50%, with the remainder being Fe and impurities. The surface has a Vickers hardness of 210 HV or higher. A stainless steel material for blackening heat treatment, wherein the difference between the Vickers hardness at a position 1 / 8 of the thickness from the surface and the Vickers hardness at a position 1 / 2 of the thickness from the surface is 40 HV or less.

2. The stainless steel material for blackening heat treatment according to claim 1, further comprising at least one selected from Nb: 1.00% or less, Al: 1.00% or less, Zr: 1.00% or less, Co: 1.00% or less, V: 1.00% or less, W: 1.00% or less, REM: 0.100% or less, Ca: 0.100% or less, Sn: 0.100% or less, and B: 0.0100% or less, on a mass basis.

3. The stainless steel material for blackening heat treatment according to claim 1 or 2, 2 The concentration is 1 to 10% by volume, the water vapor concentration is 5 to 20% by volume, and the following formula (1): 2 × 0 2 Concentration + Water vapor concentration ... (1) A method for manufacturing black stainless steel material, comprising performing a blackening heat treatment at a temperature of 900 to 1100°C for 30 to 120 seconds in an atmosphere where the value represented by is 15 to 30.

4. A method for manufacturing a stainless steel material for blackening heat treatment according to claim 1, A slab having a composition by mass containing C: 0.100% or less, Si: 1.00% or less, Mn: 0.05 to 1.00%, P: 0.100% or less, S: 0.100% or less, Cr: 16.00 to 25.00%, Ni: 1.00% or less, Cu: 1.00% or less, Mo: 2.00% or less, N: 0.100% or less, Ti: 0.08 to 0.50%, with the remainder being Fe and impurities, is hot-rolled, annealed and pickled, and then subjected to one cold-rolling, or two or more cold-rolling with annealing and pickling in between. A method for manufacturing stainless steel material for blackening heat treatment, wherein the average grain size before the first cold rolling, or the average grain size before the final cold rolling among the two or more cold rollings, is controlled to be 300 μm or less.

5. A method for manufacturing a stainless steel material for blackening heat treatment according to claim 2, The method for producing stainless steel material for blackening heat treatment according to claim 4, wherein the slab further comprises at least one selected by mass from Nb: 1.00% or less, Al: 1.00% or less, Zr: 1.00% or less, Co: 1.00% or less, V: 1.00% or less, W: 1.00% or less, REM: 0.100% or less, Ca: 0.100% or less, Sn: 0.100% or less, and B: 0.0100% or less.

6. A method for manufacturing a stainless steel material for blackening heat treatment according to claim 4 or 5, wherein the rolling rate of the hot rolling is 40% or more.

7. A method for manufacturing stainless steel material for blackening heat treatment according to claim 4 or 5, wherein the rolling rate of the intermediate cold rolling among the two or more cold rollings is 40% or more.

8. A method for manufacturing stainless steel material for blackening heat treatment according to claim 4 or 5, wherein the rolling rate of the single cold rolling, or the rolling rate of the final cold rolling among the two or more cold rollings, is 40% or more.

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