JP3011077B2 - High weather resistant steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a weather-resistant steel material.

[0002]

2. Description of the Related Art Generally, P, Cu, Cr, Ni are added to ordinary steel.
By adding an appropriate amount of such elements as low alloy steel,
Corrosion resistance in the atmosphere can be improved. This low-alloy steel is called weather-resistant steel, and forms rust that protects it from corrosion outdoors in several years (usually 5 to 10 years), and does not need to be subjected to a corrosion-resistant surface treatment such as painting thereafter. , So-called maintenance-free steel.

However, the above-mentioned conventional weather-resistant steel does not form protective rust in severe atmospheric corrosion environments such as coastal areas, and even after several years, floating rust such as red rust and yellow rust and flowing rust. This is disadvantageous not only in appearance, but also in pollution of the surrounding environment.

For this reason, as a measure for preventing the occurrence of floating rust and flowing rust, there is a surface treatment method for forming a phosphate film as disclosed in, for example, Japanese Patent Application Laid-Open No. 1-142088. However, such a surface treatment method requires a large amount of a special treatment solution, requires a great deal of labor, and has a disadvantage that an increase in product cost cannot be avoided.

On the other hand, in recent years, due to labor shortages and demands for enhancing social capital, which have become social problems in recent years, the time required for the above surface treatment and painting is not required, and the product cost does not increase. In fact, there is a strong demand for the development of maintenance-free high weather resistance steel.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and an object thereof is to form an excellent protective rust layer on the surface of a steel material by a relatively simple treatment. It is another object of the present invention to provide a highly weather-resistant steel material which can not only generate floating rust and flowing rust which impair the appearance but also reduce the product cost even in a severe atmospheric corrosion environment such as a coastal zone.

[0007]

According to the present invention, an iron sesquioxide (α-Fe ₂ O ₃ ) film of a predetermined thickness, especially an α-type iron sesquioxide, that is, a hematite (α-Fe ₂ O ₃ ) film is formed on the surface of a steel material. .

The present inventors have conducted various experimental studies to obtain a highly weather-resistant steel material exhibiting excellent weather resistance under severe atmospheric corrosion environments such as coastal areas. As a result, the steel material surface has a thickness of 30 nm to 200 μm. Hematite (α-Fe
Only when a ₂ O ₃ ) film was formed, it was found that floating rust and flowing rust which impair the appearance did not occur to the extent of exaggeration, and there was almost no corrosion thinning.

The present invention has been made based on the above findings, and the gist lies in the following highly weather-resistant steel material.

A highly weather-resistant steel material characterized in that its surface is covered with hematite (α-Fe ₂ O ₃ ) having a thickness of 30 nm to 200 μm.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The hematite film formed on the surface of a weather-resistant steel material and the reason why the thickness thereof is limited to the above range will be described together with the function and effect.

Hematite, among iron oxides, oxyhydroxides and hydroxides, is the most thermodynamically stable oxide under normal temperature atmospheric conditions, and is transformed even in the presence of water and oxygen. It exists stably without doing.

[0013] For this reason, the steel material on which the hematite film is formed has extremely excellent protection against corrosion from the beginning of use, and it is said that there is no floating rust such as red rust or yellow rust or flow rust. It does not occur as much as possible. In other words, when a hematite film is formed in close contact with the steel surface,
This hematite film not only acts as a resistance barrier that suppresses corrosion factors such as water, oxygen, sulfurous acid gas, and salts in the atmospheric corrosion environment from reaching the metal surface, but also these corrosion factors contribute to the hematite itself. No longer adsorbs on the surface.
As a result, almost no floating rust such as red rust or yellow rust or flowing rust is generated, and excellent weather resistance is exhibited.

However, when the thickness is less than 30 nm, the reactivity between the above-mentioned corrosion factor and hematite increases, so that when the hematite film has a partial fine defect, dust or sea salt particles may be removed from the hematite film. If it adheres to the surface of
Corrosion factors reach the surface of the steel material, and the corrosion resistance is not maintained.

When the thickness exceeds 200 μm,
Defects such as voids and voids (cracks) in the hematite film are likely to occur, or the reactivity with each of the above-mentioned corrosion factors is increased. As a result, the corrosion factors reach the metal surface during prolonged exposure to the atmosphere. Reaches and causes floating rust such as red rust and yellow rust and flow rust.

Therefore, the thickness of the hematite film to be formed is 30 nm to 200 μm.

The base steel constituting the high weathering steel of the present invention is austenitic or ferritic stainless steel or a Ni-based alloy, and does not usually include a steel type which does not form a so-called rust which is a precipitation type coating. For example, JIS-G31
Low-alloy steel or carbon steel such as a weathering steel specified in No. 25, steel for welding structure specified in G3106, and its equivalent steel; and pure iron such as Armco iron. That is,
Any steel type can be used as long as it is a steel type that generates the so-called rust, which is the above-mentioned precipitation type coating.

The above-mentioned hematite film is made of the above-mentioned sedimentation type film, that is, a kind of steel that generates rust. For example, a steel material formed into various shapes such as a shaped steel, a bar, a plate, and a tube may be used. 0.1 ℃ in air atmosphere furnace at ℃
It can be formed by performing a heat treatment for 1000 hours. That is, the heating and holding temperature is 200 ° C.
When the holding time is less than 0.1 hours or when the holding time is less than 0.1 hour, not only a hematite film having a sufficient thickness is not uniformly formed, but also a portion not covered with the hematite film is formed, and the above-mentioned performance cannot be exhibited. On the other hand, when the heating holding temperature is 400
If the temperature exceeds 100 ° C. or the holding time exceeds 1000 hours, the hematite film becomes too thick, and the above performance cannot be satisfied. Therefore, it is preferable that the heat treatment is performed under the condition of holding in an atmosphere furnace at 200 to 400 ° C. for 0.1 to 1000 hours.

As described above, the highly weather-resistant steel material of the present invention can be easily obtained only by performing a low-temperature heat treatment, so that the product cost can be reduced.

[0020]

EXAMPLE A test piece having a length of 100 mm, a width of 60 mm and a thickness of 3 mm was sampled from three types of test materials A to C having the component compositions shown in Table 1, and the surface thereof was emery polished paper (# 1200). ) To adjust the polishing. Thereafter, each test piece was kept in a temperature range of 150 to 450 ° C. in the air atmosphere for a predetermined time (0 to 1200 hr) to form hematite layers of various thicknesses. Each of the obtained test pieces was
It is located at a height of 2 m, 5 m away from the coastline of the coastal area of Kitakyushu City, which is in a severely corrosive environment.
It was installed at an angle and exposed for 300 days.

[0021]

[Table 1]

After the above-described exposure test, the presence or absence of rust is visually observed to confirm the presence or absence of rust. If there is even a small amount of rust, the rust layer is removed and the thickness reduction (average corrosion depth on both front and back surfaces) (Mean value). Table 2 shows the measurement results.

[0023]

[Table 2]

As is evident from the results shown in Table 2, the steel materials (Nos. 4 to 15, Nos. 21 to 32 and Nos. 38 to 38) having a hematite layer formed in a thickness within the range specified in the present invention.
No. 49) showed virtually no occurrence of floating rust or flowing rust in any of the steel types, and the corrosion thinning amount was 0.03 μm or less, which was very slight and showed good weather resistance.

On the other hand, a steel material of a comparative example (No. 1 to No. 1) having a hematite layer having a thickness outside the range specified in the present invention was formed.
3, No. 16-17, No. 18-20, No. 33-3
No. 35, No. 35-37 and No. 50-51), all steel types showed rust generation and had a corrosion thinning amount of 48.32 μm.
m or more, and the weather resistance was poor.

[0026]

As described above, since the surface of the highly weatherable steel material of the present invention is covered with a stable anti-corrosive hematite film,
No rust is generated even under severe atmospheric corrosion environment. As a result, the appearance of a building or a structure using the steel material of the present invention is not impaired for a long time. In addition, its production can be obtained only by performing a simple heat treatment, and since it has excellent maintenance-free properties, it is economical.

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Claims (1)

(57) [Claims] 1. A highly weather-resistant steel material whose surface is covered with hematite (α-Fe ₂ O ₃ ) having a thickness of 30 nm to 200 μm.