JP2003315290A - Evaluation method of crystal structure of materials and steel materials with excellent weather resistance - Google Patents

Abstract

(57) Abstract: The present invention provides a method for evaluating a crystal structure of a material, particularly a rust, and a steel material having excellent weather resistance based on the method. In a method for evaluating the arrangement of atoms in a material, a vector V _ij connecting two adjacent atoms i and j in an arbitrary crystal unit X in the material and a proximity in another arbitrary crystal unit Y are provided. _Assuming that the distance between the two vectors k and l is r and the angle is θ, the vector V _ij and the vector V _kl are positioned at positions satisfying the distance r and the angle θ. A crystal structure evaluation method for a material in which the arrangement state of atoms is evaluated by a function A (r, θ) indicating a probability of finding i, j, k, and l.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for evaluating the crystal structure of a material in which the periodicity of atoms in the material is incomplete, and more particularly to a contact rust layer which is stable in an atmospheric corrosive environment. The present invention relates to a method for evaluating a rust structure of a weather resistant steel having the above and a steel material having excellent weather resistance.

[0002]

2. Description of the Related Art Generally, as a method for evaluating the arrangement of atoms in a material, if the periodicity is high and there is no disorder,
A unique description method is adopted by specifying the space group of the crystal and the positions of the constituent atoms in the unit cell (eg, X-ray diffraction / scattering technology, Kikuta, The University of Tokyo Press, 1
992, Chapter 1). On the other hand, if the periodicity of the atoms in the material is low or absent, such as amorphous, it cannot be expressed by the classification of the periodicity, and therefore the correlation between the two atoms is represented. It is evaluated by a body distribution function or a radial distribution function (for example, X-ray diffraction / scattering technique, written by Kikuta, The University of Tokyo Press, 1992, Chapter 3).

However, when the periodicity of the arrangement of the atoms is moderate, such as rust formed on the surface of steel, and the structure cannot be evaluated correctly using either of the above-mentioned methods. However, there are many actual materials. By exposing a steel material containing a small amount of elements such as Cu, Ni, Cr, and P to the atmosphere for a period of several years or more, the steel material adheres directly to the iron core and is exposed to atmospheric corrosion. A so-called weather-resistant steel that does not require a corrosion-resistant treatment such as painting is formed by using a protective rust layer having a function of protecting the steel, and is used as a structural material for bridges and the like.

Conventionally, the structure of the protective rust layer of such weather resistant steel is said to be composed mainly of fine-grained goethite (α-FeOOH) having an average crystal grain size of 200 nm or less. . However, such fine-grained goethite having an average grain size of 200 nm or less is Cu, N
It is also present in the rust layer of ordinary steel to which elements such as i, Cr, and P are not added, and the presence of such fine-grained goethite exerts the weather resistance function of the protective rust layer of the weather resistant steel. It was clarified that not all of the factors are due to the structural analysis of rust layer particles by transmission electron microscopy.

That is, in weather-resistant steel and ordinary steel, there are crystal grains of various grain sizes in the rust layer that adheres directly on the base steel, and the crystal structure is according to the analysis results of electron beam diffraction and the like. , Is mainly composed of goethite (α-FeOOH), but in addition, depending on additional elements and exposure period, acaganate (β-FeOOH), lepidocrocite (γ-FeOOH), magnetite (Fe ₃ O ₄₎ It has been found that there are cases where it is difficult to determine the crystal structure because there are crystal structures such as), and these crystal structures are fine. Further, there may be a case where the phase has a structure that cannot be identified in these phases due to defects in the crystal structure.

Originally, the function required of the surface layer of the weather resistant steel which does not require the corrosion resistance treatment is that the surface layer itself has a corrosion preventing function. The anticorrosion function is a function of preventing the base iron from further corroding after the formation of the protective rust layer that is the surface anticorrosion layer, so the first requirement for the surface anticorrosion layer is Cl, which causes corrosion of the layer, It is to have an environment blocking function capable of preventing the invasion 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, it has environmental stability.

In order to have an environmental barrier property, it is first necessary that the rust itself be dense and have a structure in which it is difficult for the elements that are expected to enter from the environment as described above to permeate. Therefore, the arrangement of atoms forming the rust layer, that is, the atomic correlation becomes 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.However, as described above, the evaluation of the structure of the rust layer is applied to materials with perfect periodicity. Of the weather resistant steel and the structure of the rust layer formed on the weather resistant steel because the evaluation was performed only by the evaluation method applied to a material having a remarkably low periodicity. The correlation with was unclear.

[0008]

DISCLOSURE OF THE INVENTION The present invention provides a method for evaluating the structure of a material having incomplete periodicity by using an X-ray analysis method, and particularly by analyzing the crystal structure of the rust layer of weathering steel. It is an object of the present invention to provide a method for evaluating the weather resistance thereof, and to reliably provide a steel material having excellent weather resistance having a protective rust layer whose atomic correlation is controlled by the evaluation method.

[0009]

The inventors of the present invention can evaluate the structure of a material having a medium periodicity by a characteristic atomic arrangement, which allows the crystal structure of a rust layer to be evaluated. Found. Further, it was found that the above evaluation method can provide a steel material having a protective rust layer whose atomic correlation is controlled and having excellent weather resistance. The present invention has been completed based on such findings, and the gist thereof is as follows. (1) In a method of evaluating the arrangement of atoms of a material, a vector V _ij connecting two adjacent atoms i and j in an arbitrary crystal unit X of the material and an adjacent vector in another arbitrary crystal unit Y Vector V _kl connecting two atoms k and l
Regarding, regarding the distance between the two vectors r and the angle θ, the vector V _ij and the vector V _kl are the distance r and the angle θ.
A crystal structure evaluation method for a material, which evaluates the arrangement state of atoms by a function A (r, θ) indicating the probability of finding atoms i, j, k, and l at a position satisfying the relation.

(2) The crystal structure evaluation method for a material according to (1), wherein the material is a rust layer of steel. (3) A steel material having a rust layer formed on the surface of the steel material and in close contact with the base steel, wherein the steel material is in a mass% of Cu:
0.1% or more and 3% or less, Ni: 0.1% or more and 6% or less, either or both of Cu and Ni are contained, and the balance consists of iron and inevitable impurities.
The rust layer connects a vector V _ab connecting two adjacent atoms a and b in any crystal unit X of the rust layer and a vector connecting two adjacent atoms c and d in another crystal unit Y. For V _cd , the distance between the two vectors is r,
When the angle is θ, the atoms a, b, c, d are located at positions where the vector V _ab and the vector V _cd satisfy the relationship with the distance r and the angle θ.
Θ = 0 in the function A (r, θ) indicating the probability of finding
A steel material excellent in weather resistance steel, which is a steel material having peak values at ° and 60 °.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention can be applied to evaluate the atomic correlation of various materials. Here, the structural analysis of rust formed on the surface of weathering steel, which is a low alloy steel, is taken as an example. The form will be described. As shown in FIG. 1, a goethite (a-FeOOH) crystal structure, which is one of the structures of rust formed on weather-resistant steel, has a total of 6 O around Fe,
Fe (O, OH) ₆ structural units (hereinafter referred to as Fe (O, OH) ₆ units) coordinated with OH share a ligand or a bond with each other to form a network {Fe (O, OH) ₆ network} is formed.

However, in the rust layer of the steel resistant steel, due to the additive elements, the exposure period and the corrosive environment, goethite (α-Fe
In addition to OOH), akaganate (β-FeOOH),
There are lepidocrocite (γ-FeOOH), magnetite (Fe ₃ O ₄ ) and the like, and the difference between these structures is Fe.
This can be understood as a difference in the network {Fe (O, OH) ₆ network} due to the difference in the arrangement of (O, OH) ₆ units. Further, in the actual structure of rust formed on the surface of the weather resistant steel, although the basic Fe (O, OH) ₆ unit itself is present, some defects are present in the unit, and Fe (O, OH) ₆ is present. ) There is a disorder in the periodicity of the _6- unit sequence.

In order to improve the weather resistance, the environmental barrier property of the rust layer is important. For that purpose, first, the rust itself is dense, and elements such as Cl, O, and H that cause corrosion are protected from the external environment. It is necessary to have an environment blocking function that can prevent intrusion. Furthermore, it is necessary to be stable and not change under the use environment, that is, to have environmental stability. For this purpose, it is necessary that Fe (O, OH) ₆ units in the rust layer have extremely few defects and that the arrangement of the units is high. The network structure of the rust layer is determined by X
This is done by quantitatively analyzing phenomena such as scattering of rays, electron beams, neutrons, and ions. For example, from information obtained by X-ray diffraction, RMC (Reverse-Monte-C
The network structure may be determined using the (arlo) method. The method is shown below.

With respect to the collected rust sample, a powder diffraction pattern is measured by an X-ray diffractometer and an X-ray anomalous scattering measurement is performed by using a radiation source. Next, after correcting the measured diffraction intensity such as absorption, a radial distribution analysis is performed, and RMC (Reverse-M
The onte-Carlo method is used to determine the network structure of the rust sample. The RMC (Reverse)
The procedure of the -Monte-Carlo) method is shown below.

A tentative initial structural model of rust is set.
Here, assuming that there are N particles in a cell of volume V,
This number density ρ ₀ is the same as the experimental value. A partial two-body distribution function g _ij (r) is calculated from the initial arrangement under a periodic boundary condition, and an inverse Fourier transform is performed on the partial structure factor a _ij (Q) by the equation (1). The concept of the partial two-body distribution function and the Fourier transform are
There is a feature that it can be applied regardless of whether it is amorphous or not.

[0016]

[Equation 1]

Where g _ij (r) is a partial two-body distribution function,
Q is the scattering vector, r is the interatomic distance, and ρ ₀ is the density. Further, interference from the partial structure factor function: i (Q), and elemental surrounding environment interference function: △ i _a (Q) is calculated, and statistical fluctuation difference x between the experimental data given in equation (2)
Estimate ² .

[0018]

[Equation 2]

Here, σ ² is the statistical error in the experimental data, i (Q) is the partial structure factor obtained by the experiment, and Δ
i _a (Q) is an environmental interference function around a specific element obtained by an anomalous scattering experiment, i ^c (Q _m ) and Δi ^c _a (Q _m ) are partial structure factors calculated based on the structural model, respectively.
It is an environmental interference function around a specific element. By selecting one of the arranged particles and moving it in a random direction, a virtual atomic arrangement with different energies is created. From the new arrangement obtained by the procedure of, the interference function is calculated by the same procedure as, and the statistical variation difference x ² _new is estimated. If x ² _new <x ² _old , the new placement is adopted and x ²
_{If new} > x ² _old , return to the previous arrangement. Repeat the procedure from ~ until the calculation result agrees with the experimental value within the error range.

As described above, in the actual structure of the rust formed on the surface of the weather resistant steel, the basic Fe (O, OH) ₆ unit itself exists, but some defects exist in the unit. , Fe (O, OH) ₆ units have irregularities in the periodicity. Therefore, individual Fe
By applying a parameter representative of (O, OH) ₆ units, an essential evaluation of the rust structure is possible.

FIG. 2 is a crystal structure model in which an Fe (O, OH) ₆ unit has a Fe (O, OH) ₆ network structure in which the units are arranged with complete periodicity (only some iron atoms are described). Here, any Fe (O,
OH) 2 one atom i to close in ₆ units A, and the vector V _ij connecting j, another optional Fe (O, OH) 2 one atom k close in ₆ units B, the vector V connecting l
Considering _kl , the vector V _ij is the unit A containing the atoms i and j, and the vector V _kl is the unit B containing the atoms k and l.
, Respectively, can be represented. Assuming that the distance between the two vectors is r and the angle is θ, the arrangement relationship between the two vectors is that the vector V _ij and the vector V _kl are the distance r and the angle θ.
Can be represented by a function A (r, θ) that represents the probability of finding atoms i, j, k, and l at a position that satisfies the relationship of

That is, as shown in FIG. 2, Fe (O, O
H)₆Fe in which the units are arranged with perfect periodicity
(O, OH)₆If it is a network structure,
Toru V _ijAnd vector V_klThe distance r and the angle θ are
r = (distance between adjacent units) × (integer = unit
The number of intervals) and the condition of θ = 60 ° is satisfied.
There are only atoms i, j, k, l, and under these conditions,
The function A (r, θ) will have a high peak value.

This will be described with reference to FIG. FIG. 3 shows a function A (r, θ) of rust generated when pure iron is left in salt water for 2 months, with r on the vertical axis and θ on the horizontal axis. Function A (r, θ) is θ = 0 ° and θ = 6
When it has a high peak value at 0 ° (as can be seen from the definition, θ = 0 °, θ = 180 °, θ = 60 °
And θ = 180−60 = 120 ° are essentially the same, so including them both will be expressed as θ = 60 ° below), Fe (O, OH) ₆ units have very few defects and disturbances. , Shows that they are arranged with almost perfect periodicity. That is, it can be evaluated that the rust has excellent weather resistance when it has peak values at θ = 0 ° and 60 °, and the rust having poor weather resistance when it does not have it.

The angle θ varies slightly depending on the crystal state. For example, Fe (O, O
H) ₆ units are distorted, Fe (O, OH) ₆
The peak values at θ = 0 ° and θ = 60 °, which indicate that the unit has very few defects and disturbances, may deviate by a few degrees, and in this case, a high peak appears in the angle range of θ considering the deviation. Function A (r, θ) to exist
Should be set. Whether or not the function A (r, θ) has a peak value is determined by the function A (r, θ) at a specific point (r,
θ) depending on whether or not the value is higher than the average value. The average value may be obtained by a weighted average when atoms are randomly arranged in the whole space and when atoms are completely arranged. In addition, the evaluation of the actual material is often performed based on the reference material. In this case, the function A of the test material and the reference material is used.
Difference function of (r, θ)

[0025]

[Outer 1]

When has a positive value, it is determined to have a peak value. In the present invention, the rust structure evaluation method for weather resistant steel is not particularly limited to the weather resistant steel to be evaluated. 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 base iron, and the steel material is Cu: 0.1% by mass or more and 3% by mass or more. In the following, in the range of Ni: 0.1% by mass or more and 6% by mass or less, one or both of Cu and Ni is contained,
The balance is iron and unavoidable impurities, and the rust layer is a steel material having peak values at θ = 0 ° and 60 ° in A (r, θ). Here, the steel material excellent in weather resistant steel means that the average corrosion rate is 0.01 mm / year or less.

When the rust layer is A (r, θ), θ =
Fe with rust when it has peaks at 0 ° and 60 °
The defects of (O, OH) ₆ units are extremely small, and the periodicity of the arrangement of the units is high. Therefore, the crystal grain size of the rust formed is small, the rust itself is dense, and it has an environmental barrier function capable of preventing the invasion of elements such as Cl, O, and H, which cause corrosion, from the external environment. Further, it is a steel material that is stable under the use environment and does not change and has excellent weather resistance.

The protective rust may be formed by coating the surface with a substance that promotes the formation, or may be formed naturally. As a component forming the rust layer, goethite, magnetite, inert amorphous rust and the like are preferable from the viewpoint of stability. In order to accelerate the reaction of further filling the voids after the particles are filled, the environmental barrier function can be efficiently enhanced by mixing a small amount of goethite, acaganate, lepidocrocite, active amorphous rust and the like.

In the range investigated by the present inventors, Cu: 0.
1 mass% or more and 3 mass% or less, Ni: 0.1 mass% or more 6
When one or both of Cu and Ni are contained in the range of not more than mass% and the balance consists of iron and unavoidable impurities, the rust layer has A = 0, 60 ° in A (r, θ). It is possible to obtain a steel material having a peak value in the and excellent weather resistance steel. This is due to the addition of these elements
This is because there are many reaction sites that are the starting points of the initial reaction of rust generation, and the reaction is generated from many points, so that the finally obtained structure becomes fine.

The addition of these elements also has the effect of changing the formed rust itself into one having a high ion blocking function. In order to realize these effects, Cu: 0.
It is necessary to contain one or both of 1 mass% or more and Ni: 0.1 mass% or more. On the other hand, when the amount of element added increases, the structure of rust changes significantly,
The weather resistance will deteriorate. Therefore, it is desirable that Cu: 3% or less and Ni: 6% or less.

[0031]

EXAMPLES Example 1 Pure A, pure titanium, and stainless (SUS304) were left to stand in salt water for 2 months, and the rust generated by the method was evaluated by the method of the present invention.
(R, θ) was obtained. The result of pure iron is shown in FIG. For example,
Θ = 0 °, 60 °, 12 at a distance r = 0.9 nm
High peaks were observed at 0 ° and 180 °, which indicates that the periodicity of the arrangement of Fe (O, OH) ₆ units in this rust layer is high. In the method of the present invention, the correlation between two pairs of atoms i and j and k and l is expressed by the distance r and the angle θ of the two, so that the azimuth relationship of the pairs existing at a certain distance is clear. Obvious, the relationship between structure and properties is clear.

(Comparative Example 1) Rust generated when pure iron was left in salt water for 2 months was evaluated by XAFS (X-ray Absorption Fin), which is one of the conventional structural evaluation methods.
e-structures: X-ray absorption fine structure) method, and a radial distribution function was obtained. The result is shown in FIG. In this conventional method, only the correlation between the two atoms at the distance r is known, and it is difficult to assign the peaks, so the detailed structure is not known, and further, the information on the azimuth relationship between the atom pairs cannot be obtained.

(Example 2) The weather resistant steel of the present invention (Fe-
0.6 mass% Cr-0.3 mass% Cu-0.5 mass% M
n-0.5 mass% Si-0.08 mass% P) was used in the present invention, with the rust formed when the steel was left standing in the rural area for 31 years as the standard material. Function by evaluating by the method of

[0034]

[Outside 2]

Was obtained. It is shown in FIG. Correlation between two pairs of atoms i and j, k and l is shown as distance r and angle θ between them.
Since it is expressed by, the azimuth relation of the pair existing at a certain distance can be clearly understood, and the relation between the structure and the characteristic is clear. For example, at a distance r = 0.9 nm, θ = 0
High peaks were observed at °, 60 °, 120 ° and 180 °, indicating that the arrangement of Fe (O, OH) ₆ units in this rust layer has a high periodicity.

(Example 3) Steel materials having different amounts of Cu and Ni added were exposed to the beach area for 9 years in the atmosphere. After the outermost rust on the outermost surface of the steel material was mechanically scraped off with a wire brush, the adhesive rust layer adhered to the base steel was sampled and evaluated by the method of the present invention. Then, the value of the peak intensity is 20 times or more the average value (⊚), 5 to 20 times the average value (∘),
The evaluation was carried out in four grades of 2 to 5 times the average value (Δ) and 2 times or less the average value (x) (Table 1). Here, the average value was obtained by a weighted average when atoms were randomly arranged in the entire space and when atoms were completely arranged. All of the weather resistant steels of the present invention (Invention Example Nos. 6 to 13) have θ in A (r, θ).
The evaluation of the peak intensity near = 0 ° and 60 ° is ◎ +
◎, ◎ + ○, or ○ ＋ ○ with an average corrosion rate of 0.0
It was 10 mm / year or less, and showed good corrosion resistance. Therefore, while the weather resistance of the weather resistant steel of the present invention is good,
It was shown that the evaluation method of the peak strength in the vicinity of θ = 0 ° and 60 ° in A (r, θ) of the present invention can be applied as the evaluation method of the rust layer of the steel material.

[0037]

[Table 1]

(Comparative Example 2) As in Example 3, Cu and N
Steels with different amounts of i were exposed to the atmosphere in the beach area. 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 adhered to the base steel was collected,
Evaluation was performed by the method of the present invention in the same manner as in Example 3 (Table 1). The weather resistant steels of Comparative Examples (Comparative Examples Nos. 1 to 5) were evaluated as A + (+, Δ), Δ + Δ, and Δ in the peak strengths near θ = 0 ° and 60 ° in A (r, θ). ++ or XXX, the average corrosion rate is greater than 0.010 mm / year, and the corrosion resistance is poor.

[0039]

As described above, according to the present invention, the structure of a material whose periodicity is incomplete can be evaluated. A steel material having a rust layer formed on the surface of the steel material and in close contact with the base steel, wherein the steel material is C in mass%, C
u: 0.1% or more and 3% or less, Ni: 0.1% or more and 6% or less, containing either or both of Cu and Ni, and the balance consisting of iron and unavoidable impurities. The rust layer was a steel material having peak values at θ = 0 ° and 60 ° in A (r, θ), and it was possible to provide a steel material having an adherent rust layer which provided a sufficiently low average corrosion rate. The steel material of the present invention can surely obtain excellent weather resistance without being subjected to special surface treatment such as painting, and is also an industrially extremely valuable invention from the viewpoint of environmental load reduction and economical efficiency. I can say.

[Brief description of drawings]

FIG. 1 is a diagram showing a schematic view of a crystal structure of a-FeOOH.

FIG. 2 is a diagram showing a schematic diagram of a crystal structure model (only a part of iron atoms is described) where 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 2 months by the method of the present invention.

FIG. 4 shows rust generated when pure iron is left in salt water for 2 months, which is one of conventional structure evaluation methods, XAFS (X-r
ay Absorption Fine-structure
ures: X-ray absorption fine structure) method and is a diagram showing a determined 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 2 months by the method of the present invention.

Continued front page    (72) Inventor Yoshio Waseda             2-1-1 Katahira, Aoba-ku, Sendai City, Miyagi Prefecture             Inside Institute for Multidisciplinary Research for Advanced Materials (72) Inventor Shigeru Suzuki             2-1-1 Katahira, Aoba-ku, Sendai City, Miyagi Prefecture             Inside Institute for Multidisciplinary Research for Advanced Materials (72) Inventor Tamaki Suzuki             20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel shares             Company Technology Development Division (72) Inventor Hiroshi Kihira             20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel shares             Company Technology Development Division F term (reference) 2G001 AA01 AA03 AA04 AA05 BA23                       CA01 KA01 KA08 KA09 LA02                       MA04 NA10 NA11 NA13 NA17

Claims (3)

[Claims] 1. A method for evaluating an arrangement of atoms of a material, wherein a vector V _ij connecting two adjacent atoms i, j in an arbitrary crystal unit X of the material and another arbitrary crystal unit Y For a vector V _kl connecting two adjacent atoms k and l, where the distance between the two vectors is r and the angle is θ, the vector V _ij and the vector V _kl are positioned so as to satisfy the relationship with the distance r and the angle θ. A crystal structure evaluation method for a material, which evaluates the arrangement state of atoms by a function A (r, θ) indicating the probability of finding atoms i, j, k, l. 2. The method for evaluating a crystal structure of a material according to claim 1, wherein the material is a rust layer of steel. 3. It is generated on the surface of steel and adheres to the base metal.
A steel material having a rubbing layer, wherein the steel material is mass%,
Cu: 0.1% to 3%, Ni: 0.1% to 6%
Either or both of Cu and Ni within the following range
And the balance consists of iron and unavoidable impurities,
In addition, the rust layer is in any crystal unit X of the rust layer.
Vector V connecting two adjacent atoms a and b of_abAnd another
Two adjacent atoms c, d in any crystal unit Y of
Vector V connecting_cdFor, the distance between the two vectors
Where r is the angle and θ is the angle, the vector V_abAnd vector V
_cdAre atoms a, b, at positions that satisfy the relationship with the distance r and the angle θ.
It is represented by a function A (r, θ) that indicates the probability of finding c and d.
At times, the function shows peak values at θ = 0 ° and 60 °.
Steel excellent in weather resistance steel characterized by being a rust layer
Material.