Stainless steel and its manufacturing method

Abstract

To provide a stainless steel that has both superior surface properties and excellent antibacterial properties.SOLUTION: A stainless steel has a properly controlled composition. On the surface of the stainless steel, the number of Ti oxide particles with a particle size of 0.5-5.0 μm is 5 or more and 100 or less per 1000 μm2. The sum X of Si, Al and Mn by atom% is 3.0 or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a stainless steel and a method for manufacturing the same, and in particular to a stainless steel that is applied to antibacterial materials used as raw materials for kitchen utensils, household items, sanitary items, medical equipment, building materials, etc., and a method for manufacturing the same. [Background technology]

[0002] Among steel materials, stainless steel is often used in applications requiring antibacterial properties, such as kitchen utensils, household goods, and building materials. For this reason, studies are being conducted to enhance the antibacterial properties of stainless steel by adding a photocatalyst, typically titanium oxide (TiO2), to stainless steel.

[0003] By absorbing light, photocatalysts exhibit the following two characteristic self-cleaning effects, which give them antibacterial properties. - A strong oxidizing power is generated, which breaks down bacteria. - Superhydrophilicity makes it difficult for water droplets to form on the material surface, preventing dirt from adhering.

[0004] Here, a commonly known method for applying a photocatalyst to stainless steel is, for example, a method in which a photocatalyst such as that disclosed in Patent Document 1 is applied to the surface of stainless steel to form a photocatalyst coating layer (hereinafter also referred to as the photocatalyst coating method).

[0005] However, the above photocatalytic coating method requires a coating step, which increases production costs.

[0006] Therefore, as another method for adding a photocatalyst to a material, a method is being considered in which a photocatalyst layer, for example, an oxide layer containing TiO2 (hereinafter also referred to as a TiO2 layer) is formed on the surface of stainless steel by heat treatment (hereinafter also referred to as an oxide layer formation method).

[0007] For example, Patent Document 2 states: "A method for producing stainless steel with an antibacterial coating, characterized by heat-treating a stainless steel material containing 0.1 to 1 weight percent Ti at a temperature of 850 to 1150°C in H2 gas or a mixed gas of 90 volume percent or more H2 and N2, with the dew point controlled at +10 to -65°C, to form a coating containing 20% ​​or more Ti as TiO2 on the surface of the material." has been disclosed.

[0008] Patent Document 3 states: "A stainless steel sheet exhibiting photocatalytic activity, characterized in that an oxide layer containing titanium oxide and having a thickness of 0.1 to 20 μm is formed on the surface of a stainless steel substrate containing 0.2 to 3 weight percent Ti, the concentration of Ti as titanium oxide being 3 atomic percent or more, and the content ratio of anatase in the titanium oxide being 1 volume percent or more." has been disclosed. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Publication No. 2018-111063 [Patent Document 2] Japanese Patent Application Publication No. 8-193218 [Patent Document 3] Japanese Patent Application Publication No. 10-121222 Summary of the Invention [Problem to be solved by the invention]

[0010] Meanwhile, in kitchen utensils, household goods, sanitary goods, medical equipment, building materials, and the like, the surface luster and color tone specific to stainless steel are often required from the standpoint of design and the like.

[0011] However, the TiO2 layer formed on the surface of stainless steel by the oxide layer formation methods typified by Patent Documents 2 and 3 may adversely affect the surface gloss and color tone (hereinafter also referred to as surface properties) of the stainless steel. Furthermore, Patent Document 3 requires a two-stage heat treatment to form the TiO2 layer. This imposes restrictions on the installation of equipment for such heat treatment, which further poses the problem of increased costs.

[0012] The present invention was developed in view of the above-mentioned current situation, and aims to provide a stainless steel that combines good surface properties with excellent antibacterial properties, together with a suitable method for producing the same.

[0013] Here, "good surface quality" means that the surface has the same surface gloss and color tone as SUS430LX.

[0014] "Excellent antibacterial properties" means a sterilization rate of 99.0% or more in an evaluation of antibacterial properties against Staphylococcus aureus. The sterilization rate is calculated from the number of viable bacteria measured in accordance with the film adhesion method specified in JIS R 1702:2020.

[0015] Detailed test methods are as described in the examples below. [Means for solving the problem]

[0016] The inventors have conducted extensive research to achieve the above object, and as a result have made the following findings. That is, by appropriately controlling the chemical composition of the stainless steel and subjecting a stainless steel material having that chemical composition to heat treatment and pickling under appropriate conditions, the following features (A) and (B) can be simultaneously obtained, thereby achieving the desired object. (A) Particle size on the surface of stainless steel: The number of Ti oxide particles with a size of 0.5 to 5.0 μm was counted over 1000 μm. 2 The number of prizes must be between 5 and 100. (B) The sum X of Si, Al and Mn in atomic percentage on the surface of the stainless steel is set to 3.0 or less. The present invention was completed based on the above findings and further investigations.

[0017] That is, the gist and configuration of the present invention are as follows. 1. By mass%, C: 0.001 to 0.030%, Si: 0.01 to 0.60%, Mn: 0.01 to 0.50%, P: 0.050% or less, S: 0.010% or less, Al: 0.001 to 0.050%, Cr: 15.0~25.0%, Ni: 0.01 to 2.00% Ti: 0.10 to 0.50% and N: 0.001 to 0.030% and the balance being Fe and unavoidable impurities, On the surface, Particle size: 0.5 to 5.0 μm Ti oxide particles count 1000 μm 2 There are 5 to 100 pieces per lot, A stainless steel in which the sum, X, of Si, Al and Mn in atomic % is 3.0 or less.

[0018] 2. The component composition further comprises, in mass%, Mo: 3.00% or less Cu: 1.00% or less, W: 0.50% or less, Co: 0.50% or less, Nb: 0.50% or less, Zr: 0.20% or less, V: 0.20% or less and B: 0.0100% or less 1. The stainless steel according to 1 above, containing one or more of the following:

[0019] 3. A step of subjecting a stainless steel material to heat treatment, the stainless steel material having the component composition described in 1 or 2 above; Next, a step of subjecting the treated material to pickling treatment; and In the heat treatment, Heat treatment atmosphere: O2 concentration of 1% by volume or more, Heat treatment temperature: 850~1000℃ and Heat treatment time: 5 seconds or more and 600 seconds or less and In the pickling treatment, Pickling dissolution amount: 1.0~12.0g / m 2 A method for manufacturing stainless steel. [Effects of the Invention]

[0020] According to the present invention, a stainless steel having both good surface properties and excellent antibacterial properties can be obtained, and is therefore particularly suitable for use as an antibacterial material for kitchen utensils, household items, sanitary items, medical equipment, building materials, etc. [Brief explanation of the drawings]

[0021] [Figure 1] 1 is a cross-sectional schematic view of a stainless steel according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0022] The present invention will be described based on the following embodiments.

[0023] [1] Stainless steel First, the chemical composition of a stainless steel according to one embodiment of the present invention will be described. Note that although the units for the chemical compositions are all "mass %," hereinafter, unless otherwise specified, they will simply be expressed as "%."

[0024] C: 0.001 to 0.030% C is an element that has the effect of increasing the strength of steel. This effect can be achieved by making the C content 0.001% or more. On the other hand, if the C content exceeds 0.030%, it is likely to lead to a decrease in workability. Therefore, the C content is set to 0.001 to 0.030%. The C content is preferably 0.002% or more. The C content is preferably 0.020% or less.

[0025] Si: 0.01 to 0.60% Si is an element useful for deoxidation. This effect can be achieved by setting the Si content to 0.01% or more. However, if the Si content exceeds 0.60%, a thick oxide film containing Si is formed during heat treatment. This makes it more likely that the oxide film will remain on the surface of the stainless steel even after pickling. If such an oxide film remains on the surface of the stainless steel, the antibacterial effect of the Ti oxide particles will decrease, and sufficient antibacterial properties will not be obtained. Therefore, the Si content is set to 0.01 to 0.60%. The Si content is preferably 0.30% or less.

[0026] Mn: 0.01 to 0.50% Mn is an element that has the effect of increasing the strength of steel. This effect can be achieved by setting the Mn content to 0.01% or more. However, if the Mn content exceeds 0.50%, a thick oxide film containing Mn is formed during heat treatment. This makes it more likely that the oxide film will remain on the surface of the stainless steel even after pickling. If such an oxide film remains on the surface of the stainless steel, the antibacterial effect of the Ti oxide particles will decrease, and sufficient antibacterial properties will not be obtained. Therefore, the Mn content is set to 0.01 to 0.50%. The Mn content is preferably 0.05% or more. The Mn content is preferably 0.30% or less.

[0027] P:0.050% or less P is an element that is inevitably contained in steel and reduces the corrosion resistance of stainless steel. Therefore, the lower the P content, the better. Therefore, the P content is set to 0.050% or less. The P content is preferably 0.040% or less, and more preferably 0.030% or less. There is no particular restriction on the lower limit of the P content. However, excessive dephosphorization leads to increased costs. Therefore, the P content is preferably set to 0.010% or more.

[0028] S: 0.010% or less S is an element that is inevitably contained in steel and reduces the corrosion resistance of stainless steel. In particular, if the S content exceeds 0.010%, the formation of water-soluble sulfides such as CaS and MnS is promoted, which is likely to lead to a decrease in corrosion resistance. Therefore, the S content is set to 0.010% or less. There is no particular lower limit for the S content. However, excessive desulfurization leads to an increase in costs. Therefore, it is preferable that the S content be 0.001% or more.

[0029] Al: 0.001 to 0.050% Al is an element useful for deoxidation. This effect can be achieved by setting the Al content to 0.001% or more. However, if the Al content exceeds 0.050%, a thick oxide film containing Al is formed during heat treatment. This makes it more likely that the oxide film will remain on the surface of the stainless steel even after pickling. If such an oxide film remains on the surface of the stainless steel, the antibacterial effect of the Ti oxide particles will be reduced, and sufficient antibacterial properties will not be obtained. Therefore, the Al content is set to 0.001 to 0.050%. The Al content is preferably 0.030% or less.

[0030] Cr: 15.0~25.0% Cr is an element that improves the corrosion resistance of stainless steel. To obtain the corrosion resistance required for use in kitchen utensils, household goods, building materials, etc., the Cr content is set to 15.0% or more. On the other hand, if the Cr content exceeds 25.0%, the corrosion resistance becomes excessively high. As a result, it becomes necessary to increase the acid concentration and temperature of the treatment solution during pickling treatment, making it difficult to control the amount of pickling dissolved, and ultimately, sufficient antibacterial properties may not be obtained. Therefore, the Cr content is set to 15.0 to 25.0%. The Cr content is preferably 18.0% or more. The Cr content is preferably 23.0% or less.

[0031] Ni: 0.01 to 2.00% Ni is an element that improves the corrosion resistance of stainless steel. This effect can be achieved by making the Ni content 0.01% or more. However, if the Ni content exceeds 2.00%, the risk of stress corrosion cracking increases. Therefore, the Ni content is set to 0.01 to 2.00%. The Ni content is preferably 0.05% or more. The Ni content is preferably 0.60% or less.

[0032] Ti: 0.10 to 0.50% Ti is an element necessary for enhancing antibacterial properties. In particular, to ensure a sufficient number of Ti oxide particles of a predetermined size, the Ti content is set to 0.10% or more. On the other hand, if the Ti content exceeds 0.50%, the toughness of the steel decreases, making it difficult to manufacture the steel. Therefore, the Ti content is set to 0.10 to 0.50%. The Ti content is preferably 0.20% or more. The Ti content is preferably 0.35% or less.

[0033] N: 0.001 to 0.030% N is an element that has the effect of increasing the strength of steel through solid solution strengthening. This effect can be achieved by setting the N content to 0.001% or more. However, if the N content exceeds 0.030%, workability is likely to decrease. Therefore, the N content is set to 0.001 to 0.030%. The N content is preferably 0.002% or more. The N content is preferably 0.020% or less.

[0034] The basic component composition of a stainless steel according to one embodiment of the present invention has been described above. However, the component composition of a stainless steel according to one embodiment of the present invention may further contain at least one of the following optional additional elements, either alone or in combination. Mo: 3.00% or less Cu: 1.00% or less, W: 0.50% or less, Co: 0.50% or less, Nb: 0.50% or less, Zr: 0.20% or less, V: 0.20% or less and B: 0.0100% or less

[0035] Mo: 3.00% or less Mo is an element that improves the corrosion resistance of stainless steel. From the viewpoint of obtaining this effect, the Mo content is preferably 0.01% or more. However, if the Mo content exceeds 3.00%, the strength increases excessively, which tends to lead to a decrease in workability. Therefore, when Mo is contained, the content is preferably 3.00% or less. The Mo content is more preferably 2.00% or less.

[0036] Cu:1.00% or less Cu is an element that improves the corrosion resistance of stainless steel. From the viewpoint of obtaining this effect, it is preferable that the Cu content be 0.01% or more. However, if the Cu content exceeds 1.00%, it may induce the formation of coarse ε-Cu, which may result in a decrease in corrosion resistance. Therefore, when Cu is contained, its content is preferably 1.00% or less. The Cu content is more preferably 0.60% or less.

[0037] W: 0.50% or less Like Mo, W is an element that improves the corrosion resistance of stainless steel. From the viewpoint of obtaining this effect, it is preferable that the W content be 0.01% or more. However, if the W content exceeds 0.50%, the strength increases excessively, which is likely to lead to a decrease in workability. Therefore, when W is contained, its content is preferably 0.50% or less.

[0038] Co:0.50% or less Co is an element that improves toughness. From the viewpoint of obtaining this effect, the Co content is preferably 0.01% or more. However, if the Co content exceeds 0.50%, workability is likely to be reduced. Therefore, when Co is contained, the content is preferably 0.50% or less.

[0039] Nb: 0.50% or less Nb combines with C and N to suppress sensitization. From the viewpoint of obtaining this effect, the Nb content is preferably 0.01% or more. However, if the Nb content exceeds 0.50%, it is likely to cause a decrease in workability. Therefore, when Nb is contained, its content is preferably 0.50% or less. The Nb content is more preferably 0.20% or less, and further preferably 0.10% or less.

[0040] Zr: 0.20% or less Zr combines with C and N to suppress sensitization. From the viewpoint of obtaining this effect, the Zr content is preferably 0.01% or more. However, if the Zr content exceeds 0.20%, it is likely to cause a decrease in workability. Therefore, when Zr is contained, the content is preferably 0.20% or less. The Zr content is more preferably 0.10% or less.

[0041] V:0.20% or less V is an element that forms VN and thereby suppresses the deterioration of corrosion resistance due to the precipitation of Cr nitrides. From the viewpoint of obtaining this effect, the V content is preferably 0.01% or more. However, if the V content exceeds 0.20%, it is likely to cause a deterioration in workability. Therefore, when V is contained, its content is preferably 0.20% or less. The V content is more preferably 0.10% or less.

[0042] B: 0.0100% or less B is an element that improves secondary work embrittlement. From the viewpoint of obtaining this effect, the B content is preferably 0.0001% or more. However, if the B content exceeds 0.0100%, solid solution strengthening tends to cause a decrease in workability. Therefore, when B is contained, the content is preferably 0.0100% or less. The B content is more preferably 0.0030% or less.

[0043] The balance other than the above elements is Fe and inevitable impurities. Note that any of the above optional added elements may be 0%. Furthermore, when the content of each of the above optional added elements is less than the preferable lower limit, it can be said that the element is contained as an inevitable impurity.

[0044] As described above, it is extremely important that the stainless steel according to one embodiment of the present invention simultaneously satisfies the following requirements (A) and (B). (A) Particle size on the surface of stainless steel: The number of Ti oxide particles with a size of 0.5 to 5.0 μm was counted over 1000 μm. 2 The number of prizes must be between 5 and 100. (B) The sum X of Si, Al and Mn in atomic percentage on the surface of the stainless steel is set to 3.0 or less.

[0045] The number of Ti oxide particles with a particle size of 0.5 to 5.0 μm on the surface of stainless steel (hereinafter also referred to as the number of surface Ti oxide particles): 1000 μm 2 5 to 100 winning pieces FIG. 1 shows a schematic cross-sectional view of a stainless steel according to one embodiment of the present invention. In the figure, reference numeral 1 denotes stainless steel, 2 denotes Ti oxide particles, and 3 denotes light. As shown in FIG. 1, Ti oxide particles are dispersed near the surface of the stainless steel. When light is irradiated onto the Ti oxide particles exposed on the surface of the stainless steel (hereinafter also referred to as surface Ti oxide particles), excellent antibacterial properties are exhibited. From the viewpoint of obtaining the above-mentioned effect, the number of surface Ti oxide particles is set to 1000 μm 2 The number of surface Ti oxide particles is 5 or more per 1000 μm. 2If the number of surface Ti oxide particles is less than 5 per 1000 μm, the antibacterial effect is not fully exerted and excellent antibacterial properties cannot be obtained. 2 On the other hand, if the number of surface Ti oxide particles becomes excessive, good surface properties cannot be obtained. 2 The number of surface Ti oxide particles is preferably 100 or less per 1000 μm 2 There are 50 or fewer per item.

[0046] Here, the reason why particles having a particle size of 0.5 to 5.0 μm are counted as surface Ti oxide particles is as follows. That is, particles with a particle size of less than 0.5 μm are easily affected by surface properties such as passive films, oxide films, and deposits, and do not exhibit sufficient antibacterial effects. Furthermore, there is a risk that the particles may be lost due to wear during use. On the other hand, particles with a particle size of more than 5.0 μm may fall off during processing into products or during use. Furthermore, if a large number of particles with a particle size of more than 5.0 μm are produced, the number of particles decreases and the distance between particles increases. As a result, the antibacterial effect is not exhibited sufficiently, and excellent antibacterial properties cannot be obtained. Therefore, particles with a particle size of 0.5 to 5.0 μm are counted as surface Ti oxide particles.

[0047] In addition, in the stainless steel according to one embodiment of the present invention, the particle size of the surface Ti oxide particles is smaller and the number of particles is smaller than in the TiO2 layer formed on the surface of stainless steel by the oxide layer formation methods typified by Patent Documents 2 and 3. This makes it possible for the stainless steel according to one embodiment of the present invention to achieve both the surface properties unique to stainless steel and excellent antibacterial properties.

[0048] The number of surface Ti oxide particles is measured using a field emission electron probe microanalyzer (hereinafter also referred to as FE-EPMA) as follows, for example. The surface of the stainless steel is degreased and washed with ethanol. Then, the surface composition of the stainless steel is measured by FE-EPMA at an acceleration voltage of 10 kV and an irradiation current of 2 × 10-7 An elemental map is created under the conditions of A and measurement field of view: 100 × 90 μm. From the obtained elemental map, the distribution positions of Ti and O are superimposed, and the overlapping region is identified as Ti oxide particles. Next, by image processing, the number of particles with a particle size of 0.5 to 5.0 μm is counted within the region identified as Ti oxide particles, and a 1000 μm area is calculated from the area of ​​the measurement field of view. 2 The particle size is calculated by converting the area of ​​the region into a circle-equivalent diameter. This measurement is carried out at three arbitrary locations on the surface of the stainless steel, and the average value of the three locations is taken as the number of Ti oxide particles on the stainless steel.

[0049] Surface Ti oxide particles may contain impurity elements, such as N, Fe, Cr, Zr, V, Mo, W, Ni, and Co. Surface Ti oxide particles measured by FE-EPMA may contain Ti carbides or Ti nitrides when observed on the nanometer order using a transmission electron microscope or the like.

[0050] The sum of Si, Al, and Mn in atomic percent on the surface of stainless steel, X: 3.0 or less The condition of the oxide film on the surface of stainless steel is important for the antibacterial effect of surface Ti oxide particles. As described below, heat treatment in an O2-containing atmosphere is required to generate Ti oxide particles in stainless steel. However, heat treatment in such an O2-containing atmosphere oxidizes the Cr, Si, Al, and Mn contained in the stainless steel, forming an oxide film containing these elements on the stainless steel surface. The presence of such an oxide film on the stainless steel surface prevents light from reaching the Ti oxide particles, preventing the antibacterial effect of the Ti oxide particles. Furthermore, contact or proximity between bacteria and the Ti oxide particles may be prevented, potentially preventing the antibacterial effect of the Ti oxide particles. Therefore, it is important to reduce the amount of such oxide film to a certain level, for example, by pickling. The inventors conducted extensive research into the relationship between the surface analysis of stainless steel, the amount of oxide film remaining after pickling, and photocatalytic properties, and obtained the following findings. That is, the amount of oxide film remaining on the surface of stainless steel can be expressed as the sum X of Si, Al, and Mn on the surface of the stainless steel in atomic % (X = Si + Al + Mn, where Si, Al, and Mn are the concentrations (atomic %) of each element present on the surface of the stainless steel). By setting X to 3.0 or less, a predetermined amount of surface Ti oxide particles can be secured that are irradiated with light, which promotes the prevention of bacterial growth and sterilization, making it possible to obtain an excellent antibacterial effect. Therefore, X is set to 3.0 or less. X is preferably less than 1.0. There is no particular restriction on the lower limit of X. For example, X is preferably 0.1 or more. Furthermore, X is more preferably 0.4 or more.

[0051] Here, X is measured by X-ray photoelectron spectroscopy (hereinafter also referred to as XPS) as follows, for example. The stainless steel surface is degreased and cleaned with ethanol. Next, a wide spectrum is obtained from the stainless steel surface using X-ray photoelectron spectroscopy (XPS). The evaluation area may be, for example, Φ600 μm. Next, background processing is performed on each confirmed peak using the iterated Shierley method, and the area intensity of each element detected by qualitative analysis is calculated. Next, the area intensity of each element is calculated (by correction calculation) using general-purpose analysis software, and the quantitative value (atomic %) of each element is calculated. The total atomic % of Si, Al, and Mn is then defined as X. Note that the quantitative value of elements below the lower limit of detection in qualitative analysis may be set to 0.

[0052] Furthermore, examples of the shape of the stainless steel according to one embodiment of the present invention include a steel plate (stainless steel plate) and a rod (stainless steel rod and wire).

[0053] Furthermore, the thickness of the stainless steel according to one embodiment of the present invention (meaning the plate thickness for a steel plate and the diameter for a wire rod) is not particularly limited, but is preferably 0.10 to 3.00 mm. The thickness of the stainless steel according to one embodiment of the present invention is more preferably 0.30 mm or more. The thickness of the stainless steel according to one embodiment of the present invention is more preferably 2.00 mm or less.

[0054] The thickness direction is the thickness direction for steel plates and the radial direction for wire rods (the direction from the surface toward the center of a circular cross section perpendicular to the axial direction). The surface does not include end faces (for example, surfaces other than the flat surface of a steel plate or the surfaces at both ends of a wire rod).

[0055] Additionally, the properties of the stainless steel according to one embodiment of the present invention are as described above.

[0056] [2] Stainless steel manufacturing method Next, a method for producing stainless steel according to one embodiment of the present invention will be described.

[0057] A method for producing stainless steel according to one embodiment of the present invention includes: A step of subjecting a stainless steel workpiece having the above-mentioned component composition to heat treatment; Next, a step of subjecting the treated material to pickling treatment; and In the heat treatment, Heat treatment atmosphere: O2 concentration of 1% by volume or more, Heat treatment temperature: 850~1000℃ and Heat treatment time: 5 seconds or more and 600 seconds or less and In the pickling treatment, Pickling dissolution amount: 1.0~12.0g / m 2 That is what it is. Also, a method for producing stainless steel according to one embodiment of the present invention is a method for producing stainless steel according to the above-described one embodiment of the present invention. Unless otherwise specified, the temperatures in the manufacturing methods are based on the surface temperatures of steel slabs, hot-rolled steel sheets, cold-rolled steel sheets, materials to be treated, etc.

[0058] [Preparation process] First, a stainless steel material to be treated (a material to be subjected to the heat treatment described below) having the above-mentioned composition is prepared. The method for preparing the material to be treated is not particularly limited, and the material may be prepared according to a conventional method. Here, an example will be described in which a stainless steel plate is prepared as the material to be treated. First, a steel slab having the above-described chemical composition is heated to 1100 to 1300°C. Next, the steel slab is hot-rolled to produce a hot-rolled steel plate having a thickness of, for example, 2.0 to 15.0 mm. Next, the hot-rolled steel plate is annealed in a temperature range of 800 to 1100°C (hot-rolled plate annealing). Next, the hot-rolled steel plate is pickled to remove scale. In the pickling, a descaling treatment using a mechanical action such as shot blasting may be performed. Alternatively, scale may be removed by grinding using a grinder or an abrasive belt instead of the pickling. Next, the hot-rolled steel plate is cold-rolled to produce a cold-rolled steel plate having a thickness of, for example, 0.3 to 3.0 mm. In the cold rolling, the reduction is preferably 50% or more, more preferably 70% or more. In this way, a stainless steel plate can be prepared as the material to be treated. Also, when preparing a stainless steel bar or wire as the material to be treated, the stainless steel bar or wire can be prepared by subjecting a steel material having the above-mentioned composition to hot working and cold working under the same conditions as above.

[0059] [Heat treatment process] Next, the material to be treated prepared as described above is subjected to a heat treatment under the following conditions.

[0060] Heat treatment atmosphere: O2 concentration of 1% by volume or more This heat treatment internally oxidizes Ti contained in the stainless steel material to form dispersed Ti oxide particles near the surface of the stainless steel. If the O2 concentration in the heat treatment atmosphere is less than 1% by volume, Ti oxide particles of the desired size are not generated in sufficient numbers, and excellent antibacterial properties cannot be obtained. Therefore, the O2 concentration in the heat treatment atmosphere is set to 1% by volume or higher. The O2 concentration in the heat treatment atmosphere is preferably 5% by volume or higher. On the other hand, if the O2 concentration in the heat treatment atmosphere exceeds 25% by volume, an excessive oxide film may be formed, making it difficult to remove the oxide film by the pickling treatment described below. Therefore, the O2 concentration in the heat treatment atmosphere is preferably 25% by volume or lower. The O2 concentration in the heat treatment atmosphere is more preferably 15% by volume or lower. The gas in the heat treatment atmosphere may contain N2, CO2, rare gases, and water vapor. In other words, gases other than O2 in the heat treatment atmosphere include, for example, N2, CO2, rare gases, and water vapor.

[0061] Heat treatment temperature: 850~1000℃ If the heat treatment temperature is less than 850°C, the generation of Ti oxide particles does not proceed, and a sufficient number of Ti oxide particles of the desired size cannot be obtained. On the other hand, if the heat treatment temperature exceeds 1000°C, the Ti oxide particles become coarse, and a sufficient number of Ti oxide particles of the desired size cannot be obtained. Furthermore, if the heat treatment temperature exceeds 1000°C, a thick oxide film is formed, making it difficult to remove the oxide film in the pickling treatment described below. Therefore, the heat treatment temperature is set to 850°C to 1000°C. The heat treatment temperature is preferably 900°C or higher. The heat treatment temperature is preferably 950°C or lower.

[0062] Heat treatment time: 5 seconds or more and 600 seconds or less If the heat treatment time is less than 5 seconds, the generation of Ti oxide particles does not proceed, and a sufficient number of Ti oxide particles of the desired size cannot be obtained. Therefore, the heat treatment time is set to 5 seconds or more. The heat treatment time is preferably set to 10 seconds or more. On the other hand, if the heat treatment time exceeds 600 seconds, Ti oxide particles of the desired size are generated in excess, and good surface properties cannot be obtained. Therefore, the heat treatment time is set to 600 seconds or less. The heat treatment time is preferably set to 90 seconds or less.

[0063] [Acid washing process] Next, the material to be treated is subjected to pickling treatment under the following conditions.

[0064] Pickling dissolution amount: 1.0~12.0g / m 2 Pickling is an important process for obtaining the antibacterial effect (photocatalytic effect) of Ti oxide particles. As mentioned above, Ti oxide particles exhibit excellent antibacterial properties when irradiated with light. Therefore, the Ti oxide particles must be exposed and dispersed on the surface of the stainless steel. In other words, if a thick oxide film exists on the surface of the stainless steel, excellent antibacterial properties cannot be obtained. Therefore, it is important to remove the oxide film by pickling. Here, the pickling dissolution amount is 1.0 g / m 2 If the amount of acid dissolved in the pickling is less than 12.0 g / m, the oxide film is not sufficiently removed and excellent antibacterial properties cannot be obtained. 2 If the amount of dissolution in pickling exceeds 1.0 to 12.0 g / m, the Ti oxide particles formed in the heat treatment will also be dissolved and removed. As a result, the antibacterial effect will be reduced and excellent antibacterial properties will not be obtained. Therefore, the amount of dissolution in pickling should be 1.0 to 12.0 g / m 2 The amount of acid to be dissolved in the pickling is preferably 2.0 g / m 2 The amount of acid to be dissolved in the pickling is preferably 4.0 g / m 2 The following is the result.

[0065] The amount of acid dissolved in the pickling is calculated by the following formula. Pickling dissolution amount (g / m 2 ) = (mass of the material before pickling (g) - mass of the material after pickling (g)) / surface area of ​​the material (m 2 ) The surface area of ​​the material to be treated does not include the area of ​​the end faces.

[0066] As the pickling method, for example, a neutral salt electrolysis-nitric hydrofluoric acid pickling method or a nitric hydrochloric acid electrolysis method can be used. In the neutral salt electrolysis-nitric hydrofluoric acid pickling method, for example, the material to be treated is immersed in a neutral salt solution of Na2SO4: 100-300g / L at a temperature of 50-90°C, with a current density of ±5-±100C / dm 2 Then, the material is immersed in a nitric hydrofluoric acid aqueous solution containing 20 to 100 g / L of HNO3 and 5 to 50 g / L of HF at a temperature of 40 to 80°C for 10 to 150 seconds. In the nitric acid electrolysis method, for example, the material to be treated is immersed in a mixed acid solution of 100 to 160 g / L nitric acid and 1 to 20 g / L hydrochloric acid at a temperature of 25 to 55°C, and heated to +1 to 50°C / dm 2 →-1~50C / dm 2 The electrolytic pickling is carried out multiple times.

[0067] The amount of acid dissolved in the pickling can be controlled by adjusting, within the above-mentioned ranges, the temperature, concentration, current density, and immersion time of the treatment solution in the neutral salt electrolysis-nitric hydrofluoric acid pickling method and the nitric hydrochloric acid electrolysis method exemplified above.

[0068] Conditions other than those mentioned above are not particularly limited, and may be carried out in accordance with conventional methods. [Example]

[0069] Steel having the chemical composition shown in Table 1 (the balance being Fe and unavoidable impurities) was produced. The resulting steel ingot was then heated at 1250°C for 1 hour and hot-rolled to produce a hot-rolled steel sheet with a thickness of 3.0 mm. This hot-rolled steel sheet was then annealed at 950°C for 10 minutes. Oxide scale was then removed from the surface of the hot-rolled steel sheet by grinding. This hot-rolled steel sheet was then cold-rolled to produce a cold-rolled steel sheet with a thickness of 1.0 mm. The cold-rolled steel sheet thus prepared was used as the material to be treated and subjected to heat treatment and pickling under the conditions shown in Table 2 to obtain the final stainless steel sheet. The pickling was performed using a neutral salt electrolysis-nitric hydrofluoric acid pickling method. In neutral salt electrolysis, a neutral salt solution of Na2SO4:200g / L is used, and the current density is ±5 to ±80C / dm 2For the nitric hydrofluoric acid pickling, a nitric hydrofluoric acid aqueous solution of 50 g / L HNO3 and 34 g / L HF was used. The amount of pickling dissolved was controlled by adjusting the temperature and current density of the neutral salt electrolysis treatment solution, and the temperature and immersion time of the nitric hydrofluoric acid pickling treatment solution. Conditions not specified were those according to standard methods.

[0070] The stainless steel sheets thus obtained were subjected to the evaluation of surface properties (1) described below, and then the number of surface Ti oxide particles and X were measured in the same manner as above. The measurement results are shown in Table 2.

[0071] In addition, (1) surface properties and (2) antibacterial properties were evaluated according to the following test methods. The evaluation results are shown in Table 2.

[0072] (1)Surface texture The appearance of the obtained stainless steel sheet was visually inspected, and the surface properties were evaluated according to the following criteria while comparing with the appearance of separately prepared SUS430LX (No. 2D surface finish). Good (pass): Has the same gloss and color as SUS430LX (No. 2D surface finish). Poor (failed): Compared to SUS430LX (No. 2D surface finish), at least one of the gloss and color tone is inferior.

[0073] (2) Antibacterial properties The antibacterial properties were evaluated in accordance with the film adhesion method specified in JIS R 1702:2020 (Fine ceramics - Antibacterial test method and antibacterial effect of photocatalytic antibacterial processed materials). The specific procedure is as follows. The surface of the test piece cut out from the obtained stainless steel (30 mm 2 ) and degrease with 99.5% ethanol. Staphylococcus aureus (2.2 × 10 6 0.4 mL of the solution (particles / mL) is dropped onto the surface of the test piece. -Cover the surface of the test piece with PE film. - UV black light (0.25mW / cm) is applied to the surface of the test specimen. 2 ) for 6 hours. 10 mL of the SCDLP medium used for washing out live bacteria is collected and diluted 10 times with saline. Then, it is cultured on agar medium at 35°C for 40 hours. - Measure the bacterial population after cultivation, i.e., the viable cell count. The antibacterial activity was evaluated according to the following evaluation criteria, using the sterilization rate defined by the following formula as an index. Bacterial sterilization rate (%) = (control bacterial count - post-test bacterial count) / control bacterial count x 100 Here, the control bacterial count is the number of live bacteria measured using a sterilized glass plate in the same manner as above. The viable bacterial count was measured three times, and the average value was used as the control bacterial count. The control bacterial count was 1.5 x 10 6 The number of bacteria after the test was the number of viable bacteria measured on each test piece. Evaluation criteria Excellent (pass, particularly excellent): sterilization rate of 99.9% or more Good (pass, excellent): Sterilization rate of 99.0% or more but less than 99.9% Poor (failed): Sterilization rate less than 99.0%

[0074] [Table 1]

[0075] [Table 2]

[0076] As shown in Table 2, all of the inventive examples had good surface properties and excellent antibacterial properties.

[0077] On the other hand, in the comparative examples, at least one of the surface properties and the antibacterial properties was not sufficient. That is, in No. 10, the Ti content was below the appropriate range, so the number of surface Ti oxide particles was insufficient, and the antibacterial property was unacceptable. In No. 11, the O2 concentration in the heat treatment atmosphere was below the appropriate range, so the number of Ti oxide particles on the surface was insufficient, and the antibacterial properties were unacceptable. In No. 12, the heat treatment temperature exceeded the appropriate range, so the number of Ti oxide particles on the surface was insufficient, and the antibacterial properties were unacceptable. In No. 13, the heat treatment temperature did not satisfy the appropriate range, so the number of Ti oxide particles on the surface was insufficient, and the antibacterial properties were unacceptable. In No. 14, the heat treatment time did not satisfy the appropriate range, so the number of Ti oxide particles on the surface was insufficient, and the antibacterial property was unacceptable. In No. 15, the amount of pickling dissolved did not satisfy the appropriate range, so a thick oxide film remained on the surface of the stainless steel, resulting in unacceptable surface properties and antibacterial properties. For No. 16, the amount of pickling dissolved exceeded the appropriate range, resulting in a rough surface and a failure in surface quality. Furthermore, the titanium oxide particles in the stainless steel were dissolved and removed, resulting in an insufficient number of titanium oxide particles on the surface, and the antibacterial properties also failed. For No. 17, the heat treatment time exceeded the appropriate range, causing the number of Ti oxide particles on the surface to exceed the appropriate range, resulting in a surface quality that was unacceptable.

[0078] Furthermore, the Staphylococcus aureus used in the above (2) antibacterial evaluation does not have an outer membrane. Such bacteria are classified as Gram-positive bacteria. On the other hand, bacteria with an outer membrane are classified as Gram-negative bacteria. A representative example of Gram-negative bacteria is Escherichia coli. For reference, the antibacterial activity of the above-mentioned Example No. 1 against Escherichia coli was evaluated.

[0079] That is, similar to the evaluation of antibacterial properties in (2) above, the antibacterial properties against E. coli were evaluated in accordance with the film adhesion method specified in JIS R 1702:2020 (Fine ceramics - Antibacterial test method and antibacterial effect of photocatalytic antibacterial processed materials). The specific procedure is as follows. The surface of the test piece cut out from the obtained stainless steel (30 mm 2 ) and degrease with 99.5% ethanol. E. coli (1.8 × 10 6 0.4 mL of the solution (particles / mL) is dropped onto the surface of the test piece. -Cover the surface of the test piece with PE film. - UV black light (0.25mW / cm) is applied to the surface of the test specimen. 2 ) for 6 hours. 10 mL of the SCDLP medium used for washing out live bacteria is collected and diluted 10 times with saline. Then, it is cultured on agar medium at 37°C for 40 hours. - Measure the bacterial population after cultivation, i.e., the viable cell count. The antibacterial activity was evaluated according to the following evaluation criteria, using the sterilization rate defined by the following formula as an index. Bacterial sterilization rate (%) = (control bacterial count - post-test bacterial count) / control bacterial count x 100 Here, the control bacterial count is the number of live bacteria measured using a sterilized glass plate in the same manner as above. The viable bacterial count was measured three times, and the average value was used as the control bacterial count. The control bacterial count was 1.8 x 10 6 The number of bacteria after the test was the number of viable bacteria measured on each test piece. Evaluation criteria Good (pass, excellent): Sterilization rate of 99.0% or more Poor (failed): Sterilization rate less than 99.0%

[0080] As a result of the above evaluation, No. 1 exhibited excellent antibacterial properties against Escherichia coli as well.

[0081] Furthermore, when the antibacterial properties of the above-mentioned Nos. 2 to 9, which are inventive examples, against E. coli were evaluated in the same manner as above, excellent antibacterial properties were also obtained against E. coli. [Industrial Applicability]

[0082] According to the present invention, a stainless steel having both good surface properties and excellent antibacterial properties can be obtained. The stainless steel of the present invention is particularly suitable for applications requiring antibacterial properties, such as kitchen utensils, household goods, sanitary goods, medical equipment, and building materials. [Explanation of symbols]

[0083] 1: Stainless steel 2: Ti oxide particles 3: light

Claims

1. In mass%, C：0.001～0.030％、 Si: 0.01 to 0.60%, Mn: 0.01 to 0.50%, P: 0.050% or less, S: 0.010% or less, Al：0.001～0.050％、 Cr：15.0～25.0％、 Ni: 0.01-2.00%, Ti: 0.10 to 0.50% and N：0.001～0.030％ and the balance being Fe and unavoidable impurities, On the surface, Particle size: 0.5 to 5.0 μm Ti oxide particles, number of which is 1000 μm 2 There are 5 to 100 pieces per lot, A stainless steel in which the sum, X, of Si, Al and Mn in atomic percent is 3.0 or less.

2. The component composition further comprises, in mass%, Mo: 3.00% or less, Cu: 1.00% or less, W: 0.50% or less, Co: 0.50% or less, Nb: 0.50% or less, Zr: 0.20% or less, V: 0.20% or less and B: 0.0100% or less The stainless steel according to claim 1, containing one or more of the following:

3. A method for producing stainless steel, comprising: The stainless steel is On the surface, Particle size: The number of Ti oxide particles having a particle size of 0.5 to 5.0 μm is 5 to 100 per 1000 μm 2 , The sum X of Si, Al and Mn in atomic % is 3.0 or less, The method comprises: a step of subjecting a material stainless steel having the chemical composition according to claim 1 or 2 to a heat treatment; Next, a step of subjecting the treated material to pickling treatment to obtain the stainless steel; and In the heat treatment, Heat treatment atmosphere: O 2 Concentration of 1% by volume or more, Heat treatment temperature: 850-1000°C and Heat treatment time: 5 seconds to 600 seconds and In the pickling treatment, Pickling solubility: 1.0~12.0g / m 2 A method for manufacturing stainless steel.

Images