Austenitic stainless steel section and method for manufacturing same

Abstract

To provide a shape steel of austenitic stainless steel having high surface glossiness and less anisotropy of surface gloss.SOLUTION: A shape steel of austenitic stainless steel extends in the longitudinal direction and has a plane parallel to the longitudinal direction. In measurement of specular glossiness at Gs (60°) of the shape steel, the specular glossiness L1 in the longitudinal direction and the specular glossiness L2 in the vertical direction to the longitudinal direction are 10 or more and the relationship between L1 and L2 satisfies [|L1-L2|≤5]. The relationship between Vickers hardness HS on the surface and Vickers hardness HC at the center of thickness of a cross section vertical to the surface satisfies [30≤HS-HC≤150].SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an austenitic stainless steel section and a method for producing the same. [Background technology]

[0002] Stainless steel has excellent corrosion resistance, and for this reason, stainless steel sections are used as structural materials for social infrastructure facilities such as water treatment plants.

[0003] Stainless steel sections are usually subjected to a surface treatment called No. 1 finish, as described in Non-Patent Document 1. No. 1 finish refers to surface treatments such as heat treatment and pickling, and by performing No. 1 finish, stainless steel sections with a beautiful silvery-white appearance can be obtained. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] Stainless Steel Association "Stainless Steel Basics" Revision Committee, "Stainless Steel Basics 2015", 2015, p. 20 Summary of the Invention [Problem to be solved by the invention]

[0005] However, as a result of research by the present inventors, it was found that when austenitic stainless steel sections are manufactured using the No. 1 finish, the surface gloss may decrease, resulting in poor design. While mechanical polishing of the surface can be considered to increase the surface gloss, this requires polishing that matches the shape of the section steel. In this case, polishing is performed in one direction, resulting in a large difference in surface gloss between the polishing direction and other directions. This results in increased anisotropy of the surface gloss, which in turn results in poor design.

[0006] An object of the present invention is to solve the above problems and to provide an austenitic stainless steel section having high surface gloss and little anisotropy of the surface gloss. [Means for solving the problem]

[0007] The present invention has been made to solve the above-mentioned problems, and is summarized as follows: an austenitic stainless steel section and a method for producing the same.

[0008] (1) An austenitic stainless steel section having a plane extending in the longitudinal direction and parallel to the longitudinal direction, When the specular gloss is measured on the flat surface at Gs (60°) as defined in JIS Z 8741:1997, the specular gloss L1 in the longitudinal direction and the specular gloss L2 in the direction perpendicular to the longitudinal direction are 10 or more, and the relationship between L1 and L2 satisfies the following formula (i): The austenitic stainless steel section has a relationship between a Vickers hardness HS on the surface of the plane and a Vickers hardness HC at the center of thickness of a cross section perpendicular to the surface, which satisfies the following formula (ii): |L1-L2|≦5 (i) 30≦HS-HC≦150 (ii) However, HS and HC in the above formula (ii) are hardness (HV1) when the test force is 1 kgf.

[0009] (2) An austenitic stainless steel section according to (1) above, in which a dry-wet cyclic test is carried out for nine cycles, in which one cycle is sprayed with a 5 wt% NaCl solution at 35°C for two hours, followed by holding in a dry atmosphere at 60°C and a relative humidity of 30% for four hours, and then holding in a wet atmosphere at 50°C and a relative humidity of 95% for two hours, and the rating number specified in JIS Z 2371:2015 on the surface after nine cycles has been determined to be 8 or higher.

[0010] (3) A method for producing an austenitic stainless steel section according to (1) above, The method includes a hot working step, an annealing step, and a descaling step, A method for producing austenitic stainless steel sections, wherein the descaling step involves blasting a slurry of metal and / or ceramic blasting material with an average particle size of 50 to 300 μm mixed with water at a ratio of 1.5 to 6.0 vol.% using an impeller or air pressure.

[0011] (4) A method for producing an austenitic stainless steel section according to (2) above, The method for producing an austenitic stainless steel section according to (3) above, wherein the annealing step uses a gas burner furnace and maintains the steel for 5 to 30 minutes in a furnace atmosphere in which the air-fuel ratio of the combustion gas is greater than 1.0 and the amount of water vapor is 12 vol.% or more so that the surface temperature of the steel reaches 1100 to 1300°C. [Effects of the Invention]

[0012] According to the present invention, it is possible to obtain an austenitic stainless steel section having high surface gloss and little anisotropy of the surface gloss. DETAILED DESCRIPTION OF THE INVENTION

[0013] The present inventors have investigated the surface gloss and other properties of austenitic stainless steel sections and have obtained the following findings (a) to (e).

[0014] (a) In the production of austenitic stainless steel sections, an annealing process is carried out to control the metal structure. The annealing process is a heat treatment process in an air atmosphere at a material temperature of 1000 to 1100°C. In this annealing process, a strong oxide scale is formed on the surface of the steel, so to remove the scale, for example, shot blasting using steel particles as projectiles is carried out, followed by pickling using an acid containing hydrofluoric acid. This series of processes from the annealing process to the pickling process is called No. 1 finishing.

[0015] When austenitic stainless steel sections are manufactured using the No. 1 finish described above, they lose sufficient surface gloss. The inventors have discovered that this is due to the Cr-depleted zone.

[0016] (b) When austenitic stainless steel is annealed, the Cr in the steel is preferentially oxidized, forming an oxide scale containing a large amount of Cr oxide, known as chromia scale. Meanwhile, a Cr-depleted layer with a reduced Cr content forms in the steel sheet base material directly below the chromia scale. This Cr-depleted layer is exposed on the surface after the scale is removed. During the subsequent pickling process, the Cr-depleted layer dissolves preferentially, forming surface irregularities and reducing surface gloss.

[0017] (c) Therefore, the inventors investigated descaling without pickling. They found that blasting, which involves blasting a slurry made of a mixture of water and blasting material with a predetermined average particle size, is effective for descaling without pickling. This allows sufficient descaling without pickling, thereby suppressing preferential dissolution of the Cr-depleted layer.

[0018] (d) Furthermore, blasting treatment using a slurry can increase the surface hardness to a moderate level. This improves scratch resistance and prevents deterioration of the appearance during long-term use. Furthermore, residual compressive stress can be imparted to the surface, improving SCC resistance. Meanwhile, compared to shot blasting using steel particles, the surface hardness does not increase excessively, preventing excessive deterioration of workability.

[0019] (e) Furthermore, if a Cr-depleted layer remains after descaling, this is undesirable from the viewpoint of corrosion resistance. For this reason, the inventors have also found that it is desirable to form a scale containing mixed oxides of Fe and Cr by controlling the annealing atmosphere.

[0020] An embodiment of the present invention has been made based on the above findings. Hereinafter, each requirement of the austenitic stainless steel section of this embodiment will be described in detail.

[0021] 1. Shapes of austenitic stainless steel sections The austenitic stainless steel section of this embodiment may be any austenitic stainless steel section that extends in the longitudinal direction and has a plane parallel to the longitudinal direction. Typical examples of such section steel include long steel members with irregular cross sections, such as angle steel, channel steel, and H-shaped steel.

[0022] 2. Types of austenitic stainless steel sections The type of steel for the austenitic stainless steel section of this embodiment is not particularly limited as long as it is a type specified by JIS G 4317:2013 or ASTM A276. For example, in JIS, austenitic stainless steel types such as SUS304, SUS304L, ASTM304L, SUS316, and SUS316L are desirable. In ASTM, austenitic stainless steel types such as Type 304, Type 304L, Type 316, and Type 3016L are desirable. The following describes the preferred ranges for the content of each element contained in the steel. In the following description, "%" in the content means "% by mass."

[0023] C: 0.01 to 0.15% Carbon (C) increases the strength of stainless steel and is effective in adjusting the stability of austenite as a metal structure. The content is adjusted depending on the desired strength and metal structure, but excessively reducing the C content increases refining costs. For this reason, the C content is preferably 0.01% or more. However, excessive C content may cause carbides to crystallize in the metal structure, impairing corrosion resistance. For this reason, the C content is preferably 0.15% or less, and more preferably 0.08% or less.

[0024] Si: 0.2 to 1.5% Silicon (Si) is an element primarily used to adjust the strength of stainless steel. For this reason, the Si content is preferably 0.2% or more. However, excessive Si content can cause the steel to become excessively hard, potentially impairing the toughness of the material. For this reason, the Si content is preferably 1.5% or less, and more preferably 1.0% or less.

[0025] Mn: 0.5 to 2.5% Manganese (Mn) is an element used to strengthen stainless steel through solid solution and to adjust the austenite stability of the metal structure. Therefore, the Mn content is preferably 0.5% or more. However, excessive Mn content not only increases compounding costs but also may form coarse compounds with the impurity element S. Therefore, the Mn content is preferably 2.5% or less, and more preferably 2.0% or less.

[0026] P:0.045% or less P (phosphorus) is an impurity element contained in steel. If the P content is excessive, it may segregate at grain boundaries and impair grain boundary strength during hot rolling. However, if the P content is excessively reduced, refining costs increase. Therefore, the P content is preferably 0.045% or less.

[0027] S: 0.03% or less S (sulfur) is an impurity element contained in steel. If the S content is excessive, it may form compounds with Mn and the like to form coarse inclusions. However, if the S content is excessively reduced, the refining cost increases. Therefore, the S content is preferably 0.03% or less.

[0028] Ni: 8-22% Ni (nickel) is used to strengthen stainless steel through solid solution and stabilize the austenite phase in the metal structure. For this reason, the Ni content is preferably 8% or more. However, excessive Ni content increases the compounding cost. For this reason, the Ni content is preferably 22% or less, and more preferably 15% or less.

[0029] Cr: 16~26% Cr (chromium) strengthens the solid solution of stainless steel and has the effect of increasing corrosion resistance. Therefore, the Cr content is preferably 16% or more. However, excessive Cr content increases the compounding cost. Therefore, the Cr content is preferably 26% or less, and more preferably 20% or less.

[0030] Mo: 0-3.0% Mo (molybdenum) strengthens stainless steel through solid solution and has the effect of suppressing the occurrence of pitting corrosion that occurs in austenitic stainless steel exposed to a saltwater environment. Therefore, it may be added as needed. However, excessive Mo content increases the formulation cost. For this reason, the Mo content is preferably 3.0% or less. On the other hand, to obtain the above effects, the Mo content is preferably 2.0% or more.

[0031] Other elements that may be included include Cu, N, and Al. Cu may be included when scrap is used as the raw material, and the Cu content is preferably 0.75% or less. N may be included for the purpose of adjusting strength, and the N content is preferably 0.10% or less. Al may be included because it is used as a deoxidizer, and the Al content is preferably 0.03% or less.

[0032] It is preferable that the alloy contains the above-mentioned elements, with the balance being Fe and impurities. Here, "impurities" refer to components that are mixed in during the industrial production of austenitic stainless steel sections due to various factors in the raw materials, such as ores and scrap, and in the manufacturing process, and are acceptable within a range that does not adversely affect the present embodiment. A total amount of 0.5% or less is at a level that does not cause any problems.

[0033] 3. Specular gloss In the austenitic stainless steel section of this embodiment, the specular glossiness L1 and L2 described below are controlled to fall within the following ranges.

[0034] In a plane parallel to the longitudinal direction, the longitudinal specular gloss L1 is 10 or more. If the longitudinal specular gloss L1 is less than 10, it is undesirable from the viewpoint of design. Furthermore, the surface quality will be poor and corrosion will be more likely to occur. For this reason, the longitudinal specular gloss L1 is 10 or more. The longitudinal specular gloss L1 is preferably 12 or more. There is no particular upper limit to the longitudinal specular gloss L1, but from the viewpoint of antiglare properties, it is preferably 50 or less, and more preferably 30 or less.

[0035] Similarly, in a plane parallel to the longitudinal direction, the specular gloss L2 in the direction perpendicular to the longitudinal direction is set to 10 or more. If the specular gloss L2 in the direction perpendicular to the longitudinal direction is less than 10, this is undesirable from the standpoint of design. Furthermore, the surface quality will be poor and corrosion will be more likely to occur. For this reason, the specular gloss L2 in the direction perpendicular to the longitudinal direction is set to 10 or more. The specular gloss L2 in the direction perpendicular to the longitudinal direction is preferably set to 12 or more. There is no particular upper limit to the specular gloss L2 in the direction perpendicular to the longitudinal direction, but from the standpoint of antiglare properties, it is preferably set to 50 or less, and more preferably 30 or less.

[0036] Furthermore, in a plane parallel to the longitudinal direction, the relationship between L1 and L2 described above satisfies the following formula (i). |L1-L2|≦5 (i)

[0037] If the value of the left side of equation (i), which is the absolute value of the difference between L1 and L2, exceeds 5, the difference in surface gloss depending on the direction becomes large. In other words, anisotropy of gloss occurs, and as a result, gloss varies depending on the direction, resulting in a decrease in design quality. For this reason, the value of the left side of equation (i) is set to 5 or less. It is preferable that the value of the left side of equation (i) is set to 3 or less.

[0038] The specular gloss is specified in JIS Z 8741:1997, and the specular glosses L1 and L2 are measured using a gloss meter. For example, a gloss meter GM-1 manufactured by Suga Test Instruments Co., Ltd. may be used as the gloss meter. In the austenitic stainless steel section of this embodiment, the 60° specular gloss Gs(60°) at an incident angle of 60° is used as an index of specular gloss.

[0039] 4. Vickers hardness In the austenitic stainless steel section of this embodiment, the Vickers hardness is controlled at the following positions: Specifically, the relationship between the Vickers hardness HS on a surface of a plane parallel to the longitudinal direction and the Vickers hardness HC at the center of the thickness of a cross section perpendicular to the surface satisfies the following formula (ii):

[0040] 30≦HS-HC≦150 (ii) However, HS and HC in the above formula (ii) are hardness (HV1) when the test force is 1 kgf.

[0041] If the difference between HS, which is the Vickers hardness on a plane surface parallel to the longitudinal direction, and HC, which is the Vickers hardness at the center of the thickness of a cross section perpendicular to the surface (hereinafter also referred to as the "value in equation (ii)") is less than 30 HVl, scratch resistance decreases and the appearance deteriorates. Therefore, the value in equation (ii) is set to 30 HVl or more. The value in equation (ii) is preferably set to 60 HVl or more.

[0042] On the other hand, if the value in equation (ii) exceeds 150HVl, it becomes difficult to drill bolt holes, etc., and punching processes become difficult, resulting in reduced workability. For this reason, the value in equation (ii) is set to 150HVl or less.

[0043] The above-mentioned HS and HC can be measured using a Vickers hardness tester in accordance with JIS Z 2244:2009, with a test force of 1 kgf. The hardness is measured at five points on a flat surface parallel to the longitudinal direction, and the average value is taken as HS. Similarly, the hardness is measured at five points at the center of the thickness of a cross section perpendicular to the surface, and the average value is taken as HC. The measurement points are spaced at intervals of at least three times the diagonal length of the indentation to avoid interference between each other.

[0044] 5. Residual compressive stress As will be described later, the austenitic stainless steel section of this embodiment is subjected to a blasting treatment in which a slurry, which is a mixture of blasting material and water, is projected instead of shot blasting. Therefore, the austenitic stainless steel section of this embodiment has a residual compressive stress of 0 (N / mm 2 It is preferable that the residual compressive stress is greater than 0 (N / mm 2 ) or more, it becomes possible to improve SCC resistance.

[0045] 6.Corrosion resistance As described above, annealing of austenitic stainless steel sections results in the formation of a Cr-depleted zone. Since Cr is the element primarily responsible for the corrosion resistance of stainless steel, the formation of a Cr-depleted zone tends to reduce corrosion resistance. Therefore, it is preferable to suppress the formation of the Cr-depleted zone itself. In particular, in austenitic stainless steel sections, attempts to improve surface gloss can sometimes result in a decrease in corrosion resistance. Therefore, it is preferable to suppress the formation of the Cr-depleted zone and improve corrosion resistance by controlling the annealing conditions, as described below.

[0046] Therefore, it is preferable to perform nine cycles of wet-dry cycling tests, each cycle consisting of spraying a 5 wt% NaCl solution at 35°C for two hours, holding in a dry atmosphere at 60°C and a relative humidity of 30% for four hours, and holding in a humid atmosphere at 50°C and a relative humidity of 95% for two hours, and to ensure that the rating number on the surface after nine cycles is 8 or higher. It is also preferable that the rating number after the wet-dry cycling test be 9 or higher.

[0047] That is, the following (a) to (c) are considered one cycle, and the rating number is evaluated after nine cycles of dry-wet cyclic testing of (a) to (c) (72 hours later). (a) The temperature in the spray chamber was set to 35°C, and a 5 wt% NaCl solution was sprayed for 2 hours. (b) 60°C, 30% relative humidity, dry atmosphere for 4 hours (c) 50°C, 95% relative humidity, humid atmosphere for 2 hours

[0048] The rating number is an index of corrosion resistance and is specified in JIS Z 2371:2015. The size of the test specimens used in the salt spray test and wet-dry cycle test is, for example, 40 mm x 120 mm in the case of angle steel, and can be obtained by cutting off 5 mm of each end from a 50 mm-wide flat surface that forms one side of the angle steel. The four periphery of the test specimen and the surface that corresponds to the inner surface of the angle steel are protected by covering them with vinyl adhesive film or the like, and the corrosion resistance of only the surface that corresponds to the outer surface of the angle steel is evaluated.

[0049] 7. Manufacturing method A preferred method for producing the austenitic stainless steel section of this embodiment will now be described. The austenitic stainless steel section of this embodiment can be stably produced by using the following production method.

[0050] The method for producing an austenitic stainless steel section according to this embodiment includes a hot working step, an annealing step, and a descaling step. Each step will be described in detail below.

[0051] 7-1.Hot processing process Austenitic stainless steel with the above-mentioned chemical composition is melted and cast, or hot forged and hot bloomed to produce an intermediate material in the shape of a billet or bloom. The resulting intermediate material is heated to a hot working temperature and hot worked. Hot working is usually hot rolling, in which the intermediate material is passed through grooved rolls multiple times. This hot working produces the desired cross-sectional shape. The conditions for hot working are not particularly limited; conventional methods can be used. After hot working, the metal structure of the intermediate material is a hot-worked structure. Furthermore, the surface of the material is covered with oxide scale formed during hot working.

[0052] 7-2. Annealing process Next, the hot-worked intermediate material is annealed. By annealing, the hot-worked structure is restored and recrystallized to form a recrystallized structure. By forming the metal structure into such a recrystallized structure, properties such as strength and ductility can be adjusted.

[0053] The annealing process is a heat treatment process that typically uses a gas burner furnace or an electric furnace in the atmosphere, where the material is held at a temperature of 1000 to 1100°C for 3 to 15 minutes. During this process, a new oxide scale forms at the interface between the oxide scale formed during hot working and the metal part of the base material (parent material). In the case of ordinary austenitic stainless steel, the Cr contained in the steel is preferentially oxidized, forming a dense, uniform chromia scale that contains a lot of Cr.

[0054] On the other hand, when Cr diffuses into the scale and forms chromia scale, a Cr-depleted layer with a reduced Cr concentration forms beneath the scale. Since most of the Cr diffuses along large grain boundaries, the Cr-depleted layer is significantly formed near the grain boundaries. This Cr-depleted layer dissolves preferentially during conventional pickling, resulting in the formation of surface irregularities. As a result, the silvery metallic luster is lost, resulting in a white surface and reduced surface luster. Furthermore, some of the Cr-depleted layer remains even after pickling, reducing corrosion resistance. Therefore, it is preferable to suppress the formation of the Cr-depleted layer.

[0055] Therefore, in the austenitic stainless steel section of this embodiment, it is preferable to destroy the chromia scale, which is one cause of the formation of the Cr-depleted zone, and form a different oxide scale. Chromia scale is formed when Cr is oxidized, resulting in the formation of dense, uniform Cr2O3, or chromia, which forms the scale. It is also preferable to further promote the oxidation of the chromia scale and also promote the oxidation of Fe, which is inherently difficult to oxidize, to form a scale made of a mixed oxide of Fe and Cr with a (Fe,Cr)3O4 structure (hereinafter referred to as "Fe-Cr-based scale").

[0056] To form this Fe-Cr-based scale, it is preferable to use a gas burner furnace in the annealing step, and to create a furnace atmosphere in which the air-fuel ratio of the combustion gas exceeds 1.0 and the amount of water vapor is 12 vol.% or more. The amount of water vapor is more preferably 16 vol.% or more. If the atmosphere in the furnace is not within the above range, Fe is not sufficiently oxidized, making it difficult for Fe-Cr-based scale to form.

[0057] In addition, it is preferable to hold the steel in the above atmosphere for 5 to 30 minutes so that the surface temperature of the steel reaches 1100 to 1300°C. If the surface temperature of the steel is less than 1100°C, Fe-Cr scale does not form sufficiently. The same applies if the holding time is less than 5 minutes. On the other hand, if the surface temperature of the steel is 1300°C, coarsening of the crystal grains occurs, and the desired properties cannot be obtained. If the holding time exceeds 30 minutes, the effect of scale formation saturates, impeding productivity in industrial production, and coarsening of the crystal grains may occur, preventing the desired properties from being obtained.

[0058] As described above, the formation of an Fe-Cr-based scale suppresses the formation of a Cr-depleted layer, thereby suppressing the deterioration of surface gloss and corrosion resistance. Furthermore, because the formation of a Cr-depleted layer is suppressed, there is no need to further perform spalling or cutting of the substrate after removing the scale.

[0059] 7-3. Descaling process The hot-worked and annealed intermediate material is descaled. In the production of the austenitic stainless steel section of this embodiment, Fe-Cr scale is removed by a blasting treatment in which a slurry of metal and / or ceramic blast material with an average particle size of 50 to 300 μm mixed with water at a ratio of 1.5 to 6.0 vol.% is blasted onto the material using an impeller or air pressure.

[0060] Typically, when producing austenitic stainless steel sections using the No. 1 finish, shot blasting is followed by pickling. Shot blasting involves using an impeller to project shot material made of metal such as cast iron or stainless steel with an average particle size of 300 to 1200 μm. Shot blasting causes the shot material to collide with the oxide scale, creating cracks in the scale. These cracks allow the acid to penetrate deep into the oxide scale during subsequent pickling, removing the scale.

[0061] In shot blasting, a shot material with a large average particle size is used to increase the energy during projection. However, because the average particle size is large and the contact area upon impact is large, scale cannot be completely removed by shot blasting alone. Shot blasting is an auxiliary process that generates cracks to enable complete removal of scale during subsequent pickling.

[0062] On the other hand, for the austenitic stainless steel section of this embodiment, a small shot material with an average particle size of 50 to 300 μm is selected, and a slurry made by mixing this shot material with water is projected onto the scale. If a shot material with a small average particle size is used, the energy at the time of projection will be reduced due to the air resistance experienced before it hits the scale. Therefore, by mixing water with the shot material, it is possible to project the slurry onto the scale while maintaining the energy at the time of projection, and the scale can be removed by this blasting process alone.

[0063] Specifically, it is preferable to use a slurry in which shot material with an average particle size of 50 to 300 μm is mixed with water at a ratio of 1.5 to 6.0 vol.%. Furthermore, when using such a slurry, if the average particle size is large, it may actually push the scale into the metal surface, making it difficult to remove. For this reason, it is more preferable that the average particle size of the shot material be in the range of 100 to 200 μm.

[0064] Furthermore, if the volume ratio of the shot material in the slurry becomes large, it becomes difficult for the shot material to obtain kinetic energy from the water flow, making it difficult to achieve the scale removal effect. For this reason, it is preferable to mix the shot material with water at a ratio of 1.5 to 6.0 vol.% to form a slurry. It is more preferable to mix the shot material with water at a ratio of 2.0 to 5.5 vol.% to form a slurry.

[0065] Thus, the blasting process using the above-described slurry eliminates the need for pickling. As a result, the decrease in surface gloss due to the Cr-depleted layer can be suppressed. That is, the above-described L1 and L2 can be within the range of the requirements of this embodiment. Furthermore, since an acid solution, which is a mixture of hydrofluoric acid and nitric acid, is not used, the environmental load can be reduced.

[0066] It should be noted that methods for descaling without pickling include cutting, but this reduces productivity and yield. Furthermore, descaled shaped steel can be polished using belt polishing or the like to improve surface gloss, but in this case, polishing is performed in one direction, which is likely to result in differences in gloss depending on the direction, making this undesirable from the standpoint of design. Therefore, it can be seen that descaling by the above-mentioned blasting treatment is superior in terms of productivity, design, etc.

[0067] Furthermore, when the annealing conditions are controlled in addition to the blasting treatment to form an Fe-Cr scale, the Cr-depleted layer itself is not formed, thereby improving corrosion resistance. Specifically, a 9-cycle dry-wet cycle test, in which a 5 wt% NaCl solution is sprayed at 35°C for 2 hours, followed by a 4-hour hold in a dry atmosphere at 60°C and a relative humidity of 30%, and a 2-hour hold in a wet atmosphere at 50°C and a relative humidity of 95%, is performed, and the surface after the 9 cycles has a rating number of 8 or higher as specified in JIS Z 2371:2015.

[0068] EXAMPLES The austenitic stainless steel section according to the present invention will be described in more detail below with reference to examples, but the present embodiment is not limited to these examples. [Example]

[0069] A rectangular cast billet with a square cross section and a side length of 140 mm was prepared from austenitic stainless steel having the chemical composition shown in Table 1. After heating this billet to 1150°C, it was hot rolled for a total of 16 passes using a six-stand grooved rolling mill to produce an intermediate material in the shape of an equal-leg angle iron with a thickness of 5 mm, a side length of 50 mm, and a length of 4000 mm.

[0070] [Table 1]

[0071] The resulting intermediate material was annealed. This was done using a gas burner furnace or an electric furnace in the atmosphere. When a gas burner furnace was used for annealing, the air-fuel ratio was calculated from the flow rate of natural gas and the flow rate of introduced air. Gas was also sampled from the atmosphere inside the furnace, and the water vapor content at room temperature was measured in vol.% using a moisture meter. The heat treatment temperature was measured using a thermocouple attached to the material, and the holding time at the reached temperature was recorded.

[0072] After annealing, the steel was cooled to obtain austenitic stainless steel, and then descaling was carried out by a combination of various methods as shown in Table 2 to obtain austenitic stainless steel sections. The conditions for each method are described below.

[0073] Shot blasting: Spherical cast iron shot material with an average particle size of 500 μm was projected using an impeller at a velocity of 64 m / s.

[0074] Pickling: The steel was pickled by spraying a mixed acid of 6.5 mass.% hydrofluoric acid and 11.5 mass.% nitric acid, adjusted to 40°C, from a nozzle.

[0075] Belt grinding: A strip of emery paper containing alumina abrasive grains with a grit size of #80 was attached to the rotary tool, and the surface was finished by grinding in one direction.

[0076] Blasting with blasting slurry: Using either or both of square shot materials made of stainless steel with an average particle size of 140 to 350 μm (SUS grit shot materials) or spherical shot materials made of zirconia beads with an average particle size of 100 μm (spherical zirconia shot materials), each shot material was mixed with water at a ratio of 2.5 to 7 vol.%, and the resulting slurry was sprayed from a nozzle using compressed air at a spray pressure of 0.4 MPa.

[0077] The specular gloss and Vickers hardness of the austenitic stainless steel sections obtained after each treatment were measured in the following manner. Tests to evaluate corrosion resistance were also conducted.

[0078] (Specular gloss) The specular glossiness L1 and L2 were measured using a gloss meter GM-1 manufactured by Suga Test Instruments Co., Ltd. The measured specular glossiness was Gs(60°), which is the 60° specular glossiness when the incident angle was 60°.

[0079] (Vickers hardness) HS and HC were measured using a Vickers hardness tester in accordance with JIS Z 2244:2009, with a test force of 1 kgf. Hardness was measured at five points on a flat surface parallel to the longitudinal direction, and the average value was taken as HS. Similarly, hardness was measured at five points at the center of the thickness of a cross section perpendicular to the surface, and the average value was taken as HC. Each measurement point was spaced at an interval of at least three times the diagonal length of the indentation to avoid interference with other measurements.

[0080] (Corrosion resistance evaluation test) After a salt spray test in which a 5 wt% NaCl solution was sprayed at 35°C for 2 hours, nine cycles of wet-dry cycles were performed, each consisting of a 4-hour hold in a dry atmosphere at 60°C and 30% relative humidity, followed by a 2-hour hold in a wet atmosphere at 50°C and 95% relative humidity. The rating number was then determined for each specimen after nine cycles. The specimens measured 40 mm x 120 mm and were obtained by cutting off 5 mm from each end of a 50 mm-wide flat surface that formed one side of the angle iron. The periphery of the specimens and the inner surface of the angle iron were protected with vinyl adhesive film, and only the outer surface of the angle iron was evaluated for corrosion resistance. The results are summarized in Table 2.

[0081] [Table 2]

[0082] Inventive examples Nos. 1 to 16 had good surface gloss and small gloss anisotropy. They also satisfied the hardness requirements and had good scratch resistance. In particular, Nos. 1 to 7 had favorable descaling conditions as well as favorable annealing conditions, resulting in good corrosion resistance. On the other hand, Nos. 8 to 16 had unfavorable annealing conditions, resulting in the formation of a Cr-depleted layer and poor corrosion resistance.

[0083] The comparative examples, Nos. 17 to 22, had poor surface gloss or significant gloss anisotropy. They also did not meet the hardness requirements. No. 17 was annealed at 1,080°C according to a typical conventional manufacturing method, and descaling was performed by shot blasting followed by pickling. In this case, pickling was performed while a Cr-depleted layer was still formed, resulting in a low specular gloss. Furthermore, the impact of shot blasting increased the difference between HS and HC, resulting in excessive hardening of the product surface and reduced workability, such as drilling. Similarly, Nos. 18 and 19 were descaled by shot blasting followed by pickling, but they also had low specular gloss and a large difference between HS and HC, resulting in reduced workability.

[0084] In Nos. 20 and 21, the average particle size or volume fraction of the shot material did not satisfy the preferred conditions of this embodiment, so the specular gloss decreased and the hardness requirements were not met. In addition, uneven scale remained and could not be sufficiently removed, so the rating number could not be measured.

[0085] No. 22 is an example in which, after annealing, the descaling method was shot blasting followed by belt polishing, without using pickling. In this case, the specular gloss was too high, which is undesirable from the perspective of anti-glare properties. Also, the difference in specular gloss was large, resulting in a large anisotropy of gloss. Furthermore, the rating number in the wet-dry cycle test was low at 5, which is thought to be due to the fact that descaling was insufficient with the belt polishing finish.

Claims

1. An austenitic stainless steel section extending in a longitudinal direction and having a plane parallel to the longitudinal direction, When measuring the specular gloss at Gs (60 °) defined in JIS Z 8741:1997 on the plane, the specular gloss in the longitudinal direction L １ , and the specular gloss L in the direction perpendicular to the longitudinal direction ２ is 10 or more, and L １ and L ２ The relationship satisfies the following formula (i): The relationship between the Vickers hardness HS on the surface of the plane and the Vickers hardness HC at the center of the thickness of a cross section perpendicular to the surface satisfies the following formula (ii): |L １ -L ２ |≦5 ・・・(i) 30≦HS-HC≦150...(ii) However, HS and HC in the above formula (ii) are hardness (HV1) when the test force is 1 kgf.

2. 2. The austenitic stainless steel section according to claim 1, wherein a dry-wet cycling test is carried out for nine cycles, each cycle consisting of spraying a 5 wt % NaCl solution at 35°C for two hours, holding in a dry atmosphere at 60°C and a relative humidity of 30% for four hours, and holding in a wet atmosphere at 50°C and a relative humidity of 95% for two hours, and the rating number specified in JIS Z 2371:2015 on the surface after the nine cycles has passed is 8 or more.

3. 2. A method for producing an austenitic stainless steel section according to claim 1, The method includes a hot working step, an annealing step, and a descaling step, In the descaling step, a blasting treatment is carried out in which a slurry of blasting material made of metal and / or ceramic and having an average particle size of 50 to 300 μm mixed with water in a ratio of 1.5 to 6.0 vol. % is blasted by an impeller or air pressure.

4. 3. A method for producing an austenitic stainless steel section according to claim 2, 4. The method for producing an austenitic stainless steel section according to claim 3, wherein the annealing step uses a gas burner furnace and maintains the steel for 5 to 30 minutes in a furnace atmosphere in which the air-fuel ratio of the combustion gas is greater than 1.0 and the amount of water vapor is 12 vol.% or more so that the surface temperature of the steel is 1100 to 1300°C.

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