JPH09125224A - High weather resisting steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high weather rosion resisting steel free from the occurrence of loose rust and slobbery rust in a severe atmospheric corrosion environment by coating the surface of a steel with hematite of specific thickness. SOLUTION: The surface of a steel is coated with hematite (α-Fe2 O3 ) of 30nm to 200μm thickness. By this method, a rust layer with excellent protective property can be formed on the surface of the steel by relatively simplified treatment, and the highly atmospheric corrosion resisting steel, in which the occurrence of loose rust and slobbery rust, deteriorating external appearance, is prevented even under a severe atmospheric corrosion environment such as coastal area, can be obtained at a low cost.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD 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 against corrosion in a few years outdoors (usually 5 to 10 years), and it is not necessary to subsequently apply corrosion resistant surface treatment such as painting. , So-called maintenance-free steel.

However, the above-mentioned conventional weathering steel does not form rust having a protective property in a severe atmospheric corrosive environment such as a coastal area, and even after several years, floating rust such as red rust and yellow rust and flow rust. Has the disadvantage that not only is it undesirable from the outside, it also causes pollution of the surrounding environment.

Therefore, as a measure for preventing the floating rust and the flow rust from occurring, there is a surface treatment method for forming a phosphate film as disclosed in, for example, Japanese Patent Laid-Open No. 1-142088. However, such a surface treatment method has a drawback in that a large amount of special treatment solution is required and a great deal of labor is required, and an increase in product cost cannot be avoided.

On the other hand, in recent years, due to the labor shortage, which has become a social problem, and the demand for enhancing social capital, the above-mentioned surface treatment and painting are not required, and the product cost is not increased. 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 it is an object of the present invention to form a rust layer having an excellent protective property on the surface of a steel material by a relatively simple treatment. The present invention is to provide a high weather resistance steel material capable of reducing the product cost as well as causing no floating rust or flow rust that deteriorates the appearance even in a severe atmospheric corrosion environment such as a coastal area.

[0007]

In the present invention, a ferric sesquioxide having a predetermined thickness, particularly an α-type ferric sesquioxide coating, that is, a hematite (α-Fe ₂ O ₃ ) coating is formed on the surface of a steel material. .

The inventors of the present invention conducted various experimental studies in order to obtain a high weather resistance steel material exhibiting excellent weather resistance in a severe atmospheric corrosion environment such as a coastal area. As a result, the surface of the steel material was 30 nm to 200 μm. Thickness of hematite (α-Fe
_It has been found that only when a ₂ O ₃ ) film is formed, floating rust or flow rust that impairs the appearance does not occur to the extent that it is exaggerated and corrosion thinning is almost eliminated.

The present invention has been made on the basis of the above findings, and its gist resides in the following high weather resistance steel material.

A high weather resistance steel material having a surface 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 reason why the hematite film formed on the surface of a weather-resistant steel material and the thickness thereof are limited to the above range will be described together with the function and effect.

Among iron oxides, oxyhydroxides and hydroxides, hematite is the thermodynamically most stable oxide in the atmospheric environment at room temperature, and is transformed even in the presence of water and oxygen. Existence stable without doing.

Therefore, the steel material on which the hematite film is formed has an extremely excellent protection against corrosion from the beginning of its use, and it is said that floating rust such as red rust and yellow rust and flow rust are completely absent. It doesn't occur as much as you can. That is, when a hematite film is closely formed on the steel surface,
This hematite film not only acts as a resistance barrier that suppresses and prevents corrosion factors such as water, oxygen, sulfurous acid gas, and salt in the atmospheric corrosive environment from reaching the metal surface, but these corrosion factors also affect the hematite itself. No longer sticks to the surface.
As a result, almost no floating rust or flow rust such as red rust or yellow rust is generated, and excellent weather resistance is exhibited.

However, if the thickness is less than 30 nm, the reactivity with the above-mentioned corrosion factors and hematite is increased, and therefore, if the hematite coating has a partial fine defect, dust or sea salt particles are not contained in the hematite coating. When it adheres to the surface of
Corrosion factors reach the surface of the steel material, and corrosion resistance cannot be maintained.

If the thickness exceeds 200 μm,
Defects such as voids or voids (cracks) in the hematite film are more likely to occur, or the reactivity with the above corrosion factors increases, and as a result, the corrosion factors reach the metal surface during long-term exposure to the atmosphere. It reaches and causes floating rust and flow rust such as red rust and yellow rust.

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

The base steel constituting the high weather resistance steel of the present invention is austenitic or ferritic stainless steel, Ni-based alloy, etc., and does not normally include a so-called rust-free steel type which is a precipitation type film. , For example, JIS-G31
The low-alloy steels and carbon steels such as the weathering steel defined in No. 25, the welded structural steels defined in G3106, and their equivalent steels, and pure iron such as Armco iron. That is,
Any kind of steel may be used as long as it is a kind of steel that produces so-called rust which is the above-mentioned precipitation type film.

The above-mentioned hematite film is made of a so-called rust-forming steel species which is the above-mentioned precipitation-type film, and shaped steel, steel bars, plates, and steel materials formed into various tubular shapes, for example, 200 to 400. In an atmospheric furnace at
It can be formed by performing a heat treatment for holding for 1000 hours. That is, the heating and holding temperature is 200 ° C.
If it is less than 1 hour, or if 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 part not covered with the hematite film is formed and the above performance cannot be exhibited. On the other hand, the heating and holding temperature is 400
If the temperature exceeds ℃ or the holding time exceeds 1000 hours, the hematite coating becomes too thick and the above performance cannot be satisfied. Therefore, the heat treatment is preferably carried out under the condition of holding in an air atmosphere furnace at 200 to 400 ° C. for 0.1 to 1000 hours.

As described above, the high weather resistance steel material of the present invention can be easily obtained only by performing the 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 taken from each of three types of test material steel materials A to C having the composition shown in Table 1, and the surface thereof was emery-polished paper (# 1200). ) Was used for polishing adjustment. Thereafter, each test piece was held in a temperature range of 150 to 450 ° C. in an air atmosphere for a predetermined time (0 to 1200 hr) to form hematite layers having various thicknesses. Each of the obtained test pieces,
At a height of 2m, 5m away from the coastline of the coastal area of Kitakyushu in a severe atmospheric corrosive environment, 30
It was installed at an angle of inclination and exposed for 300 days.

[0021]

[Table 1]

After the above-mentioned exposure test, the presence or absence of rust was confirmed by visual observation, and the rust layer was removed if even a slight amount of rust was generated, and the thickness reduction amount (average corrosion depth on both front and back surfaces) Was measured. The measurement results are shown in Table 2.

[0023]

[Table 2]

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

On the other hand, the steel materials of the comparative examples (No.
3, No. 16-17, No. 18-20, No. 33-3
No. 4, No. 35-37 and No. 50-51), rust generation was observed in all steel types, and the corrosion thinning amount was 48.32μ.
It was as high as m or more and was inferior in weather resistance.

[0026]

The high weather resistance steel material of the present invention has its surface covered with a stable hematite film having high anticorrosion property.
No rust is generated even under severe atmospheric corrosion environment. As a result, the appearance of the building or structure using the steel material of the present invention will not be impaired for a long period of time. In addition, the manufacturing thereof is obtained only by performing a simple heat treatment, and since it has excellent maintenance-free property, it is excellent in economic efficiency.

Claims (1)

[Claims] 1. A high weather resistance steel material, the surface of which is covered with hematite (α-Fe ₂ O ₃ ) having a thickness of 30 nm to 200 μm.