JP3854533B2 - Evaluation method of weatherability of rust layer of steel - Google Patents

Description

【００１７】
ここで、ｇ_ij（ｒ）は部分２体分布関数、Ｑは散乱ベクトル、ｒは原子間距離、ρ₀は密度を示す。
さらに、部分構造因子から干渉関数：ｉ（Ｑ）、および元素まわりの環境干渉関数：△ｉ_a （Ｑ）を計算し、式（２）で与えられる実験データとの統計変動差ｘ²を見積もる。
【００１８】
【数２】

【００１９】
なお、ここでσ² は実験データにおける統計誤差、ｉ（Ｑ）は実験により求めた部分構造因子、△ｉ_a （Ｑ）は異常散乱実験により求めた特定の元素まわりの環境干渉関数、ｉ^c （Ｑ_m ）、△ｉ^c _a （Ｑ_m ）はそれぞれ構造モデルに基いて計算された部分構造因子、特定の元素まわりの環境干渉関数である。
▲３▼配置された粒子のうち１つを選びランダムな方向に動かすことにより、エネルギーの異なる仮想的な原子配列をつくる。
▲４▼▲３▼の手順により得られた新しい配置から、▲２▼と同様の手順で干渉関数を計算し、統計変動差ｘ² _newを見積もる。
▲５▼もしｘ² _new＜ｘ² _oldならば新しい配置を採用し、ｘ² _new＞ｘ² _oldならば前の配置に戻す。
▲６▼▲３▼〜▲５▼の手順を繰り返し、計算結果が実験値と誤差の範囲内で一致するまで行う。
【００２０】
上記のように、実際の耐候性鋼表面に形成されるさびの構造は、基本となるＦｅ（Ｏ，ＯＨ）₆ ユニット自体は存在するものの、そのユニットには一部欠陥が存在し、Ｆｅ（Ｏ，ＯＨ）₆ ユニットの配列の周期性には乱れが存在している。そのため、個々のＦｅ（Ｏ，ＯＨ）₆ ユニットを代表するパラメータを適用することにより、さびの構造の本質的な評価が可能になる。
【００２１】
図２は、Ｆｅ（Ｏ，ＯＨ）₆ ユニットが完全な周期性をもって配列したＦｅ（Ｏ，ＯＨ）₆ ネットワーク構造である場合の結晶構造モデル（一部分について鉄原子のみ記述）である。ここで、任意のＦｅ（Ｏ，ＯＨ）₆ ユニットＡ内の近接する２つの原子ｉ、jを結ぶベクトルＶ_ijと、別の任意のＦｅ（Ｏ，ＯＨ）₆ ユニットＢ内の近接する２つの原子ｋ、ｌを結ぶベクトルＶ_klを考えると、ベクトルＶ_ijは原子ｉ、ｊを含むユニットＡの、ベクトルＶ_klは原子ｋ、ｌを含むユニットＢの、それぞれ代表とすることができる。両ベクトル間の距離をｒ、角度をθとすると、両ベクトルの配列関係は、ベクトルＶ_ijとベクトルＶ_klが距離ｒおよび角度θの関係を満たす位置に原子ｉ、ｊ、ｋ、ｌを見出す確率を表す関数Ａ（ｒ，θ）により表記することができる。
【００２２】
すなわち、図２に示すようなＦｅ（Ｏ，ＯＨ）₆ ユニットが完全な周期性を持って配列したＦｅ（Ｏ，ＯＨ）₆ ネットワーク構造である場合には、ベクトルＶ_ijとベクトルＶ_klの距離ｒおよび角度θが、距離ｒ＝（隣接するユニット間の距離）×（整数＝ユニット間の数）で、かつθ＝６０°の条件を満たす場合にのみ、原子ｉ、ｊ、ｋ、ｌが存在し、この条件において、関数Ａ（ｒ，θ）は高いピーク値をもつことになる。
【００２３】
これを図３を例に説明する。図３は、純鉄を塩水中に２ヶ月放置した際に生じたさびについて、縦軸にｒ、横軸にθをとり、関数Ａ（ｒ，θ）を図示したものである。関数Ａ（ｒ，θ）がθ＝０°およびθ＝６０°に高いピーク値を有する場合（なお、その定義からわかるように、θ＝０°と、θ＝１８０°、θ＝６０°とθ＝１８０−６０＝１２０°は、それぞれ本質的に同じであるため、両者を含めて、以下θ＝６０°と表現する）はＦｅ（Ｏ，ＯＨ）₆ ユニットに欠陥および乱れが極めて少なく、ほぼ完全な周期性をもって配列していることを示す。つまり、θ＝０°および６０°にピーク値を有する場合は耐候性に優れたさびであり、有さない場合は耐候性の劣るさびであると評価できる。
【００２４】
なお、前記角度θは、結晶の状態により若干変動する。例えば、添加元素等によりＦｅ（Ｏ，ＯＨ）₆ ユニットが歪んだ場合には、Ｆｅ（Ｏ，ＯＨ）₆ ユニットに欠陥および乱れが極めてすくないことを示すθ＝０°およびθ＝６０°のピーク値は、数度程度ずれることがあり、この場合にはそのずれを考慮したθの角度範囲に高いピークが存在するように関数Ａ（ｒ，θ）を設定すればよい。
関数Ａ（ｒ，θ）がピーク値を有するか否かの判断は、関数Ａ（ｒ，θ）が特定の点（ｒ，θ）において平均値より高い値を示すか否かにより行う。平均値は全空間に原子がランダムに配列した場合と、完全に配列した場合の加重平均により求めればよい。また、実際の材料の評価は、基準材料を基に行うことが多く、この場合は試験材料と基準材料の関数Ａ（ｒ，θ）の差分関数
【００２５】
【外１】

【００２６】
が正の値をもつ場合にピーク値を有するものと判断する。また、本発明は耐候性鋼のさび構造評価法において、評価対象とする耐候性鋼は特に限定するものではない。本発明の耐候性鋼に優れた鋼材とは、鋼材表面に生成され、地鉄に密着しているさび層を有する鋼材であって、該鋼材が、Ｃｕ：０．１質量％以上３質量％以下、Ｎｉ：０．１質量％以上６質量％以下の範囲で、Ｃｕ、Ｎｉのいずれか一方もしくは両方を含有し、残部が鉄および不可避的不純物からなり、さらに、該さび層がＡ（ｒ，θ）において、θ＝０°および６０°にピーク値を有する鋼材である。ここで耐候性鋼に優れた鋼材とは、平均腐食速度が０．０１ｍｍ／年以下であることをいう。
【００２７】
前記さび層がＡ（ｒ，θ）において、θ＝０°および６０°にピークを有する場合、さびのＦｅ（Ｏ，ＯＨ）₆ ユニットの欠陥が極めて少なく、かつそのユニットの配列の周期性が高い。そのため、形成されるさびの結晶粒径も小さく、さび自体が緻密で、腐食の原因となるＣｌ、Ｏ、Ｈ等の元素の外部環境からの侵入を防止できる環境遮断機能を有することになる。さらに、使用環境下で安定であって変化しないことに対応し、耐候性に優れた鋼材となる。
【００２８】
また、保護性さびの生成方法は、表面に生成を促進する物質を塗布することによっても良いし、自然に生成させても良い。さび層を構成する成分については、ゲーサイト、マグネタイト、不活性非晶質さびなどが安定性の観点から望ましい。粒子充填後の空隙をさらに埋める反応を促進するためには、ゲーサイト、アカガネイト、レピドクロサイト、活性非晶質さび等が微量に混在した方が効率的に環境遮断機能を高めることができる。
【００２９】
本発明者が調査した範囲では、Ｃｕ：０．１質量％以上３質量％以下、Ｎｉ：０．１質量％以上６質量％以下の範囲で、Ｃｕ、Ｎｉのいずれか一方もしくは両方を含有し、残部が鉄および不可避的不純物からなる場合、該さび層がＡ（ｒ，θ）において、θ＝０°および６０°にピーク値を有し耐候性鋼に優れた鋼材を得ることができる。これは、これらの元素の添加により、さびの発生の初期反応の起点となる反応サイトが多くなり多数の点から反応が生成する結果、最終的に得られる組織が微細となるからである。
【００３０】
また、これらの元素の添加は、形成されるさびそのものをイオン遮断機能の高いものに変化させる効果もある。これらの効果の発現のためにはＣｕ：０．１質量％以上、Ｎｉ：０．１質量％以上のいずれか一方もしくは両方を含有させることが必要になる。一方、元素の添加量が多くなると、さびの構造が大きく変化し、耐候性が劣化するようになる。そのため、Ｃｕ：３％以下、Ｎｉ：６％以下であることが望ましい。
【００３１】
【実施例】
（実施例１）
純鉄、純チタン、ステンレス（ＳＵＳ３０４）を塩水中に２ヶ月放置した際に生じたさびを本発明の方法により評価することにより関数Ａ（ｒ，θ）を得た。純鉄の結果を図３に示す。例えば、距離ｒ＝０．９ｎｍにおいて、θ＝０°、６０°、１２０°、１８０°に高いピークに観察され、このさび層におけるＦｅ（Ｏ，ＯＨ）₆ ユニットの配列の周期性が高いことを示している。
本発明の方法では、ある２組の原子のペアｉとｊ、ｋとｌの相関を、両者の距離ｒと角度θにより表現しているために、ある距離に存在するペアの方位関係がはっきりわかり、構造と特性の関係が明瞭である。
【００３２】
（比較例１）
純鉄を塩水中に２ヶ月放置した際に生じたさびを、従来の構造評価法のひとつであるＸＡＦＳ（Ｘ−ｒａｙ Ａｂｓｏｒｐｔｉｏｎ Ｆｉｎｅ−ｓｔｒｕｃｔｕｒｅｓ：Ｘ線吸収微細構造）法により評価し、動径分布関数を求めた。その結果を図４に示す。この従来法では、距離ｒにある２つの原子間の相関のみしかわからず、ピークの帰属は困難であるため詳細な構造がわからず、さらに原子対の方位関係に関する情報は得られなかった。
【００３３】
（実施例２）
本発明の耐候性鋼（Ｆｅ−０．６質量％Ｃｒ−０．３質量％Ｃｕ−０．５質量％Ｍｎ−０．５質量％Ｓｉ−０．０８質量％Ｐ）を田園地域に３１年放置した際に生じたさびを、同様の条件で放置した普通鋼に生じたさびを標準材料として、本発明の方法により評価することにより関数
【００３４】
【外２】

【００３５】
を得た。それを図５に示す。ある２組の原子のペアｉとｊ、ｋとｌの相関を両者の距離ｒと角度θにより表現しているために、ある距離に存在するペアの方位関係がはっきりわかり、構造と特性の関係を明瞭である。例えば、距離ｒ＝０．９ｎｍにおいて、θ＝０°、６０°、１２０°、１８０°に高いピークに観察され、このさび層におけるＦｅ（Ｏ，ＯＨ）₆ ユニットの配列の周期性が高いことを示している。
【００３６】
（実施例３）
ＣｕやＮｉ添加量の異なる鋼材を海浜地帯に９年間大気暴露した。鋼材表面の最表面の外層さびをワイアーブラシで機械的に削ぎ落とした後に、地鉄に密着した密着さび層を採取し、本発明の方法により評価を行った。そして、ピーク強度の値を、平均値より２０倍以上（◎）、平均値の５〜２０倍（○）、平均値の２〜５倍（△）、平均値の２倍以下（×）、の四段階で評価した（表１）。ここで、平均値は全空間に原子がランダムに配列した場合と、完全に配列した場合の加重平均により求めた。本発明の耐候性鋼（本発明例Ｎｏ．６〜１３）は、いずれもＡ（ｒ，θ）においてθ＝０゜および６０゜付近のピーク強度の評価が、◎＋◎、◎＋○、又は○＋○であり、平均腐食速度が０．０１０ｍｍ／年以下であり、良好な耐食性を示した。従って、本発明の耐候性鋼の耐候性が良好であるとともに、本発明のＡ（ｒ，θ）におけるθ＝０゜および６０゜付近のピーク強度の評価法が、鋼材のさび層の評価法として適用可能であることが示された。
【００３７】
【表１】

【００３８】
（比較例２）
実施例３と同様に、ＣｕやＮｉ添加量の異なる鋼材を海浜地帯に大気暴露した。鋼材表面の最表面の外層さびをワイアーブラシで機械的に削ぎ落とした後に、地鉄に密着した密着さび層を採取し、実施例３と同様に本発明の方法により評価を行った（表１）。比較例の耐候性鋼（比較例Ｎｏ．１〜５）は、いずれもＡ（ｒ，θ）においてθ＝０゜および６０゜付近のピーク強度の評価が、○＋△、△＋△、△＋×、または×＋×であり、平均腐食速度が０．０１０ｍｍ／年より大きく、耐食性が悪い。
【００３９】
【発明の効果】
以上述べたように、本発明により、その周期性が不完全である材料の構造を評価することができた。そして、鋼材表面に生成され、地鉄に密着しているさび層を有する鋼材であって、該鋼材が質量％にて、Ｃｕ：０．１％以上３％以下、Ｎｉ：０．１％以上６％以下の範囲で、Ｃｕ、Ｎｉのいずれか一方もしくは両方を含有し、残部が鉄および不可避的不純物からなり、さらに、該さび層がＡ（ｒ，θ）において、θ＝０゜および６０゜にピーク値を有する鋼材であり、十分に低い平均腐食速度をもたらす密着さび層を有する鋼材を提供できた。本発明の鋼材は、塗装等の特別な表面処理を施さなくとも確実に優れた耐候性を得ることができ、環境負荷低減や経済性の観点からも、産業上極めて価値の高い発明であるといえる。
【図面の簡単な説明】
【図１】ａ−ＦｅＯＯＨの結晶構造の模式図を示す図である。
【図２】Ｆｅ（Ｏ，ＯＨ）₆ ユニットが完全な周期性をもって配列した場合の結晶構造モデル（一部分について鉄原子のみ記述）の模式図を示す図である。
【図３】純鉄を塩水中に２ヶ月放置した際に生じたさびを本発明の方法により評価することにより得られた関数Ａ（ｒ，θ）を示す図である。
【図４】純鉄を塩水中に２ヶ月放置した際に生じたさびを、従来の構造評価法のひとつであるＸＡＦＳ（Ｘ−ｒａｙ Ａｂｓｏｒｐｔｉｏｎ Ｆｉｎｅ−ｓｔｒｕｃｔｕｒｅｓ：Ｘ線吸収微細構造）法により評価し、求められた動径分布関数を示す図である。
【図５】純鉄を塩水中に２ヶ月放置した際に生じたさびを本発明の方法により評価することにより得られた関数Ａ（ｒ，θ）を示す図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for evaluating the crystal structure of a material in which the periodicity of atomic arrangement in the material is incomplete, particularly rust of weathering steel having a stable adhesion rust layer in an atmospheric corrosion environment. The present invention relates to a structural evaluation method and a steel material excellent in weather resistance.
[0002]
[Prior art]
Generally, as a method for evaluating the arrangement of atoms in a material, when the periodicity is high and there is no disturbance, it is uniquely described by specifying the space group of the crystal and the position of the constituent atoms in the unit cell. The method is taken (for example, X-ray diffraction / scattering technique, written by Kikuta, University of Tokyo Press, 1992, Chapter 1). On the other hand, when the periodicity of the arrangement of atoms in the material is low or lacking, such as amorphous, it cannot be expressed by the periodicity classification, and thus the correlation between two atoms is expressed. It is evaluated by a body distribution function or a radial distribution function (for example, X-ray diffraction / scattering technique, written by Kikuta, University of Tokyo Press, 1992, Chapter 3).
[0003]
However, for example, rust formed on the steel surface, the periodicity of the arrangement of the atoms is moderate, and the structure cannot be correctly evaluated using either method described above. There are many materials. By exposing a steel material to which a small amount of elements such as Cu, Ni, Cr, P and the like is exposed to the atmosphere for a period of several years or more, the steel material is in close contact with the steel and directly above the steel. A protective rust layer having a protective function is formed, so-called weathering steel that does not require a corrosion-resistant treatment such as painting is manufactured, and is used as a structural material such as a bridge.
[0004]
Conventionally, the structure of the protective rust layer of such a weather-resistant steel has been considered to be composed of fine-grained goethite (α-FeOOH) whose main part has an average crystal grain size of 200 nm or less. However, such fine-grained goethite having an average particle diameter of 200 nm or less is also present in a rust layer of ordinary steel to which elements such as Cu, Ni, Cr, and P are not added. It was clarified from the structural analysis of the rust layer particles using a transmission electron microscope that the presence of the grain goethite is not all of the manifestation factors of the weathering function of the protective rust layer of the weathering steel.
[0005]
That is, in weathering steel and ordinary steel, there are crystal grains of various grain sizes in the rust layer that is in close contact with the base metal, and the crystal structure is determined according to analysis results such as electron beam diffraction. Although it is said that (α-FeOOH) is the main component, other elements such as akaganeate (β-FeOOH), lipidocrosite (γ-FeOOH), magnetite (Fe ₃ O ₄ ), etc. It has been found that since crystal structures exist and these crystal structures are fine, it is sometimes difficult to determine the crystal structure. Furthermore, it may have a structure that is not identified in these phases due to defects in the crystal structure.
[0006]
Originally, the function required for the surface layer of the weathering steel that does not require the anticorrosion treatment work is that it has an anticorrosion function. Since the anticorrosion function is a function to prevent further corrosion of the base iron after the formation of the protective rust layer that is the surface anticorrosion layer, the first requirement for the surface anticorrosion layer is Cl, which causes corrosion of the layer, It has an environment blocking function capable of preventing the entry of elements such as O and H from the external environment. The second requirement is that it is stable and does not change under the usage environment, that is, has environmental stability.
[0007]
In order to have environmental barrier properties, it is necessary to have a structure in which the rust itself is dense and does not easily transmit the elements that are expected to enter from the environment as described above. Therefore, the arrangement of atoms constituting the rust layer, that is, the atomic correlation is important.
Therefore, it can be said that by analyzing the atomic correlation of the rust layer, it is possible to evaluate the characteristics of weathering steel, etc., but as described above, the evaluation of the structure of the rust layer is applied to materials with perfect periodicity. Of the weathering steel and the structure of the rust layer formed on the weathering steel. The correlation with was unclear.
[0008]
[Problems to be solved by the invention]
The present invention relates to a method for evaluating the structure of a material having incomplete periodicity using X-ray analysis, particularly a method for evaluating the weather resistance by analyzing the crystal structure of a rust layer of weathering steel. It is an object of the present invention to reliably provide a steel material excellent in weather resistance having a protective rust layer whose atomic correlation is controlled by the evaluation method.
[0009]
[Means for Solving the Problems]
The inventors have found that the structure evaluation of a material having a medium periodicity can be evaluated by a characteristic atomic arrangement, whereby the crystal structure of the rust layer can be evaluated. Furthermore, it has been found that the evaluation method can provide a steel material having a protective rust layer with controlled atomic correlation and excellent weather resistance. The present invention has been completed on the basis of such knowledge, and the gist thereof is as follows .
[0010]
(1) In a method for evaluating the weather resistance of a rust layer formed on a steel material surface, Fe (of the rust layer determined by RMC (Reverse-Monte-Carlo) method from information obtained by X-ray diffraction) A vector Vij connecting two adjacent atoms i and j in an arbitrary Fe (O, OH) ₆ crystal unit X in an O, OH) ₆ network structure and another arbitrary Fe (O, OH) ₆ crystal unit Y And the vector Vkl connecting two adjacent atoms k and l in the vector, r is the distance between the midpoints of the two vectors, θ is the angle formed by the two vectors, and the two atoms i and j and the two Using the function A (r, θ) expressing the atomic correlation of atoms k and l by the relationship between the distance r and the angle θ, r = 0.9 nm, θ = 0 ° and r = 0. Function at 9nm, θ = 60 ° (R, theta) wherein the value of Fe (O, OH) ₆ to evaluate the arrangement of the crystal unit, are weatherproof Evaluation of rust layer of steel and evaluating the weather resistance of the rust layer .
[0011]
DETAILED DESCRIPTION OF THE INVENTION
The present invention can be applied to evaluate the atomic correlation of various materials. Here, the form will be described by taking as an example the structural analysis of rust formed on the surface of weathering steel, which is a low alloy steel.
As shown in FIG. 1, the goethite (a-FeOOH) crystal structure, which is one of the rust structures formed in weathering steel, has a total of 6 O and OH coordinated around Fe (O, O, OH) ₆ structural units (hereinafter referred to as Fe (O, OH) ₆ units) share a ligand or a connector to form a network {Fe (O, OH) ₆ network}. .
[0012]
However, in the rust layer of steel-resistant steel, in addition to goethite (α-FeOOH), akaganeate (β-FeOOH), lipid docrosite (γ-FeOOH), magnetite, depending on the additive element, exposure period, and corrosive environment (Fe ₃ O ₄ ) and the like exist, and the difference in these structures can be understood as the difference in the network {Fe (O, OH) ₆ network} due to the difference in the arrangement of Fe (O, OH) ₆ units. In addition, the structure of rust formed on the surface of the actual weatherproof steel has a basic Fe (O, OH) ₆ unit itself, but some defects exist in the unit, and Fe (O, OH) ) Disturbances exist in the periodicity of _6- unit arrays.
[0013]
In order to improve the weather resistance, the environmental barrier by the rust layer is important. To that end, the rust itself is dense and prevents the entry of elements such as Cl, O, and H that cause corrosion from the external environment. It is necessary to have a function to shut off the environment. Furthermore, it is necessary to be stable and not change under the usage environment, that is, to have environmental stability. For this purpose, it is necessary that the Fe (O, OH) ₆ unit of the rust layer has very few defects and the unit arrangement is high. The network structure of the rust layer is determined by quantitatively analyzing phenomena such as X-ray, electron beam, neutron and ion scattering. For example, the network structure may be determined from information obtained by X-ray diffraction using an RMC (Reverse-Monte-Carlo) method. The method is shown below.
[0014]
The collected rust sample is subjected to measurement of a powder diffraction pattern using an X-ray diffractometer and measurement of abnormal X-ray scattering using a radiation light source. Next, after correcting the measured diffraction intensity such as absorption, a radial distribution analysis is performed, and the network structure of the rust sample is determined from the result using the RMC (Reverse-Monte-Carlo) method. . The procedure of the RMC (Reverse-Monte-Carlo) method is shown below.
[0015]
(1) Set a temporary initial structural model of rust. Here, it is assumed that N particles exist in a cell of volume V, and the number density ρ ₀ is set to be the same as the experimental value.
{ _{Circle around} (2)} The partial two-body distribution function g _ij (r) is calculated from the initial arrangement under the periodic boundary condition, and inverse Fourier transform is performed to the partial structure factor a _ij (Q) by the equation (1). Note that the concept of the partial two-body distribution function and the Fourier transform are applicable regardless of crystal or non-crystal.
[0016]
[Expression 1]

[0017]
Here, g _ij (r) is a partial two-body distribution function, Q is a scattering vector, r is an interatomic distance, and ρ ₀ is a density.
Further, the interference function: i (Q) and the environmental interference function around the element: Δi _a (Q) are calculated from the partial structure factor, and the statistical variation difference x ² from the experimental data given by the equation (2) is estimated. .
[0018]
[Expression 2]

[0019]
Here, σ ² is a statistical error in experimental data, i (Q) is a partial structure factor obtained by experiment, Δi _a (Q) is an environmental interference function around a specific element obtained by anomalous scattering experiment, and i ^c (Q _m ) and Δi ^c _a (Q _m ) are a partial structure factor calculated based on the structural model and an environmental interference function around a specific element, respectively.
(3) A virtual atomic arrangement with different energies is created by selecting one of the arranged particles and moving it in a random direction.
From the new arrangement obtained by the procedures of (4) and (3), an interference function is calculated in the same procedure as in ( ²⁾ , and the statistical fluctuation difference x ² _new is estimated.
(5) If x ² _new <x ² _old , the new arrangement is adopted, and if x ² _new > x ² _old , the previous arrangement is restored.
The procedures of (6) (3) to (5) are repeated until the calculation results agree with the experimental values within the error range.
[0020]
As described above, the structure of rust formed on the actual weather-resistant steel surface has a basic Fe (O, OH) ₆ unit itself, but some defects exist in the unit, and Fe ( Disturbances exist in the periodicity of the arrangement of O, OH) ₆ units. Therefore, by applying parameters representative of individual Fe (O, OH) ₆ units, the rust structure can be essentially evaluated.
[0021]
FIG. 2 is a crystal structure model (only iron atoms are partially described) in the case of an Fe (O, OH) ₆ network structure in which Fe (O, OH) ₆ units are arranged with perfect periodicity. Here, a vector V _ij connecting two adjacent atoms i, j in an arbitrary Fe (O, OH) ₆ unit A and two adjacent _ones in another arbitrary Fe (O, OH) ₆ unit B Considering the vector V _kl connecting the atoms k and l, the vector V _ij can be representative of the unit A including the atoms i and j, and the vector V _kl can be representative of the unit B including the atoms k and l. Assuming that the distance between the two vectors is r and the angle is θ, the arrangement relationship between the two vectors is to find atoms i, j, k, and l at positions where the vector V _ij and the vector V _kl satisfy the relationship between the distance r and the angle θ. It can be expressed by a function A (r, θ) representing the probability.
[0022]
That is, such Fe as shown in FIG. 2 (O, OH) ₆ unit is arranged with a complete period of Fe (O, OH) ₆ in the case of network structure, the distance of the vector V _ij and the vector V _kl Only when r and the angle θ satisfy the condition of distance r = (distance between adjacent units) × (integer = number of units) and θ = 60 °, atoms i, j, k, and l are In this condition, the function A (r, θ) will have a high peak value.
[0023]
This will be described with reference to FIG. FIG. 3 shows a function A (r, θ), with r on the vertical axis and θ on the horizontal axis for rust generated when pure iron is left in salt water for two months. When the function A (r, θ) has high peak values at θ = 0 ° and θ = 60 ° (note that θ = 0 °, θ = 180 °, θ = 60 ° Since θ = 180−60 = 120 ° is essentially the same, including both, θ = 60 ° is hereinafter expressed as θ = 60 °), and Fe (O, OH) ₆ units have very few defects and disturbances. It shows that it is arranged with almost perfect periodicity. That is, when it has a peak value at θ = 0 ° and 60 °, it can be evaluated as rust having excellent weather resistance, and when it does not have it, it can be evaluated as rust having poor weather resistance.
[0024]
The angle θ varies slightly depending on the crystal state. For example, when the Fe (O, OH) ₆ unit is distorted by an additive element or the like, peaks at θ = 0 ° and θ = 60 ° indicating that the Fe (O, OH) ₆ unit is not very defective and disordered. The value may deviate by several degrees. In this case, the function A (r, θ) may be set so that a high peak exists in the angle range of θ considering the deviation.
Whether or not the function A (r, θ) has a peak value is determined by whether or not the function A (r, θ) shows a value higher than the average value at a specific point (r, θ). The average value may be obtained by a weighted average when the atoms are randomly arranged in the entire space and when the atoms are completely arranged. In addition, the actual material is often evaluated based on the reference material. In this case, a difference function between the test material and the reference material function A (r, θ)
[Outside 1]

[0026]
Is determined to have a peak value when it has a positive value. Further, in the rust structure evaluation method for weather resistant steel according to the present invention, the weather resistant steel to be evaluated is not particularly limited. The steel material excellent in the weather resistant steel of the present invention is a steel material having a rust layer formed on the surface of the steel material and in close contact with the ground iron, and the steel material is Cu: 0.1% by mass or more and 3% by mass. Hereinafter, Ni: In the range of 0.1% by mass to 6% by mass, either or both of Cu and Ni are contained, the balance is made of iron and inevitable impurities, and the rust layer is made of A (r , Θ) is a steel material having peak values at θ = 0 ° and 60 °. Here, the steel material excellent in weathering steel means that the average corrosion rate is 0.01 mm / year or less.
[0027]
When the rust layer has a peak at θ = 0 ° and 60 ° in A (r, θ), the number of defects in the rust Fe (O, OH) ₆ unit is extremely small, and the periodicity of the arrangement of the units is high. Therefore, the formed rust has a small crystal grain size, the rust itself is dense, and has an environment blocking function that can prevent the entry of elements such as Cl, O, and H that cause corrosion from the external environment. Further, the steel material is stable in the use environment and does not change, and the steel material has excellent weather resistance.
[0028]
Moreover, the production | generation method of a protective rust may be apply | coated by the substance which accelerates | stimulates production | generation on the surface, and may be produced | generated naturally. As for the components constituting the rust layer, goethite, magnetite, inert amorphous rust and the like are desirable from the viewpoint of stability. In order to promote the reaction to further fill the voids after the particles are filled, the environment blocking function can be efficiently enhanced when a small amount of goethite, akaganeate, lipidocrosite, active amorphous rust and the like are mixed.
[0029]
In the range investigated by the present inventor, Cu: 0.1% by mass or more and 3% by mass or less, Ni: 0.1% by mass or more and 6% by mass or less, and either or both of Cu and Ni are contained. When the balance is made of iron and inevitable impurities, the rust layer has peak values at θ = 0 ° and 60 ° in A (r, θ), and a steel material excellent in weathering steel can be obtained. This is because the addition of these elements increases the number of reaction sites that are the starting point for the initial reaction of rust generation, and the reaction is generated from a number of points, resulting in a fine structure.
[0030]
In addition, the addition of these elements also has an effect of changing the formed rust itself to one having a high ion blocking function. In order to exhibit these effects, it is necessary to contain one or both of Cu: 0.1% by mass or more and Ni: 0.1% by mass or more. On the other hand, when the amount of element added increases, the structure of rust changes greatly, and the weather resistance deteriorates. Therefore, it is desirable that Cu: 3% or less and Ni: 6% or less.
[0031]
【Example】
Example 1
The function A (r, θ) was obtained by evaluating the rust generated when pure iron, pure titanium, and stainless steel (SUS304) were left in salt water for 2 months by the method of the present invention. The result of pure iron is shown in FIG. For example, at a distance r = 0.9 nm, high peaks are observed at θ = 0 °, 60 °, 120 °, and 180 °, and the periodicity of the arrangement of Fe (O, OH) ₆ units in this rust layer is high. Is shown.
In the method of the present invention, since the correlation between two pairs of atoms i and j and k and l is expressed by the distance r and angle θ between them, the azimuthal relationship between pairs existing at a certain distance is clear. Clearly, the relationship between structure and properties is clear.
[0032]
(Comparative Example 1)
Rust generated when pure iron is left in salt water for two months is evaluated by the XAFS (X-ray Absorption Fine-structures) method, which is one of the conventional structural evaluation methods. I asked for a function. The result is shown in FIG. In this conventional method, only the correlation between two atoms at a distance r is known, and it is difficult to assign a peak. Therefore, the detailed structure is not known, and information on the azimuthal relationship between atom pairs cannot be obtained.
[0033]
(Example 2)
31 years of weather resistant steel of the present invention (Fe-0.6 mass% Cr-0.3 mass% Cu-0.5 mass% Mn-0.5 mass% Si-0.08 mass% P) in rural areas The function of the rust generated when left standing is evaluated by the method of the present invention using the rust generated in ordinary steel left under the same conditions as a standard material.
[Outside 2]

[0035]
Got. This is shown in FIG. Since the correlation between a pair of atoms i and j and k and l is expressed by their distance r and angle θ, the azimuthal relationship of pairs existing at a certain distance is clearly understood, and the relationship between structure and properties Is clear. For example, at a distance r = 0.9 nm, high peaks are observed at θ = 0 °, 60 °, 120 °, and 180 °, and the periodicity of the arrangement of Fe (O, OH) ₆ units in this rust layer is high. Is shown.
[0036]
Example 3
Steel materials with different amounts of Cu and Ni were exposed to the beach for 9 years. After the outer layer rust on the outermost surface of the steel material was mechanically scraped off with a wire brush, an adhesion rust layer closely adhered to the ground iron was collected and evaluated by the method of the present invention. The peak intensity value is 20 times or more than the average value (◎), 5 to 20 times the average value (◯), 2 to 5 times the average value (Δ), 2 times or less the average value (×), (Table 1). Here, the average value was obtained by a weighted average when the atoms were randomly arranged in the entire space and when the atoms were completely arranged. The weather resistant steels of the present invention (Invention Examples Nos. 6 to 13) all have an evaluation of peak strengths near θ = 0 ° and 60 ° in A (r, θ), ◎ + ◎, ◎ + ○, Or it is (circle) + (circle), an average corrosion rate is 0.010 mm / year or less, and showed favorable corrosion resistance. Therefore, the weather resistance steel of the present invention has good weather resistance, and the evaluation method of the peak intensity in the vicinity of θ = 0 ° and 60 ° in A (r, θ) of the present invention is the evaluation method of the rust layer of the steel material. As shown to be applicable.
[0037]
[Table 1]

[0038]
(Comparative Example 2)
Similarly to Example 3, steel materials with different amounts of Cu and Ni added were exposed to the beach area in the atmosphere. After the outer layer rust on the outermost surface of the steel material was mechanically scraped off with a wire brush, the adhesion rust layer closely adhered to the ground iron was collected and evaluated by the method of the present invention in the same manner as in Example 3 (Table 1). ). The comparative weather resistant steels (Comparative Examples Nos. 1 to 5) were evaluated for peak intensities near θ = 0 ° and 60 ° in A (r, θ) as ○ + Δ, Δ + Δ, Δ ++ or xxx, the average corrosion rate is greater than 0.010 mm / year, and the corrosion resistance is poor.
[0039]
【The invention's effect】
As described above, according to the present invention, the structure of a material having incomplete periodicity could be evaluated. And it is a steel material which has a rust layer which is generated on the steel material surface and is in close contact with the ground iron, and the steel material is in mass%, Cu: 0.1% or more, 3% or less, Ni: 0.1% or more In the range of 6% or less, one or both of Cu and Ni are contained, the balance is made of iron and inevitable impurities, and the rust layer is A = 0 (r, θ) and θ = 0 ° and 60 It was possible to provide a steel material having an adhesion rust layer that has a peak value at ° and that has a sufficiently low average corrosion rate. The steel material of the present invention can surely obtain excellent weather resistance without performing special surface treatment such as painting, and is an industrially extremely valuable invention from the viewpoint of reducing environmental burden and economy. I can say that.
[Brief description of the drawings]
FIG. 1 is a diagram showing a schematic diagram of the crystal structure of a-FeOOH.
FIG. 2 is a diagram showing a schematic diagram of a crystal structure model (only iron atoms are partially described) when Fe (O, OH) ₆ units are arranged with complete periodicity.
FIG. 3 is a diagram showing a function A (r, θ) obtained by evaluating the rust generated when pure iron is left in salt water for two months by the method of the present invention.
[Fig. 4] Rust generated when pure iron is left in salt water for 2 months was evaluated by XAFS (X-ray Absorption Fine-structures) method, which is one of the conventional structural evaluation methods. It is a figure which shows the obtained radial distribution function.
FIG. 5 is a diagram showing a function A (r, θ) obtained by evaluating the rust generated when pure iron is left in salt water for two months by the method of the present invention.

Claims (1)

In the method for evaluating the weather resistance of a rust layer formed on the surface of a steel material, Fe (O, OH) of the rust layer determined by RMC (Reverse-Monte-Carlo) method from information obtained by X-ray diffraction ) A vector Vij connecting two adjacent atoms i, j in an arbitrary Fe (O, OH) ₆ crystal unit X in a ₆ network structure and an adjacent in another arbitrary Fe (O, OH) ₆ crystal unit Y two atoms k to, on the vectors Vkl connecting l, the distance between the midpoints of the two vectors r, the angle of both said vector forms a theta, the two atoms i, j and the two atoms k L = 0.9 nm, θ = 0 ° and r = 0.9 nm of the rust layer using a function A (r, θ) expressing the atomic correlation of l by the relationship between the distance r and the angle θ . Function A (r at θ = 60 ° The Fe (O from the value of theta), OH) ₆ to evaluate the arrangement of the crystal unit, the weather resistance evaluation of rust layer of steel and evaluating the weather resistance of the rust layer.

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