JP2004332098A - Steel having excellent weatherability - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel coated with a surface layer in which the condition (valence) of iron is controlled, and having excellent weatherability. <P>SOLUTION: In the steel having excellent weatherability, the surface of an Ni-containing iron based alloy material comprising, by mass, 0.2 to 10% Ni, and the balance Fe with inevitable impurities is coated with a film mainly consisting of FeOOH and Fe<SB>3</SB>O<SB>4</SB>, and the average valence of iron in the film is >2.77 to <2.99. Further, one or more kinds of metals selected from Cu, Cr, Ti, W, Mo, Sb, Al, Nb and Ta by 0.01 to 8% in total are incorporated into the Ni-containing iron based alloy material. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a steel material excellent in weather resistance, and more particularly to a steel material excellent in weather resistance having a stable adhesion rust layer in an atmospheric corrosion environment.

A so-called weathering steel is a steel material in which a small amount of elements such as Cu, Ni, Cr, and P are added to a base metal. Since a protective rust layer having a function of protecting the ground steel layer against corrosion is formed directly above the ground steel, no corrosion-resistant work such as painting is required, and it is used as a structural material for bridges and the like. However, in an area where the amount of incoming salt is large, there is a problem that it is difficult to prevent the ingress of Cl ^- ions by the protective rust layer and it is difficult to form the protective rust layer.

Essentially, the function required of the surface layer of weather-resistant steel that does not require a corrosion-resistant treatment is that it has a corrosion-proof function itself. Since the anticorrosion function is a function of preventing further corrosion of the ground iron after the formation of the protective rust layer, which is a surface anticorrosion layer, the first requirement of the surface anticorrosion layer is that the layer is a corrosion-resistant Cl , O, H, and the like from the outside environment. The second requirement is to be stable under the use environment and not change, that is, to have environmental stability. In order to have an environmental barrier property, first, it is necessary that the rust itself is dense and has a structure in which elements expected to invade from the environment do not easily penetrate. In particular, in the airborne salt amount is large area, the surface of the steel material, just covered with rust layer only is dense with fine grains, Cl ^- it is difficult to prevent the ingress of ions, protective rust There was a problem that it was difficult to form a layer.

  Various attempts have conventionally been made to solve this problem. Japanese Patent Application Laid-Open No. 10-251797 (Patent Document 1) discloses that, by mass%, C: 0.15% or less, Si: 0.7% or less, Mn: 0.2 to 1.5%, P: 0. 03 to 0.15%, S: 0.02% or less, Al: 0.01 to 0.1%, Cr: 0.1% or less, Ni: 0.4 to 4%, Cu: 0.4% or less , Mo: 0.05-1%, Sn: 0.01-0.5%, Sb: One or two of 0.01-3%, the balance being Fe and Disclosed are welded structural steels having excellent weather resistance and consisting of unavoidable impurities.

Here, Mo and Ni are important addition elements for improving the corrosion resistance, and Mo is enriched in the vicinity of the rust base iron interface, increases the density of rust near the base iron interface, and causes corrosion factors such as moisture and salt content in the steel. Although it is said that it has an effect of preventing contact with the surface, it does not sufficiently prevent Cl ^- ions from entering the rust layer from the outside. The method of Patent Document 1 is expected to prevent the contact between the external Cl ^- ions and the ground iron interface by attaching ions and the like generated by the additional elements such as Mo to the rust near the ground iron interface. However, since rust itself does not have a sufficient function to prevent the ingress of Cl ^- ions from the outside, it is difficult to stably improve the chloride corrosion resistance.

Further, Patent Document 2 discloses a weather resistant steel material containing ultrafine particles having a particle volume of 3 × 10 ⁻²⁴ m ³ or less in a rust layer on the surface of a steel sheet at a volume fraction of 10 to 30%. However, in order to withstand use in a severe corrosive environment with more flying salt, it is difficult to sufficiently improve the chloride corrosion resistance only by controlling the distribution of crystal grains such as ultrafine grains.
JP-A-10-251797 JP 2001-123246 A

  An object of the present invention is to solve the above-mentioned problems and to provide a steel material excellent in weather resistance covered with a stable adhesion rust layer in an atmospheric corrosive environment and a method for producing the same.

  Conventionally, the structure of the protective rust layer of weathering steel has been described as having a main portion composed of fine-grained goethite (α-FeOOH) having an average crystal grain size of 200 nm or less. However, as a result of a structural analysis of the protective rust layer of the weathering steel by a transmission electron microscope, the present inventors found that fine-grained goethite having an average crystal grain size of 200 nm or less was Cu, Ni, Cr, It is clarified that elements such as P are also present in the rust layer of ordinary steel to which no element is added, and the presence of such fine-grained goethite indicates that the protective rust layer of the weather-resistant steel has a weather-resistant function. It was newly obtained that not all of the expression factors were expressed.

Furthermore, the present inventors have newly obtained the knowledge that the rust layer itself should have a negative charge under the donor environment in order to effectively prevent Cl ⁻ ions from entering the rust layer. . In weathering steel and ordinary steel, there are various sizes of crystal grains in the rust layer in close contact with the ground iron. According to the analysis results of electron beam diffraction and the like, the crystal structure shows that the geite (α-FeOOH) ) Is predominant, but the crystal structure of akaganate (β-FeOOH), lepidocrocite (γ-FeOOH), magnetite (Fe ₃ O ₄ ), etc., depends on the added elements and the exposure period. It has been found that these crystal structures are fine crystal grains having an average particle size of about several tens of nanometers, so that it is sometimes difficult to determine the crystal structure.

  Further, it has been found that these rust layers sometimes have a crystal structure that has not been clarified conventionally due to a defect in the crystal structure or the like. That is, the present inventors show that the rust layer locally deviates from electrical neutrality at the atomic level, that is, the valence number of iron is partially changed by its crystal structure, It has been newly obtained that the macroscopic control of the state makes it possible to realize a state in which the rust layer itself has a negative charge.

The present inventors have found that in order to achieve a state of rust layer itself is negatively charged, the atomic valence of the iron is changed by a change in the crystal structure, Cl to rust layer of this time ^- ion penetration As a result of various studies on the effect of prevention, it was newly found that if the valence number of iron is controlled to be more than 2.77 and less than 2.99, a sufficient effect can be obtained for improving the corrosion resistance to chloride. . According to this requirement, Fe ³⁺ and Fe ²⁺ in the crystal structure constituting the rust layer are mixed at a desirable abundance ratio. That is, the presence of Fe ^{3+ in} the crystal structure forms the FeOOH phase necessary to maintain the denseness of the rust layer, and the presence of Fe ²⁺ prevents the entry of Cl ⁻ ions from the outside. The formation of a rust layer with a sufficient negative charge is realized.

The present invention has been completed based on such findings, and the gist thereof is as follows.
(1) mass% Ni: contained 0.2 to 10%, the balance is Fe and the surface of the Ni-containing iron-based alloy material consisting of unavoidable impurities, covering with a film consisting mainly FeOOH and Fe ₃ O ₄ A steel material excellent in weather resistance, characterized in that the average valence of iron in the coating is more than 2.77 and less than 2.99.
(2) One or more of Cu, Cr, Ti, W, Mo, Sb, Al, Nb, and Ta may be further added to the Ni-containing iron-based alloy material in a mass percentage of 0.01 to 8 in total. %, The steel material having excellent weather resistance according to (1).

(3) The steel material according to (1) or (2), wherein the mole fraction of Fe ₃ O _{4 in} the coating is 4 to 35%.
(4) The Ni atoms in the film and one or more of Cu, Cr, Ti, W, Mo, Sb, Al, Nb, and Ta atoms are contained in the Fe ₃ O ₄ crystal lattice in the film. The steel material having excellent weather resistance according to any one of (1) to (3), wherein 0.4 to 25% is present in the octahedral site coordinated to the six oxygen atoms.
(5) Any of (1) to (4), wherein the volume fraction of fine crystal grains having a particle volume of 18 × 10 ⁻²⁴ m ³ or less in the crystal grains of the film is more than 20% and less than 90%. A steel material excellent in weather resistance described in Crab.
(6) The steel material according to any one of (1) to (5), wherein the coating has a thickness of 0.01 to 200 μm.

According to the present invention, it is possible to provide a steel material excellent in weather resistance covered with a surface layer in which the state (valence) of iron is controlled. The weathering steel of the present invention can prevent Cl ^- ions from entering the inside of the rust layer, and can be said to be extremely valuable industrially from the viewpoint of reducing environmental load and economic efficiency.

Embodiments of the present invention will be described in detail below.
The weathering steel according to the present invention is an iron-based alloy containing 0.2 to 10% of Ni as an additive element, and further includes Cu, Cr, Ti, W, Mo, Sb, Al, One or two or more of Nb and Ta are contained in a total amount of 0.01 to 8%. Further, the surface of the weather-resistant steel is covered with a film mainly composed of FeOOH and Fe ₃ O ₄ (hereinafter, this film may also be referred to as a rust layer), and the average valence of iron in this film. The number was controlled to be more than 2.77 and less than 2.99. Further, the molar fraction of Fe ₃ O ₄ in the film during is a 4 to 35%, Ni atoms, and, Fe ₃ of one or two or more kinds coating of additive elements other than Ni (rust layer) It is present at 0.4 to 25% at octahedral sites coordinated to six oxygen atoms in O ₄ . Further, the volume fraction of fine crystal grains having a particle volume of 18 × 10 ⁻²⁴ m ³ or less in the crystal grains of the coating is more than 20% and less than 90%, and the thickness of the coating is 0.01 to 200 μm.

The additive components in the weathering steel according to the present invention will be described. In addition, "%" shown below means "% by mass" unless otherwise specified.
The weathering steel according to the present invention is an iron-based alloy containing 0.2 to 10% of Ni. Ni has the effect of lowering the corrosion potential of the entire iron-based alloy and reduces the amount of metal ions eluted from the steel. Furthermore, the Ni is present partially enters the Fe ₃ O ₄ phase in the coating, and energetically stabilizes the Fe ₃ O ₄ phase, more stable abundant the Fe ₃ O ₄ phase in the film It is possible to do. While the iron atoms in the FeOOH of the film are in the state of Fe ³⁺ , the iron atoms in the Fe ₃ O ₄ phase have both the Fe ³⁺ and Fe ²⁺ states. ^It is considered that the Fe ₃ O ₄ phase is energetically stabilized by replacing the atom in the ²⁺ state with the same divalent Ni ²⁺ .
If the Ni content is less than 0.2%, the effect of lowering the corrosion potential of the steel material and the effect of suppressing the elution amount of metal ions are significantly reduced, and the effect does not increase even if added over 10%. Therefore, the Ni content is set to 0.2 to 10%.

  In the present invention, there are Cu, Cr, Ti, W, Mo, Sb, Al, Nb, and Ta as additional elements having the same function and effect as Ni described above. It is preferable to contain 0.01 to 8%. When these elements are added, if the total content is less than 0.01%, the effect of lowering the corrosion potential and the effect of suppressing the elution amount of ions are significantly reduced, and the total content is 8%. Since the effect does not increase even if it is added in excess, the total of the preferable contents is set to 0.01 to 8%.

In the weathering steel according to the present invention, the average valence number M of iron in the crystal structure of the film (rust layer) formed on the surface is more than 2.77 and less than 2.99, so that the chloride corrosion resistance is sufficiently improved. It is necessary to improve the average valence number M. The control of the average valence number M is performed by a method described later. When the average valence number M of iron in the crystal structure is in the range of more than 2.77 and less than 2.99, Fe ³⁺ and Fe ²⁺ are mixed in the film, and the denseness of the film is caused by the presence of Fe ³⁺ . After the formation of the FeOOH phase necessary to maintain the rust layer, the rust layer having a negative charge sufficient to prevent the ingress of Cl ⁻ ions from the outside due to the presence of Fe ²⁺ is formed on the steel material surface. Is generated.

When the average valence number M of iron is 2.99 or more, the Fe ₃ O ₄ phase cannot be stably present in the crystal structure of the film, so that the film prevents the Cl ⁻ ions from entering from the outside. It becomes difficult to carry a sufficient negative charge. On the other hand, when the average valence number M of iron is 2.77 or less, the Fe ₃ O ₄ phase is stably present in the crystal structure of the film, but the generation of the FeOOH phase is reduced. A dense film is not formed, and the corrosion resistance is remarkably deteriorated. The iron atoms in the FeOOH of the coating are in the state of Fe ³⁺ , while the iron atoms in the Fe ₃ O ₄ phase have both the states of Fe ³⁺ and Fe ²⁺ . Therefore, when the average valence number M of iron is 2.99 or more, the Fe ₃ O ₄ phase cannot be stably present in the crystal structure of the film. On the other hand, when the average valence number M of iron is 2.77 or less, although the Fe ₃ O ₄ phase is stably present in the crystal structure of the film, the FeOOH phase is not stably present.

Further, in the present invention, in the above-mentioned film (rust layer) mainly composed of FeOOH and Fe ₃ O ₄ , the average valence number M is more than 2.77 and less than 2.99. By setting the molar fraction of Fe ₃ O ₄ therein to 4% or more and 35% or less, the chlorine corrosion resistance is further improved. This is because even if a layer having a crystal structure other than Fe ₃ O ₄ and FeOOH in the film, for example, FeO or Fe ₂ O ₃ is present, if the Fe ₃ O ₄ in the film is within the above range, chlorine is more stably formed. This is because the corrosion resistance is improved. If the molar fraction of Fe ₃ O ₄ is less than 4%, the film prevents Cl ^- ions from entering from the outside when the number of layers having a crystal structure other than Fe ₃ O ₄ and FeOOH increases in the film. In some cases, it is difficult to take a sufficient negative charge. On the other hand, if the molar fraction of Fe ₃ O ₄ exceeds 35%, the FeOOH phase is not formed, and the denseness may be deteriorated.

The Fe ₃ O ₄ type crystal structure in the film is called a spinel structure, and is a structure in which the valence of iron stably assumes both Fe ³⁺ and Fe ²⁺ states. FIG. 1 is a diagram schematically showing an Fe ₃ O ₄ type spinel structure, in which iron atoms are formed at the center of an octahedral site composed of six oxygen atoms surrounding the iron atom, and four iron atoms are formed around the octahedron. Is located at the center of the tetrahedral site composed of oxygen atoms. The iron atoms of Fe ³⁺ and Fe ²⁺ are stably present by occupying these sites. When the average valence number M of iron in the film specified in the present invention is more than 2.77 and less than 2.99, it is more stable for the iron atom to have a Fe ₃ O ₄ type spinel structure, It is less affected by pH, oxygen, ions and the like in the use environment, and the corrosion resistance is further improved.

Still further, in order to improve the salt corrosion resistance of the weatherable steel according to the present invention, one or more of the Ni atoms in the coating and one or more of the added elements have an Fe ₃ O ₄ type spinel structure. Is preferably present at 0.4 to 25% at the octahedral site to which the oxygen atom coordinates. If it is less than 0.4%, the film will not be sufficiently negatively charged to prevent Cl ^- ions from entering. On the other hand, if it exceeds 25%, the FeOOH phase is not formed and the denseness is deteriorated.
When one or more of the Ni atoms and the added elements are present in the octahedral site where six oxygen atoms of the Fe ₃ O ₄ type spinel structure are coordinated, the average valence number of iron in the film The effect of keeping M at more than 2.77 and less than 2.99 is high, and it is hardly affected by pH, oxygen, ions and the like in the use environment.

The iron atoms in the Fe ₃ O ₄ phase coexist in both Fe ³⁺ and Fe ²⁺ states. In an ideal Fe ₃ O ₄ type spinel structure, the atomic positions where iron atoms are considered to exist are an octahedral site coordinated by 6 oxygen atoms and a tetrahedral coordinated by 4 oxygen atoms There are two types of sites, with half of Fe ³⁺ being at tetrahedral sites and the other half of Fe ³⁺ and Fe ²⁺ being at octahedral sites. Since Ni atoms have a stable structure of Ni ²⁺ , the same divalent Ni ²⁺ replaces the atoms in the Fe ²⁺ state, so that the Ni atoms exist preferentially at the octahedral site, and the Fe ₃ O ₄ phase is energetically active. To stabilize. As a result, the effect of keeping the average valence number M of iron in the coating at more than 2.77 and less than 2.99 increases. Ni ²⁺ substituted on the octahedral site is energetically stable and is less susceptible to pH, oxygen, ions and the like in the use environment.

Further, in order to improve the salt corrosion resistance of the weatherable steel according to the present invention, the coating is mainly composed of fine crystal grains having a particle volume of 18 × 10 ⁻²⁴ m ³ or less, and the volume of the fine crystal grains is Desirably, the rate is greater than 20% and less than 90%. If the volume fraction of the fine crystal grains is more than 20% and less than 90%, a finer film is formed by filling the space between the grain boundaries of the coarse crystal grains constituting the film with a finer crystal film, and from the corrosive environment. of Cl ^- intrusion such as ion and oxygen and water or the like can be prevented. Boundaries of the fine crystal grains and coarse crystal grains of the coating will vary by the respective volume fractions, generally because it is the particle volume 7~18 × 10 ^-24 m ^3, the particle volume 18 × 10 ^-24 m ³ or less It is desirable that more than 20% and less than 90% of fine crystal grains be present in the coating. If the volume fraction of the fine crystal grains is 20% or less, it becomes difficult for the fine grains to sufficiently fill the voids between the grain boundaries of the coarse crystal grains. On the other hand, the upper limit of the volume fraction of the fine crystal grains is not particularly limited, but it is industrially difficult to set the upper limit to 90% or more. Therefore, the upper limit is set to less than 90%.
Furthermore, in the weathering steel according to the present invention, it is preferable that the thickness of the coating existing near the interface with the ground iron in which the corrosion reaction proceeds is 0.01 to 200 μm. If the coating in the vicinity of the interface with the base iron is less than 0.01 μm, there is no sufficient ion-blocking property.

Next, a method for forming a film formed on the surface of the weatherable steel according to the present invention and a method for controlling average valence number M of iron in the film, crystal structure of the film, volume distribution of crystal grains, film thickness of the film, and the like. Will be described. In order to produce a film according to the present invention, one or more of the following film production methods are combined.
As one of methods for controlling the average valence number M of iron in the coating, there is a method of heating steel in a humid atmosphere (hereinafter, referred to as a wetting / heating method). By maintaining the surface of the weatherable steel according to the present invention in a wet state, a dissolution reaction of metal ions on the steel surface is caused, and then, by gradually changing the state to a dry state, a compound of the metal ion and oxygen is formed. A production reaction occurs. By repeating this reaction cycle, a surface layer is produced.

  The desirable condition range of this cycle is as follows: after being left in a wet state at a temperature of 5 to 40 ° C. and a relative humidity of 30 to 100% for 1 to 10 hours, a dry state at a temperature of 10 to 50 ° C. and a relative humidity of 0 to 90% This is to repeat the cycle of leaving for 10 hours 1 to 100 times. The reason why the wet state is a temperature of 5 to 40 ° C. and a relative humidity of 30 to 100% is that under this condition, the surface of the material becomes sufficiently wet. The reason for setting the drying state to a temperature of 10 to 50 ° C. and a relative humidity of 0 to 90% is that under this condition, the surface of the material is sufficiently dried, and the film forming reaction proceeds in a state where oxygen is supplied to the interface with the base iron. That's why.

  The wet / dry reaction cycle is a cycle in which the substrate is left in the above-mentioned wet state for 1 to 10 hours, and then left in the above-mentioned dry state for 1 to 10 hours, and this cycle is desirably repeated 1 to 100 times. If the wet state time is shorter than 1 hour, there is no leaching of ions necessary for forming a good quality film, and if the wet state is longer than 10 hours, only leaching of ions precedes and a film with adhesion is formed. Not done. On the other hand, if the drying time is shorter than 1 hour, the film-forming reaction in a situation where oxygen is supplied to the ferrous metal interface is insufficient. A film with a crack is not formed. Further, if the number of cycles is less than 1, no film is formed, and even if the number of cycles exceeds 100, there is no remarkable improvement in the quality of the film, and the film quality may be deteriorated due to non-uniformity in the film surface.

  As an example of a specific cycle, for example, after holding steel in a wet state (temperature 35 ° C., relative humidity 95%) for 4 hours, the temperature is fixed at 35 ° C. → 50 ° C., and the relative humidity is fixed at 95% → 40%. Is changed over a period of 4 hours, and a series of processes in which the steel plate is kept in a dry state (temperature 50 ° C., relative humidity 40%) for 4 hours is defined as one cycle, and this cycle is repeated 100 to 300 times. A film of about several to 200 μm is formed. Further, as another method for controlling the average valence number M of iron in the film, there is a film forming method by sputtering (hereinafter, referred to as a sputtering method). Sputtering is performed on the surface of the weatherable steel according to the present invention using iron and an oxide of an additive element or an alloy of iron and the additive element as a target to form a film on the steel surface. The average valence number M is adjusted by the amount of oxygen in the gas atmosphere during sputtering.

  Preferred gas atmosphere and sputtering time are to perform sputtering in a gas atmosphere having an oxygen content of 3 to 20% by volume for 2 minutes to 5 hours. If the oxygen content is less than 3% by volume, an oxide cannot be formed, and if it is more than 20% by volume, the oxidation proceeds too much, and in either case, it is difficult to keep the average valence number M within a predetermined range. When the sputtering time is shorter than 2 minutes, the thickness of the oxide layer is too small, and when the sputtering time is longer than 5 hours, the thickness of the oxide layer becomes non-uniform. Film cannot be obtained.

  Further, as another method of controlling the average valence number M of iron in the film, there is a film forming method by ion plating (hereinafter, referred to as an ion plating method). On the surface of the weathering steel according to the present invention, a mixture of powders such as iron, nickel, and cobalt is placed in a crucible at a ratio such that the average valence number M of iron falls within the above-mentioned predetermined range. Heating and evaporating it by using a high-frequency coil and attaching it to the steel surface. At this time, by keeping the gas atmosphere in a state in which a slight amount of oxygen is mixed with argon, a film having a thickness of about 10 to several tens μm in which the average valence number M is controlled can be produced.

  Preferred gas atmosphere and sputtering time are to perform sputtering in a gas atmosphere having an oxygen content of 3 to 20% by volume for 2 minutes to 8 hours. If the oxygen content is less than 3% by volume, an oxide cannot be formed, and if it is more than 20% by volume, the oxidation proceeds too much, and in either case, it is difficult to keep the average valence number M within a predetermined range. When the sputtering time is shorter than 2 minutes, the thickness of the oxide layer is too small, and when the sputtering time is longer than 8 hours, the thickness of the oxide layer becomes non-uniform. Film cannot be obtained.

  Further, as another method for controlling the average valence number M of iron in the film, there is a method using a sol-gel reaction (hereinafter, referred to as a sol-gel reaction method). On the surface of the weatherable steel according to the present invention, iron and the alkoxide of the additional element in the alloy material are dissolved in alcohol, and further, a solution containing a small amount of acid is prepared, and the solution is sufficiently stirred to form a metal sol. The contained viscous solution may be applied.

  The concentration of the alkoxide solution of the additive element in iron and the alloy material is preferably 10 to 40% by volume. If it is less than 10% by volume, corrosion by the sol-gel solution proceeds and a good film is not formed. If it exceeds 40% by volume, the sol-gel reaction does not progress uniformly and a good film is not formed. As the acid to be added, acetic acid, hydrochloric acid, or the like having a concentration of 0.01 to 10 M is preferably used because the sol-gel reaction can be promoted. The stirring may be performed so that the solution becomes uniform, and may be performed, for example, at a temperature of 30 to 80 ° C. for 10 minutes to 5 hours.

  This solution is applied to the steel material and dried to form a film. A coating film whose average valence number M is controlled can be produced by the application amount and the heating conditions (temperature, oxygen atmosphere) at the time of drying. The coating on the steel material may be performed so as to form a uniform film with a film thickness in the range of 1 to 200 μm, and a roll coater, a bar coater, or the like generally used as a method for applying the viscous solution may be used. When the film thickness is less than 1 μm, there is no sufficient ion-blocking property. In this method, it is easy to control the film thickness.

  The drying is preferably performed so that the applied viscous solution reacts uniformly to form a good quality film, and is dried at 150 to 600 ° C. for 1 to 60 hours. If the temperature is lower than 150 ° C., the sol-gel reaction does not sufficiently proceed, and if the temperature exceeds 600 ° C., the formed film may be decomposed and become non-uniform. If the drying time is shorter than 1 hour, there is no reaction necessary for forming a good quality film, and if the drying time is longer than 60 hours, the film may be non-uniform due to excessive heating.

  Further, as another method of controlling the average valence number M of iron in the film, there is a method of directly applying an oxide to a steel material (hereinafter, referred to as a coating method). The surface of the weatherable steel according to the present invention is finely pulverized by mixing iron and an oxide of an additive element in an alloy material at a ratio such that the average valence number M of iron falls within a predetermined range. The preferred average particle size is from 0.1 μm to 200 μm. If it is less than 0.1 μm, the structure of the oxide may be destroyed, and if it is more than 200 μm, mixing of particles becomes insufficient.

  Thereafter, the mixture is ground for several hours in a planetary ball mill in, for example, ethanol. Thereafter, the sufficiently dried powder is dispersed in a suitable thickening organic material solution, and then applied to the steel material and dried to obtain a film. As the organic solution, alcohols such as polyvinyl alcohol, which can uniformly disperse the powder, are preferable. The coating on the steel material may be performed so as to form a uniform film in a thickness range of 1 to 200 μm, and a roll coater, a bar coater, or the like generally used as a method for applying a viscous solution may be used. When the film thickness is less than 1 μm, there is no sufficient ion-blocking property, and when the film thickness is more than 200 μm, the film tends to be uneven.

  The drying is preferably performed so that the applied powder solution uniformly forms a good quality film, and is dried at 120 to 700 ° C. for 20 minutes to 40 hours. If the temperature is lower than 120 ° C., the drying and the film-forming reaction do not sufficiently proceed. If the temperature exceeds 700 ° C., the formed film may be decomposed and become non-uniform. If the drying time is shorter than 20 minutes, there is no reaction necessary for forming a good-quality film, and if the drying time is longer than 40 hours, the film may be non-uniform due to excessive heating.

Next, a method for confirming whether or not the film formed by the method according to the present invention satisfies the requirements of the present invention will be described.
For example, the XAFS (X-ray Absorption Fine-structures) method may be used. When the absorptance of a material is measured while increasing the energy of X-rays, the absorptivity of the material decreases in response to the increase in the energy of X-rays, but at a specific X-ray energy (X-ray absorption edge), There is a portion where the absorptance increases sharply, and a part of the photoelectrons generated by the absorption of X-rays is reflected as structural information on the amount of X-ray absorption by scattering and interference by a plurality of atoms.

That is, by monitoring the amount of X-ray absorption, information on the atomic structure can be obtained (for example, edited by Yasuo Udagawa, X-ray absorption fine structure, Gakkai Shuppan Center (1993)). This is the principle of the structure analysis by the XAFS method. When the XAFS method is used, the average valence M of the iron atom and the existing state of the added element (Ni exists at the site where Ni is coordinated to oxygen in Fe ₃ O ₄ ) Or not) can be easily obtained. If there is a variation depending on the location, it is sufficient to measure at three to five points and obtain an average value. If it is less than three points, it is difficult to obtain the average information, and if it is more than five, it takes more time for measurement than necessary.
The fraction of each phase contained in the film may be determined by X-ray diffraction or magnetic measurement, and the average valence number M may be determined by the following equation.

Here, w _i is the mole fraction of i-phase, m _i is the average valence of the iron in the i phase. C _i is a correction coefficient for each phase and is a constant determined by normalizing effects such as density and crystal orientation using a standard sample. Since the structures of the coating such as α-FeOOH, β-FeOOH, γ-FeOOH, α-FeOOH, and Fe ₃ O ₄ are different from each other, an X-ray diffraction pattern is measured and the fraction of each layer is determined from the relative intensity of the peak. Can be estimated. Also, since Fe ₃ O ₄ is a ferromagnetic material, only the amount of Fe ₃ O ₄ in the sample can be selectively estimated by measuring the magnetization.

Further, measurement is performed by X-ray photoelectron spectroscopy (for example, Toshiro Yamashina / Shin Fukuda, Basics and Applications of Surface Analysis, University of Tokyo Press (1991)). The sample is irradiated with X-rays of several KeV or more and the emitted electrons are spectrally analyzed to determine its binding energy, and compared with a standard sample to determine the average valence number M of iron atoms and the presence of added elements. The state can be determined.
Furthermore, the target film is removed, and the mass fraction of constituent elements such as iron, oxygen, etc. is determined by combining instrumental analysis such as chemical analysis and X-ray fluorescence analysis, and the average atomic atom of iron The valence M can also be determined.

In addition, the presence of the added element (the site where Ni is coordinated to oxygen in Fe ₃ O ₄₎ is also measured by powder diffraction pattern measurement using an X-ray diffractometer or X-ray anomalous scattering measurement using a radiation light source. To see if they exist). If there is a variation depending on the location, it is sufficient to measure at three to five points and obtain an average value. If it is less than three points, it is difficult to obtain the average information, and if it is more than five, it takes more time for measurement than necessary.

Further, in order to confirm the volume distribution of the particles constituting the film, for example, Mossbauer spectroscopy may be used. Mossbauer spectroscopy is a technique for measuring the resonance absorption of γ-rays by nuclei in a solid, and is widely applied in the fields of physical science and material science as a method capable of identifying the magnetic properties of materials. There are many nuclei that can be subjected to Moessbauer spectroscopy, and of these, the ⁵⁷ Fe nucleus is particularly effective. That is, goethite (α-FeOOH), akaganate (β-FeOOH), lepidocrocite (γ-FeOOH), magnetite (Fe ₃ O ₄ ), etc., which are considered to constitute the weatherable steel protective rust layer of the present invention, Since each has a regular arrangement of magnetization such as ferromagnetism and antiferromagnetism below a specific phase transition temperature, the Fe nucleus feels a strong internal magnetic field in the direction of the atomic magnetic moment, and its Mossbauer absorption spectrum Results in six separate spectra.

If this spectrum is analyzed, for example, Jpn. J. Appl. Phys. Vol. 38 (199) L189, it is known that their composition ratio can be quantitatively determined. In addition, the magnetic anisotropy energy that stabilizes the direction of magnetization decreases as the particle volume of such magnetic particles decreases, and the magnetization becomes unstable when the energy becomes about the same as the energy of thermal fluctuation. Is known to occur. Such a superparamagnetic phenomenon is characterized by a characteristic physical quantity called a relaxation time. Usually, the relaxation time τ of superparamagnetism is expressed as follows.
τ = τ ₀ exp (KV / kT) (1)
Here, τ ₀ is a constant independent of temperature of about 10 ⁻¹⁰ (sec), K is a magnetic anisotropy constant, V is a particle volume, k is a Boltzmann constant, and T is a temperature.
That is, by measuring the relaxation time τ at various temperatures (for example, 15 to 300 k), the volume distribution of the particles can be measured for each phase.

  Since α-FeOOH, β-FeOOH, and γ-FeOOH, which are main constituent particles of the protective rust layer of the present invention, are antiferromagnetic substances, they do not exhibit macroscopic magnetization but have a magnetic ordered structure. Due to its presence, a clear spectral split is observed in the Mossbauer spectrum. The splitting is six at a low temperature, but becomes two at a temperature higher than a certain temperature (Neel temperature). The Neel temperature is different for each phase (in the case of an ideal bulk sample, α-FeOOH, β-FeOOH, and γ-FeOOH are about 405, 280, and 70 k, respectively). As can be seen from equation (1), this temperature further shifts to a lower temperature side when the volume of the particles constituting the film becomes smaller. Thus, by measuring the spectrum at various temperatures (for example, 15 to 300 k) and performing quantitative analysis, the volume distribution of the particles can be measured for each phase.

Specifically, it is performed as follows.
(1) Measure spectra at various temperatures (for example, 15 to 300 k).
(2) Peaks of each spectral line using physical parameters such as internal magnetic field, isomer shift, anisotropy constant, and initial values of volume distribution of particles considered appropriate with reference to TEM observation and X-ray diffraction data. Determine the position.
(3) A relaxation time corresponding to each volume is calculated, and each relaxation time is multiplied by a weight corresponding to the particle volume distribution assumed in (2), and the spectrum is calculated.
(4) The spectrum calculated in (3) is compared with the spectrum actually measured in (1). If the error is equal to or smaller than a preset error, the initial value of the volume distribution of the particles is determined as the determined value. If not, return to (2) and repeat the process of correcting the volume distribution and performing the calculation.

  In addition, the volume distribution of the particles can also be examined by observing the crystal grain structure using an electron microscope or measuring the powder diffraction pattern using an X-ray diffractometer. If there is a variation depending on the location, it is sufficient to measure at three to five points and obtain an average value. If it is less than three points, it is difficult to obtain the average information, and if it is more than five, it takes more time for measurement than necessary.

(Example 1)
Next, the operation and effect of the present invention will be described more specifically with reference to examples. However, these are merely examples, and the present invention is not limited thereto.
First, in Examples 1 to 3, the fraction of fine crystal grains in the crystal grains constituting the coating is 45%, the molar fraction of Fe ₃ O _{4 in} the coating is 10%, and the film thickness is constant at 100 μm. As described above, the coating was formed by the following methods, and the corrosion resistance was evaluated.
After melting and manufacturing the Ni-containing iron-based alloy described in Table 1, high-temperature rolling at an initial temperature of 1100 ° C. was repeated five times. From the material, a plurality of plates of a predetermined size were cut out and subjected to various treatments to form a film (rust layer) on the surface, which was used as a sample, and the corrosion resistance was evaluated.
The film was formed by a wet / heat method, a sputtering method, an ion plating method, a sol-gel reaction method, and a coating method.
The wet / heat method is a method of heating steel in a humid atmosphere. A sample having a size of 20 × 50 × 5 mm was kept in a wet state (35 ° C., relative humidity 95%, 4 hours), and then linearly changed to a temperature of 50 ° C. and a relative humidity of 40% over 4 hours. This cycle was repeated 150 times, with the process of maintaining the sample in a dry state at 40 ° C. and a relative humidity of 40% for 4 hours as a cycle, to form a film of several to 100 μm on the sample surface.

The sputtering method was performed as follows. Sputtering is performed on the surface of a sample having a size of 10 × 10 × 2 mm, using any one of an oxide of iron and an additional element, an alloy of iron and an additional element, or an oxide containing iron and an additional element as a target. A film was prepared. Sputtering is performed using high frequency, argon gas is flowed at a flow rate of 60 s / cm ³ , oxygen gas is flowed at a flow rate of ³ to 10 s / cm ³ , the pressure of the entire system is maintained at 1 to 10 mTorr, and sputtering is performed. A thick film was formed.
The ion plating method was performed as follows. A sample having a size of 10 × 10 × 2 mm is held in a crucible containing powder of iron and an additive element, the gas atmosphere is kept in a state in which some oxygen is mixed with argon, and heated and evaporated by an electron beam or the like. The sample was accelerated by a sample high-frequency coil and adhered to the sample surface.

The sol-gel method was performed as follows. A viscous solution obtained by mixing a sample having a size of 20 × 50 × 5 mm with 1 ml of an ethanol solution of iron and the additive element trimethoxide, 3 ml of water and 0.01 ml of acetic acid, and stirring at 50 ° C. for 1 hour. Was applied to the steel material. The coating was performed with a roll coater or a bar coater. Thereafter, by drying in a nitrogen atmosphere at 400 ° C. for 32 hours, a film having a thickness of about 100 μm was formed.
The coating method was performed as follows. A sample having a size of 20 × 50 × 5 mm was prepared. Next, a mixture of iron and oxides of the additional elements at a ratio such that the average valence number M of iron is more than 2.77 and less than 2.99 is put in ethanol and ground for 3 hours by a planetary ball mill. did. Thereafter, the sufficiently dried powder was dispersed in polyvinyl alcohol, and then applied to a sample and dried to form a film having a thickness of about 100 μm.

The average valence number M of the film of the sample was determined by using the XAFS method. FeO, and measure the energy of Fe-Ka absorption edge powder samples of Fe ₂ O _3, respectively, were used as M = 2, M = 3 standard sample. Next, a film in the range of 5 μm was collected from the surface of the ground iron of the sample, the energy at the Fe—Ka absorption edge was measured, and the average valence number M was determined. The average was calculated by measuring at three points in consideration of the variation depending on the place. When the three points have large variations, the fraction of Fe ₃ O _{4 is} determined by measuring the magnetization, and the fraction of each phase of α-FeOOH, β-FeOOH, γ-FeOOH, and α-FeOOH is determined by X-ray diffraction. The average valence number M of the iron atom was estimated from the weighted average.

Further, the XAFS method, with determining the radial distribution function from XAFS vibration of high-energy region than the absorption edge, subjected to X-ray anomalous scattering measurement using a radiation source, from both results, the Ni in the Fe ₃ O ₄ The position of the existing atoms was determined. From both of these results, it was determined whether Ni was present at octahedral sites coordinated to six oxygen atoms of the Fe ₃ O ₄ crystal lattice. The average was calculated by measuring at three points in consideration of the variation depending on the place. A case where Ni atoms in the film are present in a site where oxygen atoms in Fe ₃ O ₄ are coordinated with 0.4% or more and 25% or less is defined as good (〇 mark), while less than 0.4% or 25% If there is more than one, it is determined to be defective (x mark).

Next, each test piece was coated with 0.3% by mass of brine at 500 ml / cm ² , and was wet (35 ° C., relative humidity 95%, 6 hr) → dry (40 ° C., relative humidity 40%, 18 hr). ) Was defined as one cycle, and a 50-cycle corrosion test was repeated. The corrosion resistance of the test sample was evaluated from the corrosion weight loss of each test piece based on the corrosion weight loss of pure iron subjected to a corrosion test under the same conditions. The corrosion resistance evaluation is performed in the following five stages, and a score of 3 or more is regarded as good.

Rating 1: The corrosion weight loss of the test sample is greater than that of pure iron (more than 100%).
Rating 2: The corrosion weight loss of the test sample is equivalent to that of pure iron (more than 90% and 100% or less). Rating 3: The corrosion weight loss of the test sample is more than 50% and 90% or less of the corrosion weight loss of pure iron.
Rating 4: The corrosion weight loss of the test sample is more than 30% and 50% or less of the corrosion weight loss of pure iron.
Rating 5: The corrosion weight loss of the test sample is 30% or less of the corrosion weight loss of pure iron.
Table 1 shows the results. In the weathering steel of the present invention, the Ni and the additional element atoms in the film are present at octahedral sites where the atoms of the Fe and the additional elements are coordinated to six oxygen atoms of the Fe ₃ O ₄ crystal lattice. The valency M was more than 2.77 and less than 2.99. Further, as a result of the corrosion test, all the scores were 3 to 5, and it was confirmed that the present invention can provide a steel material having excellent corrosion resistance.

(Example 2)
After melting and producing the Ni-containing iron-based alloy described in Table 2, rolling was performed in the same manner as in Example 1, and a plurality of sheets of a predetermined size were cut out from the material, and the same surface finishing as in Example 1 was performed. Was done. Next, a film (rust layer) formed on the surface by performing various treatments similar to those in Example 1 was used as a sample. In the same manner as in Example 1, Ni and the atoms of the additional elements in the film were Fe _3. The measurement was performed to determine whether or not there was an octahedral site coordinated to six oxygen atoms of the O ₄ crystal lattice, the average valence number M of iron in the film was measured, and the corrosion resistance was evaluated by a corrosion test. .
Table 2 shows the results. In the weathering steel of the present invention, the Ni and the additional element atoms in the film are present at octahedral sites where the atoms of the Fe and the additional elements are coordinated to six oxygen atoms of the Fe ₃ O ₄ crystal lattice. The valency M was more than 2.77 and less than 2.99. Further, as a result of the corrosion test, all the scores were 3 to 5, and it was confirmed that the present invention can provide a steel material having excellent corrosion resistance.

(Example 3)
After melting and producing the Ni-containing iron-based alloy described in Table 3, rolling was performed in the same manner as in Example 1, and a plurality of sheets of a predetermined size were cut out from the material, and the same surface finish as in Example 1 was obtained. Was done. Next, a film (rust layer) formed on the surface by performing various treatments similar to those in Example 1 was used as a sample. In the same manner as in Example 1, Ni and the atoms of the additional elements in the film were Fe _3. The measurement was performed to determine whether or not there was an octahedral site coordinated to six oxygen atoms of the O ₄ crystal lattice, the average valence number M of iron in the film was measured, and the corrosion resistance was evaluated by a corrosion test. .
Table 3 shows the results. In the weathering steel of the present invention, the Ni and the additional element atoms in the film are present at octahedral sites where the atoms of the Fe and the additional elements are coordinated to six oxygen atoms of the Fe ₃ O ₄ crystal lattice. The valency M was more than 2.77 and less than 2.99. Further, as a result of the corrosion test, all the scores were 3 to 5, and it was confirmed that the present invention can provide a steel material having excellent corrosion resistance.

(Example 4)
Then, in this embodiment 4-6, the molar fraction of Fe ₃ O ₄ in the film is 10%, Ni atoms, and / or, the one or more atoms other additive elements, in the coating A film is formed by each of the following methods so that 10% is present at octahedral sites coordinated to six oxygen atoms of the Fe ₃ O ₄ crystal lattice and the film thickness is constant at 100 μm. An evaluation was performed.
After melting and producing the Ni-containing iron-based alloy described in Table 4, high-temperature rolling at an initial temperature of 1100 ° C. was repeated five times. From the material, a plurality of plates of a predetermined size were cut out and subjected to various treatments to form a film (rust layer) on the surface, which was used as a sample, and the corrosion resistance was evaluated.

The film was formed by a dry-wet cycle method and a sol-gel reaction method.
The wet / heat method is a method of heating steel in a humid atmosphere. After holding a sample of 20 × 50 × 5 mm in a wet state (temperature 30 ° C., relative humidity 95%) for 2 hours, the temperature is fixed at 30 ° C. → 55 ° C. and the relative humidity is fixed at 95% → 40%. A series of processes in which the rate of change is changed over 1 hour and held in a dry state (temperature 55 ° C., relative humidity 40%) for 2 hours is defined as one cycle, and this cycle is repeated 250 times, whereby several tens to A film of about 100 μm was formed.

  The sol-gel method was performed as follows. A 20 × 50 × 5 mm sample was mixed with 1 ml of an ethanol solution of iron and the additive element trimethoxide, 3 ml of water, and 0.02 ml of acetic acid, and stirred at 70 ° C. for 1 hour to obtain a viscous solution. Was applied to the steel material. The coating was performed with a roll coater or a bar coater. Thereafter, by drying in a nitrogen atmosphere at 400 ° C. for 40 hours, a film having a thickness of about 90 μm was formed.

The volume distribution of the particles constituting the film of the sample was examined using Mossbauer spectroscopy. A sample was taken, inserted into an Al foil, and measured for a Mossbauer spectrum at a temperature of 15 to 300 K using a g-line from 57Fe. The method for obtaining the volume distribution of the particles in the film from the Mossbauer spectrum is as follows.
(1) Calculate parameters necessary for analyzing a spectrum such as an internal magnetic field, an isomer shift, and an anisotropy constant.
(2) Based on (1), the temperature of the spectrum to be further calculated is determined, and the peak position of each spectral line is determined.
(3) As the initial value of the volume distribution of particles, a value that is considered appropriate is set with reference to TEM observation, X-ray diffraction data, and the like.

(4) The relaxation time corresponding to each volume is calculated, three sets of motional narrowing are calculated for each relaxation time, and a weight corresponding to the particle volume distribution assumed in (3) is added to the calculation result. Multiply and add to calculate and display the spectrum.
(5) The spectrum calculated in (4) is compared with the actually measured spectrum, and the processes (1) to (4) are repeated so that the error is equal to or less than a preset error. FIG. 2 shows an example of the determined volume distribution. The volume distribution of the particles in the film was measured, and the corrosion resistance was evaluated by a corrosion test.

Table 4 shows the results. The weathering steel of the present invention is covered with a coating mainly composed of FeOOH and Fe ₃ O ₄ , and the particles constituting the coating determined by Mossbauer spectroscopy are composed of fine grains and coarse grains, and The volume fraction of fine grains was more than 20% and less than 90%. Further, as a result of the corrosion test, all the scores were 3 to 5, and it was confirmed that the present invention can provide a steel material having excellent corrosion resistance.

(Example 5)
After melting and producing the Ni-containing iron-based alloy described in Table 5, rolling was performed in the same manner as in Example 1, and a plurality of sheets of a predetermined size were cut out from the material, and the same surface finishing as in Example 1 was performed. Was done. Next, a sample obtained by forming a film (rust layer) on the surface by performing various treatments similar to Example 4 was used as a sample, and the volume distribution of particles in the film and a corrosion test were performed in the same manner as in Example 4. The corrosion resistance was evaluated.
Table 5 shows the results. In the weathering steel of the present invention, the particles constituting the film determined by Mossbauer spectroscopy were composed of fine grains and coarse grains, and the volume fraction of the fine grains was more than 20% and less than 90%. Further, as a result of the corrosion test, all the scores were 3 to 5, and it was confirmed that the present invention can provide a steel material having excellent corrosion resistance.

(Example 6)
After melting and producing the Ni-containing iron-based alloy described in Table 6, rolling was performed in the same manner as in Example 1, and a plurality of sheets of a predetermined size were cut out of the material, and the same surface finishing as in Example 1 was performed. Was done. Next, a sample obtained by forming a film (rust layer) on the surface by performing various treatments similar to Example 4 was used as a sample, and the volume distribution of particles in the film and a corrosion test were performed in the same manner as in Example 4. The corrosion resistance was evaluated.
Table 6 shows the results. In the weathering steel of the present invention, the particles constituting the film determined by Mossbauer spectroscopy were composed of fine grains and coarse grains, and the volume fraction of the fine grains was more than 20% and less than 90%. Further, as a result of the corrosion test, all the scores were 3 to 5, and it was confirmed that the present invention can provide a steel material having excellent corrosion resistance.

(Comparative example)
After melting and producing the Ni-containing iron-based alloy described in Table 7, rolling was performed in the same manner as in Example 1, and a plurality of sheets of a predetermined size were cut out from the material, and the same surface finish as in Example 1 was obtained. Was done. Next, various treatments were performed on each alloy surface to form a film (rust layer) on the alloy surface, which was used as a sample. The film was formed by a wetting / heating method, a sputtering method, an ion plating method, a sol-gel reaction method, and a coating method, but these treatment conditions were outside the scope of the present invention. For these samples, in the same manner as in Examples 1 and 4, Ni and the additional element atoms in the coating are present at octahedral sites coordinated to six oxygen elements of the Fe ₃ O ₄ crystal lattice. The measurement was performed to determine whether or not the average valence number M of iron in the coating, the volume distribution of the particles, and the corrosion resistance by a corrosion test were evaluated. Table 7 shows the results. All of the weathering steels of the comparative examples have any condition such as the valence number M of iron in the coating and the volume distribution of the grains outside the scope of the present invention, and have a score of 1 or 2 and have poor corrosion resistance. Was confirmed.
FIG. 3 is a graph showing the results of the examples and the comparative examples described above.

It is a schematic diagram of a spinel structure. It is an example of the volume distribution in the particle film | membrane of this invention determined by Mossbauer spectroscopy. It is a graph of the result of Examples 1-6 and a comparative example.

Claims (6)

By mass% Ni: contained from 0.2 to 10%, the surface of the Ni-containing iron-base alloy material and the balance of Fe and unavoidable impurities, is covered with a film consisting mainly FeOOH and Fe ₃ O ₄ A steel material having excellent weather resistance, wherein the average valence of iron in the coating is more than 2.77 and less than 2.99. The Ni-containing iron-based alloy material further contains, in mass%, one or more of Cu, Cr, Ti, W, Mo, Sb, Al, Nb, and Ta in a total amount of 0.01 to 8%. The steel material excellent in weather resistance according to claim 1, characterized in that: Steel molar fraction of Fe ₃ O ₄ in the coating, excellent weather resistance according to claim 1 or 2, characterized in that 4 to 35%. Ni atoms in the film and one or more of Cu, Cr, Ti, W, Mo, Sb, Al, Nb, and Ta atoms in the film have six Fe ₃ O ₄ crystal lattices. The steel material excellent in weather resistance according to any one of claims 1 to 3, wherein 0.4 to 25% is present in the octahedral site coordinated to the oxygen atom. Weathering according to any one of the preceding claims, characterized in that the volume fraction of particle volume 18 × 10 ^-24 m ³ or less of the fine crystal grains in the crystal grains of said film is less than 20 percent 90 percent Steel with excellent properties. The steel material having excellent weather resistance according to any one of claims 1 to 5, wherein the coating has a thickness of 0.01 to 200 µm.

Images