KR20130126729A - Steel material with rust layer, exhibiting excellent weather resistance even in high-salt environments - Google Patents

Abstract

In particular, even in a strict corrosive environment where a large amount of fly salt is present in coastal areas and the like, there is provided a steel material having a rust layer having excellent corrosion resistance without coating. Specifically, the steel material in which the rust layer of this invention was formed is formed by forming a rust layer on the surface of a base steel material. The rust layer contains Nb and Sn, and the number of Nb atoms in the rust layer is 0.01 or more with a maximum value of 100 Fe atoms, and the number of atoms of Sn in the rust layer has a maximum value with respect to 100 Fe atoms. By 0.005 or more.

Description

STEEL MATERIAL WITH RUST LAYER, EXHIBITING EXCELLENT WEATHER RESISTANCE EVEN IN HIGH-SALT ENVIRONMENTS}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to steel materials mainly used for steel structures used outdoors, such as bridges, and more particularly to areas of coastal or coastal environments (hereinafter, simply referred to as "seaboard region"). The present invention relates to steel materials having a rust layer having excellent paint resistance and corrosion resistance (also referred to as bare corrosion resistance) even under severe corrosion environments in which high salts are present.

Conventionally, weathering steel has been used in steel structures used outdoors such as bridges. Weather resistant steels exhibit a corrosion rate by covering a surface in a highly protective rust layer in which alloying elements such as Cu, P, Cr, and Ni are concentrated in an atmospheric exposure environment. It is the steel material which it reduced considerably. Because of its excellent weather resistance, it is known that a bridge using weather resistant steel usually withstands use for several decades in an unpainted state. However, in a severe corrosive environment with a large amount of air-borne salt, such as in coastal areas, such a highly protective rust layer is difficult to form on the surface of the steel, and there is a problem that practical weather resistance is not obtained well. In addition, the amount of flying salts referred to in this application means the average amount of flying salts for one year, and measures the amount of flying salts of the month over a year according to "measurement of chloride by dry gauze method" described in JIS Z 2382 (1998), The mean annual average salinity is calculated.

According to Non-Patent Document 1, conventional weathering steel (JIS G 3114: weathering hot rolled steel for welding structure) has a conventional salt content of 0.05 mg NaCl / dm 2 / day (hereinafter, the unit display “mg · NaCl / dm 2 / day ”may be simply abbreviated as“ mdd ”).

Therefore, it is common to use anticorrosive measures, such as painting, on the surface of steel materials (JIS G 3106: welded structural rolled steel) in the severe corrosive environment with a large amount of flying salts in coastal areas and the like. In addition, dm is a meaning of a decimeter.

However, coating has a deterioration in coating film over time, requires regular maintenance and repair, and in addition, labor cost rises and repainting There is a problem that recoating is difficult. For this reason, there is now a strong demand for developing steels which can be used for unpainting even in severe corrosive environments with high amounts of fly salt.

In response to such a situation, recently, steel materials containing a large amount of various alloying elements, especially Ni, have been developed as steel materials which can be used unpainted even in a strict corrosive environment such as coastal areas where a large amount of fly salt is present.

For example, Patent Document 1 discloses a high weather resistant steel material in which Cu and 1% or more of Ni are added as a weather resistance improving element. Moreover, in patent document 2, the steel material excellent in the weather resistance which added 1% or more of Ni and Mo is disclosed. Moreover, in patent document 3, the weld structural steel which is excellent in the weather resistance which contains a large amount of Ni and contains Mo, Sn, Sb, P, etc. further is disclosed. In addition, Patent Document 4 discloses a steel material having excellent corrosion resistance, wherein a rust layer having a fine crystallite size of β-FeOOH is formed by Ti addition.

Japanese Patent Publication No. 3785271 (Japanese Patent Application Laid-Open No. 11-172370) Japanese Patent Publication No. 3846218 (JP-A-2002-309340) Japanese Patent Application Laid-Open No. 10-251797 JP 2001-152374 A

 Joint Research Report on the Application of Weathering Steel to Bridges (XX), 1993.3, Institute of Construction Civil Engineering, Steel Club, Nippon Bridge Construction Association

However, as in patent documents 1 and 2, when the content of expensive Ni increases, there exists a problem that the price of steel materials will rise by the increase of alloy cost. Moreover, like patent document 3, expensive Ni content is increased and further, in steel materials containing Mo, Sn, Sb, P, etc., the price of steel materials rises by the increase of alloy cost. Moreover, although patent document 4 improves corrosion resistance by refinement | miniaturization of the rust structure containing (beta) -FeOOH by Ti addition, there exists a problem that it is difficult to exhibit sufficient corrosion resistance in a high salt environment.

An object of the present invention is to provide a steel material in which a rust layer excellent in corrosion resistance is formed by unpainting even in a strict corrosive environment having a large amount of fly salt in a coastal area or the like.

MEANS TO SOLVE THE PROBLEM As a result of earnestly examining corrosion resistance improvement in the severe corrosive environment with abundant salt content, the present inventors formed the rust layer containing the element of Nb and Sn of arbitrary appropriate content on the surface of a base steel material, and is excellent in corrosion resistance with no coating. Succeeded in developing steel with rust.

That is, the structure of the present invention is as follows.

[1] A steel material having a rust layer formed by forming a rust layer on the surface of the base steel, wherein the rust layer contains Nb and Sn, and the number of Nb atoms in the rust layer is 0.01 to the maximum of 100 Fe atoms. The steel material in which the rust layer which was excellent in corrosion resistance characterized by the above-mentioned, The number of Sn atoms in the said rust layer being 0.005 or more as the maximum with respect to 100 Fe atoms.

[2] The rust layer having excellent corrosion resistance according to the above [1], wherein the base steel contains, in mass%, Nb: 0.005% or more and 0.200% or less and Sn: 0.005% or more and 0.200% or less. Formed steels.

[3] The method of [1] or [2], wherein the base steel side portion of the rust layer contains β-oxyferric hydroxide (β-FeOOH) containing elements of Nb and Sn. Steel formed with a rust layer excellent in corrosion resistance.

Advantageous Effects of Invention According to the present invention, it is possible to provide a steel material having a rust layer which is low in cost and excellent in corrosion resistance without coating. The steel material in which the rust layer of the present invention is formed has an excellent weather resistance even in a strict corrosive environment in which the salt content is high at low cost without containing a large amount of expensive elements such as Ni, by effectively containing an appropriate amount of elements effective for improving corrosion resistance. Can be exercised. The present invention can exert a particularly remarkable effect in an environment of high aerobic salts where the amount of aerobic salts exceeds 0.05 mdd.

BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which shows the relationship between Sn atom number and Nb atom number with respect to Fe atom number 100 with respect to content of Nb and Sn in the rust layer of the steel material with a rust layer which concerns on this invention.

Next, embodiment of this invention is described in detail below.

The steel material in which the rust layer which concerns on this invention was formed is a steel material in which the rust layer formed by forming a rust layer on the surface of a base steel material, The number of atoms of Nb and Sn of the rust layer is the maximum with respect to the number of Fe atoms 100, respectively. 0.01 or more and 0.005 or more.

First, the steel material in which the rust layer excellent in corrosion resistance of this invention was formed is demonstrated. In addition, "%" which is a unit of content of the component shown below shall mean "mass%" unless there is particular notice.

Although it does not specifically limit as a base steel, For example, in mass%, the said base steel is hot rolled which has a component composition containing Nb: 0.005% or more and 0.200% or less and Sn: 0.005% or more and 0.200% or less. Steel materials, such as a steel plate, are mentioned.

The reason for limitation of the preferable component composition of a base steel is described below. Unless otherwise stated,% of a composition means the mass%.

Nb: 0.005% or more and 0.200% or less

Nb, as one of the most important components in the present invention, is concentrated in Sn and β-FeOOH, thereby having an effect of remarkably improving weather resistance of steel materials in a high salt environment. Enriched layers of Nb and Sn are different in position, and are intended to increase and enhance the site of inhibition of intrusion of Cl ⁻ ions. Moreover, it concentrates in the vicinity of the interface of a rust layer and a base steel material (ferrous iron) in an anode part, and suppresses an anode reaction and a cathode reaction. In order to fully acquire these effects, it is preferable to contain Nb content 0.005% or more. On the other hand, when it exceeds 0.200%, toughness will tend to fall. Therefore, it is preferable to make Nb content into 0.005% or more and 0.200% or less. Preferably, they are 0.010% or more and 0.030% or less.

Sn: 0.005% or more and 0.200% or less

Sn is one of the most important components in the present invention, and by being concentrated in Nb and β-FeOOH, Sn has an effect of remarkably improving the weather resistance of steel materials in a high salt environment. Moreover, the weather resistance of structural steel materials is improved by forming the oxide film containing Sn on the steel surface, and suppressing anode reaction and cathode reaction of steel materials. In order to fully acquire these effects, it is preferable to contain Sn content 0.005% or more. On the other hand, when Sn content exceeds 0.200%, there exists a tendency which causes deterioration of ductility and toughness of steel. Therefore, it is preferable to make Sn content into 0.005% or more and 0.200% or less. More preferably, they are 0.010% or more and 0.100% or less.

As mentioned above, although the basic component contained in the steel of this invention was demonstrated, it is C: 0.020% or more and less than 0.140% and Si: 0.05% or more and 2.00% or less by mass% as other arbitrary containing components, for example in this invention. , Mn: 0.20% or more, 2.00% or less, P: 0.005% or more and 0.030% or less, S: 0.0001% or more and 0.0200% or less, Al: 0.001% or more and 0.100% or less, Cu: 0.10% or more and 1.00% or less and Ni: 0.10% It may contain less than 0.65% or more.

If necessary, Mo: 0.001% or more, 1.000% or less, Cr: 0.2% or more and 1.0% or less, Co: 0.01% or more and 1.00% or less, REM: 0.0001% or more and 0.1000% or less, Sn: 0.005% or more and 0.200% or less, Ti: 0.005% or more and 0.200% or less, V: 0.005% or more and 0.200% or less, Zr: 0.005% or more and 0.200% or less, B: 0.0001% or more and 0.0050% or less, and Mg: 0.0001% or more and 0.0100% or less.

The reason for limitation of the preferable arbitrary containing component of a base steel is described below.

C: 0.020% or more and less than 0.140%

C is an element which improves the strength of the structural steel, and it is preferable to contain 0.020% or more in order to secure a predetermined strength. On the other hand, when C content is 0.140% or more, there exists a tendency for weldability and toughness to deteriorate. Therefore, it is preferable to make C content into 0.020% or more and less than 0.140%. Preferably it is 0.08% or more in terms of strength ensuring, and more preferably less than 0.10% in terms of weldability and toughness.

Si: 0.05% or more and 2.00% or less

Si is preferably contained at least 0.05% as a deoxidizer during steelmaking and as an element that improves the strength of the structural steel and secures a predetermined strength. On the other hand, when Si content exceeds 2.00% and contains excessively, there exists a tendency for toughness and weldability to deteriorate remarkably. Therefore, it is preferable to make Si content into 0.05% or more and 2.00% or less. Preferably, they are 0.10% or more and 0.80% or less.

Mn: 0.20% or more and 2.00% or less

Mn is an element which improves the strength of structural steel, and it is preferable to contain M0% or more in order to ensure predetermined strength. On the other hand, when Mn content exceeds 2.00% and contains excessively, there exists a tendency for toughness and weldability to deteriorate. Therefore, it is preferable to make Mn content into 0.20% or more and 2.00% or less. Preferably, they are 0.20% or more and 1.50% or less.

P: 0.005% or more and 0.030% or less

P is an element which improves the weather resistance of structural steel materials. In order to acquire such an effect, it is preferable to contain P content 0.005% or more. On the other hand, when P content exceeds 0.030%, there exists a tendency for weldability to deteriorate. Therefore, it is preferable to make P content into 0.005% or more and 0.030% or less. Preferably, they are 0.005% or more and 0.025% or less.

S: 0.0001% or more and 0.0200% or less

When S contains exceeding 0.0200%, there exists a tendency for weldability and toughness to deteriorate. On the other hand, reducing the S content to less than 0.0001% causes an increase in the production cost. Therefore, it is preferable to make S content into 0.0001% or more and 0.0200% or less. Preferably, they are 0.0003% or more and 0.0050% or less.

Al: 0.001% or more and 0.100% or less

Al is an element required for deoxidation during steelmaking. In order to acquire such an effect, it is preferable to contain 0.001% or more as Al content. On the other hand, when Al content exceeds 0.100%, there exists a tendency which adversely affects weldability. Therefore, it is preferable to make Al content into 0.001% or more and 0.100% or less. Preferably, they are 0.010% or more and 0.050% or less. In addition, Al content measured acid soluble Al.

Cu: 0.10% or more and 1.00% or less

Cu has the effect of forming a dense rust layer by miniaturizing rust particles and improving the weather resistance of the structural steel. Such an effect is obtained at Cu content of 0.10% or more. On the other hand, when Cu content exceeds 1.00%, it will only raise the cost accompanying Cu increase. Therefore, it is preferable to make Cu content into 0.10% or more and 1.00% or less. Preferably, they are 0.20% or more and 0.50% or less.

Ni: 0.10% or more and less than 0.65%

Ni has the effect of forming a dense rust layer by making rust particle | grains fine, and improving the weather resistance of structural steel materials. In order to fully acquire this effect, it is preferable to make Ni content into 0.10% or more. On the other hand, when Ni content is 0.65% or more, it will only raise the cost accompanying Ni increase. Therefore, it is preferable to make Ni content into 0.10% or more and less than 0.65%. Preferably, they are 0.15% or more and 0.50% or less.

Mo: 0.001% or more and 1.000% or less

Mo coexists with Nb, and there exists an effect which improves the weather resistance of steel materials in a high salt environment, and adds it as needed. Further, by forming molybdate ions in the rust layer, chloride ions of the corrosion promoting factor are prevented from penetrating the rust layer and reaching the ground iron. Moreover, MoO ₄ ^{2 -} elutes with the anode reaction of the steel material, and the compound containing Mo precipitates on the steel surface, thereby suppressing the anode reaction of the steel material. In order to fully acquire these effects, it is necessary to contain 0.001% or more. On the other hand, when it exceeds 1.000%, the cost will increase with increasing Mo consumption. Therefore, Mo amount is made into 0.001% or more and 1.000% or less of range. Preferably, they are 0.005% or more and 1.000% or less, More preferably, they are 0.10% or more and 0.70% or less.

Nb: 0.005% or more and 0.200% or less

By coexisting with Mo, Nb has the effect of improving the weather resistance of steel materials in a high salt environment, and is added as needed. Nb is concentrated in the rust layer near the steel surface, and has an effect which suppresses the anode reaction of steel materials. In order to fully acquire these effects, it is preferable to contain 0.005% or more. On the other hand, when it exceeds 0.200%, the toughness of steel will deteriorate. Therefore, Nb amount is taken as 0.005% or more and 0.200% or less of range. Preferably, they are 0.010% or more and 0.030% or less.

Furthermore, when improving a desired characteristic, 1 type, or 2 or more types of Cr, Co, REM, Sn can be contained as a selection element.

Cr: 0.2% or more and 1.0% or less

Cr is effective for forming a dense rust layer by miniaturizing rust particles and improving weather resistance. When it contains 0.2% or more, Cr exerts its effect, and when it exceeds 1.0%, weldability is reduced. Therefore, when it contains Cr, it is preferable to make the amount into 0.2% or more and 1.0% or less of range. More preferably, they are 0.2% or more and 0.7% or less.

Co: 0.01% or more and 1.00% or less

Co is distributed throughout the rust layer and is effective for forming a dense rust layer by miniaturizing the rust particles, improving the weather resistance of structural steel materials, and when it contains 0.01% or more, the effect is exhibited, and when it exceeds 1.00%, This leads to an increase in costs accompanied by an increase in Co consumption. Therefore, when it contains Co, it is preferable to make the amount into 0.01% or more and 1.00% or less of range. More preferably, they are 0.10% or more and 0.50% or less.

REM: 0.0001% or more and 0.1000% or less

REM is distributed throughout the rust layer, and it is effective for forming a dense rust layer by miniaturizing rust particles, and improving the weather resistance of structural steel materials. When it contains 0.0001% or more, the effect is exhibited, and when it exceeds 0.1000%, the effect is Is saturated. Therefore, when it contains REM, it is preferable to make the quantity into 0.0001% or more and 0.1000% or less of range. More preferably, they are 0.0010% or more and 0.0100% or less.

Sn: 0.005% or more and 0.200% or less

Sn is concentrated in the rust layer, and is effective for suppressing the anode reaction of steel materials, and when it contains 0.005% or more, the effect is exhibited. When it exceeds 0.200%, the toughness is deteriorated. Therefore, when it contains Sn, it is preferable to make the amount into 0.005% or more and 0.200% or less of range. More preferably, they are 0.010% or more and 0.100% or less.

In addition, in this invention, 1 type, or 2 or more types of Ti, V, Zr, B, Mg can be contained as a selection element.

Ti: 0.005% or more and 0.200% or less

Ti is an effective element for increasing the strength of the steel material, and when Ti is contained in an amount of 0.005% or more, the effect is exhibited. Therefore, when it contains Ti, it is preferable to make the amount into 0.005% or more and 0.200% or less of range. More preferably, they are 0.010% or more and 0.100% or less.

V: 0.005% or more and 0.200% or less

V is an effective element for increasing the strength, and when V is contained in 0.005% or more, the effect is exhibited, and when it exceeds 0.200%, the effect is saturated. Therefore, when it contains V, it is preferable to make the amount into 0.005% or more and 0.200% or less of range. More preferably, they are 0.010% or more and 0.100% or less.

Zr: 0.005% or more and 0.200% or less

Zr is an effective element for increasing strength, and when Zr is contained in 0.005% or more, the effect is exhibited, and when it exceeds 0.200%, the effect is saturated. Therefore, when it contains Zr, it is preferable to make the amount into 0.005% or more and 0.200% or less of range. More preferably, they are 0.010% or more and 0.100% or less.

B: 0.0001% or more and 0.0050% or less

B is an element necessary for increasing the strength, but if the amount is less than 0.0001%, the effect is not sufficiently obtained. On the other hand, when it exceeds 0.0050%, deterioration of toughness will be caused. Therefore, when it contains B, it is preferable to make the amount into 0.0001 to 0.0050% of range. More preferably, they are 0.0005% or more and 0.0040% or less.

Mg: 0.0001% or more and 0.0100% or less

Mg is an element effective in improving the toughness of the weld heat affected zone by fixing S in the steel. When Mg is contained in an amount of 0.0001% or more, Mg exerts its effect. When the content exceeds 0.0100%, Mg increases the amount of inclusions in the steel and deteriorates toughness. Cause. Therefore, when it contains Mg, it is preferable to make the amount into 0.0001% or more and 0.0100% or less of range. More preferably, they are 0.0005% or more and 0.0030% or less.

In addition, the balance is Fe and unavoidable impurities. As inevitable impurities, N: 0.010% or less, O: 0.010% or less, and Ca: 0.0010% or less can be allowed here. Particularly, when Ca contained as an unavoidable impurity is present in a large amount in steel, in addition to deteriorating the toughness of the weld heat affected zone, the formation of the rust layer described later is preferably 0.0010% or less.

The base steel according to the present invention is a steel plate or shaped steel by hot rolling a slab obtained by a conventional continuous casting or a pulverization method. It is manufactured and obtained from steel materials such as steel, steel sheet and bar steel. Heating and rolling conditions may be appropriately determined depending on the required material, and combinations of controlled rolling, accelerated cooling, or reheating may be used.

In this invention, in order to improve corrosion resistance in a high salt environment, it is necessary to form the rust layer aimed at the said base steel material.

The rust layer formed in the surface of the base steel material which has the said component composition is demonstrated.

In general, as the kind of components constituting the rust layer formed on the surface of the base steel, crystalline oxyferrihydrite such as α-FeOOH, β-FeOOH, and γ-FeOOH, Fe ₃ O ₄ , X There is X-ray noncrystalline material. In an environment of high salinity, unstable β-FeOOH containing a large amount of Fe ₃ O ₄ , β-FeOOH coexists, and contains chloride ions (Cl ⁻ ) and (hereinafter, referred to as Cl ⁻ ions) in the voids. As a starting point, corrosion proceeds. In addition, due to the influence of Cl ^- ions, crystallization of these rusts is promoted, and a rusted layer with many defects is reduced, so that the density is lowered, so that a large amount of Cl ^- ions penetrates deep into the rusted layer in the coastal region, etc. It becomes. Therefore, in order to increase the corrosion resistance in a harsh corrosion environment with a large amount of salt, how to suppress the progress of corrosion generated from β-FeOOH as a starting point, or the corrosion accelerating agent (Cr ^- ion) as a corrosion accelerating agent for the ferrous interface It becomes important to prevent intrusions.

The inventors of the present invention conducted a study for improving the corrosion resistance under an environment having a large amount of salt, and as a result, formation of a rust layer containing only one of Nb and Sn could not achieve improvement in corrosion resistance, and thus a rust layer containing an appropriate amount of Nb and Sn was obtained. By forming, it was found that the corrosion resistance was greatly improved. Here, the "nokcheung" is, α-FeOOH, β-FeOOH, γ-FeOOH of the crystalline oxy iron hydroxide, Fe ₃ O _4, composed by any one or a plurality of types of X shipment amorphous material.

Furthermore, the proper content of Nb and Sn was examined. Using a hot rolled steel sheet having a thickness of 6 mm as the base steel material having the above-described composition, a test piece having a size of 35 mm × 35 mm × 5 mm was taken from the hot rolled steel sheet, and an arithmetic mean roughness (arithmetic-average) was applied to the surface of the steel sheet. roughness) The grinding process was performed so that Ra might be 1.6 micrometers or less. Next, this test piece was left for 11 hours in a dry atmosphere having a temperature of 40 ° C. and a relative humidity of 40% RH, after which a transition time of 1 hour was taken, followed by a temperature of 25 ° C. 11 cycles in a wet atmosphere with a relative humidity of 95% RH, followed by a cycle of 24 hours taking 1 hour of transition time, 1 cycle per day, 1 cycle per day Repeat the year (365 days) and add an artificial seawater solution (an artificial seawater solution with an amount of salinity of 1.4 mg / dm 2) to 0.2 mdd of salt attached to the surface of the specimen. Once a week, rust formation was performed by apply | coating to the surface of a test piece during a drying process. At this time, the preferable form which promotes the formation of an effective rust layer with respect to corrosion resistance is 0.1 mdd or more of adhesion salt content. Further, when the amount of attached salt is more than 0.05 mdd, the rust layer of the present invention is formed. However, it takes time to form the rust layer of this invention in the case of the atmosphere with low adhesion salt amount, ie, when the adhesion salt content is 0.05 mdd or less.

On the other hand, it is necessary to form a rust layer at 0.30 mdd or less of adhesion salt amount. At more than 0.30 mdd, a large amount of Cl penetrates into the rust layer, so that dense rust having a high concentration of Nb and Sn is hardly formed in the rust layer. As a result, the rust layer of this invention will not be formed and it will become steel materials which have a rust layer with bad weather resistance.

The test piece which formed this rust layer by the above was further subjected to the corrosion test in the high salt environment of 0.2 mdd for 6 months after that. After the end of the corrosion test, the test piece was immersed in an aqueous solution in which hexamethylenetetramine was added to hydrochloric acid, derusted, and then weighed. The difference between the initial weight of the test piece before rust formation and the weight of the test piece after derusting was measured. The average corrosion rate (micrometer / year) per single side was calculated | required. When this average corrosion rate was 60 micrometers / year or less, it evaluated as having excellent unpainted corrosion resistance compared with the conventional weather resistant steel.

Next, the relationship between the average corrosion rate and content of Nb and Sn in a rust layer was investigated. Although the content (number of atoms with respect to 100 Fe atoms) of Nb and Sn in a green layer can be calculated | required by various methods, the method calculated | required by the electron probe microanalyzer (EPMA) as an example is shown below.

First, the cross section sample of the steel material in which the rust layer was formed is manufactured. After shearing the test piece of the steel material on which the rust layer was formed, it was embedded in a resin (25 mm in diameter), and then polished to 4000 finish was performed by using ethanol (not using water). The measurement conditions of the electron beam microanalyzer (EPMA) are accelerating voltage 15 kV, radiation current 2 x 10 ^-7 A, beam diameter 2 m, scanning area 1.5 mm X 0.5 mm.

Here, the calculation method of content is demonstrated using the case of Nb.

Subjected to hold the steel (base material) measured for five points an arbitrary point on, and obtains the average of the Fe, Nb, each of the X-ray intensity, and a I _FeStnd, I _NbStnd respectively, a reference value that obtained by dividing the I _NbStnd as I _FeStnd It was set as. In addition, it is measured for 100,000 points at any place in the _{rust layer} , and the average of X-ray intensity of Fe and Nb is calculated for the upper 30 points from the higher X-ray intensity of Nb, and I _FeAve , in that I _NbAve and divided by I to I _{NbAve FeAve} was concentrated to index (enrichment index). The concentration of Nb in the rust layer (Nb atoms relative to 100 Fe atoms) was calculated by multiplying the obtained concentration index by a reference value and multiplying the number of Nb atoms to 100 atoms of Fe in the base steel (base metal). By this procedure, content of Sn can also be calculated | required similarly. The number of atoms of Nb and Sn with respect to the number of Fe atoms in the base steel (base material) is 100. The addition ratio of each component to Fe in the production of the base steel (base material), or the wet analysis of the base steel (base material) ) (Priorly known). Moreover, in calculating | requiring the X-ray intensity of each element, you may appropriately perform correction of a background (back ground).

The result obtained by the above is shown in FIG. By the said corrosion evaluation method, the thing excellent in corrosion resistance was described in the figure as "(circle)" and the inferior thing as "x".

From the result of FIG. 1, it turns out that corrosion resistance is excellent when the number of atoms of Nb and Sn in a rust layer is Nb: 0.01 or more and Sn: 0.005 or more at the maximum with respect to the number 100 of Fe atoms.

Therefore, in this invention, a rust layer contains Nb and Sn, and the number of Nb atoms in the said rust layer is 0.01 or more as the maximum with respect to the number of Fe atoms, and the number of Sn atoms in the said rust layer is Fe It is made into 0.005 or more the maximum with respect to the number of atoms 100.

In addition, if the number of Nb and Sn atoms in the rust layer is set to a maximum value of 0.5 with respect to the number of Fe atoms, the cost is increased in addition to problems such as ductility and toughness of the base steel (base metal). In particular, if Nb is to be increased, the amount of niobium (amont of solute niobium) is reduced by increasing the amount of precipitated niobium carbide (NbC), which affects the corrosion resistance. Therefore, the number of upper limit atoms of Nb and Sn is preferably 0.5 or less as a maximum with respect to the number of Fe atoms 100.

On the other hand, if the number of Nb and Sn atoms in the rust layer is set to be greater than 0.5, respectively, with the maximum value relative to the number of Fe atoms 100, it is necessary to increase Nb or Sn in the component of the base steel (base metal). This causes cost increases in addition to problems such as deterioration of ductility and toughness. In particular, when Nb in the base steel (base material) is increased, the amount of dissolved niobium (amont of solute niobium) is reduced by the increase in the deposited niobium carbide (NbC), which adversely affects the corrosion resistance. Therefore, the number of upper limit atoms of Nb and Sn in the rust layer is preferably 0.5 or less as the maximum with respect to the number of Fe atoms 100.

In addition, a rust test piece manufactured by focused ion beam processing (FIB) from any point of the rust layer portion of less than 100 µm from the base steel is used by a transmission electron microscope (TEM). In addition, the identification of the green layer was performed at the positions of concentrated Nb and Sn by an electron diffraction pattern. As a result, the components of Nb and Sn are all located in β-iron hydroxide (β-FeOOH), and the base steel side portion of the rust layer contains β-oxyhydroxide (β-FeOOH) containing elements of Nb and Sn. It was found to contain. It is estimated that this promotes rust stabilization and improves corrosion resistance even under high salt content.

Therefore, it is preferable that the base steel-side part of a rust layer contains (beta) -FeOOH) which contains the elements of Nb and Sn.

In addition, in addition to the electron diffraction pattern using TEM, raman spectroscopy etc. are mentioned for identification of the rust of the position which Nb and Sn exist, Especially, in identification of the rust layer in high position resolution, Diffraction pattern analysis is recommended.

In addition, by line analysis of the depth direction of the electron beam microanalyzer (EPMA), Nb and Sn of the official pit and rust area where erosion progression is considered to be a significant part of the rust layer. The number (concentration) of the concentrated layer in which the element was concentrated was also investigated. Here, the concentrated layer means Nb and Sn with respect to the number of Fe atoms 100 based on the number of atoms of Nb and Sn based on the number of atoms of Nb and Sn in the base steel (base metal) in the content of Nb and Sn in the official rusted portion. The number of atoms of: is higher than 0.03 and 0.02, which means that the thickness (peak half-value width on the line profile) of the thickened layer is 15 µm or less. And the average number of concentrated layers was calculated | required and it was set as the number (number) of concentrated layers. The conditions for measuring the number of concentrated layers are acceleration voltage: 15 kV, irradiation current: 2x10 <^-7> A, beam diameter: 2 micrometers. The average number of the concentrated layers of Nb and Sn of the official rust part was computed from the result of the line analysis about the official rust part of 5 points chosen arbitrarily.

As an example, C: 0.091%, Si: 0.20%, Mn: 0.70%, P: 0.019%, S: 0.0034%, sol.Al: 0.031%, N: 0.0032%, O: 0.0026%, Cu: 0.30%, On the surface of the base steel containing Ni: 0.21%, Nb: 0.052%, and Sn: 0.052%, a line is formed on the official rust of the rust layer of the steel material (base steel material) in which a rust layer is formed by the above-described rust forming method. Analysis was performed. As a result, the content of Nb in the rust layer was 0.111, the content of Sn was 0.060 with respect to the number of Fe atoms 100, and as a result of the above-mentioned corrosion test, the average corrosion rate was 54.6 µm / year, and it was confirmed that the corrosion resistance was excellent. . The presence points of the concentrated layer of Nb and Sn in this official green part were 8.1 pieces and 4.2 pieces, respectively. It can be seen that as the concentration point of the thickened layer increases, the corrosion resistance can be improved by the diffusion-inhibiting action of the corrosion promoting substance on the steel.

In view of the above results, in the present invention, in addition to the definition of the number of Nb atoms and the number of Sn atoms in the rust layer, from the viewpoint of improving the corrosion resistance by inhibiting the penetration of the corrosion promoting substance into the steel, Nb is 5 or more, Sn It is preferable that these are three or more. In addition, since it is possible to increase and enhance the diffusion-inhibiting sites of corrosion factors by overlapping the thickening layers of Nb and Sn, it is more preferable that the peak half widths overlap each other. )

Example 1

No.1-14 which has the component composition shown in Table 1 was heated, it heated at 1150 degreeC, hot-rolled, air-cooled to room temperature, and started the hot rolled steel plate of thickness 6mm. Next, the test piece of size: 35 mm x 35 mm x 5 mm was extract | collected from the obtained steel plate. The test piece was ground to a surface so that the arithmetic mean roughness Ra was 1.6 µm or less, the tape was sealed on the end face and the back face, and the area of the surface exposed area of the test piece was 25 mm × 25. The surface was also sealed with tape so that it might be mm.

The test piece obtained by the above was left to stand for 11 hours in a drying atmosphere of temperature 40 ° C and a relative humidity of 40% RH, after which the transition time of 1 hour was taken, and then the temperature was 25 ° C and a humidity of 95% RH. After leaving for 11 hours in the atmosphere and then repeating one cycle per day for one year (365 days) with a cycle of 24 hours in total, taking one hour of transition time, it is adhered to the surface of the test piece. An artificial seawater solution in an amount of 0.2 mdd, which is desirable for forming a rust layer effective for corrosion resistance, once a week in an amount of artificial seawater solution (1.4 mg / dm &lt; 2 &gt; Rust formation was performed by apply | coating to the surface. The test piece which performed this rust formation was further subjected to the corrosion test in the high salt environment of 0.2 mdd of adhesion salt amount for further 6 months after that. After the completion of the corrosion test, the test piece was immersed in an aqueous solution in which hexamethylenetetramine was added to hydrochloric acid, and then demelted, and the weight thereof was measured. The velocity (µm / year) was obtained. When this average corrosion rate was 60 micrometers / year or less, it evaluated as being excellent in corrosion resistance.

Moreover, about each test piece, about the official rust part of 5 points chosen arbitrarily, the progress of corrosion is considered to be a remarkable part in a rust layer by the line analysis about the depth direction of the electron beam microanalyzer (EPMA) mentioned above. The number (number) of enriched layers in which the Nb and Sn elements of the official green areas were concentrated was also investigated. When there was no concentrated layer of Nb or Sn, it indicated in Table 2 by "x".

Table 2 shows the results of the presence or absence of Nb and Sn in the rust layer, the content of the Nb and Sn components (the number of Nb and Sn atoms relative to the number of Fe atoms), the average number of concentrated layers, and the average corrosion rate. . In addition, these measuring methods are the same as the above-mentioned method.

From the results of Table 2, the average corrosion rates of the invention examples 1 to 6 in which the number of atoms of Nb and Sn in the rust layer are in the range of Nb: 0.01 or more and Sn: 0.005 or more at maximum with respect to the number of Fe atoms 100 are all 57.2. Even under a 占 퐉 / year, even in a strict corrosive environment in which high salts are present, the corrosion resistance is superior to conventional weather resistant steels. In addition, as for the average number of the concentrated layers in Inventive Examples 1-6, Nb is five or more, and Sn is three or more.

On the other hand, Comparative Examples 1-7 whose content of Nb and Sn components are outside the range of this invention in the rust layer were all inferior to corrosion resistance in the severe corrosion environment in which high salt exists because the average corrosion rate is more than 65 micrometers / year.

Moreover, although the corrosion resistance was the same level as invention examples 1-6, the reference example 1 which contained expensive Ni in 1.53% in large quantities in steel materials, the product cost became about 30% or more higher than invention examples 1-6.

Example 2

The test piece of the steel plate which has the component composition shown in Table 1 was manufactured by the method similar to Example 1. The test piece was ground to the surface so that arithmetic mean roughness Ra might be 1.6 micrometers or less, and tape-sealed the cross section and the back surface, and the surface was also tape-sealed so that the area of the surface exposed part of a test piece might be set to 25 mm x 25 mm.

The test piece obtained by the above was left to stand for 11 hours in a drying atmosphere of temperature 40 ° C and a relative humidity of 40% RH, after which the transition time of 1 hour was taken, and then the temperature was 25 ° C and a humidity of 95% RH. After leaving for 11 hours in the atmosphere and then repeating one cycle per day for one year (365 days) with a cycle of 24 hours in total, taking one hour of transition time, it is adhered to the surface of the test piece. Rust formation was performed by apply | coating to the surface of a test piece once a week the artificial seawater solution of the amount which adheres salt quantity becomes 0.10-0.40 mdd in a drying process. In the application of the artificial seawater solution, for example, in the case of 0.3 mdd, an artificial seawater solution in an amount of 2.1 mg / dm 2 of salt attached to the test piece surface was applied to the test piece surface. The test piece which performed this rust formation was further subjected to the corrosion test in the high salt environment of 0.2 mdd of adhesion salt amount for further 6 months after that. After the completion of the corrosion test, the average corrosion rate (µm / year) was determined in the same manner as in Example 1. When this average corrosion rate was 60 micrometers / year or less, it evaluated as being excellent in corrosion resistance.

Moreover, about each test piece, the number (number) of the concentrated layers in which Nb and Sn elements were concentrated by the method similar to Example 1 was also investigated. When there was no concentrated layer of Nb or Sn, it indicated in Table 3 by "x".

Table 3 shows the results of the presence or absence of Nb and Sn in the rust layer, the content of Nb and Sn components (number of atoms of Nb and Sn to 100 atoms of Fe), the average number of concentrated layers, and the average corrosion rate. In addition, these measuring methods are the same as the above-mentioned method.

From the results in Table 3, the average corrosion rates of the inventions 7 to 15 in the range of Nb: 0.01 or more and Sn: 0.005 or more are the maximum values of the number of atoms of Nb and Sn in the rust layer with respect to the number of Fe atoms 100, 57.3 µm / Less than a year, even in the severe corrosive environment in which high salt exists, it is excellent in corrosion resistance compared with the conventional weather-resistant steel. Moreover, as for the average number of the concentrated layers in invention examples 7-15, Nb is five or more and Sn or more is three or more.

On the other hand, the comparative examples 8-17 whose content of Nb and Sn components outside the range of this invention in the rust layer were all inferior to corrosion resistance in the severe corrosion environment in which high salt exists, with the average corrosion rate more than 63.9 micrometers / year.

Industrial availability

According to the present invention, it is possible to provide a steel material having a rust layer which is low in cost and excellent in corrosion resistance without coating. The steel material of the present invention can exhibit excellent weather resistance even in a strict corrosive environment having a large amount of fly salt at low cost without containing a large amount of expensive elements such as Ni by effectively and effectively containing an element effective for improving corrosion resistance. . The present invention can exert a particularly remarkable effect in an environment of high aerobic salts where the amount of aerobic salts exceeds 0.05 mdd.

[Table 1]

[Table 2]

[Table 3]

Claims (3)

In a steel material having a rust layer formed by forming a rust layer on the surface of the base steel,
The rust layer contains Nb and Sn, and the number of Nb atoms in the rust layer is 0.01 or more with a maximum value of 100 Fe atoms, and the number of Sn atoms in the rust layer is relative to the number of Fe atoms 100. Steel with a rust layer with a maximum value of 0.005 or more. The method of claim 1,
The said base steel is the steel material with a rust layer which contains the component of Nb: 0.005% or more and 0.200% or less and Sn: 0.005% or more and 0.200% or less by mass%. 3. The method according to claim 1 or 2,
The steel material side part of the said rust layer is a steel material in which the rust layer containing (beta) -FeOOH) containing the element of Nb and Sn was formed.

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