WO2012133908A1

WO2012133908A1 - Steel material with rust layer, exhibiting excellent weather resistance even in high-salt environments

Info

Publication number: WO2012133908A1
Application number: PCT/JP2012/059123
Authority: WO
Inventors: 和彦馬場; 正泰名越; 勇鹿毛; 進一三浦
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2011-03-29
Filing date: 2012-03-28
Publication date: 2012-10-04
Also published as: CN103459670B; JP2013147729A; JP5891892B2; KR20130126729A; CN103459670A; KR101609430B1

Abstract

Provided is a steel material with a rust layer, said steel material being highly resistant to corrosion even in a paintless state and even in a severe corrosive environment where a large amount of air-borne salt is present, particularly, in a seaboard region. Specifically, this steel material with a rust layer can be obtained 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 in terms of the maximum is 0.01 or more relative to 100 Fe atoms, while the number of Sn atoms therein in terms of the maximum is 0.005 or more relative to 100 Fe atoms.

Description

Steel with rust layer with excellent weather resistance in high salinity environment

The present invention relates to a steel material mainly used for steel structures (Steel Structures) used outdoors such as a bridge, and particularly, along a coastal area or in the vicinity of a coastal environment (hereinafter simply referred to as a “coastal area”). As in the case of harsh corrosive environments where high salinity is present, the rust layer is excellent in paint resistance and corrosion resistance (also referred to as bare corrosion resistance). It relates to the steel material.

Conventionally, in steel structures used outdoors such as bridges, weathering steel has been used. The weathering steel has a corrosion rate (rust layer) that is covered with a highly protective rust layer enriched with alloy elements such as Cu, P, Cr, Ni, etc. in an atmospheric environment (atmospheric environment). It is a steel material with a significantly reduced corrosion rate. Due to its excellent weather resistance, it is known that bridges made of weathering steel usually withstand use for decades without being painted. However, in a severe corrosive environment where the amount of airborne salt is high, such as in coastal areas, the above highly protective rust layer is unlikely to form on the steel surface and has practical weather resistance. There is a problem that it is difficult to obtain. In addition, the amount of incoming salt in this application means the average amount of incoming salt for one year, and the amount of incoming salt every month for one year according to “Measurement of chloride by dry gauze method” described in JIS Z2382 (1998). The salinity is measured and averaged to calculate the annual average incoming salinity.

According to Non-Patent Document 1, conventional weathering steel (JIS G 3114: weathering hot rolled steel for welded structure) has an incoming salt content of 0.05 mg · NaCl / dm ² / day (hereinafter, unit display “ “mg · NaCl / dm ² / day” may be simply abbreviated as “mdd”.) Only in the following areas, it can be used without painting.

Therefore, in severe corrosive environments with a large amount of salt in the coastal areas, etc., it is common to use anti-corrosion measures such as painting on the surface of ordinary steel (JIS G 3106: rolled steel for welded structures). . Note that dm means a decimator.

However, in coating, the coating film deteriorates over time, and periodic maintenance and repairs are required. In addition, labor costs increase and labor costs (labor costs) increase. There is a problem that it is difficult to recoat. For these reasons, it has been strongly demanded to develop a steel material that can be used without painting even in a severe corrosive environment with a large amount of incoming salt.

In contrast to the current situation, in recent years, steel materials containing a large amount of various alloy elements, particularly Ni, have been used as steel materials that can be used without coating even in severe corrosive environments, such as coastal areas where there is a large amount of incoming salt. Has been developed.

For example, Patent Document 1 discloses a highly weather-resistant steel material to which Cu and 1% or more of Ni are added as a weather resistance improving element. Patent Document 2 discloses a steel material excellent in weather resistance to which 1% or more of Ni and Mo are added. Furthermore, Patent Document 3 discloses a steel for welded structures that contains a large amount of Ni and, in addition, contains Mo, Sn, Sb, P, etc. and has excellent weather resistance. In addition, Patent Document 4 discloses a steel material with excellent corrosion resistance, characterized in that a rust layer having a fine crystallite size of β-FeOOH is formed by addition of Ti.

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

However, as in Patent Documents 1 and 2, when the content of expensive Ni is increased, there is a problem that the price of the steel material increases due to an increase in alloy cost. Further, as in Patent Document 3, in steel materials that increase the expensive Ni content and additionally contain Mo, Sn, Sb, P, etc., the price of the steel materials increases due to an increase in alloy costs. Furthermore, Patent Document 4 improves the corrosion resistance by refining the rust structure containing β-FeOOH by adding Ti, but has a problem that it is difficult to exhibit sufficient corrosion resistance in a higher salt environment.

An object of the present invention is to provide a steel material with a rust layer that is unpainted and excellent in corrosion resistance even in a severe corrosive environment with a large amount of salinity, particularly in coastal areas.

As a result of intensive studies on improving corrosion resistance in a severe corrosive environment with a large amount of salinity, the present inventors have formed a rust layer containing Nb and Sn elements with a certain appropriate content on the surface of the base steel material. We succeeded in developing a steel material with a rust layer that is unpainted and has excellent corrosion resistance.

That is, the gist of the present invention is as follows.
[1] In the steel material with a rust layer formed by forming a rust layer on the surface of the base steel material, the rust layer contains Nb and Sn, and the number of Nb atoms in the rust layer is 100 Fe atoms A steel material with a rust layer excellent in corrosion resistance, wherein the maximum value is 0.01 or more, and the number of Sn atoms in the rust layer is 0.005 or more with respect to 100 Fe atoms. .

[2] In the above [1], the base steel material contains, by mass%, Nb: 0.005% to 0.200% and Sn: 0.005% to 0.200%. A steel material with a rust layer with excellent corrosion resistance.

[3] In the above [1] or [2], the base steel material side portion of the rust layer contains β-iron oxyhydroxide (β-FeOOH) containing elements of Nb and Sn. A steel material with a rust layer with excellent corrosion resistance.

According to the present invention, it is possible to provide a steel material with a rust layer that is low-cost, non-painted, and excellent in corrosion resistance. The steel material with a rust layer of the present invention contains an element effective for improving the corrosion resistance in an appropriate amount and effectively, and is low in cost without containing a large amount of expensive elements such as Ni and has a large amount of incoming salt, severe corrosion Excellent weather resistance can be exhibited even in an environment. The present invention can exert a particularly remarkable effect in a high flying salt environment where the flying salt content exceeds 0.05 mdd.

It is the figure which showed the relationship between the number of Sn atoms with respect to the number of Fe atoms of 100, and the number of Nb atoms about content of Nb and Sn in the rust layer of the steel material with a rust layer according to this invention.

Next, embodiments of the present invention will be described in detail below.
The steel material with a rust layer according to the present invention is a steel material with a rust layer formed by forming a rust layer on the surface of a base steel material, and the number of Nb and Sn atoms in the rust layer is the maximum with respect to the number of Fe atoms of 100. The values are 0.01 or more and 0.005 or more, respectively.

First, the steel material with a rust layer having excellent corrosion resistance according to the present invention will be described. Note that “%”, which is a unit of content of components shown below, means “% by mass” unless otherwise specified.

Although it does not specifically limit as a base steel material, For example, it is the mass%, the said base steel material is the mass%, Nb: 0.005% or more and 0.200% or less and Sn: 0.005% or more and 0.200% Examples of the steel material include a hot rolled steel sheet having a component composition containing the following components.

The reasons for limiting suitable compositional components of the base steel are described below. Unless otherwise specified,% of composition means mass%.

Nb: 0.005% or more and 0.200% or less Nb is one of the most important components in the present invention. By concentrating to Sn and β-FeOOH, the weather resistance of the steel material in a high salinity environment. Has the effect of significantly improving the performance. The Nb and Sn enriched layers have different positions, and are intended to increase and strengthen the Cl ^- ion intrusion suppression sites. Further, the anode portion is concentrated near the interface between the rust layer and the base steel material (base iron) to suppress the anode reaction and the cathode reaction. In order to obtain these effects sufficiently, the Nb content is preferably 0.005% or more. On the other hand, if it exceeds 0.200%, the toughness tends to decrease. Therefore, the Nb content is preferably 0.005% or more and 0.200% or less. Preferably, it is 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. By concentrating to Nb and β-FeOOH, the weather resistance of the steel material in a high salinity environment is improved. It has the effect of significantly improving. Further, an oxidation film containing Sn is formed on the surface of the steel material, and the rust resistance of the structural steel material is improved by suppressing the anode reaction and cathode reaction of the steel material. In order to sufficiently obtain these effects, the Sn content is preferably 0.005% or more. On the other hand, if the Sn content exceeds 0.200%, the ductility and toughness of the steel tend to be deteriorated. Therefore, the Sn content is preferably 0.005% or more and 0.200% or less. More preferably, it is 0.010% or more and 0.100% or less.

As described above, the basic components contained in the steel of the present invention have been described. In the present invention, as other optional components, for example, by mass, C: 0.020% or more and less than 0.140%, Si: 0.00. 05% to 2.00%, Mn: 0.20% to 2.00%, P: 0.005% to 0.030%, S: 0.0001% to 0.0200%, Al: 0.001% or more and 0.100% or less, Cu: 0.10% or more and 1.00% or less, and Ni: 0.10% or more and less than 0.65% can be contained.
Further, if necessary, Mo: 0.001% to 1.000%, Cr: 0.2% to 1.0%, Co: 0.01% to 1.00%, REM: 0.0001 %: 0.15% 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 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.

Below, the reason for limitation of the suitable arbitrary content component of a base steel material is described.
C: 0.020% or more and less than 0.140% C is an element that improves the strength of the structural steel material, and is preferably contained in an amount of 0.020% or more in order to ensure a predetermined strength. On the other hand, when the C content is 0.140% or more, weldability and toughness tend to deteriorate. Therefore, the C content is preferably 0.020% or more and less than 0.140%. Preferably, it is 0.08% or more from the viewpoint of securing strength, more preferably less than 0.10% from the viewpoint of weldability and toughness.

Si 0.05% or more and 2.00% or less Si may be contained in an amount of 0.05% or more as a deoxidizer during steelmaking and as an element for improving the strength of structural steel materials and ensuring a predetermined strength. preferable. On the other hand, when the Si content exceeds 2.00% and is contained excessively, the toughness and weldability tend to deteriorate significantly. Therefore, the Si content is preferably 0.05% or more and 2.00% or less. Preferably, it is 0.10% or more and 0.80% or less.

Mn: 0.20% or more and 2.00% or less Mn is an element that improves the strength of the structural steel material, and is preferably contained in an amount of 0.20% or more in order to ensure a predetermined strength. On the other hand, when Mn content exceeds 2.00% and it contains excessively, there exists a tendency for toughness and weldability to deteriorate. Therefore, the Mn content is preferably 0.20% or more and 2.00% or less. Preferably, it is 0.20% or more and 1.50% or less.

P: 0.005% or more and 0.030% or less P is an element that improves the weather resistance of the structural steel material. In order to obtain such an effect, the P content is preferably 0.005% or more. On the other hand, if the P content exceeds 0.030%, the weldability tends to deteriorate. Therefore, the P content is preferably 0.005% or more and 0.030% or less. Preferably, it is 0.005% or more and 0.025% or less.

S: 0.0001% or more and 0.0200% or less When S exceeds 0.0200%, weldability and toughness tend to deteriorate. On the other hand, reducing the S content to less than 0.0001% leads to an increase in production cost. Therefore, the S content is preferably 0.0001% or more and 0.0200% or less. Preferably, it is 0.0003% or more and 0.0050% or less.

Al: 0.001% or more and 0.100% or less Al is an element necessary 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, if the Al content exceeds 0.100%, the weldability tends to be adversely affected. Therefore, the Al content is preferably 0.001% or more and 0.100% or less. Preferably, it is 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 an effect of forming a dense rust layer by refining rust grains and improving the weather resistance of the structural steel material. Such an effect is obtained when the Cu content is 0.10% or more. On the other hand, if the Cu content exceeds 1.00%, only an increase in cost due to an increase in Cu consumption is caused. Therefore, the Cu content is preferably 0.10% or more and 1.00% or less. Preferably, it is 0.20% or more and 0.50% or less.

Ni: 0.10% or more and less than 0.65% Ni has an effect of forming a dense rust layer by refining rust grains and improving the weather resistance of the structural steel material. In order to sufficiently obtain this effect, the Ni content is preferably set to 0.10% or more. On the other hand, if the Ni content is 0.65% or more, only an increase in cost associated with an increase in Ni consumption is caused. Therefore, the Ni content is preferably 0.10% or more and less than 0.65%. Preferably, it is 0.15% or more and 0.50 or less.

Mo: 0.001% or more and 1.000% or less Mo coexists with Nb, thereby improving the weather resistance of the steel material in a high salinity environment, and is added as necessary. Moreover, the formation of molybdate ions in the rust layer prevents chloride ions, which are corrosion-promoting factors, from passing through the rust layer and reaching the base iron. Further, MoO ₄ ²⁻ is eluted with the anode reaction of the steel material, and the compound containing Mo is precipitated on the steel material surface, thereby suppressing the anode reaction of the steel material. In order to obtain these effects sufficiently, it is necessary to contain 0.001% or more. On the other hand, if it exceeds 1.000%, the cost will increase with the increase in Mo consumption. Therefore, the Mo amount is set to a range of 0.001% to 1.000%. 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 Nb has the effect of improving the weather resistance of a steel material in a high salinity environment by coexisting with Mo, and is added as necessary. Nb has the effect of concentrating in the rust layer near the steel surface and suppressing the anode reaction of the steel. In order to obtain these effects sufficiently, it is preferable to contain 0.005% or more. On the other hand, if it exceeds 0.200%, the toughness of the steel is deteriorated. Therefore, the Nb content is in the range of 0.005% to 0.200%. Preferably, it is 0.010% or more and 0.030% or less.

In order to further improve desired characteristics, one or more of Cr, Co, REM, and Sn can be included as a selective element.

Cr: 0.2% or more and 1.0% or less Cr is effective for forming a dense rust layer by refining rust grains and improving weather resistance. When the effect is exhibited and the content exceeds 1.0%, the weldability is deteriorated. Therefore, when it contains Cr, it is preferable to make the quantity into the range of 0.2% or more and 1.0% or less. More preferably, it is 0.2% or more and 0.7% or less.

Co: 0.01% or more and 1.00% or less Co is distributed over the entire rust layer, and is effective in improving the weather resistance of structural steel by forming a fine rust layer by refining rust grains. If it is contained in an amount of 0.01% or more, the effect is exhibited, and if it is contained in excess of 1.00%, the cost is increased due to an increase in Co consumption. Therefore, when it contains Co, it is preferable to make the quantity into the range of 0.01% or more and 1.00% or less. More preferably, it is 0.10% or more and 0.50% or less.

REM: 0.0001% or more and 0.1000% or less REM is distributed over the entire rust layer and is effective in improving the weather resistance of structural steel by forming a dense rust layer by refining rust grains. Yes, when the content is 0.0001% or more, the effect is exhibited, and when the content exceeds 0.1000%, the effect is saturated. Therefore, when it contains REM, it is preferable to make the quantity into the range of 0.0001% or more and 0.1000% or less. More preferably, it is 0.0010% or more and 0.0100% or less.

Sn: 0.005% or more and 0.200% or less Sn concentrates in the rust lower layer and is effective in suppressing the anode reaction of the steel material, and when 0.005% or more is contained, the effect is exhibited, 0.200% Exceeding this causes deterioration of toughness. Therefore, when it contains Sn, it is preferable to make the quantity into 0.005% or more and 0.200% or less of range. More preferably, it is 0.010% or more and 0.100% or less.

Furthermore, in the present invention, one or more of Ti, V, Zr, B, and Mg can be included as a selective 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 0.005% or more is contained, the effect is exhibited. It causes deterioration. Therefore, when Ti is contained, the amount is preferably in the range of 0.005% or more and 0.200% or less. More preferably, it is 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 0.005% or more is contained, the effect is exhibited, and when it exceeds 0.200%, the effect is saturated. . Therefore, when V is contained, the amount is preferably in the range of 0.005% to 0.200%. More preferably, it is 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 the strength, and when 0.005% or more is contained, the effect is exhibited, and when it exceeds 0.200%, the effect is saturated. . Therefore, when Zr is contained, the amount is preferably in the range of 0.005% to 0.200%. More preferably, it is 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 cannot be sufficiently obtained. On the other hand, if it exceeds 0.0050%, the toughness is deteriorated. Therefore, when it contains B, it is preferable to make the quantity into 0.0001 to 0.0050% of range. More preferably, it is 0.0005% or more and 0.0040% or less.

Mg: 0.0001% or more and 0.0100% or less Mg is an element effective for fixing the S in the steel and improving the toughness of the weld heat affected zone, and exhibits the effect of containing 0.0001 or more. If it exceeds 0.0100%, the amount of inclusions in the steel increases, but the toughness deteriorates. Therefore, when it contains Mg, it is preferable to make the quantity into 0.0001% or more and 0.0100% or less of range. More preferably, it is 0.0005% or more and 0.0030% or less.

Note that the balance is Fe and inevitable impurities. Here, N: 0.010% or less, 0: 0.010% or less, and Ca: 0.0010% or less are acceptable as inevitable impurities. In particular, Ca contained as an unavoidable impurity, when present in a large amount in steel, deteriorates the toughness of the weld heat-affected zone and affects the formation of a rust layer, which will be described later. It is preferable.

The base steel material according to the present invention is a steel plate obtained by subjecting a steel having the above composition to hot rolling a slab obtained by ordinary continuous casting or slabbing. ), Shaped steel, steel sheet, bar steel, and the like. The heating and rolling conditions may be appropriately determined according to the required material, and a combination of heat treatment such as controlled rolling, accelerated cooling, or reheating is also possible.

In the present invention, in order to improve the corrosion resistance in a high salinity environment, it is necessary to form a rust layer that is optimized in the base steel material.

The rust layer on the surface of the base steel having the above composition will be described.

As a kind of component which generally comprises the rust layer formed in the base steel material surface, for example, crystalline iron oxyhydroxide such as α-FeOOH, β-FeOOH, γ-FeOOH, Fe ₃ O ₄ , X There is an X-ray non-crystalline material. In a high salinity environment, Fe ₃ O ₄ is abundant and β-FeOOH coexists, and the voids contain chloride ions (Cl ⁻ ) (hereinafter referred to as Cl ⁻ ions). Corrosion proceeds starting from stable β-FeOOH. In addition, since the crystallization of these rusts is promoted by the influence of Cl ⁻ ions and the rust layer with many defects is formed and the denseness is lowered, a large amount of Cl ⁻ ions penetrate into the rust layer deeply in the coastal area. , It will be concentrated at the iron and steel interface. Therefore, in order to enhance the corrosion resistance in a severe corrosive environment with a large amount of salt, how to suppress the progress of corrosion generated from β-FeOOH, or a corrosion promoting agent for the iron-iron interface (corrosion accelerating agent). It is important to prevent intrusion of Cl ⁻ ions.

The inventors of the present invention have studied to improve the corrosion resistance under an environment having a large amount of salt. As a result, the formation of a rust layer containing only one of Nb and Sn cannot improve the corrosion resistance. It has been found that the corrosion resistance is remarkably improved by forming a rust layer containing an appropriate amount of Sn. Here, the “rust layer” is composed of one or more of α-FeOOH, β-FeOOH, γ-FeOOH crystalline iron oxyhydroxide, Fe ₃ O ₄ , and X-ray amorphous material. The

Furthermore, the proper content of Nb and Sn was examined. A hot rolled steel sheet having a thickness of 6 mm having the above composition was used as a base steel material, and a test piece of size: 35 mm × 35 mm × 5 mm was taken from the hot rolled steel sheet, and an arithmetic average roughness (arithmetic- The grinding process was carried out so that Ra was 1.6 μm or less. Next, the test piece was allowed to stand for 11 hours in a dry atmosphere having a temperature of 40 ° C. and a relative humidity of 40% RH, and after taking a transition time of 1 hour, the temperature was 25 ° C. , Left in a wet atmosphere with a relative humidity of 95% RH for 11 hours, then take 1 hour of transition time, with a total of 24 hours of the process as 1 cycle, 1 cycle per day for 1 year (365 days) Repeatedly, an artificial seawater solution (artificial seawater solution) such that the salt content attached to the surface of the test piece is 0.2 mdd (artificial amount such that 1.4 mg / dm ² of salt solution is attached) (Sea water solution) is applied to the surface of the test piece once a week during the drying process. It was Risabi formation. At this time, a desirable form for promoting the formation of a rust layer effective for corrosion resistance is an adhering salt content of 0.1 mdd or more. Note that the rust layer of the present invention is formed when the amount of adhering salt exceeds 0.05 mdd. However, in an atmosphere with a low amount of adhering salt, that is, when the amount of adhering salt is 0.05 mdd or less, it takes time to form the rust layer of the present invention.
On the other hand, it is necessary to form a rust layer with an attached salt content of 0.30 mdd or less. If it exceeds 0.30 mdd, a large amount of Cl penetrates into the rust layer, so that it becomes difficult to form a dense rust with high Nb and Sn concentrations in the rust layer. As a result, the rust layer of the present invention is not formed, and the steel material has a rust layer with poor weather resistance.

The test piece on which the rust was formed as described above was subjected to a corrosion test in a high salt environment of 0.2 mdd for another 6 months. After completion of the corrosion test, the test piece was immersed in an aqueous solution of hexamethylenetetramine added to hydrochloric acid and derusted to measure the weight, and the initial weight of the test piece before rust formation and desorption were measured. The average corrosion rate (μm / year) per side was determined by calculating the difference from the weight of the test piece after rusting. When this average corrosion rate was 60 μm / year or less, it was evaluated that the bare corrosion resistance was superior to that of conventional weathering steel.

Next, the relationship between the average corrosion rate and the contents of Nb and Sn in the rust layer was investigated. The contents of Nb and Sn in the rust layer (the number of atoms with respect to 100 Fe atoms) can be determined by various methods. As an example, the method is determined by using an electron probe microanalyzer (EPMA). Is shown below.
First, a cross-section sample of a steel material with a rust layer is created. A test piece of steel material with a rust layer was sheared, embedded in a resin (diameter 25 mm), and then polished with a # 4000 finish using ethanol (no water). The measurement conditions of the electron beam microanalyzer (EPMA) are as follows: acceleration voltage 15 kV, irradiation current 2 × 10 ⁻⁷ A, beam diameter 2 μm, scanning area 1.5 mm. × 0.5 mm.
Here, the calculation method of content is demonstrated using the case of Nb.
Was measured every arbitrary point five points per matrix steel (base material), Fe, obtains the average of the Nb respective X-ray intensity, respectively _I _FeStnd, and _{I _NbStnd,} a reference value obtained by dividing the _{I NbStnd} in _{I FeStnd} did. Furthermore, measurement was performed at 100,000 points in the rust layer, and the average of the X-ray intensities of Fe and Nb was determined for the top 30 points from the highest Nb X-ray intensity (X-ray intensity). _FeAve, and _{I _NbAve,} was thickening index (enrichment index) obtained by dividing the _{I NbAve} in _{I FeAve.} The obtained concentration index divided by the reference value is multiplied by the number of Nb atoms with respect to 100 Fe atoms of the base steel (base material), and the concentration of Nb in the rust layer (the number of Nb atoms with respect to 100 Fe atoms) Was calculated. By this procedure, the Sn content can be obtained in the same manner. The number of Nb and Sn atoms with respect to the number of Fe atoms in the base steel (base material) 100 is the addition ratio of each component to Fe when the base steel (base material) is produced, or wet analysis of the base steel (base material). (Wet analysis) (conventionally known). Further, in obtaining the X-ray intensity of each element, correction of the background (back ground) or the like may be appropriately performed.

The results obtained as described above are shown in FIG. According to the above corrosion resistance evaluation method (corrosion evaluation method), those having excellent corrosion resistance are indicated as “◯” and those having inferiority as “X” in the figure.

From the results shown in FIG. 1, when the number of Nb and Sn atoms in the rust layer is Nb: 0.01 or more and Sn: 0.005 or more with respect to the number of Fe atoms 100, the corrosion resistance is excellent. You can see that
Therefore, in the present invention, the rust layer contains Nb and Sn, and the number of Nb atoms in the rust layer is 0.01 or more at maximum with respect to the number of Fe atoms 100, and the rust layer The number of Sn atoms therein is set to 0.005 or more as a maximum value with respect to the number 100 of Fe atoms.
If the maximum number of Nb and Sn atoms in the rust layer is more than 0.5 with respect to the number of Fe atoms of 100, in addition to problems such as deterioration of ductility and toughness of the base steel (base material) Cost. In particular, when Nb is increased, the amount of precipitated niobium carbide (NbC) decreases the amount of dissolved niobium (amount of solution niobium), which rather affects the corrosion resistance. Therefore, the maximum number of upper limit atoms of Nb and Sn is preferably 0.5 or less with respect to the maximum number of Fe atoms of 100.
On the other hand, if the maximum number of Nb and Sn atoms in the rust layer is set to more than 0.5 with respect to the number of Fe atoms 100, it is necessary to increase Nb and Sn in the base steel (base material) component. There is. This leads to an increase in cost in addition to problems such as ductility and toughness deterioration. In particular, when Nb in the base steel material (base material) is increased, the amount of niobium carbide (NbC) that is precipitated decreases the amount of solid solution niobium, 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 with respect to the number of Fe atoms being 100.
In addition, a rust specimen produced by focused ion beam processing (FIB) from an arbitrary portion of the rust layer portion of less than 100 μm from the base steel material is used with a transmission electron microscope (TEM). In addition, the rust layer was identified at the positions of concentrated Nb and Sn by an electron diffraction pattern (electron diffraction pattern). As a result, the components of Nb and Sn are both located in β-iron oxyhydroxide (β-FeOOH), and the base steel material side portion of the rust layer is β-iron oxyhydroxide containing the elements of Nb and Sn. It was found to contain (β-FeOOH). It is presumed that this improves rust stabilization and improves corrosion resistance even in an environment with a large amount of salt.
Therefore, the base steel material side portion of the rust layer preferably contains β-iron oxyhydroxide (β-FeOOH) containing elements of Nb and Sn.
In addition, the identification of the rust at the position where Nb and Sn exist includes not only an electron diffraction pattern using a TEM but also Raman spectroscopy, and particularly a rust layer at a high position resolution. In the identification, diffraction pattern analysis (diffraction pattern analysis) is recommended.

Further, by line analysis with respect to the depth direction of an electron beam microanalyzer (EPMA), a pitting corrosion rust portion (pit) that is considered to be a portion in which erosion progression is remarkable in the rust layer. and the number of the concentrated layers in which the Nb and Sn elements of (and area) are concentrated. Here, the enriched layer is an Fe atom in which the content of Nb and Sn in the pitting rust portion is determined based on the number of Nb and Sn atoms relative to the number of Fe atoms of the base steel (base material) 100 The number of Nb and Sn atoms relative to the number 100: 0.03 and 0.02 higher than the thickness of the concentrated layer (peak half-value on the line profile) (width) means 15 μm or less, and the average number of concentrated layers was determined and used as the number of concentrated layers (pieces) The conditions for measuring the number of concentrated layers were: acceleration voltage: 15 kV, Irradiation current: 2 × 10 ⁻⁷ A, beam diameter: 2 μm From the results of line analysis of 5 arbitrarily selected pitting corrosion rust parts, the concentration of Nb and Sn in the pitting corrosion rust part Calculate the average number It was.

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%, Ni: 0.21%, Nb: 0.052% and Sn: 0.052% Line analysis was performed on the pitting corrosion rust portion of the rust layer of the steel material with rust layer (base steel material) in which the rust layer was formed on the surface of the base steel material by the rust formation method described above. As a result, the content of Nb in the rust layer with respect to Fe atoms 100 was 0.111 and the content of Sn was 0.060. When the above-described corrosion test was performed, the average corrosion rate was 54. It was 6 μm / year, and it was confirmed that the corrosion resistance was excellent. In this pitting corrosion rust portion, there were 8.1 and 4.2, respectively, where Nb and Sn concentrated layers existed. It can be seen that the more the concentrated layer is present, the more the corrosion resistance is improved by the diffusion-inhibiting action of the corrosion promoting substance into the steel.
From the above results, in the present invention, in addition to the regulation of the number of Nb atoms and the number of Sn atoms in the rust layer, the Nb content is 5 or more from the viewpoint of improving the corrosion resistance by suppressing the penetration of corrosion promoting substances into the steel. , Sn is preferably 3 or more. In addition, since the Nb and Sn concentrated layers overlap each other, it is possible to increase and strengthen the corrosion-inhibiting sites, and therefore it is more desirable that the respective peak half-value widths overlap. It becomes the configuration.

No. having the component composition shown in Table 1. 1 to 14 were melted and heated to 1150 ° C., followed by hot rolling, and air-cooled to room temperature to produce a 6 mm thick hot rolled steel sheet. Next, a test piece of size: 35 mm × 35 mm × 5 mm was taken from the obtained steel plate. The test piece is ground so that the arithmetic average roughness Ra is 1.6 μm or less, the end face and the back face are tape-sealed, and the surface exposed area of the test piece has an area of 25 mm × 25 mm. The surface was also sealed with tape.
The test piece obtained as described above was left in a dry atmosphere at a temperature of 40 ° C. and a relative humidity of 40% RH for 11 hours, and after taking a transition time of 1 hour, the temperature of 25 ° C. and a relative humidity of 95% RH was obtained. The sample is left in a humid atmosphere for 11 hours, and then takes a transition time of 1 hour. A total of 24 hours is taken as 1 cycle, and 1 cycle per day is repeated for 1 year (365 days) and adheres to the surface of the test piece. An artificial seawater solution in an amount such that the salinity is 0.2 mdd suitable for forming a rust layer effective for corrosion resistance (an artificial seawater solution in an amount such that 1.4 mg / dm ² of salinity adheres) per week. Rust formation was performed once by applying to the surface of the test piece during the drying process. A corrosion test was performed on the test piece on which the rust had been formed in a high salinity environment with an attached salt content of 0.2 mdd for another 6 months. After completion of the corrosion test, the test piece is immersed in an aqueous solution of hexamethylenetetramine in hydrochloric acid and derusted, and then the weight is measured. The initial weight of the test piece before rust formation, the weight of the test piece after derusting, and The average corrosion rate per side (μm / year) was determined by determining the difference between the two. If this average corrosion rate was 60 μm / year or less, the corrosion resistance was evaluated as excellent.
Further, for each test piece, the progress of corrosion is remarkable in the rust layer by the line analysis in the depth direction of the electron beam microanalyzer (EPMA) described above for five arbitrarily selected pitting rust portions. The number (number) of concentrated layers in which the Nb and Sn elements of the pitting rust portion considered to be a portion were concentrated was also investigated. When there was no Nb or Sn thickened layer, it was indicated in Table 2 as “x”.
Table 2 shows the presence or absence of Nb and Sn in the rust layer and the content of Nb and Sn components (the number of Nb and Sn atoms with respect to the number of Fe atoms of 100), the average number of concentrated layers, and the average corrosion rate. Results are shown. Note that these measurement methods are the same as those described above.

From the results of Table 2, the number of Nb and Sn atoms in the rust layer is within the range of Nb: 0.01 or more and Sn: 0.005 or more at the maximum with respect to the number 100 of Fe atoms. Examples 1 to 6 all have an average corrosion rate of 57.2 μm / year or less, and are excellent in corrosion resistance compared to conventional weathering steel even in a severe corrosive environment where high salinity exists. Furthermore, the average number of concentrated layers in Invention Examples 1 to 6 is 5 or more for Nb and 3 or more for Sn.
On the other hand, in Comparative Examples 1 to 7 in which the contents of Nb and Sn components are outside the scope of the present invention in the rust layer, the average corrosion rate is over 65 μm / year, and in a severe corrosive environment where high salinity exists. Corrosion resistance at is poor.
Further, in Reference Example 1 in which expensive Ni is contained in a large amount of 1.53% in the steel material, the corrosion resistance is the same level as that of Invention Examples 1 to 6, but the product cost is about as compared with Invention Examples 1 to 6. More than 30% higher.

In the same manner as in Example 1, steel sheet test pieces having the component compositions shown in Table 1 were prepared. The test piece is ground so that the arithmetic average roughness Ra is 1.6 μm or less, the end face and the back face are tape-sealed, and the surface of the test piece is exposed to an area of 25 mm × 25 mm. Tape sealed.
The test piece obtained as described above was left in a dry atmosphere at a temperature of 40 ° C. and a relative humidity of 40% RH for 11 hours, and after taking a transition time of 1 hour, the temperature of 25 ° C. and a relative humidity of 95% RH The sample is left in a humid atmosphere for 11 hours, and then takes a transition time of 1 hour. A total of 24 hours is taken as 1 cycle, and 1 cycle per day is repeated for 1 year (365 days) and adheres to the surface of the test piece. Rust formation was performed by applying an artificial seawater solution in such an amount that the amount of adhered salt was 0.10 to 0.40 mdd once a week to the surface of the test piece during the drying process. For example, in the case of 0.3 mdd, the artificial seawater solution was applied to the surface of the test piece in such an amount that 2.1 mg / dm ² of salt adhered to the surface of the test piece. A corrosion test was performed on the test piece on which the rust had been formed in a high salinity environment with an attached salt content of 0.2 mdd for another 6 months. After the corrosion test, the average corrosion rate (μm / year) was determined in the same manner as in Example 1. If this average corrosion rate was 60 μm / year or less, the corrosion resistance was evaluated as excellent.
Further, for each test piece, the number (number) of concentrated layers in which the Nb and Sn elements were concentrated was also examined by the same method as in Example 1. When there was no Nb or Sn enriched layer, it was indicated in Table 3 as “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 (the number of Nb and Sn atoms with respect to the number of Fe atoms of 100), the average number of concentrated layers, and the average corrosion rate. Show. Note that these measurement methods are the same as those described above.
From the results of Table 3, Invention Example 7 in which the number of Nb and Sn atoms in the rust layer is within the range of Nb: 0.01 or more and Sn: 0.005 or more with respect to the number of Fe atoms of 100. Each of Nos. 15 to 15 has an average corrosion rate of 57.3 μm / year or less, and is excellent in corrosion resistance as compared with conventional weathering steel even in a severe corrosive environment where high salinity exists. Furthermore, the average number of concentrated layers in Invention Examples 7 to 15 is 5 or more for Nb and 3 or more for Sn.
On the other hand, in Comparative Examples 8 to 17 in which the contents of the Nb and Sn components are outside the range of the present invention in the rust layer, the average corrosion rate exceeds 63.9 μm / year, and severe corrosion in which high salinity exists is present. Corrosion resistance in the environment is poor.

According to the present invention, it is possible to provide a steel material with a rust layer that is low-cost, non-painted, and excellent in corrosion resistance. The steel material of the present invention contains an element effective for improving the corrosion resistance in an appropriate amount and effectively, so that it is low-cost without containing a large amount of expensive elements such as Ni, under a severe corrosive environment with a large amount of incoming salt. Even if it exists, it can exhibit excellent weather resistance. The present invention can exert a particularly remarkable effect in a high flying salt environment where the flying salt content exceeds 0.05 mdd.

Claims

In the steel material with a rust layer formed by forming a rust layer on the surface of the 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 respect to 100 Fe atoms, and the number of Sn atoms in the rust layer Is a steel material with a rust layer having a maximum value of 0.005 or more with respect to hundreds of Fe atoms.
2. The steel material with a rust layer according to claim 1, wherein the base steel material contains components of Nb: 0.005% to 0.200% and Sn: 0.005% to 0.200% in mass%.
3. The steel material with a rust layer according to claim 1, wherein the base steel material side portion of the rust layer contains β-iron oxyhydroxide (β-FeOOH) containing elements of Nb and Sn.