JP7727197B2 - Austenitic stainless steel section and method for manufacturing same - Google Patents

Description

The present invention relates to an austenitic stainless steel section and a manufacturing method thereof.

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

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 No. 1 finish results in stainless steel sections with a beautiful silvery-white appearance.

Stainless Steel Association "Stainless Steel Basics" Revision Committee, "Stainless Steel Basics 2015", 2015, p. 20

However, as a result of research by the inventors, it was found that when austenitic stainless steel sections are manufactured using the No. 1 finish, the surface gloss decreases, which can lead to poor design. While mechanical polishing of the surface could be considered to increase surface gloss, this requires polishing to match 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 increases the anisotropy of the surface gloss, which in turn leads to a problem of poor design.

The present invention aims to solve the above problems and provide an austenitic stainless steel section with high surface gloss and little anisotropy in surface gloss.

The present invention was made to solve the above problems, and is summarized as follows: The austenitic stainless steel section and its manufacturing method.

(1) An austenitic stainless steel section extending in a longitudinal direction and having a plane 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 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) An austenitic stainless steel section as described in (1) above, in which a dry-wet cycle test is carried out for nine cycles, in which one cycle consists of spraying a 5 wt% NaCl solution at 35°C for two hours, then 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 surface after nine cycles has a rating number of 8 or higher as specified in JIS Z 2371:2015.

(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,
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) 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.

The present invention makes it possible to obtain austenitic stainless steel sections with high surface gloss and little anisotropy in surface gloss.

The inventors have investigated the surface gloss and other properties of austenitic stainless steel sections and have discovered the following (a) to (e):

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

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

(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 base steel directly below the chromia scale. This Cr-depleted layer is exposed on the surface after the scale is removed. Then, in the subsequent pickling process, the Cr-depleted layer dissolves preferentially, forming irregularities on the surface and reducing the surface gloss.

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

(d) Furthermore, blasting, which involves projecting a slurry, can moderately increase the surface hardness. This improves scratch resistance and suppresses deterioration of appearance over 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 in workability.

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

One embodiment of the present invention has been made based on the above findings. Below, we will explain in detail each requirement of the austenitic stainless steel section steel of this embodiment.

1. Shape of Austenitic Stainless Steel Section The austenitic stainless steel section of this embodiment may be any austenitic stainless steel section that extends in the longitudinal direction and has a flat surface 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.

2. Types of Austenitic Stainless Steel Sections The steel type of the austenitic stainless steel section of this embodiment is not particularly limited as long as it is a type specified in 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 preferred ranges for the content of each element contained in the steel are described below. In the following description, "%" in the content means "mass %."

C: 0.01-0.15%
C (carbon) is effective in increasing the strength of stainless steel and adjusting the stability of austenite as a metal structure. The content is adjusted depending on the target strength and metal structure, but excessive reduction of 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.

Si: 0.2-1.5%
Silicon (Si) is an element primarily used to adjust the strength of stainless steel. Therefore, 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. Therefore, the Si content is preferably 1.5% or less, and more preferably 1.0% or less.

Mn: 0.5-2.5%
Mn (manganese) is an element used to solid-solution strengthen stainless steel and 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 the compounding cost 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.

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 working. However, if the P content is excessively reduced, refining costs increase. Therefore, the P content is preferably 0.045% or less.

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.

Ni: 8 to 22%
Ni (nickel) is used to strengthen the solid solution of stainless steel and to stabilize the austenite phase as a 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.

Cr: 16 to 26%
Cr (chromium) has the effect of solid solution strengthening stainless steel and 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.

Mo: 0 to 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 compounding cost. For this reason, the Mo content is preferably 3.0% or less. On the other hand, to obtain the above effect, the Mo content is preferably 2.0% or more.

Other elements may be contained: Cu, N, and Al. Cu may be contained when scrap is used as the raw material, and the Cu content is preferably 0.75% or less. N may be contained for the purpose of adjusting strength, and the N content is preferably 0.10% or less. Al may be contained as it is used as a deoxidizer, and the Al content is preferably 0.03% or less.

It preferably contains the above elements, with the balance being Fe and impurities. Here, "impurities" refers 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 ore and scrap, and the manufacturing process, and are acceptable within a range that does not adversely affect this embodiment. A total amount of 0.5% or less is considered to be at a level that does not cause any problems.

3. Specular Glossiness In the austenitic stainless steel section of this embodiment, the specular glossiness _L1 and _L2 described below are controlled within the following ranges.

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, this 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.

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 viewpoint 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 viewpoint of antiglare properties, it is preferably set to 50 or less, and more preferably 30 or less.

Furthermore, in a plane parallel to the longitudinal direction, the relationship between _L1 and _L2 satisfies the following formula (i).
|L ₁ -L ₂ |≦5...(i)

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, the gloss differs depending on the direction, resulting in a decrease in designability. 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.

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.

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):

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

If the difference between HS, the Vickers hardness on a flat surface parallel to the longitudinal direction, and HC, the Vickers hardness at the center of the thickness of a cross section perpendicular to that surface (hereinafter also referred to as the "value in equation (ii)") is less than 30 HV1, scratch resistance will decrease and the appearance will deteriorate. For this reason, the value in equation (ii) should be 30 HV1 or more. Preferably, the value in equation (ii) should be 60 HV1 or more.

On the other hand, if the side 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 side value in equation (ii) should be 150HVl or less.

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. Hardness is measured at five points on a flat surface parallel to the longitudinal direction, and the average value is taken as HS. Similarly, 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. Each measurement point should be spaced at an interval of at least three times the diagonal length of the indentation to avoid interference from other measurements.

5. Residual Compressive Stress In the austenitic stainless steel section of this embodiment, as described below, a blasting treatment is performed in which a slurry, which is a mixture of blasting material and water, is blasted, rather than shot blasting. Therefore, in the austenitic stainless steel section of this embodiment, it is preferable that the residual compressive stress exceeds 0 (N/mm ² ). By making the residual compressive stress exceed 0 (N/mm ² ), it becomes possible to improve SCC resistance.

6. Corrosion Resistance As described above, a Cr-depleted layer is formed by annealing an austenitic stainless steel section. Cr is the element that is primarily responsible for the corrosion resistance of stainless steel, and the formation of a Cr-depleted layer tends to reduce corrosion resistance. For this reason, it is preferable to suppress the formation of the Cr-depleted layer itself. In particular, in austenitic stainless steel sections, attempts to improve surface gloss can sometimes result in a reduction in corrosion resistance. Therefore, it is preferable to suppress the formation of a Cr-depleted layer and improve corrosion resistance by controlling the annealing conditions as described below.

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, then holding in a dry atmosphere at 60°C and 30% relative humidity for four hours, and then holding in a humid atmosphere at 50°C and 95% relative humidity for two hours, and to ensure that the surface rating after nine cycles is 8 or higher. It is also preferable that the rating after the above wet-dry cycling test be 9 or higher.

That is, the following (a) to (c) are counted as one cycle, and the rating number is evaluated after nine cycles of dry and wet cycle tests 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) The sample was kept in a dry atmosphere at 60°C and a relative humidity of 30% for 4 hours. (c) The sample was kept in a humid atmosphere at 50°C and a relative humidity of 95% for 2 hours.

The rating number is an index of corrosion resistance and is specified in JIS Z 2371:2015. The size of the test specimen used in the salt spray test and wet-dry cycle test is, for example, 40 mm x 120 mm in the case of angle iron, and can be obtained by cutting out 5 mm of each end from a 50 mm-wide flat surface that forms one side of the angle iron. The four periphery of the test specimen and the surface that corresponds to the inner surface of the angle iron 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 iron is evaluated.

7. Manufacturing Method A preferred manufacturing method for the austenitic stainless steel section of this embodiment will be described. The austenitic stainless steel section of this embodiment can be stably manufactured by using the following manufacturing method.

The method for manufacturing austenitic stainless steel sections in this embodiment includes a hot working process, an annealing process, and a descaling process. Each process will be described in detail below.

7-1. Hot Working Process Austenitic stainless steel having the above-described 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 the 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 may be followed. After hot working, the metal structure of the intermediate material is a hot-worked structure. Furthermore, oxide scale formed during hot working adheres to the surface of the material.

7-2. Annealing Step Next, the hot-worked intermediate material is annealed. By annealing, the hot-worked structure described above 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.

The annealing process is a heat treatment process that typically uses a gas burner furnace or an electric furnace in air, where the material is held at a temperature of 1000-1100°C for 3-15 minutes. During this process, 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 material is preferentially oxidized, resulting in the formation of a dense, uniform chromia scale that is rich in Cr.

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. Because most of the Cr diffuses along large grain boundaries, a Cr-depleted layer is formed prominently near the grain boundaries. This Cr-depleted layer dissolves preferentially during standard pickling, causing the surface to become uneven. As a result, the silver 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. For this reason, it is preferable to suppress the formation of the Cr-depleted layer.

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 Cr ₂ O ₃ (i.e., chromia). It is then 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 consisting of a mixed oxide of Fe and Cr having a structure of (Fe, Cr) ₃ O ₄ (hereinafter referred to as "Fe-Cr-based scale").

To form this Fe-Cr-based scale, it is preferable to use a gas burner furnace in the annealing process, with the combustion gas having an air-fuel ratio of greater than 1.0 and a furnace atmosphere with a water vapor content of 12 vol. % or more. It is even more preferable to set the water vapor content to 16 vol. % or more. If the furnace atmosphere is not within this range, the Fe will not be sufficiently oxidized, making it difficult for Fe-Cr-based scale to form.

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 below 1100°C, Fe-Cr scale will not form sufficiently. The same is true for holding times of 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 will saturate, impeding industrial productivity, and coarsening of the crystal grains may occur, preventing the desired properties from being obtained.

As mentioned above, the formation of an Fe-Cr scale suppresses the formation of a Cr-deficient layer, thereby preventing a decrease in surface gloss and corrosion resistance. Furthermore, by suppressing the formation of a Cr-deficient layer, there is no need to further scarify or cut the base material after removing the scale.

7-3. Descaling Step The hot-worked and annealed intermediate material is descaled. In the production of austenitic stainless steel sections according to this embodiment, Fe-Cr scale is removed by a blasting process 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.

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.

In shot blasting, a large average particle size of shot material 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 creates cracks to enable complete removal of scale during subsequent pickling.

On the other hand, for the austenitic stainless steel structural steel of this embodiment, a small shot material with an average particle size of 50 to 300 μm is selected, and this shot material is mixed with water to form a slurry that 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 it experiences before colliding with the scale. Therefore, by mixing water into 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.

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 in a ratio of 1.5 to 6.0 vol. %. Furthermore, when using such a slurry, if the average particle size is too large, it may actually push the scale into the metal surface, making it difficult to remove. For this reason, it is more preferable for the average particle size of the shot material to be in the range of 100 to 200 μm.

Furthermore, if the volumetric proportion 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 scale removal effects. 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 even more preferable to mix the shot material with water at a ratio of 2.0 to 5.5 vol. % to form a slurry.

In this way, 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 mixed acid of hydrofluoric acid and nitric acid, is not used, the environmental load can be reduced.

Methods of descaling without pickling include cutting, but this reduces productivity and yield. Also, descaled structural steel can be polished using belt polishing or other methods to improve surface gloss, but in this case, polishing is done in one direction, which can lead to differences in gloss depending on the direction, making this undesirable from a design perspective. Therefore, it can be seen that descaling using the above-mentioned blasting process is superior in terms of productivity, design, etc.

Furthermore, if the annealing conditions are controlled in addition to the above-mentioned blasting treatment to form an Fe-Cr-based scale, the Cr-deficient layer itself is not formed, thereby improving corrosion resistance. Specifically, after spraying a 5 wt% NaCl solution at 35°C for 2 hours, the material is then held in a dry atmosphere at 60°C and 30% relative humidity for 4 hours, followed by a humid atmosphere at 50°C and 95% relative humidity for 2 hours. This cycle constitutes nine cycles of repeated wet-dry testing, and the surface after nine cycles can achieve a rating of 8 or higher as specified in JIS Z 2371:2015.

The following examples will explain the austenitic stainless steel section according to the present invention in more detail, but the present embodiment is not limited to these examples.

A rectangular cast billet of austenitic stainless steel with the chemical composition shown in Table 1 and a square cross section with sides of 140 mm was prepared. This billet was heated to 1150°C and then hot rolled 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 steel with a thickness of 5 mm, sides of 50 mm, and a length of 4000 mm.

The resulting intermediate material was annealed. This was done using either 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 furnace atmosphere, and the amount of water vapor 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.

After annealing, the steel was cooled to produce austenitic stainless steel. The steel was then descaled using a combination of various methods as shown in Table 2 to obtain austenitic stainless steel structural steel. The conditions for each method are described below.

Shot blasting:
Spherical shot material made of cast iron and having an average particle size of 500 μm was projected using an impeller at a velocity of 64 m/s.

Pickling:
Pickling was carried out by spraying an acid solution adjusted to 40° C. using a mixed acid of 6.5 mass % hydrofluoric acid and 11.5 mass % nitric acid from a nozzle.

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 polished in one direction for finishing.

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) and 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. % to form a slurry, which was then sprayed from a nozzle using compressed air at a spray pressure of 0.4 MPa.

The specular gloss and Vickers hardness of the austenitic stainless steel sections obtained after each treatment were measured using the following procedure. Tests to evaluate corrosion resistance were also conducted.

(Specular gloss)
The specular glossiness _L1 and _L2 were measured using a glossmeter GM-1 manufactured by Suga Test Instruments Co., Ltd. The measured specular glossiness is Gs (60°), which is the 60° specular glossiness when the incident angle is 60°.

(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 mutual influence of the measurements.

(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 cycling tests were performed, each cycle consisting of 4 hours in a dry atmosphere at 60°C and 30% relative humidity, followed by 2 hours in a wet atmosphere at 50°C and 95% relative humidity. The rating number was then determined on the surface of each test specimen after nine cycles. The test specimens measured 40 mm x 120 mm and were obtained by cutting off 5 mm of each end from the 50 mm-wide flat surface that formed one side of the angle iron. The periphery of the test specimens and the surface corresponding to the inner surface of the angle iron were protected with vinyl adhesive film or the like, and the corrosion resistance was evaluated only on the surface corresponding to the outer surface of the angle iron. The results are summarized in Table 2.

Nos. 1 to 16, which are inventive examples, had good surface gloss and little gloss anisotropy. They also met 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 less than favorable annealing conditions, resulting in the formation of a Cr-deficient layer and poor corrosion resistance.

Comparative examples Nos. 17 to 22 had poor surface gloss or significant gloss anisotropy. They also did not meet hardness requirements. No. 17 was annealed at 1,080°C in accordance with a typical conventional manufacturing method, and descaling was performed by shot blasting followed by pickling. In this case, pickling was performed while a Cr-deficient layer was still formed, resulting in low specular gloss. Furthermore, the impact of shot blasting increased the difference between HS and HC, causing the product surface to harden excessively, reducing workability in drilling and other processes. Similarly, Nos. 18 and 19 were descaling by shot blasting followed by pickling, but they also had low specular gloss and a large difference between HS and HC, reducing workability.

For Nos. 20 and 21, the average particle size or volume fraction of the shot material did not satisfy the preferred conditions of this embodiment, resulting in a decrease in specular gloss and not meeting the hardness requirements. Furthermore, uneven scale remained, and the scale could not be sufficiently removed, making it impossible to measure the rating number.

No. 22 is an example in which, after annealing, shot blasting was used as a descaling method, followed by belt polishing without pickling. In this case, the specular gloss was too high, which is undesirable from the perspective of anti-glare properties. There was also a large difference in specular gloss, resulting in significant gloss anisotropy. 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 belt polishing.

Claims (4)

An austenitic stainless steel section extending in a longitudinal direction and having a plane 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 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. The austenitic stainless steel section according to claim 1, wherein the test specimen is subjected to nine cycles of wet-dry cycling testing, 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 then holding in a wet atmosphere at 50°C and a relative humidity of 95% for two hours. After nine cycles, the surface has a rating number of 8 or higher as specified in JIS Z 2371:2015. 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. 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.