JP3648085B2 - Steel material with excellent corrosion resistance and structure using this steel material - Google Patents

Description

【００７８】
【表２】

【００７９】
【発明の効果】
本発明によれば、特に塩分腐食環境下での耐食性が優れた鋼材を提供することができる。したがって、特に、この種耐食性が優れた鋼の用途を新規に、しかも大幅に拡大するものであり、工業的な価値は大きい。
【図面の簡単な説明】
【図１】本発明で規定する錆中のTi含有量と板厚減少量との関係を示す説明図である。
【図２】本発明で規定する錆中のTi含有量 (微量な範囲) と板厚減少量との関係を示す説明図である。
【図３】本発明で規定する錆中のTi含有量 (微量な範囲) と板厚減少量との関係を示す説明図である。
【図４】本発明で規定する錆の細孔径と板厚減少量との関係を示す説明図である。[0001]
[Industrial application fields]
TECHNICAL FIELD The present invention relates to a steel material having excellent corrosion resistance suitable for a structural material that is difficult to perform maintenance management, such as a bridge, and is used with or without painting, or a structure using this steel material. .
[0002]
[Prior art]
For example, steel materials used for bridge structures such as road bridges in salty corrosive environments, such as mountainous areas and coastal areas where salt water and snow melting salt come in, are conventionally painted and used to improve corrosion resistance. . However, since this coating film always deteriorates with time, there is a need for maintenance management in which the coating film is repainted at regular intervals in order to maintain corrosion resistance.
[0003]
On the other hand, in recent years, in place of conventional multi-girder bridges, few main girder bridges with a small number of main girder, such as a two-main girder bridge, have been used for these bridges. This minority main girder bridge can reduce the amount of steel material used (steel weight) and the number of bridge material pieces, has good workability, and has advantages in terms of environmental protection and shortening the construction period, compared to a large girder bridge. Such minority main girder bridges are strongly required to minimize the maintenance load and cost after the installation of the bridge and to increase the life of the bridge itself.
[0004]
Therefore, steel materials used for structural materials such as steel towers and buildings, including such minority main girder bridges, can be used without coating (bare use) in the above-mentioned salt corrosion environment. Therefore, there is a strong demand for a steel material that maintains high corrosion resistance so that maintenance and management after installation of a bridge is unnecessary in any case even if the coating film is deteriorated or destroyed during use.
[0005]
Conventionally, in order to improve the corrosion resistance of this type of steel material, various improvement techniques from the steel material side as a base material have been proposed. For example, typical examples include weather resistant steels including P: 0.15% or less, Cu: 0.2 to 0.6%, Cr: 0.3 to 1.25%, Ni: 0.65% or less. This weather resistant steel is standardized in two types: JIS G 3114 (weather resistant hot rolled steel for welded structures) or JIS G 3125 (high weather resistant rolled steel). In this weather resistant steel, the rust generated on the surface of the steel during use of the steel material becomes a dense stable rust layer (weather resistant rust) having high corrosion resistance represented by bare weather resistance due to the action of the trace elements. Has a self-corrosion prevention function. Due to such properties, weathering steel has been basically used without coating as a maintenance-free structural material for various structures such as bridges.
[0006]
However, in the salt corrosion environment, the stable rust layer, which is a feature of the weather resistant steel, is hardly formed due to the influence of the salt. And if this stable rust layer is no longer formed, the corrosion resistance of the weathering steel will be significantly reduced. This is due to the fact that the pH in the rust film is particularly lowered with the corrosion of steel in the above-mentioned corrosive environment with much salt. In other words, when steel corrosion starts even slightly, first, Fe → Fe ²⁺ + 2e ^- And the following Fe ²⁺ + 2H ₂ O → Fe (OH) ₂ + 2H ⁺ By this reaction, the pH of the steel surface is lowered, and the pH of the rust film or at the interface between the rust film and the steel is also lowered. And once these pH falls, in order to maintain electrical neutrality, the transport number of the chlorine ion in a rust film | membrane increases, and the concentration of a chlorine ion arises in the interface of a rust film | membrane and steel. As a result, a hydrochloric acid atmosphere is formed at the interface portion, which promotes corrosion of the steel. At the same time, a decrease in pH in the rust film increases the solubility of iron ions, which also causes a phenomenon that inhibits the formation of the stable rust layer, which is the key to the anticorrosion mechanism of corrosion resistant low alloy steels such as weather resistant steel. Corrosion acceleration situation is formed.
[0007]
For this reason, in order to prevent the pH in the rust film from being lowered, a technique for alkalizing the surface of the weathering steel and preventing the formation of the accelerated corrosion state has been proposed. More specifically, oxides (chemical species) such as Be, Mg, Ca, Sr, and Ba that alkalize the surface of the weathering steel are previously dispersed in the steel, and simultaneously with the corrosion reaction of the steel. For example, Japanese Patent Application Laid-Open No. 58-25458 and Japanese Patent No. 2572447 propose methods for causing these chemical species to act to suppress the decrease in the pH of the steel surface.
[0008]
The technique of adding these oxides to prevent the formation of the accelerated corrosion state is surely effective in suppressing the influence of salt content from the outside. However, the formation of the stable rust layer itself is difficult or limited in the same manner as the weather resistant steel, and the fact is that sufficient corrosion resistance is not obtained. In addition, there is a concern that the oxide itself added to the steel adversely affects properties such as weldability and strength.
[0009]
For this reason, various techniques for forming the stable rust layer itself by surface treatment of the steel material have been proposed for the problem of improving the corrosion resistance of the steel material, not by improving the component composition of the steel material. For example, Japanese Patent Laid-Open No. 06-93467 discloses that a steel surface is coated with rust made of α-FeOOH containing 0.3 wt% or more of Cr, Cu, P, Ni, or two or more, and this rust. A technique for applying an aqueous solution containing Cr, Cu, P 2, and Ni ions to the surface of a steel material in order to form the steel is disclosed.
[0010]
Japanese Patent Application Laid-Open No. 09-125224 discloses that the steel surface is heated to 30 nm to 200 μm in hematite (αFe ₂ O _Three A technique for coating with rust consisting of) is disclosed.
[0011]
[Problems to be solved by the invention]
These conventional techniques for forming the stable rust layer itself by the surface treatment or heat treatment of the steel material are notable techniques from the viewpoint of focusing on the components and the composition of the stable rust layer. That is, the above-mentioned weathering steel and oxide dispersion steel contain a large amount of additive elements, and the weldability during construction of steel materials and the efficiency at the time of steel production such as melting and rolling are inevitably compared to those of ordinary steel. Decline. In addition, the steel production cost due to a decrease in production efficiency and a large amount of additive elements is high, and the steel construction cost is high due to the decrease in weldability. Therefore, if the high corrosion resistance of the steel material can be realized by using the normal carbon steel or low alloy steel without using these weathering steels and the components and composition of the stable rust layer, the production efficiency and cost are reduced. There are many advantages in terms of construction efficiency and cost.
[0012]
However, as disclosed in Japanese Patent Laid-Open No. 06-93467, rust composed of α-FeOOH containing 0.3% by weight or more of Cr, Cu, P, Ni, or two or more, and Japanese Patent Laid-Open No. 09-125224 described above. Hematite (αFe ₂ O _Three Rust consisting of) always exhibits high corrosion resistance when used without coating (bare use) in the above-mentioned salt corrosion environment, or when the coating film is deteriorated or destroyed when used after coating. The inventor has found that this is not done. As one reason for this, it has also been found that the method for producing the respective films (rust layers) is not easy, and even if chemical treatment or heat treatment for production is performed, reproducibility is not high and high corrosion resistance is not exhibited.
[0013]
Therefore, in view of the problem of the technology of forming a stable rust layer itself by surface treatment or heat treatment of these conventional steel materials, the present invention is used even if it is ordinary carbon steel or low alloy steel even if it is painted or unpainted. An object of the present invention is to provide a steel material that is suitable for a structural material and has excellent reproducibility and corrosion resistance.
[0014]
[Means for solving problems]
The gist of the present invention for this purpose is that the steel surface is coated with rust containing one or more of Ti, Nb, Ta, Zr, V, and Hf in a total of 0.01 wt% or more, preferably 0.05 wt% or more. It is that.
[0015]
By adopting such a gist, the rust generated on the surface of the steel material during use as a structure can be made into a dense stable rust layer even under a salt corrosion environment, and can have high corrosion resistance. .
[0016]
The present inventors examined the relationship between the composition and composition of rust generated on the steel material surface and the corrosion resistance under a salt corrosion environment. As a result, high corrosion resistance in a salt corrosion environment can be exhibited with good reproducibility by including or presenting one or more of Ti, Nb, Ta, Zr, V, and Hf on the steel surface or the steel material rust layer. I found out.
[0017]
In other words, if one or more of Ti, Nb, Ta, Zr, V, and Hf are contained or present on the steel surface or the steel material rust layer, then the steel surface or the steel material rust layer is generated in an atmospheric environment. By containing these elements, the generated rust is generated as fine and dense α-FeOOH rust or amorphous rust even in a salt corrosion environment. In this process, β-FeOOH is generated as much as possible. It was found that it was suppressed.
[0018]
The effect of these Ti and other substances on rust formation is that during the rust formation / growth stage, ions, colloidal fine compound particles or fine precipitates (Ti or Ti ions formed by oxidation, hydrolysis, etc.) Such as hydroxides, oxyhydroxides, oxides, or reaction products with other substance elements), disturbing the crystal structure of rust, suppressing growth, and filling rust defects. It is presumed that the above-mentioned action suppresses the starting point of corrosion and peeling.
[0019]
However, as a result of the X-ray diffraction method for determining the composition of rust having excellent corrosion resistance containing Ti and the like, fine and dense α-FeOOH rust and amorphous rust are generated on the steel surface or the steel rust layer. At the same time, since the generation of β-FeOOH is suppressed as much as possible, it was found that high corrosion resistance under a salt corrosion environment can be exhibited particularly with good reproducibility. More specifically, the contributions to improving corrosion resistance are (1) β-FeOOH suppression, (2) amorphous rust fraction, and (3) α-FeOOH rust fraction. Particularly, the effects (1) and (2) are great.
[0020]
That is, among the rust, rust having an α-FeOOH component and an amorphous component of preferably 35 wt% or more and a β-FeOOH component of preferably 20 wt% or less in a salt corrosion environment. High corrosion resistance can be demonstrated with good reproducibility. However, even if the fraction of the α-FeOOH component and the amorphous component is the same 35 wt% or more, the higher the fraction of amorphous rust, the better the corrosion resistance. Further, the fraction of β-FeOOH component is preferably 20 wt% or less, rather than the fraction of α-FeOOH component and amorphous component being 35 wt% or more, and the higher the corrosion resistance of rust. Contributes to
[0021]
Japanese Patent Laid-Open No. 06-93467 discloses that the component of the rust layer formed on the steel material surface is preferably dense α-FeOOH. However, in order to exhibit high corrosion resistance with better reproducibility, as described above, it is more important not to have β-FeOOH in this dense rust, in other words, to suppress the formation of β-FeOOH as much as possible. It is. The major features of the present invention are that this β-FeOOH does not exist, that it does not adversely affect the high corrosion resistance, and that the formation and growth of β-FeOOH is suppressed as much as possible to promote the formation of dense rust. is there.
[0022]
Therefore, even in the above-mentioned Japanese Patent Application Laid-Open No. 06-93467, although attention is paid to α-FeOOH rust, when a steel material is used in the salt corrosion environment or when it is used after being coated, the coating film is deteriorated or deteriorated. The reason why the high corrosion resistance is not necessarily exhibited in the case of destruction is considered to be due to the generation of β-FeOOH in the rust, which is not noted in the publication.
[0023]
That is, no matter how high the component ratio of amorphous or α-FeOOH in rust is, if β-FeOOH that easily promotes corrosion exists, corrosion proceeds from this β-FeOOH. This phenomenon becomes remarkable particularly in a salt corrosion environment. Therefore, how to suppress this β-FeOOH is the key to whether or not the stable rust layer exhibits high corrosion resistance.
[0024]
In this regard, one or more of Ti, Nb, Ta, Zr, V, and Hf, and especially among them, Ti is 0.01 wt% or more, preferably 0.05 wt% or more, more preferably 0.1 wt% or more. Contained in the steel, the ratio of amorphous rust and α-FeOOH generated during use of the steel is increased to form fine and dense rust, and a stable rust layer that suppresses β-FeOOH is formed. To do. As a result, high corrosion resistance is exhibited particularly in a salt corrosion environment.
[0025]
DETAILED DESCRIPTION OF THE INVENTION
First, the significance of Ti, Nb, Ta, Zr, V, and Hf contained in the rust in the present invention will be described in detail. Inclusion of 0.01 wt% or more of Ti, Nb, Ta, Zr, V, and Hf in the rust increases the proportion of amorphous rust and α-FeOOH generated during the use of steel. Thus, it is possible to form a fine and dense rust and to form a stable rust layer in which β-FeOOH is suppressed. As a result, high corrosion resistance is exhibited particularly in a salt corrosion environment. The denser the rust generated during use of the steel material, the higher the effect of preventing the entry of corrosion factors such as chloride ions.
[0026]
The mechanism of the fine and dense rust formation effect of Ti, Nb, Ta, Zr, V, and Hf and the suppression effect of β-FeOOH are still unclear. However, (1) fine carbide and nitride fine particles of these metals formed in the steel are discharged into the solution during the corrosion and dissolution of the base iron, and these particles act as iron rust (FeOOH) nuclei. And / or (2) when the steel at the time of rust dissolution, Ti and the like are also eluted as ions, and these metal ions form fine colloids or hydroxides by oxidation, hydrolysis, etc. These are considered to be the core of the rust generated. That is, the presence of nuclei such as (1) and (2) described above inhibits and suppresses the generation and growth of coarse, brittle and easy-to-peel crystalline rust such as β-FeOOH. It is thought to promote the formation of crystalline rust.
[0027]
The effect of these Ti, Nb, Ta, Zr, V, and Hf is that the total (total amount) of one or more of these elements is 0.01 wt% or more, preferably 0.05 wt% or more, more preferably 0.1 wt% It is exhibited when contained in rust in an amount of at least%. However, even if the content exceeds 50 wt%, the effect is the same, and depending on the use conditions of the steel material, there is a possibility that the corrosion resistance may be lowered, such as reducing the adhesion between the rust and the steel material surface. Accordingly, the upper limit amount is preferably about 50 wt% in total (total amount). Among these elements, Ti has the highest effect of improving the corrosion resistance, as will be described later. Therefore, when these elements are contained alone or in combination in rust, it is preferable to make Ti essential. In addition, when Ti does not contain Ti and one or more of Nb, Ta, Zr, V, and Hf is contained in rust, it may contain more than the content when set based on Ti, It is preferable for reliably exhibiting the corrosion resistance effect.
[0028]
In addition, as for impurities contained in elements other than Ti, Nb, Ta, Zr, V, and Hf of the rust of the present invention, as impurities of other elements within a range that does not inhibit the effects of these elements or the intended rust formation of the present invention The inclusion of is acceptable. Among these elements, one or more of Cr, Cu, P, and Ni disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 06-93467 may be contained. As described above, these elements alone cannot reliably improve the corrosion resistance of rust, but in combination with the elements of Ti, Nb, Ta, Zr, V, and Hf of the present invention, 0.3 wt% By containing it as described above, it can be expected to have a composite effect that contributes to rust amorphization and β-FeOOH production suppression.
[0029]
Next, the components and composition of rust in the present invention will be described below. In the present invention, it is preferable that the main component of rust is amorphous rust and has a small amount of β-FeOOH. Usually, the main components of iron rust generated on the steel surface are α-FeOOH, β-FeOOH, γ-FeOOH and Fe. _Three O _Four There are 5 types of crystalline rust and amorphous rust. Among these, amorphous rust forms a very fine and dense stable rust layer than crystalline rust. Moreover, even if a “defect portion” is formed as a rust film due to crystalline rust during the use of steel, the amorphous rust portion fills this hole and reduces the “defect portion”. It also has a “repair function”. As a result, the long-term corrosion resistance of the steel material is ensured. Therefore, the higher the proportion of amorphous rust in the iron rust (the degree of amorphousness), the less β-FeOOH, and then the higher the proportion of fine and dense α-FeOOH among crystalline rust components. It has a high corrosion resistance. Further, these dense rusts make the adhesion to the coating film good when the steel material is further used for coating, and ensures the long-term corrosion resistance of the steel material. For this reason, in this invention, as preferable conditions, the fraction of the amorphous component and (alpha) -FeOOH component which were calculated | required by the X ray diffraction method of the rust produced | generated on the steel material surface is prescribed | regulated as 35 wt% or more.
[0030]
On the other hand, other rust, especially crystalline rust such as β-FeOOH, should be suppressed as much as possible even if the amorphous content in the rust is high. There is. For this reason, in this invention, as preferable conditions, the fraction of (beta) -FeOOH component calculated | required by the X ray diffraction method of the rust produced | generated on the steel material surface is controlled to 20 wt% or less. When the fraction of amorphous rust component to α-FeOOH component is less than 35 wt% and the fraction of β-FeOOH component exceeds 20 wt%, the β-FeOOH, γ-FeOOH and Fe _Three O _Four There is a possibility that the high corrosion resistance of the steel material cannot be assured because the coarse rust component such as the crystallinity increases and the rust on the steel material surface does not form a dense stable rust layer.
[0031]
Here, the evaluation of the denseness of the rust generated on the steel material surface is also important as an evaluation of the corrosion resistance. That is, since the actual corrosion resistance test requires a long time as is well known, the target steel material cannot be evaluated in a short time. As an evaluation of the compactness of this rust, the present inventors have also found that the smaller the rust pore diameter value measured by this molecular adsorption method, the smaller the rust pore diameter value can be evaluated as dense rust. .
[0032]
The molecular adsorption method recommended as a method for evaluating the density of rust is a kind of gas adsorption method. Nitrogen adsorption isotherm is measured at the liquid nitrogen temperature using an automatic capacity adsorption device. From this adsorption isotherm, the pore diameter and pore diameter distribution of the fine pores of the porous body are calculated by the t-plot method. This is to determine the pore size value of the micropores by the Kelvin equation, assuming that the nitrogen at the liquid nitrogen temperature (77.4K) completely wets the surface of the micropores of the porous body. This method itself is well known in “Chemistry Seminar 16, Adsorption Chemistry” (Maruzen, issued July 30, 1991) and the like.
[0033]
The denseness of rust generated on the steel material surface as referred to in the present invention is an aggregated state of rust, and this aggregated state (aggregation degree) can be evaluated by the size of an interval (gap) between rust particles. The inventors have also found that the interval between the rust particles can be measured as the above-mentioned pore diameter by a molecular adsorption method. In other words, the pore size measured using the molecular adsorption method corresponds well to the spacing between rust particles corresponding to the degree of rust aggregation, and rust can be analyzed as a three-dimensional (overall) structure. . In this respect, the pore diameter of rust referred to in the present invention is substantially the pore diameter measured by the molecular adsorption method, and essentially means the interval (gap) between the rust particles.
[0034]
The smaller the rust pore diameter, which is evaluated by this molecular adsorption method, that is, the denser the rust, the more difficult the corrosive substance enters and the corrosion resistance is improved. As a measure for this, in the present invention, the pore diameter of rust is defined as a preferred embodiment of 3 nm or less, preferably 2 nm or less, more preferably 1 nm or less. Accordingly, when the pore diameter of rust is large, a corrosive substance easily enters, and the corrosion resistance of rust is lowered.
[0035]
In the molecular adsorption method, the specific surface area that can be obtained by BET plot is also an index representing the density of rust in terms of the particle size (size) of rust. As a measure of this specific surface area, preferably 10m ² / g or more, more preferably 50m ² Corrosion resistance improves at / g or more. The rust particle size (crystallite size) required by the X-ray diffraction method is preferably 50 nm or less, more preferably 20 nm or less.
[0036]
Incidentally, it is possible to measure the distance between the rust particles with a transmission electron microscope (TEM). However, this TEM is inferior to the molecular adsorption method in that it can only obtain local region information and cannot grasp rust as a three-dimensional (overall) structure. For this reason, in order to evaluate the rust on the steel surface with TEM with good reproducibility, it is necessary to measure a large number of points, and there is a problem that much time and effort are required for this measurement and evaluation.
[0037]
In the present invention, the high corrosion resistance of the rust generated on the surface of the steel material is the corrosion resistance of the steel material in a salt corrosion environment. Therefore, it is important to evaluate the denseness of rust by the molecular adsorption method. However, as a matter of fact, such as quality assurance, in order to ensure high corrosion resistance, the steel is exposed to the atmosphere, which is also simulated in a salt corrosion environment. It is necessary to evaluate the corrosion resistance of steel after exposure to the atmosphere, including salt water spray (eg, 0.1 to 5.0% salt water once a week).
[0038]
In addition, as a means of measuring the degree of rust amorphousness, see “Corrosion protection 95 C-306 (pages 341 to 344)”, “Quantification of iron rust components by powder X-ray diffraction method and its application”. The disclosed powder X-ray diffraction method is effective. In this document, we tried to quantify the iron rust component on the steel surface by means of powder X-ray diffraction for weathering steel, and the higher the ratio of amorphous rust in the iron rust (the degree of amorphousness) It supports the corrosion resistance improvement model that becomes a dense stable rust layer. As a more specific powder X-ray diffraction method, this document describes a fixed weight ratio of CaF as an internal standard substance. ₂ Alternatively, ZnO and other rust samples taken from steel are mixed and pulverized and identified by the usual X-ray diffraction method, and the integrated intensity ratios of the unique diffraction peaks of each of the above five types of rust are determined in advance. Quantification of each crystalline rust component is performed from the calibration curve of the rust component, and the ratio of the amorphous component is calculated by subtracting the amount of each crystalline rust component from the total amount of rust. This is because the integral intensity ratio of the diffraction peaks of the amorphous component itself is difficult to obtain and is difficult to quantify. In this document, ZnO is highly reliable as an internal standard substance.
[0039]
Incidentally, as disclosed in the same document, qualitative analysis of rust components is possible with other analysis methods such as infrared spectroscopy other than X-ray diffraction, but quantitative analysis is difficult. There is no established quantitative analysis method for rust components. Therefore, the amorphous degree of rust on the steel surface referred to in the present invention was quantitatively measured by this X-ray diffraction method, particularly by the powder X-ray diffraction method using ZnO disclosed in the above literature as an internal standard substance. Say things.
[0040]
Next, a method for forming the dense stable rust layer of the present invention will be described. First, an appropriate treatment such as cleaning, cleaning, or surface polishing is performed on the surface of a steel material before or during use as a structural material. These treatments may be selectively performed as appropriate depending on the surface condition required for the steel material, such as mirroring the surface of the steel material to simple cleaning. Of course, these processes are not necessary.
[0041]
An aqueous solution or mixture containing Ti, Nb, Ta, Zr, V, and Hf ions, fine particles, or fine compounds, alone or in combination, or Cr, Cu, P, Ni in addition to these ions A solution such as an aqueous solution containing ions is applied to the surface of the steel material, or a method by chemical treatment such as immersing the steel material in a solution or mixed solution. One or more of V and Hf are contained or present. Under the present circumstances, it is preferable that a solution contains Ti ion or Ti acid ion from the predominance of the formation effect of the said dense Ti rust compared with another element. When these ions or acid ions of these elements are contained, it is preferable to use sulfates or chlorides of these elements from the viewpoint of solution stability and throwing power. When fine particles such as Ti and Nb or fine compounds are used, it is preferable that the average dimensional diameter is 50 nm or less, 25 nm or less, and further 15 nm or less from the viewpoint of improving corrosion resistance. Further, oxides, carbides, nitrides or composite compounds based on these are preferable. When each ion and fine particles or fine compounds coexist appropriately, a further corrosion resistance effect is exhibited.
[0042]
As for the chemical treatment method, the simplest method for bringing the solution into contact with the steel surface or the rust layer on the steel material is to apply the solution to the steel material. However, a normal solution treatment method such as immersing a steel material in a solution is appropriately selected according to circumstances and circumstances. In addition, in the case of Ti, Nb, Ta, Zr, V, Hf fine particles or fine compounds, there is a method of directly spraying in a solid state on the rust layer of the steel surface or steel surface, Is preferably made into a solution or a mixed solution.
[0043]
In addition, with respect to treatment methods other than this chemical treatment method, a vapor phase coating method for concentrating or existing these elements on the surface of the steel material by sputtering or vapor deposition, or containing these elements and concentrating these elements on the surface. There is a method of forming a steel material surface in which Ti, Nb, Ta, Zr, V, and Hf are present, which is a base for forming the dense stable rust layer of the present invention, by a method using a steel material.
[0044]
However, it is difficult to increase the amount of these elements and the amount of surface enrichment in the steel by the method of transferring the elements contained in the steel into rust by thermal diffusion, etc. There is a risk of disturbing other properties. Furthermore, even if the amount of elements and the amount of concentrated surface are increased, it is difficult to contain a predetermined amount (more than the lower limit amount) of elements in the rust. And the advantage that normal carbon steel and low alloy steel can be used may also be lost. Further, the vapor phase coating method is expensive in equipment and processing cost itself, and in addition, it is not efficient to process a large amount of a large steel plate itself, and it is not feasible. Therefore, among the above methods, the most simple and inexpensive method based on the chemical treatment is preferable.
[0045]
However, as described above, it is difficult to cover the necessary amount of Ti in the rust layer with Ti in the steel material component as described above, but as described later, Ti in the steel material component is not solidified as Ti ions as solute Ti. Molten Ti, as fine precipitate particles (in terms of morphology, carbide, nitride, oxide, etc.), has the effect of promoting the formation of dense stable rust. Therefore, by supplying Ti from the outside to the steel material surface or the rust layer and including Ti or the like in the steel material component, a synergistic effect of promoting the generation of dense stable rust by these single effects or a combination can be expected. These effects are also the same for Nb, Ta, Zr, V, and Hf other than Ti.
[0046]
As described above, steel with Ti, Nb, Ta, Zr, V, and Hf present on the steel surface or in the steel does not need to be treated particularly aggressively, and salt content such as salt water or snow melting salt comes in. Even in a corrosive environment, a great advantage is that a dense stable rust layer is formed in a relatively short time during use as a structural material such as a bridge. However, from the viewpoint of quality assurance to ensure reliable corrosion resistance such as bare weather resistance, after manufacturing the steel material, pre-treatment such as pickling as necessary is followed by heat treatment in an atmosphere such as a gas with controlled oxidation potential. Alternatively, the surface is chemically treated with chemicals such as phosphates, chromates, and oxidizing agents, and the rust generated during the manufacturing process of steel is amorphized to make it dense and aggressive. A stable rust layer may be formed.
[0047]
Therefore, as a target steel material for measuring the composition of elemental components on the surface of the steel material and the amorphous degree of rust, for example, even a steel material before actually used as a structural material is used as a structural material. Or steel that has been tested for atmospheric exposure (including once-weekly salt water spray).
[0048]
Moreover, this invention can be used not only for the steel material for new structures but also for improving the corrosion resistance of painted or non-painted steel materials that are in use as existing structures. That is, without peeling or removing the coating film or rust on the surface of the steel material in use as an existing structure, or if necessary, removing the coating film or rust completely or partially (for example, only the corroded portion) to clean it. By the chemical treatment, an aqueous solution containing Ti, Nb, Ta, Zr, V, Hf ions, fine particles, or fine compounds on the steel surface, or Cr, Cu, P in addition to these ions. Even if an aqueous solution containing Ni ions is applied, dense rust can be generated over time. Therefore, the present invention can also be used for repair or maintenance management of existing structures. In this respect, the coating with rust referred to in the present invention is not only when the steel surface is entirely covered, but when the steel surface is partially covered, when the coating amount is partially changed, or in the coating process Naturally, a case where an uncoated portion is inevitably generated, and a case where only a steel member requiring corrosion resistance is selectively covered in a structure are naturally included.
[0049]
Furthermore, the component composition of the steel used in the present invention will be described. First, from the viewpoint of the stable rust formation, it is desirable that the steel used in the present invention does not contain an element that inhibits the formation of stable rust. Further, since the steel material of the present invention is for structural materials such as the above-mentioned minority main girder bridges, the heat input amount is 5 KJ / mm or more by carbon dioxide arc welding or electrogas arc welding in some cases from the viewpoint of workability and shortening of work period. High heat input welding with heat input of 100 to 300KJ / mm or more is performed. Therefore, as a steel material used for this structural material, there is no need for preheating as well as mechanical properties such as strength as a structural material, and excellent welding capable of high-efficiency welding such as high heat input welding. Steel materials that have both corrosion resistance and corrosion resistance are preferred.
[0050]
In this regard, the steel used in the present invention does not include high alloy steel that does not cause rust in the environment where salt corrosion is used, and does not include rust in the environment where salt corrosion is used. Steel etc. can basically be used. Further, conventional weathering steel containing P, Cu, Cr, Ni or the like may be used.
[0051]
However, it is necessary to pay attention to elements that inhibit the formation of stable rust among the component composition of the steel under the stricter corrosion resistance specifications and the environment where salt corrosion is used. That is, S and Cr are examples of elements that inhibit stable rust formation.
[0052]
Among these, if the S content exceeds 0.02%, the formation of the stable rust layer due to the inclusion of Ti or the like in the rust may be hindered, leading to deterioration in corrosion resistance. Therefore, the S content is preferably 0.02% or less.
[0053]
In addition, Cr is recognized as an indispensable additive element for forming the stable rust layer together with P, Cu, and Ni in conventional weathering steel, and as described above, 0.30 to 1.25% is also contained in JIS standards. ing. In addition, even in the above-mentioned Japanese Patent Application Laid-Open No. 58-25458 and Japanese Patent No. 2572447, although addition of Cr is not specified, it is necessarily contained by 0.05% or more as an impurity from an iron raw material or a steelmaking process. .
[0054]
However, when Cr is contained in the steel in an amount of 0.05% or more, if corrosion starts even slightly at the micro surface defects of the steel, Cr ions that dissolve in a small amount along with the iron atoms in chemical equilibrium from the steel, especially Cl In an environment where ions exist, it causes a decrease in pH in the micro surface defect portion of the steel, and promotes the oxidation of condensed water in the defect and has the effect of inducing corrosion. Therefore, even if the dense stable rust layer is formed, Cr has an action of promoting the corrosion of the steel at the lower part of the stable rust layer, and inhibits the adhesion between the rust layer and the steel, thereby peeling the rust layer. And as a result, there is a possibility that the formation or maintenance of a dense stable rust layer is hindered. Therefore, it is preferable to reduce the Cr content in the steel as much as possible, and considering the economy of reducing the Cr content, the upper limit is preferably made less than 0.05%.
[0055]
On the other hand, Ti is preferably contained in the steel as an element for promoting the formation of a stable rust layer in place of Cr. Ti has the effect of promoting the formation of the stable rust layer without causing the decrease in the pH like Cr, and it is a unique feature that synergistically enhances the effect of promoting the formation of the stable rust layer of Ti in the rust layer. Has properties. Specifically, the ratio of amorphous in iron rust and the ratio of α-FeOOH rust is increased, and the generation of β-FeOOH, which easily promotes corrosion among crystalline rust components, is suppressed. Promotes the formation of a fine and dense stable rust layer. As described above, such an effect is exhibited in any form of fine precipitates such as carbides, nitrides, oxides, etc., as Ti ions in solid solution Ti and in solid solution Ti. As a result, entry of corrosion factors such as chloride ions into the rust layer is prevented, and a dense stable rust layer is maintained to improve the corrosion resistance. If the Ti content is less than 0.01%, this effect is not obtained, and if it exceeds 1.0%, the effect is saturated and not economical. In this respect, in order to exert the effect of Ti more, it is preferable to contain 0.03% or more, more preferably 0.05% or more of Ti in the steel, and when Ti in the steel exceeds 0.5%, the steel becomes brittle. It can be problematic and not economical. Therefore, when Ti is contained in the steel, the Ti content is preferably in the range of 0.03 to 1.0%, and more preferably in the range of 0.05 to 0.5%.
[0056]
As a steel containing Ti, which restricts S and Cr in the steel which inhibits the formation of this stable rust and has the effect of promoting the formation of a stable rust layer, the present inventors have previously made a patent application. No. 09-330173, C: 0.15% or less, Si: 0.10 to 1.0%, Mn: 1.5% or less, S: 0.02% or less, P: 0.05% or less, Cr: 0.05% or less, Ti: 0.01 to 1.0%, A steel material containing one or two of Ca: 0.0001 to 0.01% and Cu: 0.05 to 3.0% and Ni: 0.05 to 6.0% and having a basic composition composed of the balance Fe and inevitable impurities was filed. This steel material also has good weldability, and is optimal as a preferred embodiment of the steel material used in the present invention.
[0057]
In addition to this basic composition, the following elements having the effect of promoting the formation of a stable rust layer other than Ti may be included in the steel. One or two of Mo: 0.05-3.0% and W: 0.05-3.0%. One or more of Al: 0.05 to 0.50%, La: 0.0001 to 0.05%, Ce: 0.0001 to 0.05%, Mg 0.0001 to 0.05%. 0.5% or less total of one or more of Zr, Ta, Nb, V and Hf. Here, the effects of Zr, Ta, Nb, V, and Hf are the same as the effects of Ti described above, both as metal ions in the case of solid solution, and as fine precipitate particles in the case of non-solid solution. It promotes the formation of dense stable rust.
[0058]
Further, as the structure of the steel material, a ferrite amount of 90% or more, or a mixed structure of ferrite and pearlite is preferable from the viewpoint of corrosion resistance. On the other hand, for example, the structural material strength of the bridge is 500 N / mm ² A bainitic structure or a bainitic + ferritic structure is preferable in order to ensure a strength or toughness of a grade or higher and improve the corrosion resistance of the steel material itself.
[0059]
Next, the manufacturing method of this invention steel material is demonstrated. The steel material of the present invention can be manufactured by a method for manufacturing a thick steel plate having a normal thickness of 50 mm or more. That is, after a steel is continuously cast or melted by an ingot-making method, hot working such as ingot rolling or hot forging or thick plate rolling is performed to produce a predetermined product plate thickness. These hot working conditions and conditions for cooling and heat treatment after hot working are 390 to 630 N / mm for steel, for example, as a structural material for bridges. ² It is determined as appropriate according to the requirements and specifications of mechanical properties such as the strength of grade or higher. Therefore, in addition to the usual hot working, after securing low alloying or low carbon equivalent to ensure weldability, the mechanical properties such as the strength are ensured, and the steel structure of the present invention is described above. Preferably, forced cooling such as accelerated cooling after hot working or controlled rolling is applied so that the ferrite content is 90% or more, or a mixed structure of ferrite + pearlite, and further a bainite structure or a bainite + ferrite structure. May be. In addition, the heat treatment after the hot working is appropriately subjected to rolling on-line direct quenching (DQ) or off-line quenching and tempering (QT) as necessary.
[0060]
【Example】
(Example 1)
Next, the significance of each requirement of the steel material of the present invention described above will be described with reference to examples. Steel ingots having chemical components shown in Table 1 were melted, and these steel ingots were forcibly cooled by accelerated cooling after hot rolling to produce steel plates having a thickness of 50 mm. In Table 1, No. 1 is a low carbon steel, No. 2 is a Ti-containing low alloy steel, No. 3 is a weather-resistant steel containing 0.05% or more of Cr, and No. 4 is a weather resistance of Japanese Patent Application No. 09-330173. Stainless steel. And cut out test pieces from these thick steel plates, the test piece surface is made into a mirror surface by emery paper polishing and buff polishing, and ions such as sulfates of Ti, Nb, Ta, Zr, V, Hf, Alternatively, an aqueous solution or mixture containing these fine particles, fine compounds such as carbide and nitride, and an aqueous solution containing a sulfate such as Cr in addition to these were applied to the surface of the steel material.
[0061]
These chemical-treated test specimens were applied to bare test specimens simulating unpainted use, and coating tests in which 50 μm of phthalic acid resin, which is commonly used for painting bridges and the like, was applied. Both pieces were prepared and subjected to a corrosion resistance test. The corrosion resistance test simulates the actual salt-corrosion environment, sprays 5% salt water once a week, and installs the specimens facing south and inclined at 30 ° to the horizontal for 15 months. An atmospheric exposure test was conducted. In addition, in the presence of relatively short-term corrosion acceleration tests such as the salt spray test, the atmospheric exposure test including salt spray for 15 months was performed because the steel material of the present invention was used especially in a salt corrosion environment. This is because it is a structural material such as a bridge, so that it cannot be evaluated to ensure corrosion resistance unless it is a test that is compatible with the corrosion under the actual salt corrosion environment (chloride atmosphere).
[0062]
Then, the long-term durability of the test piece after the atmospheric exposure test was evaluated by measuring the reduction amount of the average plate thickness for the bare test piece (evaluation of the reduction amount of the plate thickness as the corrosion weight loss). Moreover, about the coating test piece, it evaluated by the measurement of the swelling width | variety of the artificial coating-film defect part. Of these, the average plate thickness reduction amount was calculated by measuring the average plate thickness of the test material before and after the atmospheric exposure test with a micrometer and taking into account the density. And from these results, comprehensive evaluation of corrosion resistance (◎ ○ △ ×) was performed. These results are shown in Table 2.
[0063]
Moreover, about the coating test piece, while scratching a coating film beforehand and providing an artificial coating film defect, the blistering width of the artificial coating film defect part after an atmospheric exposure test was measured. And from these results, comprehensive evaluation of corrosion resistance (◎ ○ △ ×) was performed. These results are shown in Table 2. In Table 2, the amount of decrease in the average plate thickness is shown in mm, and the blister width of the artificial paint film defect portion is 0.80 mm or more A, 0.5 to 0.8 mm B, 0.5 mm or less C, and most blisters. Items that are not allowed are listed as D.
[0064]
Moreover, while measuring the element and element amount in the rust produced | generated on the test piece surface after an atmospheric exposure test by EPMA, the composition of rust was analyzed by the said X ray diffraction method. More specifically, it was carried out by the powder X-ray diffraction method disclosed in the above-mentioned “Corrosion protection 95 C-306 (pages 341 to 344)”, and ZnO of a constant weight ratio (30 wt%) was collected from steel as an internal standard. The powder mixed with the rust sample was identified by X-ray diffraction, and the α-FeOOH, β-FeOOH, γ-FeOOH and Fe _Three O _Four Each crystalline rust component was quantified from the integrated intensity ratio of the intrinsic diffraction peak of each of the four types of crystalline rust and a calibration curve of each rust component determined in advance. The ratio (%) of the amorphous component was calculated by subtracting the amount of each of these crystalline rust components from the total amount of rust. These results are also shown in Table 2.
[0065]
In Table 2, the composition of the generated rust on the surface of the test piece after the atmospheric exposure test is as follows: A: 0 to 35 wt%, B: 35 to 40 wt%, C: fraction of rust component of amorphous and α-FeOOH 41 wt% or more, the fraction of β-FeOOH rust component is shown as A: 31 wt% or more, B: 20-30 wt%, C: less than 20 wt%.
[0066]
In Table 2, Invention Example No. 4 was subjected to a corrosion resistance test in the same manner as the other invention examples without chemical treatment with an aqueous solution containing sulfates of Ti, Nb, Ta, Zr, V, and Hf. Yes. For comparison, the test steel shown in Table 1 was subjected to a corrosion resistance test in the same manner as in the invention example without chemical treatment with an aqueous solution containing sulfates of Ti, Nb, Ta, Zr, V, and Hf. Comparative Examples No. 1 to 3 performed, Comparative Example No. 5 in which the content of Ti according to the present invention deviates from the lower limit of 0.05 wt%, Cr alone was contained in rust instead of the element according to the present invention Comparative Example No. 6 (with a chromium sulfate aqueous solution applied to the test piece) was also tested in the same manner as the inventive example, and the results are shown in Table 2.
[0067]
As is apparent from the results in Table 2, Invention Examples Nos. 4 and 7 to 20 that satisfy the conditions of the present invention are excellent in corrosion resistance in both cases of no coating and coating. Among these, in comparison between Invention Examples, Invention Examples No. 7 to 10 containing Ti in rust are Invention Examples No. 11, 12, and 14 containing other Zr, Ta, V, and Hf in rust. , 15 is superior in corrosion resistance, and the superiority of Ti effect among these elements is supported. The invention example No. 13 containing Nb in the rust is superior in corrosion resistance to the invention examples No. 11, 12, 14, and 15 because of the contribution of the steel 4 itself. It is presumed that the Cr content of No. 4 is small, the corrosion resistance deterioration due to Cr is not adversely affected, and the stable rust generation effect by Ti contained in Steel 4 is caused.
[0068]
Inventive Example No. 4 is not chemically treated with an aqueous solution containing Ti or the like, but contains Ti in the rust due to diffusion of Ti contained in the steel material to the steel surface, The content of Ti in the rust is lower than that of the invention example that was chemically treated with an aqueous solution containing other Ti and the like, and the corrosion resistance is relatively poor. Therefore, it is understood that it is preferable to perform chemical treatment with an aqueous solution containing Ti or the like in order to contain a desired amount of Ti in the rust.
[0069]
In addition, Invention Example No. 10 using the weather resistant steel 4 of Table 1 containing a small amount of Cr and containing Ti is excellent in corrosion resistance despite the relatively small Ti content in rust. Invention Examples Nos. 16 to 18 using the weathering steel 4 include Ta, Zr, V and Hf in the rust, and Invention Examples No. 11 and 12 using the low carbon steel 1 and the low alloy steel 2 Compared to 14 and 15, it is superior in corrosion resistance, and this point also supports the synergistic effect of the weather resistant steel 4. In addition to Ti, Invention Examples No. 19 and 20 containing Cr and Ni have excellent corrosion resistance despite the use of plain steel 1, and the synergistic effect of Ti, Cr and Ni is supported. It is done.
[0070]
On the other hand, comparative examples No. 1 to 3 simulating a steel material usually used in a structure without chemical treatment for containing Ti in the rust, the lower content of Ti in the rust is 0.05. Comparative Example No. 5 that is less than wt%, Comparative Example No. 6 containing Cr in the rust instead of these elements according to the present invention, the thickness reduction amount is 0.80 mm or more, coating film defects The blister width is as large as A with 0.80mm or more, and the corrosion resistance is extremely inferior. In these comparative examples, the composition of rust is mainly composed of α-FeOOH and amorphous rust components, but Ti, Nb, Ta, Zr, V, and Hf are not contained in the rust or the content is Because of the small amount, the proportion (fraction) of crystalline rust of β-FeOOH increases, and corrosion progresses starting from this β-FeOOH, which is considered to be inferior in corrosion resistance. Therefore, from this result, Ti, Nb, Ta, Zr, V, Hf are used for the more preferable conditions of the present invention for the amorphization of rust on the steel surface and the suppression of the crystalline rust of β-FeOOH. The necessity of inclusion in rust is understood.
[0071]
As a result of measuring the concentration of chloride ions at the interface between the rust layer on the surface of the test piece and the ground iron using an electron beam probe microanalyzer (EPMA), the invention example shows the chloride ion concentration at the interface between the rust layer and the ground iron. Whereas the concentration was small, the comparative example had a large concentration of chloride ions at the interface between the rust layer and the base iron, confirming the results of the corrosion resistance test. Inventive Example Nos. 6 to 9 containing Ti in the rust are rust layers as compared to Inventive Nos. 10 to 14 containing Nb, Ta, Zr, V and Hf in the rust. There was less concentration of chloride ions at the interface between the steel and the iron, and this point also confirmed the superiority of the Ti effect among these elements.
[0072]
(Example 2)
Furthermore, using the low carbon steel of Table 1, under the same conditions as the invention example containing Ti in the rust of Example 1, only the Ti content in the rust was changed by changing the Ti sulfate concentration of the aqueous solution for chemical treatment. The controlled test piece was subjected to an atmospheric exposure test in the same manner as in Example 1, and the average reduction in the plate thickness was measured. From these results, the relationship between the Ti content in the generated rust and the average reduction in sheet thickness is shown in Figs. 1 and 2 (Fig. 2 shows that the Ti content in the rust is 0.12 wt% or less. Shows a very small range). As can be seen from Figs. 1 and 2, as the Ti content in the rust increases, the thickness reduction decreases, especially at around 0.05 wt%, and the thickness reduction decreases rapidly. It can be seen that there is a critical significance as a preferable condition in that the content of Ti or the like in the rust is 0.05 wt% or more.
[0073]
(Example 3)
In the above Example 2 (FIGS. 1 and 2), the content of Ti and the like in the minute range where the content of Ti and the like in the rust is 0.1 wt% or less, particularly in the portion where the thickness reduction amount rises rapidly. In order to further clarify the relationship with the thickness reduction amount, an accelerated exposure test was conducted in a range where the content of Ti and the like in the rust was very small. In the corrosion resistance test, as in Example 2, only the Ti content in rust was controlled by changing the Ti concentration of sulfuric acid in the chemical treatment aqueous solution, and a mixture of various fine particles such as carbide and nitride was applied. The prepared test piece was prepared by adding fine particles to the Ti sulfate solution to control the Ti concentration. These test pieces were simulated for 12 months under the same exposure test conditions as in Example 2 under the actual salt corrosion environment, except that the salt water spraying conditions were 0.1% salt water spraying once a week. It was. And the average reduction | decrease amount of the board thickness after a test was measured.
[0074]
From these results, Fig. 3 shows the relationship between the Ti content in the generated rust and the average reduction in sheet thickness. Figure 3 shows the relationship with the reduction in sheet thickness when the Ti content in the rust is in a very small range of 0.05 wt% or less. As can be seen from Fig. 3, as in Figs. 1 and 2, as the Ti content in the rust increases, the reduction in thickness decreases, especially at around 0.01 wt%. ing. Therefore, if the salt spray conditions in the accelerated exposure test are milder than those in Examples 1 and 2, in other words, if the salt corrosion environment is more gradual, the content of Ti, etc. in the rust can be reduced ( It can be seen that the amount of Ti can be selected depending on the salt corrosion environment. It can also be seen that there is a critical significance in that the content of Ti and the like in the rust specified in the present invention is 0.01 wt% or more.
[0075]
Further, the test pieces prepared in Example 3 with different Ti contents in rust were subjected to an accelerated exposure test for 12 months under a spraying condition of 5.0% salt water once a week. And the pore diameter of the rust of each test piece after a test was measured with the above-mentioned molecular adsorption method, respectively, and the relationship with the measurement plate | board thickness reduction | decrease amount in an accelerated exposure test was arranged. The result is shown in FIG. In the figure, X indicates a test piece with Ti content in rust of less than 0.01 wt%, ○ indicates a test piece with Ti content in rust of 0.01 to 0.05 wt%, and △ indicates the Ti content in rust. The specimen exceeds 0.05 wt%. As is apparent from FIG. 4, there is a correlation between the rust pore diameter and the reduction in plate thickness, and the smaller the pore diameter, that is, the smaller the rust particle spacing, the better the corrosion resistance. It can also be seen that with the same pore diameter, the corrosion resistance is better as the Ti content in the rust increases. Further, it can be seen from this result that Ti in the rust has an effect of reducing the interval between the rust particles, densifying the rust and improving the corrosion resistance. In FIG. 4, the triangles marked with (1) indicate the application of Ti sulfate solution and TiO with an average particle size of 10 nm. ₂ By applying the particle mixture aqueous solution, the circles marked with (2) and x marked with (3) give Ti to the surface of the test piece by applying the TiC particle mixed aqueous solution with an average particle diameter of 20 nm and 30 nm, respectively. Other than those marked with Δ, ○, and X, Ti was applied to the surface of the test piece by applying a Ti sulfate solution. And from the comparison of the △, ○, and X marked numbers from (1) to (3), in the case of fine particle coating, the smaller the particle diameter, the better the corrosion resistance. It can be said that there is an effect.
[0076]
As is clear from the results of the above examples, the steel material of the present invention is excellent in reproducibility and corrosion resistance, particularly in a salt corrosion environment, whether it is painted or unpainted, such as a minority main girder bridge. The present invention can be widely used not only for bridges but also for ordinary structures such as steel towers such as power transmission lines, buildings, buildings or buildings. Moreover, ordinary carbon steels and low alloy steels can be used as those having corrosion resistance higher than that of weathering steels, and exhibit the characteristics of ordinary carbon steels and low alloy steels such as weldability and mechanical properties. Is possible.
[0077]
[Table 1]

[0078]
[Table 2]

[0079]
【The invention's effect】
According to the present invention, it is possible to provide a steel material having excellent corrosion resistance particularly in a salt corrosion environment. Therefore, in particular, the application of steel having excellent corrosion resistance is newly and greatly expanded, and industrial value is great.
[Brief description of the drawings]
FIG. 1 is an explanatory diagram showing the relationship between the Ti content in rust and the reduction in sheet thickness defined in the present invention.
FIG. 2 is an explanatory diagram showing the relationship between the Ti content (a very small range) in rust and the thickness reduction amount defined in the present invention.
FIG. 3 is an explanatory diagram showing the relationship between the Ti content (a very small range) in rust and the thickness reduction amount defined in the present invention.
FIG. 4 is an explanatory diagram showing the relationship between the pore diameter of rust and the thickness reduction amount defined in the present invention.

Claims (13)

A steel material with excellent corrosion resistance, wherein the steel surface is coated with rust containing a total of 0.01 wt% or more of Ti, Nb, Ta, Zr, V and Hf. The steel material excellent in corrosion resistance according to claim 1, which is coated with rust containing 0.05% by weight or more of one or more of Ti, Nb, Ta, Zr, V and Hf in total. The steel material excellent in corrosion resistance according to claim 1 or 2, wherein the rust essentially contains Ti. The steel material excellent in corrosion resistance according to any one of claims 1 to 3, wherein the total content of Ti, Nb, Ta, Zr, V, and Hf in the rust is 0.1 wt% or more. The steel material excellent in corrosion resistance according to any one of claims 1 to 4, wherein at least part of Ti, Nb, Ta, Zr, V, and Hf in the rust is present as fine particles. The steel material excellent in corrosion resistance according to any one of claims 1 to 5, wherein the rust further contains 0.3 wt% or more of Cr, Ni, Cu, and P. The fraction of α-FeOOH component and amorphous component determined by X-ray diffraction of the rust is 35 wt% or more, and the fraction of β-FeOOH component is 20 wt% or less. The steel material excellent in corrosion resistance according to item 1. The steel material excellent in corrosion resistance according to any one of claims 1 to 7, wherein the pore diameter of the rust is 3 nm or less. The steel material excellent in corrosion resistance according to any one of claims 1 to 8, wherein the steel material has a thickness of 50 mm or more. The steel material excellent in corrosion resistance according to any one of claims 1 to 9, wherein the steel material is used in a salt corrosion environment. The steel material excellent in corrosion resistance according to any one of claims 1 to 10, wherein the steel material is coated and used. The steel material excellent in corrosion resistance according to any one of claims 1 to 11, wherein the steel material is used without being painted. A structure using the steel material according to any one of claims 1 to 12 as a structural material.

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