Ferritic stainless steel material, method for producing the same, and antibacterial / antiviral member

Abstract

To provide a ferritic stainless steel material with superior antibacterial and antiviral performance.SOLUTION: A ferritic stainless steel material has, at the outermost surface, a Cu-concentrated layer having a Cu content of 25 mass% or greater and an average grain size of 30 μm or below.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a ferritic stainless steel material, a method for producing the same, and an antibacterial and antiviral member. [Background technology]

[0002] Due to their excellent corrosion resistance, stainless steel materials are used in a wide range of applications, such as kitchen equipment, home appliances, medical instruments, interior building materials, and transportation equipment, and are often used in environments where bacterial proliferation and viral adhesion are likely to occur. In recent years, there has been a growing concern about the adverse effects on the human body caused by such bacterial proliferation and viral adhesion, and antibacterial and antiviral properties are being required not only for medical instruments and kitchen equipment, where cleanliness is essential, but also for various components used in buildings and transportation equipment where large numbers of people gather.

[0003] Metal elements such as Ag and Cu are known to have antibacterial and antiviral properties, and stainless steel materials that have been given antibacterial and antiviral properties by adding these metal elements, as well as methods for manufacturing such materials, have been proposed. For example, Patent Document 1 proposes a ferritic stainless steel material with excellent antibacterial properties, which contains 0.1 wt% or less of C, 2 wt% or less of Si, 2 wt% or less of Mn, 10 to 30 wt% of Cr, and 0.4 to 3 wt% of Cu, with the remainder being substantially Fe, and in which a Cu-rich phase (ε-Cu phase) precipitated by aging treatment is dispersed in the matrix at a rate of 0.2 vol% or more. Patent Document 2 proposes a stainless steel material with low contact resistance, which has as its base material an austenitic or ferritic stainless steel containing 1.0 wt % or more of Cu and in which a Cu-rich phase (ε-Cu phase) is dispersed and precipitated in the matrix at a rate of 0.2 vol % or more, and in which a passive film is formed on the surface of the base material excluding the precipitated portion of the Cu-rich phase.

[0004] Patent Document 3 proposes a stainless steel material with excellent antibacterial properties, which uses a stainless steel substrate containing 0.5% by weight or more of Cu, and has a passive film formed on the surface of the substrate after mechanical polishing, with the Fe / Cr atomic ratio adjusted to 0.4 or less. Patent Document 4 proposes an antibacterial stainless steel material in which a Cu-enriched layer is formed on the surface of a stainless steel material containing 1% or more but less than 3% Cu by weight, the maximum Cu concentration of the enriched layer being 3% by weight or more, the enriched layer including an oxide film formed by bright annealing treatment in which the layer is held at 300°C for 5 seconds or more, or by bright annealing treatment in which the layer is slowly heated between 250°C and 350°C at an average heating rate of 10°C / second or less, and the surface of the stainless steel material has #80 to #240 polishing marks.

[0005] Patent Document 5 proposes a ferritic stainless steel material for kitchen appliances containing, by mass%, C: 0.001 to 0.050%, Si: 0.01 to 1.0%, Mn: 0.01 to 2.0%, P: 0.05% or less, S: 0.01% or less, Al: 0.001 to 1.0%, Cr: 11.0 to 32.0%, Cu: 0.4 to 4.0%, N: 0.001 to 0.050%, one or two of Ti: 0.10 to 0.60%, Nb: 0.10 to 0.60%, the balance being Fe and unavoidable impurities, and further containing 0.10 to 2.97 volume% of an ε-Cu phase, the average grain size of which is 30 nm or less.

[0006] However, the ferritic stainless steel materials disclosed in the above patent documents do not have a properly controlled distribution of the ε-Cu phase, so the desired antibacterial properties may not be obtained or may be easily lost early. Furthermore, because viruses are smaller than bacteria, if viruses are attached between the ε-Cu phases, the antiviral properties may be almost completely lost. Therefore, in Patent Document 6, the present applicant proposed a ferritic stainless steel material in which the distribution of the ε-Cu phase (particularly, the area ratio of the ε-Cu phase on the surface, the average particle size of the ε-Cu phase, and the maximum interparticle distance of the ε-Cu phase) was controlled in addition to the composition. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Patent No. 3223418 [Patent Document 2] Patent No. 4368985 [Patent Document 3] Patent No. 3934244 [Patent Document 4] Patent No. 3895874 [Patent Document 5] Patent No. 6519023 [Patent Document 6] Japanese Patent Publication No. 2022-151128 Summary of the Invention [Problem to be solved by the invention]

[0008] The ferritic stainless steel material described in Patent Document 6 has a controlled distribution state of the ε-Cu phase, and therefore has improved antibacterial and antiviral properties compared to the ferritic stainless steel materials described in Patent Documents 1-5. However, the ferritic stainless steel material described in Patent Document 6 may have a small ε-Cu phase or a small amount thereof, and therefore may not have sufficiently improved antibacterial and antiviral properties.

[0009] Furthermore, because antibacterial and antiviral properties depend on the amount of Cu present on the outermost surface, they can be improved by concentrating Cu on the outermost surface. As a method for concentrating Cu on the surface, Patent Document 4 describes a method in which a Cu-enriched layer is formed on the surface by polishing the surface before bright annealing. However, with this method, the Cu-enriched layer is formed inside the oxide film, which inhibits Cu elution and results in insufficient improvement of antibacterial and antiviral properties.

[0010] An object of the present invention is to provide a ferritic stainless steel material having excellent antibacterial and antiviral properties, a method for producing the same, and an antibacterial and antiviral member. [Means for solving the problem]

[0011] The present inventors have conducted extensive research into ferritic stainless steel materials in order to solve the above problems, and have come to the following findings. By using a ferritic stainless steel material in which the ε-Cu phase is precipitated throughout and performing heat treatment at high temperatures (Cu diffusion heat treatment), Cu diffuses from the ε-Cu phase inside the ferritic stainless steel material to the surface, forming a Cu-enriched layer. When an oxide layer (a film of oxides of Fe, Cr, Si, etc.) forms on the surface of the Cu-enriched layer, it inhibits Cu dissolution, so oxidation must be suppressed as much as possible during Cu diffusion heat treatment. Since Cu diffusion proceeds more rapidly at higher temperatures and oxide layer formation is promoted at lower temperatures, it is important to appropriately control the heating rate and temperature during Cu diffusion heat treatment in a non-oxidizing atmosphere. However, because Si is an element that oxidizes extremely easily, simply controlling the atmosphere during Cu diffusion heat treatment is not enough to suppress the formation of an oxide layer. Therefore, we increase the grain boundaries of ferritic stainless steel materials to promote the formation of a Cu-enriched layer through Cu diffusion heat treatment. Because the diffusion rate of Cu is particularly fast at grain boundaries compared to within grains, pre-precipitating the ε-Cu phase at the grain boundaries allows Cu to diffuse to the surface. Furthermore, because it is difficult for a defect-free oxide layer to form on the surface grain boundaries, Cu diffusion is not impeded, making it easier to form a Cu-enriched layer on the outermost surface. The present invention has been completed under the above-mentioned circumstances.

[0012] That is, the present invention relates to a ferritic stainless steel material having a Cu-enriched layer on its outermost surface with a Cu concentration of 25 mass % or more and an average crystal grain size of 30 μm or less.

[0013] The present invention also provides a method for producing the ferritic stainless steel material, a hot rolling process in which the ferritic stainless steel slab is hot rolled at a finishing reduction rate of 50.0% or more and a finishing hot rolling temperature of 800°C or more to obtain a hot-rolled material; a cooling step of cooling the hot-rolled material to a temperature of 500°C or less at an average cooling rate of 10.0°C / sec or more; an annealing step in which the hot-rolled material cooled in the cooling step is annealed at a temperature of 750 to 900 ° C. to obtain a hot-rolled annealed material; a Cu diffusion heat treatment process in which the hot-rolled annealed material is heated in a non-oxidizing atmosphere with a dew point of -30°C or less in a temperature range up to 900°C at a temperature increase rate of 10.0°C / second or more, and then heat-treated at a temperature of 900°C or more; Including, When the annealing step is carried out in an oxidizing atmosphere, the manufacturing method further includes an oxide film removing step after the annealing step.

[0014] Furthermore, the present invention relates to an antibacterial and antiviral member containing the ferritic stainless steel material. [Effects of the Invention]

[0015] According to the present invention, it is possible to provide a ferritic stainless steel material having excellent antibacterial and antiviral properties, a method for producing the same, and an antibacterial and antiviral member. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 is an enlarged schematic view of a cross section of a typical hot-rolled annealed material. [Figure 2] 1 is a graph showing a component concentration profile in the depth direction by GD-OES in Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0017] 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 made to the following embodiments based on the ordinary knowledge of those skilled in the art without departing from the spirit of the present invention are also within the scope of the present invention.

[0018] In this specification, "%" means "% by mass" unless otherwise specified. Furthermore, in this specification, "stainless steel material" refers to a material formed from stainless steel, and the shape of the material is not particularly limited. Examples of the shape include plate (including strip), rod, and tube. Furthermore, various types of shaped steel may be used, such as T-shaped and I-shaped cross sections. In this specification, "ferritic" refers to a metal structure that is primarily ferrite at room temperature. Therefore, "ferritic" also includes those that contain small amounts of phases other than ferrite (e.g., austenite or martensite). However, "ferritic" does not include a multi-phase structure of ferrite and austenite, a multi-phase structure of ferrite and martensite, or a multi-phase structure of ferrite, austenite, and martensite.

[0019] The ferritic stainless steel material according to the embodiment of the present invention has a Cu-enriched layer on its outermost surface, with a Cu concentration of 25% or more. The presence of such a Cu-enriched layer on the outermost surface facilitates Cu elution. Therefore, for example, when a person's hand touches the surface of the ferritic stainless steel material, Cu ions can be eluted from the Cu-enriched layer due to the moisture in the hand. Therefore, even if bacteria adhere to the surface, they can be sterilized, and even if viruses adhere to the surface, they can be inactivated and eventually killed, resulting in excellent antibacterial and antiviral properties. To stably ensure this effect, the Cu concentration in the Cu-enriched layer is preferably 28% or more, more preferably 30% or more, even more preferably 32% or more, and particularly preferably 35% or more. The upper limit of the Cu concentration in the Cu-enriched layer is not particularly limited, since the higher the concentration, the easier Cu elution becomes. However, it can be, for example, 80% or less, 70% or less, 60% or less, or 50% or less.

[0020] The Cu concentration in the Cu-enriched layer can be calculated by determining the concentrations of Cu, Fe, Mn, Cr, Si, and Al at a depth of 0.4 to 0.7 nm from the surface in a component concentration profile in the depth direction obtained using glow discharge optical emission spectroscopy (GD-OES) in accordance with JIS K0144:2018, and then using the following formula: Cu concentration = Cu concentration / (Cu concentration + Fe concentration + Mn concentration + Cr concentration + Si concentration + Al concentration) × 100 The depth profile of component concentration can be determined by glow discharge optical emission spectroscopy as follows. First, a 50 mm square test piece is cut out from a ferritic stainless steel material, and the surface is degreased with acetone. Next, the surface is analyzed using glow discharge optical emission spectroscopy, thereby obtaining a depth profile of component concentration.

[0021] The thickness of the Cu-enriched layer is not particularly limited, but from the viewpoint of ensuring Cu elution by the Cu-enriched layer, it is preferably 0.7 to 10.0 nm, more preferably 0.8 to 8.0 nm, and even more preferably 0.9 to 6.0 nm. The thickness of the Cu-enriched layer means the depth from the surface of a region where the Cu concentration is 25% or more in a component concentration profile in the depth direction obtained by glow discharge optical emission spectroscopy.

[0022] The ferritic stainless steel material according to the embodiment of the present invention has an average crystal grain size of 30 μm or less. Because the ε-Cu phase easily precipitates at the grain boundaries between crystal grains, controlling the average crystal grain size to 30 μm or less can increase the size and amount of the ε-Cu phase precipitated at the grain boundaries. Therefore, it becomes possible to form a Cu-enriched layer with a Cu concentration of 25% or more on the outermost surface by Cu diffusion heat treatment. From the viewpoint of enhancing this effect, the average crystal grain size is preferably 28 μm or less, more preferably 25 μm or less, and even more preferably 20 μm or less. The lower limit of the average crystal grain size is not particularly limited, as smaller values are preferable, but it may be, for example, 1 μm, 2 μm, or 3 μm.

[0023] Here, the average crystal grain size in this specification means the average value of the crystal grain size (circle equivalent diameter) of crystal grains surrounded by crystal grain boundaries with a crystal orientation difference of 2° or more. The average grain size of the crystal grains is determined by analyzing the cross section of the ferritic stainless steel material using EBSD (Electron Backscattering Diffraction). Specifically, the average grain size of the crystal grains is determined as follows. First, a 15 mm square sample of the ferritic stainless steel material is cut out by cutting or the like, and the L-section (a cross section parallel to the rolling direction and perpendicular to the rolling surface) is mechanically polished to a mirror finish, followed by electrolytic polishing. For the L-section processed in this manner, EBSD analysis is performed on an 80 μm square area at 0.2 μm step intervals in a field of view at 1000x magnification, to identify crystal grain boundaries with a crystal orientation difference of 2° or more. The area of each crystal grain surrounded by this crystal grain boundary is then defined as S [μm 2 ], the diameter of a circle having the same area as a crystal grain is defined as D [μm], and the crystal grain size (average crystal grain size per field of view) [μm] is calculated using the following formula (1). This operation is performed for five fields of view to determine the crystal grain size for each field, and the average value of these is defined as the average crystal grain size.

[0024]

number

[0025] The ferritic stainless steel material according to the embodiment of the present invention may have an oxide layer inside the Cu-enriched layer. The presence of the oxide layer inside the Cu-enriched layer can prevent the oxide layer from inhibiting Cu elution, thereby improving antibacterial and antiviral properties. Whether or not an oxide layer exists inside the Cu-enriched layer can be determined by the presence or absence of a Si concentration peak at a position deeper than the Cu-enriched layer where the Cu concentration is 25% or more in a depth-direction component concentration profile obtained using glow discharge optical emission spectroscopy. In other words, if a Si concentration peak exists at a position deeper than the Cu-enriched layer where the Cu concentration is 25% or more, it can be said that an oxide layer exists inside the Cu-enriched layer.

[0026] The composition of the ferritic stainless steel material is not particularly limited, but preferably contains 13.00 to 32.00% Cr and 0.50 to 4.00% Cu.More preferably, the ferritic stainless steel material further contains 0.100% or less C, 1.00% or less Si, 3.00% or less Mn, 4.00% or less Ni, 0.050% or less P, 0.030% or less S, 0.100% or less N, a total of at least one of Ti and Nb: 6(C+N) to 1.00%, and 0.10% or less Al, with the balance being Fe and impurities. Furthermore, the ferritic stainless steel material may further contain one or more selected from Mo: 3.00% 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, as necessary.

[0027] In this specification, the composition of the ferritic stainless steel material means the composition of the base material (the portion other than the Cr-enriched layer and the oxide layer). In this specification, the term "impurities" refers to components that are mixed in during industrial production of ferritic stainless steel materials due to various factors in raw materials such as ores and scraps, and in the manufacturing process, and are acceptable within a range that does not adversely affect the present invention. For example, impurities also include unavoidable impurities. Regarding the content of each element, "including xx% or less" means that the content is xx% or less, but includes an amount exceeding 0% (particularly, exceeding the impurity level). Each component will be described in detail below.

[0028] <Cr:13.00~32.00%> Cr is an important element for ensuring the corrosion resistance and oxidation resistance of ferritic stainless steel materials. However, if the Cr content is too high, it will lead to an increase in refining costs, hardening due to solid solution strengthening (decrease in toughness), and a decrease in the workability of ferritic stainless steel materials. Therefore, the upper limit value of the Cr content is controlled to 32.00%, preferably 31.50%, more preferably 31.00%, and still more preferably 30.50%. On the other hand, if the Cr content is too low, sufficient corrosion resistance cannot be obtained. Therefore, the lower limit value of the Cr content is controlled to 13.00%, preferably 14.00%, more preferably 15.00%, and still more preferably 16.00%.

[0029] <Cu: 0.50~4.00%> Cu is an element necessary for the precipitation of the ε-Cu phase that provides antibacterial and antiviral properties and the formation of a Cr-concentrated layer by Cu diffusion heat treatment. To obtain this effect, the lower limit value of the Cu content is controlled to 0.50%, preferably 0.80%, more preferably 1.00%, and still more preferably 1.30%. On the other hand, if the Cu content is too high, the ferrite phase becomes unstable and the manufacturing cost also increases. Therefore, the upper limit value of the Cu content is controlled to 4.00%, preferably 3.50%, more preferably 3.00%, and still more preferably 2.50%.

[0030] <C: 0.100% or less> C is an element that improves the strength of ferritic stainless steel materials. However, if the C content is too high, in addition to becoming hard and having a decrease in workability, sensitization occurs when subjected to heat effects such as welding, and the corrosion resistance of ferritic stainless steel materials decreases. Therefore, the upper limit value of the C content is controlled to 0.100%, preferably 0.060%, more preferably 0.040%, and still more preferably 0.030%. On the other hand, the lower limit value of the C content is not particularly limited, but from the perspective of suppressing refining costs, it is preferably 0.001%, more preferably 0.003%, and still more preferably 0.005%.

[0031] <Si: 1.00% or less> Si is an element that generates a ferrite phase (α phase) and is an element effective in improving the corrosion resistance of ferritic stainless steel materials. However, if the Si content is too high, it will harden and the workability of the ferritic stainless steel material will decrease. Also, when welding is performed, the toughness of the welded part will decrease. Therefore, the upper limit value of the Si content is controlled to 1.00%, preferably 0.80%, more preferably 0.70%, and still more preferably 0.50%. On the other hand, the lower limit value of the Si content is not particularly limited, but from the viewpoint of obtaining the effect of Si, it is preferably 0.01%, more preferably 0.05%, and still more preferably 0.10%.

[0032] <Mn: 3.00% or less> Mn is an element that improves the heat resistance of ferritic stainless steel materials. However, if the Mn content is too high, MnS, which serves as a corrosion initiation point, is likely to be generated, and the ferrite phase becomes unstable. Therefore, the upper limit value of the Mn content is controlled to 3.00%, preferably 2.00%, more preferably 1.50%, and still more preferably 1.00%. On the other hand, the lower limit value of the Mn content is not particularly limited, but from the viewpoint of obtaining the effect of Mn, it is preferably 0.01%, more preferably 0.05%, and still more preferably 0.10%.

[0033] <Ni: 4.00% or less> Ni is an element that improves the corrosion resistance (especially intergranular corrosion resistance) of ferritic stainless steel materials. However, if the Ni content is too high, the ferrite phase becomes unstable, similar to Mn, and the manufacturing cost also increases. Therefore, the upper limit value of the Ni content is controlled to 4.00%, preferably 3.00%, more preferably 2.00%, and still more preferably 1.00%. On the other hand, the lower limit value of the Ni content is not particularly limited, but from the viewpoint of obtaining the effect of Ni, it is preferably 0.01%, more preferably 0.02%, and still more preferably 0.03%.

[0034] <P: 0.050% or less> Since ferritic stainless steel materials have a high Cu content, if the P content is high, it is easy to form Cu and intermetallic compounds with a low melting point. This intermetallic compound causes cracks and the like during hot rolling, so it is desirable to reduce the P content. Also, if the P content is high, it also causes a decrease in weldability and workability. Therefore, the upper limit value of the P content is controlled to 0.050%, preferably 0.045%, more preferably 0.040%. On the other hand, the lower limit value of the P content is not particularly limited, but since refining costs are incurred for reduction, it is preferably 0.001%, more preferably 0.005%, still more preferably 0.010%.

[0035] <S: 0.030% or less> Similar to P, if the S content is high, it is easy to form Cu and intermetallic compounds with a low melting point, which causes cracks and the like during hot rolling. Also, if the S content is high, MnS, which becomes a corrosion initiation point, is likely to be generated, and the toughness of the welded part decreases when welding is performed. Therefore, the upper limit value of the S content is controlled to 0.030%, preferably 0.020%, more preferably 0.010%. On the other hand, the lower limit value of the S content is not particularly limited, but since refining costs are incurred for reduction, it is preferably 0.0001%, more preferably 0.0005%, still more preferably 0.001%.

[0036] <N: 0.100% or less> N is an element effective for improving the corrosion resistance of ferritic stainless steel materials. However, if the N content is too high, in addition to becoming hard and reducing workability, sensitization occurs when subjected to the heat influence of welding or the like, and the corrosion resistance of the ferritic stainless steel material decreases. Therefore, the upper limit value of the N content is controlled to 0.100%, preferably 0.080%, more preferably 0.050%, still more preferably 0.030%. On the other hand, the lower limit value of the N content is not particularly limited, but from the viewpoint of suppressing refining costs, it is preferably 0.001%, more preferably 0.003%, still more preferably 0.005%.

[0037] <Total of at least one of Ti and Nb: 6(C + N) to 1.00%> Nb and Ti are elements that affect the intergranular corrosion resistance (sensitization suppression effect) of ferritic stainless steel materials. Also, Nb and Ti are effective elements for immobilizing C and N. Therefore, the total content of Nb and Ti is determined based on the amounts of C and N. That is, the lower limit value of the total content of Nb and Ti is 6(C + N)% from the viewpoint of obtaining the above effects. However, if the total content of Nb and Ti is too high, in addition to an increase in manufacturing cost, the workability and surface quality of the ferritic stainless steel material will deteriorate. Therefore, the upper limit value of the total content of Nb and Ti is 1.00%, preferably 0.80%, more preferably 0.60%. Also, the individual contents of Nb and Ti are preferably 0.05% or more, more preferably 0.06% or more, respectively, from the viewpoint of suppressing grain coarsening during annealing. In particular, Nb can effectively suppress grain coarsening by the pinning effect of carbonitrides such as NbC and NbN because it forms these carbonitrides.

[0038] <Al: 0.10% or less> Al is an element effective for improving the oxidation resistance of ferritic stainless steel materials. However, if the content of Al is too high, an Al oxide film is likely to be formed even in a non-oxidizing atmosphere during Cu diffusion heat treatment, and segregation also reduces productivity. Therefore, the upper limit value of the content of Al is controlled to 0.10%, preferably 0.08%, more preferably 0.05%. On the other hand, the lower limit value of the content of Al is not particularly limited, but is preferably 0.01%, more preferably 0.02%, from the viewpoint of obtaining the effect of Al.

[0039] <Mo: 3.00% or less> Mo is an element effective in improving the corrosion resistance and oxidation resistance of ferritic stainless steel materials. However, if the Mo content is too high, it will lead to a decrease in workability and an increase in manufacturing costs. Therefore, the upper limit value of the Mo content is controlled at 3.00%, preferably 2.50%, more preferably 2.00%. On the other hand, the lower limit value of the Mo content is not particularly limited, but from the perspective of obtaining the effect of Mo, it is preferably 0.01%, more preferably 0.03%, and even more preferably 0.05%.

[0040] <Zr: 1.00% or less, Co: 1.00% or less, V: 1.00% or less, W: 1.00% or less> Zr, Co, V, and W are elements effective in improving the oxidation resistance of ferritic stainless steel materials. However, if the contents of these elements are too high, it will lead to a decrease in workability and toughness and an increase in manufacturing costs. Therefore, the upper limit value of the contents of these elements is controlled at 1.00%, preferably 0.80%, more preferably 0.60% for all of them. On the other hand, the lower limit value of the contents of these elements is not particularly limited, but from the perspective of obtaining the effect of these elements, it is preferably 0.01%, more preferably 0.03%, and even more preferably 0.05% for all of them.

[0041] <REM: 0.100% or less, Ca: 0.100% or less, Sn: 0.100% or less> REM (rare earth elements), Ca, and Sn are elements effective in improving the oxidation resistance of ferritic stainless steel materials. However, if the contents of these elements are too high, it will lead to an increase in manufacturing costs. Therefore, the upper limit value of the contents of these elements is controlled at 0.100%, preferably 0.080%, more preferably 0.050% for all of them. On the other hand, the lower limit value of the contents of these elements is not particularly limited, but from the perspective of obtaining the effect of these elements, it is preferably 0.001%, more preferably 0.003%, and even more preferably 0.005% for all of them. Note that REM refers to the general term for two elements, scandium (Sc) and yttrium (Y), and 15 elements (lanthanoids) from lanthanum (La) to lutetium (Lu). These may be used alone or as a mixture.

[0042] <B: 0.0100% or less> B is an element effective for improving the secondary processing property of ferritic stainless steel. However, if the content of B is too high, it will cause a decrease in fatigue strength. Therefore, the upper limit value of the content of B is controlled to 0.0100%, preferably 0.0070%. On the other hand, the lower limit value of the content of B is not particularly limited, but from the viewpoint of obtaining the effect of B, it is preferably 0.0001%, more preferably 0.0003%.

[0043] The type of ferritic stainless steel according to the embodiment of the present invention is not particularly limited, but it is preferably a hot-rolled (hot-rolled annealed) material or a cold-rolled (cold-rolled annealed) material. In the case of a hot-rolled (hot-rolled annealed) material, its thickness is generally 3 mm or more. In the case of a cold-rolled (cold-rolled annealed) material, its thickness is generally less than 3 mm.

[0044] In the ferritic stainless steel according to the embodiment of the present invention, it is preferable that there are 100 or more ε-Cu phases with a major axis of 40 nm or more present in a cross-sectional area of 50 μm square. If the major axis of the ε-Cu phase is 40 nm or more, it can be said that the size of the ε-Cu phase is large. Also, if 100 or more ε-Cu phases of this size are present in a cross-sectional area of 五十μm square, it can be said that the amount of the ε-Cu phase is also large. Therefore, by having such characteristics, it can be said that coarse ε-Cu phases are present at the grain boundaries. The number of ε-Cu phases with a major axis of 40 nm or more present in a cross-sectional area of 50 μm square is more preferably 150 or more, and even more preferably 200 or more. On the other hand, the upper limit of the number of ε-Cu phases with a major axis of 40 nm or more present in a cross-sectional area of 50 μm square is not particularly limited, but for example, it is 600 or 500.

[0045] Here, the number of ε-Cu phases with a major axis of 40 nm or more present in a 50 μm square cross-sectional area is determined by observing the cross-section of a ferritic stainless steel material using a scanning electron microscope (SEM). Specifically, the number of ε-Cu phases with a major axis of 40 nm or more present in a 50 μm square cross-sectional area is determined as follows. First, a 15 mm square sample of the ferritic stainless steel material is cut out by cutting or the like, and the L-section (a cross-section parallel to the rolling direction and perpendicular to the rolling surface) is buffed to a mirror finish, and then finished by colloidal polishing to remove surface strain. A 50 μm square region of the L-section thus treated is observed using an SEM. ε-Cu phases are identified from the difference in contrast in the SEM backscattered electron image, and the number of ε-Cu phases with a major axis of 40 nm or more is counted. This procedure is performed in 10 fields of view to determine the number of ε-Cu phases with a major axis of 40 nm or more, and the average value is calculated. The major axis of the ε-Cu phase refers to the diameter of the longest part in a backscattered electron image of an SEM.

[0046] The ferritic stainless steel material according to the embodiment of the present invention has a Cu elution amount of 0.08 μg / cm 2 It is preferable that the amount of Cu elution is 0.10 μg / cm or more. If the amount of Cu elution is within this range, it can be said that the antibacterial and antiviral properties are excellent. The amount of Cu elution is 0.10 μg / cm or more. 2 More preferably, it is 0.15 μg / cm or more. 2 The upper limit of the amount of Cu elution is not particularly limited because the higher the amount, the better the antibacterial and antiviral properties. 2 , 0.80 μg / cm 2 or 0.50 μg / cm 2 is.

[0047] The amount of Cu elution in this specification is determined as follows. First, a 33 mm square sample is cut out from a ferritic stainless steel plate by cutting or the like, and the surface is degreased with acetone. Next, the sample is immersed in 25 mL of a 5% aqueous nitric acid solution (room temperature) for 2 hours, and the Cu concentration in the aqueous nitric acid solution after the test is analyzed by ICP analysis. The obtained Cu concentration is divided by the surface area of the sample to determine the Cu concentration per unit area (μg / cm 2 ) is calculated and this result is the amount of Cu elution.

[0048] The method for producing a ferritic stainless steel material according to an embodiment of the present invention is not particularly limited as long as it can produce a ferritic stainless steel material having the above-mentioned characteristics. A typical method for producing a ferritic stainless steel material according to an embodiment of the present invention will now be described.

[0049] A method for producing a ferritic stainless steel material according to an embodiment of the present invention includes a hot rolling step, a cooling step, an annealing step, and a Cu diffusion heat treatment step. The hot rolling process is a process in which a ferritic stainless steel slab is hot-rolled to obtain a hot-rolled material. Specifically, the ferritic stainless steel slab is subjected to rough rolling and then finish hot rolling (finish rolling) to obtain a hot-rolled material. This hot-rolled material may be wound into a coil. The ferritic stainless steel slab is not particularly limited, but can be obtained, for example, by melting stainless steel having the above composition and forging or casting it.

[0050] Rough rolling is performed at a higher temperature than finish hot rolling, and therefore has little effect on the strain applied to the hot-rolled material. Therefore, the conditions of rough rolling are not particularly limited and may be set appropriately depending on the composition of the ferritic stainless steel slab. On the other hand, the conditions of finish hot rolling affect the strain that acts as a driving force for recrystallization, so the rolling ratio and temperature of finish hot rolling are controlled from the perspective of applying sufficient strain to the hot-rolled material. The rolling ratio of the finish hot rolling (hereinafter referred to as "finish rolling ratio") is set to 50.0% or more. By controlling the finish rolling ratio within this range, sufficient strain that acts as a driving force for recrystallization is applied, making it possible to refine the crystal grains. From the viewpoint of stably ensuring this effect, the finish rolling ratio is preferably 51.0% or more, more preferably 52.0% or more. On the other hand, if the finish rolling ratio is less than 50.0%, the crystal grains will become coarse, resulting in a decrease in the amount of ε-Cu phase precipitated at the crystal grain boundaries between the crystal grains. The upper limit of the finish rolling ratio is not particularly limited, but is preferably 98.0%, more preferably 95.0%.

[0051] The finish hot rolling temperature (hereinafter referred to as "finish hot rolling temperature") is set to 800°C or higher, preferably 810°C or higher, and more preferably 820°C or higher. By controlling the finish hot rolling temperature within this range, coarsening of grains is suppressed while precipitation of the ε-Cu phase is minimized from the end of finish hot rolling to the cooling process. As a result, the ε-Cu phase at the grain boundaries grows during the annealing process, making it possible to increase the size and amount of the ε-Cu phase. If the finish hot rolling temperature is lower than 800°C, recrystallization is not completed to a certain extent at the end of finish hot rolling. As a result, the ε-Cu phase precipitates not only at the grain boundaries but also within the grains during the annealing process, resulting in insufficient size and amount of the ε-Cu phase at the grain boundaries. From the viewpoint of stably ensuring the above-mentioned effects, the upper limit of the finish hot rolling temperature is preferably 1050°C, more preferably 950°C, even more preferably 930°C, and particularly preferably 900°C.

[0052] The cooling step is performed by cooling the hot-rolled material obtained in the hot rolling step to a temperature of 500°C or less at an average cooling rate of 10.0°C / sec or more (preferably 11.0°C / sec or more, and more preferably 12.0°C / sec or more). Rapid cooling under these conditions can suppress the coarsening of crystal grains while also suppressing the precipitation of the ε-Cu phase within the crystal grains in the ε-Cu phase precipitation temperature range (900 to 500°C). On the other hand, if the average cooling rate is less than 10.0°C / sec, the crystal grains will become coarse. The upper limit of the average cooling rate is not particularly limited, but is, for example, 30.0°C / sec or 20.0°C / sec. The cooling method in the cooling step is not particularly limited, and any method known in the technical field (air cooling, water cooling, etc.) can be used.

[0053] The average grain size of the hot-rolled material after the cooling step is preferably 30 μm or less, more preferably 25 μm or less, and even more preferably 20 μm or less. With such an average grain size, it can be said that coarsening of the grains is sufficiently suppressed during the hot-rolling step and the cooling step. The lower limit of the average grain size is not particularly limited, as the smaller the better, but it is typically 1 μm, preferably 3 μm, and more preferably 5 μm. Here, the average grain size of the hot-rolled material is determined by analyzing the cross section of the hot-rolled material using EBSD, similar to the method for measuring the average grain size of the ferritic stainless steel material described above. Specifically, a 15 mm square sample of the hot-rolled material is cut out by cutting or the like, and the L-section (a section parallel to the rolling direction and perpendicular to the rolling surface) is mechanically polished to a mirror finish, followed by electrolytic polishing. For the L-section processed in this manner, EBSD analysis is performed on an 80 μm square area at 0.2 μm step intervals in a field of view of 1000x magnification, to identify grain boundaries with a crystal orientation misorientation of 2° or more. The area of each grain surrounded by this grain boundary is then defined as S [μm 2 ], the diameter of a circle having the same area as a crystal grain is defined as D [μm], and the crystal grain size (average crystal grain size per field of view) [μm] is calculated using the above formula (1). This operation is performed for five fields of view to determine the crystal grain size for each field, and the average value of these is defined as the average crystal grain size.

[0054] The annealing step is performed by annealing the hot-rolled material cooled in the cooling step at 750 to 900°C (preferably 770 to 880°C). This annealing step allows the fine "seeds" of the ε-Cu phase precipitated in the cooling step to grow. If the annealing temperature is less than 750°C, the fine "seeds" of the ε-Cu phase do not grow sufficiently, resulting in a small size of the ε-Cu phase. On the other hand, if the annealing temperature exceeds 900°C, the ε-Cu phase dissolves in the matrix phase, resulting in a small amount of the ε-Cu phase. The annealing time is not particularly limited and may be adjusted appropriately depending on the annealing temperature, but is preferably 4 hours or more, more preferably 4 to 20 hours, and even more preferably 5 to 10 hours.

[0055] The atmosphere of the annealing step is not particularly limited, and may be an oxidizing atmosphere or a non-oxidizing atmosphere. However, when the annealing step is performed in an oxidizing atmosphere, an oxide film removal step is further included after the annealing step. The oxide film removal step is not particularly limited, and may be performed according to a known method such as pickling or polishing.

[0056] Furthermore, when the ferritic stainless steel material is a cold-rolled material, the cold-rolling step is carried out after the annealing step. Note that when the oxide film removing step is carried out after the annealing step, the cold-rolling step may be carried out after the oxide film removing step, or may be carried out before the oxide film removing step. Furthermore, the cold-rolling step may be carried out after the oxide film removing step, and then the oxide film removing step may be carried out last. The cold rolling step is a step of cold rolling the hot-rolled annealed material obtained in the annealing step to obtain a cold-rolled material. The conditions of the cold rolling are not particularly limited and can be appropriately adjusted depending on the type of cold-rolled material required.

[0057] FIG. 1 shows an enlarged schematic diagram of a cross section of the hot-rolled and annealed material obtained in the annealing process. As shown in Fig. 1, the hot-rolled annealed material 10 includes crystal grains 20 and an ε-Cu phase 40 precipitated at grain boundaries 30 between the crystal grains 20. Therefore, a Cu-enriched layer can be formed on the outermost surface of the hot-rolled annealed material 10 by subjecting it to a Cu diffusion heat treatment, which will be described later.

[0058] The Cu diffusion heat treatment step is performed by heating the hot-rolled annealed material in a non-oxidizing atmosphere with a dew point of −30°C or lower at a temperature increase rate of 10.0°C / sec or higher up to 900°C, and then performing heat treatment at a temperature of 900°C or higher. This Cu diffusion heat treatment step diffuses Cu from the ε-Cu phase inside the hot-rolled annealed material, forming a Cu-enriched layer on the outermost surface. If the dew point, heating rate, and heat treatment temperature are outside the above ranges, a Cu-enriched layer with a Cu concentration of 25% or higher cannot be formed on the outermost surface, and an oxide layer is formed on the outermost surface. The dew point is preferably −40 to −100°C, more preferably −50 to −80°C. The heating rate is preferably 12.0 to 50.0°C / sec, more preferably 15.0 to 40.0°C / sec. The heat treatment temperature is preferably 920 to 1200°C, more preferably 950 to 1100°C. The non-oxidizing atmosphere is not particularly limited, but may be a hydrogen gas atmosphere, a nitrogen gas atmosphere, a mixed gas atmosphere of hydrogen gas and nitrogen gas, or the like.

[0059] The ferritic stainless steel material according to the embodiment of the present invention has excellent antibacterial and antiviral properties and can therefore be used for antibacterial and antiviral components.

[0060] An antibacterial and antiviral member according to an embodiment of the present invention includes the above-mentioned ferritic stainless steel material. The above-mentioned ferritic stainless steel material used in this antibacterial and antiviral member may be processed into various shapes by methods known in the technical field. The antibacterial and antiviral member according to the embodiment of the present invention may further include a member other than the above-mentioned ferritic stainless steel material. Examples of antibacterial and antiviral materials include, but are not limited to, various materials that are required to have antibacterial and antiviral properties and are used in kitchen equipment, home appliances, medical equipment, interior building materials for buildings, transportation equipment, laboratory equipment, sanitary equipment, etc. [Example]

[0061] 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 examples.

[0062] Ferritic stainless steels having the compositions of steel types A to H shown in Table 1 (the balance being Fe and impurities) were melted and forged to form ferritic stainless steel slabs, which were then hot-rolled to a thickness of 3 mm while controlling the finish rolling reduction and finish hot-rolling temperature as shown in Table 2 to obtain hot-rolled sheets. The rolling reduction was controlled by changing the thickness of the ferritic stainless steel slab. The finish rolling reduction was calculated by integrating the rolling reduction at a temperature at which the material temperature was 1050°C or lower. The finish hot-rolling temperature was controlled by adjusting the time from rough rolling to finish rolling. Next, the hot-rolled sheet was cooled to 500°C at the average cooling rate shown in Table 2. The average cooling rate was controlled by adjusting the conditions of air cooling and water cooling. Next, the cooled hot-rolled sheet was cut into a size of 300 mm (rolling direction) × 100 mm (width direction), placed in an EREMA electric furnace, and subjected to BAF annealing in an air atmosphere at the annealing temperature shown in Table 2 for 8 hours to obtain a hot-rolled annealed sheet. Next, the hot-rolled annealed sheet was descaled by dry honing and fluoronitric acid, and then dry polished using SiC polishing paper with a grit size of #600. Next, the hot-rolled and annealed sheet after dry polishing was cold-rolled to a thickness of 1 mm to obtain a cold-rolled sheet. Next, the cold-rolled sheet was subjected to Cu diffusion heat treatment under the conditions of dew point, heating rate, and heating rate shown in Table 2 to obtain a ferritic stainless steel sheet (cold-rolled annealed sheet). The atmosphere used was a hydrogen gas atmosphere. The dew point was controlled by changing the amount of water vapor introduced, and the heating rate was controlled by changing the distance between the cold-rolled sheet and the heating zone.

[0063] [Table 1]

[0064] [Table 2]

[0065] The above ferritic stainless steel sheets (cold-rolled and annealed sheets) were evaluated as follows.

[0066] (average grain size of crystal grains) The average grain size of the ferritic stainless steel sheets was determined using the above method. The average grain size of the grains in the hot-rolled sheets obtained during the manufacturing process of the ferritic stainless steel sheets was also determined using the same method. EBSD analysis was performed using a JXA-8900 manufactured by JEOL Ltd.

[0067] (Cu concentration in the outermost layer) The Cu concentration in the outermost layer of a ferritic stainless steel sheet was determined using the method described above. The Cu concentration in the depth direction component concentration profile was calculated at a position 0.5 nm deep from the surface. A GD-Profiler manufactured by Horiba, Ltd. was used for GD-OES. In this evaluation, if the Cu concentration in the outermost layer is 25% or more, it can be said that a Cu-enriched layer with a Cu concentration of 25 mass% or more has formed on the outermost surface.

[0068] (The number of ε-Cu phases with a major axis of 40 nm or more present in a 50 μm square cross-sectional area (hereinafter referred to as "the number of ε-Cu phases")) The number of ε-Cu phases in the ferritic stainless steel sheets was determined using the above method. The number of ε-Cu phases in the hot-rolled and annealed sheets obtained during the manufacturing process of the ferritic stainless steel sheets was also determined using the same method. A Schottky scanning electron microscope (SU5000, manufactured by Hitachi High-Technologies Corporation) was used as the SEM.

[0069] (Cu elution amount) The amount of Cu eluted from the ferritic stainless steel plate was determined according to the above method. For ICP analysis, an ICPE-9800 manufactured by Shimadzu Corporation was used.

[0070] The results of the above evaluations are shown in Table 3. As an example, a graph of the component concentration profile in the depth direction obtained by GD-OES in Example 1 is shown in FIG.

[0071] [Table 3]

[0072] As shown in Table 3, the ferritic stainless steel sheets of Examples 1 to 5 had a Cu concentration of 25% or more in the outermost layer (i.e., a Cu-enriched layer with a Cu concentration of 25% or more was formed on the outermost surface) and an average crystal grain size of 30 μm or less. Therefore, it was confirmed that the amount of Cu elution was large and that the sheets had excellent antibacterial and antiviral properties. Furthermore, as shown in the depth profile of the component concentration in Figure 2, the Si peak is located deeper than the Cu peak, confirming that an oxide layer has formed inside the Cu-enriched layer.

[0073] In contrast, the ferritic stainless steel sheets of Comparative Examples 1 to 11 had a Cu concentration of less than 25% in the outermost layer, and in some Comparative Examples the average crystal grain size exceeded 30 μm, resulting in a small amount of Cu elution and insufficient durability of the antibacterial and antiviral properties.

[0074] As can be seen from the above results, the present invention can provide a ferritic stainless steel material with excellent antibacterial and antiviral properties, a method for producing the same, and an antibacterial and antiviral member. [Explanation of symbols]

[0075] 10 Hot-rolled and annealed material 20 grains 30 Grain Boundaries 40 ε-Cu phase

Claims

1. A ferritic stainless steel material having a Cu-enriched layer on its outermost surface, the Cu concentration of which is 25 mass% or more, and an average crystal grain size of 30 μm or less.

2. The ferritic stainless steel material according to claim 1, further comprising an oxide layer on the inside of the Cu-enriched layer.

3. The ferritic stainless steel material according to claim 1 or 2, containing, by mass, Cr: 13.00 to 32.00% and Cu: 0.50 to 4.00%.

4. The ferritic stainless steel material according to claim 3, further comprising, on a mass basis, C: 0.100% or less, Si: 1.00% or less, Mn: 3.00% or less, Ni: 4.00% or less, P: 0.050% or less, S: 0.030% or less, N: 0.100% or less, a total of at least one of Ti and Nb: 6(C+N) to 1.00%, Al: 0.10% or less, and the balance consisting of Fe and impurities.

5. 5. The ferritic stainless steel material according to claim 4, further comprising, on a mass basis, one or more selected from Mo: 3.00% 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. Cu elution amount: 0.08 μg / cm 2 The ferritic stainless steel material according to claim 1 or 2.

7. The ferritic stainless steel material according to claim 1 or 2, wherein there are 100 or more ε-Cu phases having a major axis of 40 nm or more present in a 50 μm square cross-sectional area.

8. The ferritic stainless steel material according to claim 1 or 2, which is used for an antibacterial and antiviral component.

9. The method for producing a ferritic stainless steel material according to claim 1 or 2, a hot rolling step of hot rolling the ferritic stainless steel slab at a finishing rolling reduction of 50.0% or more and a finishing hot rolling temperature of 800°C or more to obtain a hot-rolled material; a cooling step of cooling the hot-rolled material to a temperature of 500°C or less at an average cooling rate of 10.0°C / sec or more; an annealing step of annealing the hot-rolled material cooled in the cooling step at a temperature of 750 to 900°C to obtain a hot-rolled annealed material; a Cu diffusion heat treatment process in which the hot-rolled annealed material is heated in a non-oxidizing atmosphere with a dew point of −30° C. or less in a temperature range up to 900° C. at a temperature increase rate of 10.0° C. / second or more, and then heat-treated at a temperature of 900° C. or more; Including, When the annealing step is carried out in an oxidizing atmosphere, the manufacturing method further comprises an oxide film removing step after the annealing step.

10. The manufacturing method according to claim 9, wherein the average grain size of the hot-rolled material after the cooling step is 30 μm or less.

11. The method according to claim 9, further comprising a cold rolling step of cold rolling the steel sheet after the annealing step to obtain a cold-rolled material.

12. An antibacterial and antiviral member comprising the ferritic stainless steel material according to claim 1 or 2.

Images