JP2765425B2 - Steel material with excellent weather resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a steel material excellent in weather resistance,
Especially, it is a stable and highly corrosion-resistant steel material whose surface is covered with rust, even in the most severe atmospheric corrosion environment where sea salt particles that adversely affect the weather resistance of the steel fly. .

[0002]

2. Description of the Related Art In general, corrosion resistance in the atmosphere can be improved by adding elements such as P, Cu, Cr and Ni to steel. These low alloy steels are called weatherable steels,
This is a so-called maintenance-free steel that forms rust (hereinafter referred to as weather-resistant rust) having protection against corrosion in a few years outdoors and does not require any corrosion-resistant treatment such as painting.

However, since it takes several years to form weather-resistant rust, floating rust such as red rust and yellow rust and flowing rust occur during the period, which is not only unfavorable in appearance, but also It still has the problem of causing pollution of the surrounding environment.

Regarding this problem, see, for example,
As described in No. 2088, a surface treatment method for forming a phosphate film has been proposed.

[0005]

However, the content of the treatment is complicated, such as the necessity of performing an appropriate pretreatment before the formation of the phosphate film, and when the welding of steel material is necessary, the welded portion is required. There is a problem that it is not easy to apply the treatment and it is difficult to apply it to a building structure.

[0006] Conventionally, a drunken surface treatment method such as applying a coating film on the surface of a weather-resistant steel or applying a coating after forming a phosphate film has been performed. There are problems such as the slow generation of rust, and the deterioration of the coating film itself to significantly impair the appearance.

Further, there is a problem that weather-resistant rust is not formed in severe environments such as a coastal zone where sea salt particles fly.

Accordingly, an object of the present invention is to provide an anticorrosive rust layer that has been artificially grown at an early stage and has excellent anticorrosive properties without causing floating rust such as red rust and yellow rust and flowing rust. An object of the present invention is to provide a steel material which is excellent in weather resistance and is used for civil engineering and construction.

[0009]

Means for Solving the Problems The present inventors have found that the surface of a steel material is covered with a rust layer having an average crystal grain size of a certain size or less. It was found that if the rust was α-FeOOH, the weather resistance of the steel material was extremely high.

That is, this application includes the following inventions. 1. A steel material excellent in weather resistance, characterized in that the surface of the steel material is covered with a rust layer having an average crystal grain size of 200 nm or less.

2. A steel material excellent in weather resistance, characterized in that the surface of the steel material is covered with α-FeOOH having an average crystal grain size of 200 nm or less.

[0012]

The present invention will be described below in more detail.
As long as the rust has few physical defects such as cracks and voids in the atmospheric corrosion environment, it will also prevent the invasion of oxygen, water, and corrosive substances in the atmosphere, and as a result, shut off the atmospheric corrosion environment And the elution of Fe ions, which is a fundamental cause of floating rust and flowing rust, is reduced.

The present inventors have found that if the average crystal grain size of the particles constituting the rust covering the surface of the steel material is equal to or less than a certain size, the rust becomes dense and performs a high anticorrosion function. As an average crystal grain size, specifically, if it is 200 nm or less,
Corrosion protection of rust is good not only in industrial areas and countryside areas, but also in coastal areas where sea salt particles fly and are the harshest environment.

In particular, it has been found that when the steel surface is covered with α-FeOOH which is a final corrosion product stable in a normal atmospheric corrosion environment, the weather resistance of the steel is further improved. This is considered to be because α-FeOOH is thermodynamically stable and suppresses the electrochemical reaction accompanying the phase transformation and dissolution of rust.

However, the steel referred to in the present invention does not include those which do not produce so-called rust which is usually a precipitation type film, such as stainless steel and Ni-based alloy, and mainly refers to low alloy steel such as weathering steel and carbon steel. .

The term "rust" as used in the present invention means that a mixture of α-FeOOH, γ-FeOOH, Fe ₃ O _{4, and the} like formed by general atmospheric corrosion of iron, unless otherwise specified. Refers to an object.

[0017]

EXAMPLES Next, the effects of the present invention will be clarified by examples. Table 1 shows the chemical components of the test steels used in the examples. In order to cover the steel surface with a rust layer having an average crystal grain size of 200 nm or less, for example, a method described in JP-A-3-320646 is used. Well, α-FeOOH of 200nm or less
In order to cover the surface, for example, an appropriate amount of an aqueous solution of chromium (III) sulfate and an aqueous solution of sodium hydroxide at an appropriate concentration may be applied to the atmosphere while applying an appropriate amount by the method described in Japanese Patent Application No. 4-199701. Further, the average crystal grain size of rust can be controlled by controlling the concentration of the aqueous solution to be applied.

The test is performed by applying an aqueous solution of 0.05 to 40 mass.% Chromium (III) sulfate once a week to the steel surface while applying it to the steel surface for three months, or after the above treatment, a pH 9 aqueous sodium hydroxide solution. Is applied to the steel surface once a week and exposed to the air for another month to obtain rust of various average grain sizes or α-FeOO of various average grain sizes.
A rust-covered exposure test piece composed of H was prepared and performed.

Approximately 100 mg of the rust layer was collected with a cutter knife until the steel surface was confirmed and ground to obtain a powder sample, which was used to measure the average grain size of the rust by transmission electron microscopy.

At this time, the constituent iron compounds were identified by taking an electron beam diffraction pattern.

The dimensions of the test piece were 150 × 60 × 3 mm ^3, and the surface before the treatment was subjected to emery paper polishing and buffing and used as a mirror surface.

Each test piece was exposed to the coastal area of Kitakyushu for 10 years under the same conditions.

The average corrosion rate during the exposure period (the thickness reduction per unit time) was evaluated by measuring the weight of the steel sheet. A concrete plate was placed immediately below the exposed test piece, and the amount of flowing rust was evaluated based on the degree of soiling of the concrete. The results are shown in Table 2, Table 3, FIG. 1 and FIG.

[0024]

[Table 1]

[0025]

[Table 2]

[0026]

[Table 3]

FIG. 1 shows the relationship between the average grain size of the rust layer and the corrosion rate. When the average crystal grain size of the rust is 200 nm or less, the corrosion rate is low, and it is clear that the rust is dense and has high corrosion resistance. Furthermore, as shown in FIG. 2, when the rust layer is composed of α-FeOOH, the corrosion resistance is particularly large if the average crystal grain size is 200 nm or less.

As shown in Tables 2 and 3, when the average crystal grain size exceeds 200 nm, the corrosion rate sharply increases and the amount of flowing rust particularly increases. Conversely, when it was 200 nm or less, almost no flow rust was observed.

[0029]

According to the present invention, the steel material surface has an average crystal grain size of 200
rust layer or a flowing rust such as red rust and yellow rust in an atmospheric corrosion environment by being covered with a rust layer of less than 200 nm or the steel material surface is covered with α-FeOOH with an average crystal grain size of less than 200 nm. A steel material having rust that is hardly generated and has high corrosion resistance is provided. Therefore, it greatly contributes to the enhancement of social capital such as buildings and structures.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the corrosion rate of a steel sheet and the average crystal grain size of surface rust.

FIG. 2 is a graph showing the relationship between the corrosion rate of a steel sheet and the average crystal grain size of surface rust composed of α-FeOOH.

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁶ , DB name) C23C 22/00-22/86 C23C 8/18

Claims (2)

(57) [Claims] 1. A steel material excellent in weather resistance, wherein the surface of the steel material is covered with a rust layer having an average crystal grain size of 200 nm or less. 2. A steel material having an α having an average crystal grain size of 200 nm or less.
-A steel material excellent in weather resistance characterized by being covered with FeOOH.