KR20150067771A - Structural steel material having excellent atmospheric corrosion resistance - Google Patents

Abstract

Provided at low cost is a structural steel excellent in weather resistance. Specifically, the structural steel contains 0.10 to 1.00% of Cu, 0.10 to less than 0.65% of W, 0.05 to 1.00% of W, and 0.005 to 0.200% of Nb in terms of mass% % Or less, and Sn: 0.005% or more and 0.200% or less. Also, the R value is? 2.0. Where, R value = (R1 value + R2 value) ^0.5, R1 value = [Cu] × 0.2 + [ Ni] × 3 + [W] × 4 + [Nb] × 20 + [Sn] × 8.5, R2 value = (Log _{1. ^{5 ([W] 2 +0.002)}} +16) × ((Log 2 ([Nb] × 0.1 + 0.001) +10) + (Log 2 ([Sn] × 0.1 + 0.001) +10) × 0.75) × 0.5 ([M] represents the content (mass%) of the element M, and is 0 when not contained).

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a structural steel material having excellent weather resistance,

The present invention relates to steel structures mainly used for outdoor use such as bridges and the like, and particularly relates to a steel structure which is excellent in weatherability under high-air-borne salt environments such as a coastal environment atmospheric corrosion resistance is required.

Background Art [0002] Weathering steels are conventionally used in steel structures used outdoors such as bridges and the like. The weathering steel is a rust layer with a high degree of corrosion, which is an enriched alloy of Cu, P, Cr, Ni, etc. in an atmospheric environment and thus has a corrosion rate It is a steel material remarkably reduced. Due to its excellent weatherability, bridges using weather-resistant steel are known to withstand decades of common use, often paintless.

However, it is known that, in an environment with a large amount of air-borne salt such as near a coast, it is difficult to produce a green layer having high protection property and it is difficult to obtain practical weatherability (hereinafter referred to as JIS Z2382 Quot;). ≪ / RTI >

According to Non-Patent Document 1, the conventional weather resistant steel (JIS G 3114: weather-resistant hot-rolled steel for welded structure) has a salt content in the air of 0.05 mg · NaCl / day) may be marked as mdd). Therefore, corrosion prevention measures are usually applied to steel (JIS G 3106: rolled steel for welding structure) in an environment where the amount of salt in the air such as the coast is large. Also, dm means a decimeter.

In coating, the coating film deteriorates with the passage of time, requiring maintenance and repair. In addition, a sudden increase in labor cost and a difficulty in recoating are added. For this reason, at present, there is a demand for a steel material which can be used as a martial art, and a demand for a steel material which can be used as a martial art is high.

With respect to this phenomenon, recently, various kinds of alloying elements, particularly, a steel material containing a large amount of Ni, have been developed as a steel material that can be used as unpainted materials in a high air saline environment such as near a coast.

For example, Patent Document 1 discloses a high-weatherability steel containing Cu and at least 1% by mass of Ni as an element for improving weather resistance.

In addition, Patent Document 2 discloses a steel material excellent in weather resistance to which 1 mass% or more of Ni and Mo is added.

Further, in Patent Document 3, a steel material excellent in weather resistance is disclosed in which Cu and Ti are added in addition to Ni.

Patent Document 4 discloses a steel for welded structure containing a large amount of Ni and further containing Cu, Mo, Sn, Sb, P and the like.

On the other hand, there is no mention of the weatherability in the environment of saline in high air such as near the coast, but the severe corrosive environment in which splashes of seawater such as ballast tanks of ships are directly applied environment, and Patent Document 5 discloses a marine corrosion-resistant steel containing W and Cr and further containing Sb, Sn, Ni, and the like.

Japanese Patent Publication No. 3785271 (JP-A-11-172370) Japanese Patent Publication No. 3846218 (Japanese Patent Application Laid-Open No. 2002-309340) Japanese Patent Publication No. 3568760 (JP-A-11-71632) Japanese Patent Application Laid-Open No. 10-251797 Japanese Patent Application Laid-Open No. 2007-254881

 Joint Research Report on the Application of Weatherable Steel to Bridges (XX), No. 88, March 1993, Japan Institute of Construction and Civil Engineering, Kang Jae Ryok Club, Japan Bridge Construction Association

However, as in Patent Documents 1 and 2, when the Ni content is increased, there is a problem that the cost of the steel is increased due to an increase in the alloy cost.

Further, in Patent Document 3, since Ti is an essential element, when a large amount of Ti is added, toughness may be deteriorated.

Also, as in Patent Document 4, in the case of a steel material containing Cu, Mo, Sn, Sb, P or the like in which the content of Ni is increased and the cost of the steel material increases due to an increase in alloy cost, The weldability is deteriorated.

Further, since the steels disclosed in Patent Document 5 have different uses, the environments in which the steels are exposed are largely different from each other. As a result, the required corrosion resistance is different, and the corrosion resistance in the high- It is not described.

In view of the above circumstances, it is an object of the present invention to provide a structural steel having excellent weather resistance even in an environment where the amount of salt in the air is about 0.4 mdd at a low cost.

In order to solve the above problems, the present invention has been studied extensively on the composition of a steel material from the viewpoint of weatherability in a high air salinity environment. As a result, it has been found that the weather resistance of the steel in an environment of high air salinity is improved by containing W and Sn and / or Nb in a base steel containing Cu and Ni.

The reduction in steel sheet thickness (84 days) due to corrosion in the wet and dry cyclic corrosion test to be described later was evaluated as follows. The steel sheet was subjected to corrosion for 84 days in an actual environment having a salt content of about 0.4 mdd in air, Which is equal to the thickness reduction amount. Therefore, in the present invention, as a method for evaluating the weather resistance in an environment in which the amount of salt in the air is about 0.4 mdd, a dry and wet repetitive corrosion test described later is carried out. It is also known that the steel type No. 18 shown in Table 1, which will be described later, containing a large amount of Ni, can be used in an environment in which the amount of salt in the air is about 0.4 mdd. In this dry type repeated corrosion test Is 14.0 占 퐉. Therefore, in the present invention, the evaluation of various steels is based on whether or not the average plate thickness reduction amount in the dry and wet repeated corrosion test described below is 14.0 占 퐉 or less.

As a result, it has been found that the steel containing W and Nb in combination or the steel containing W and Sn in combination, has a smaller amount of reduction in steel sheet thickness than that of the conventional weather resistant steel, ordinary steel and weather resistant steel using a combination of other elements And the weather resistance was excellent. Further, in the composition in which the content of the component in the steel was more limited and adjusted, even in comparison with the steel having a large Ni content, the weather resistance was better.

The reason why the excellent weather resistance is exhibited despite the low Ni content is presumed as follows.

That is, Cu and Ni densify the green layer and prevent the chloride ions, which are corrosion promoting factors, from penetrating the green layer to reach the base iron. W forms a complex oxide with Fe in the vicinity of the interface between the green layer and the base iron in the anode portion to suppress the anode reaction. Furthermore, by distributing in the green layer as the W acid ion, the cation selective permeability is expressed, and the chloride ions, which are the corrosion promoting factors, are prevented from reaching the substrate through the green layer. Nb is concentrated in the vicinity of the interface between the green layer and the base iron in the anode portion to suppress the anode reaction and the cathode reaction. Sn, like Nb, is concentrated in the vicinity of the interface between the green layer and the base iron in the anode portion, thereby suppressing the anode reaction and the cathode reaction. However, these effects are insufficient in the single-containing furnace, and the corrosion inhibiting effect of Cu, Ni, W, Nb and Sn is greatly improved by the synergistic effect of the combination of Cu, Ni, W and Nb and / . As a result, it is more excellent in weather resistance than a steel containing Ni in a high content.

On the basis of this recognition, the inventors of the present invention have further investigated the effect on the weatherability of these elements and found the weathering index R value shown below. It has been found that a steel material excellent in weather resistance can be obtained by adjusting the content of components of Cu, Ni, W, Nb and Sn so that the R value becomes 2.0 or more.

R value ≥ 2.0

R value = (R1 value + R2 value) ^0.5

R1 value = [Cu] × 0.2 + [Ni] × 3 + [W] × 4 + [Nb] × 20 + [Sn] × 8.5

R 2 value = (Log _{1 .5} ([W] ² +0.002) +16 (Log ₂ ([Nb] x0.1 + 0.001) +10) +

(Log ₂ ([Sn] x 0.1 + 0.001) +10) x 0.75) x 0.5

In the above, [M] represents the content (mass%) of the element M, and when it does not contain the element M, it is set to zero.

The R 1 value is obtained by estimating a half-life from the above-described various tests in anticipation of the effect of contributing to the improvement of the weatherability of each component alone. Also, the R2 value is intended to evaluate the synergistic effect of each component. The synergistic effect is created by semi-empirically setting constants because they are thought to be logarithmic functions because they tend to become saturated with increasing amounts. The improvement effect on the weatherability is considered as the sum of the single effect and the synergistic effect, and an equation showing the R value of the present invention is derived.

The present invention has been made based on the above recognition, and its main points are as follows.

The steel sheet according to any one of the above items [1] to [6], wherein C is 0.020% or more and less than 0.140%, Si is 0.05 to 2.00%, Mn is 0.20 to 2.00%, P is 0.005 to 0.030% 0.001 to 0.100% of Al, 0.10 to 1.00% of Cu, 0.10 to less than 0.65% of Ni, and 0.05 to 1.00% of W, % Or less, Sn: 0.005% or more and 0.200% or less, the balance being iron and inevitable impurities, and satisfying the following formula (1) Steel.

R value > Equation (1)

only,

R value = (R1 value + R2 value) ^0.5

R1 value = [Cu] × 0.2 + [Ni] × 3 + [W] × 4 + [Nb] × 20 + [Sn] × 8.5

R 2 value = (Log _{1 .5} ([W] ² +0.002) +16 (Log ₂ ([Nb] x0.1 + 0.001) +10) +

(Log ₂ ([Sn] x 0.1 + 0.001) +10) x 0.75) x 0.5

In the above, [M] represents the content (mass%) of the element M, and when it does not contain the element M, it is set to zero.

[2] The structural steel according to the above-mentioned [1], further comprising, by mass%, Cr: more than 0.1% and not more than 1.0%.

[3] The ferritic stainless steel according to the above [1] or [2], further comprising, by mass%, Co: 0.01 to 1.00%, Mo: 0.005 to 1.000%, Sb: 0.005 to 0.200% 0.0001% or more and 0.1000% or less based on the total weight of the structural steel.

[4] The steel sheet according to any one of the above items [1] to [3], further comprising Ti in an amount of 0.005 to 0.200%, V in an amount of 0.005 to 0.200%, Zr in an amount of 0.005 to 0.200% , B: 0.0001% or more and 0.0050% or less, and Mg: 0.0001% or more and 0.0100% or less.

In the present specification, the percentages representing the steel components are all% by mass. Further, in the present invention, the "structural steel material having excellent weatherability" is a structural steel material practically satisfactory in high weatherability applicable in an atmospheric salt environment of 0.4 m dd or less.

According to the present invention, a structural steel material excellent in weather resistance can be obtained at low cost.

INDUSTRIAL APPLICABILITY The structural steel according to the present invention is a composite steel containing an effective element for improvement in weatherability and thus has a practical weldability at a low cost without containing a large amount of expensive elements such as Ni or the like and also has a high salt content in the air And can have excellent weather resistance. Particularly, it has a remarkable effect in a high-salt air environment in which the salt content in the air exceeds 0.05 mdd. However, the upper limit of the amount of salt in the air is preferably 0.4 mdd or less.

1 is a schematic diagram showing the conditions and cycles of the corrosion test.

(Mode for carrying out the invention)

Hereinafter, the present invention will be described in detail.

C: 0.020% or more and less than 0.140%

C is an element for improving the strength of the structural steel, and it is required to contain 0.020% or more in order to secure a predetermined strength. On the other hand, at 0.140% or more, weldability and toughness are deteriorated. Therefore, the C content should be 0.020% or more and less than 0.140%. Preferably, it is 0.060% or more and 0.100% or less.

Si: not less than 0.05% and not more than 2.00%

Si is a deoxidizing agent at the time of steelmaking and is required to be contained in an amount of 0.05% or more as an element for securing a predetermined strength by improving the strength of the structural steel material. On the other hand, if it exceeds 2.00%, the toughness and weldability are remarkably deteriorated. Therefore, the Si content should be 0.05% or more and 2.00% or less. It is preferably not less than 0.10% and not more than 0.80%.

Mn: not less than 0.20% and not more than 2.00%

Mn is an element for improving the strength of the structural steel, and it is necessary to contain 0.20% or more of Mn in order to secure a predetermined strength. On the other hand, if it exceeds 2.00%, the toughness and weldability deteriorate. Therefore, the Mn content should be 0.20% or more and 2.00% or less. It is preferably not less than 0.20% and not more than 1.50%.

P: not less than 0.005% and not more than 0.030%

P is an element that improves the weatherability of the structural steel. In order to obtain such an effect, it is necessary to contain 0.005% or more. On the other hand, if it exceeds 0.030%, the weldability is deteriorated. Therefore, the P content should be 0.005% or more and 0.030% or less. It is preferably not less than 0.005% and not more than 0.025%.

S: 0.0001% or more and 0.0200% or less

If S is contained in excess of 0.0200%, weldability and toughness are deteriorated. On the other hand, if the content is lowered to less than 0.0001%, the production cost is increased. Therefore, the S content is 0.0001% or more and 0.0200% or less. It is preferably not less than 0.0003% and not more than 0.0050%.

Al: 0.001% or more and 0.100% or less

Al is an element necessary for deoxidation at the time of steel making. In order to obtain such an effect, it is necessary to contain Al in an amount of 0.001% or more. On the other hand, if it exceeds 0.100%, weldability is adversely affected. Therefore, the Al content should be 0.001% or more and 0.100% or less. Preferably, it is 0.010% or more and 0.050% or less. The Al content also measures acid-soluble Al.

Cu: not less than 0.10% and not more than 1.00%

Cu has an effect of improving the weatherability of the structural steel by forming a dense green layer by making the green particles fine. This effect is obtained at a content of 0.10% or more. On the other hand, if it exceeds 1.00%, the increase in the amount of Cu added leads to an increase in cost. Therefore, the Cu content was set to 0.10% or more and 1.00% or less. It is preferably not less than 0.20% and not more than 0.50%.

Further, Patent Document 5 relates to a corrosion resistant steel material for ships. From the phenomenon that the lifetime (generally 10 years) of the coating film of the ballast tank of a ship is half of the lifetime (20 years) of the ship and the corrosion resistance is maintained by carrying out maintenance painting for the remaining ten years, In the corrosion resistant steel for marine vessel, excellent corrosion resistance is exhibited without depending on the surface condition of the steel under severe corrosive environment in which seawater such as a ballast tank of a ship or its droplet is directly applied, , And further, to reduce the work of the repair painting. On the other hand, the structural steel of the present invention is applied to a steel structure used outdoors such as a bridge, and is aimed at being usable in high air salinity environment of about 0.4 mdd, 5, the environment and purpose of using the steel material are greatly different from each other. Therefore, in the steel material described in Patent Document 5, it is not necessary to contain Cu, but in the present invention, it is necessary to form a dense green layer containing Cu to improve the weatherability of the steel material. Therefore, as described above, in the present invention, Cu is contained in an amount of 0.10% or more.

Ni: 0.10% or more and less than 0.65%

Ni has the effect of improving the weatherability of the structural steel by forming a dense green layer by making the green particles fine. In order to obtain this effect sufficiently, it is necessary to contain 0.10% or more. On the other hand, if the Ni content is 0.65% or more, the increase in Ni consumption leads to an increase in cost. Therefore, the Ni content is set to be not less than 0.10% and not more than 0.65%. It is preferably not less than 0.15% and not more than 0.50%.

W: not less than 0.05% and not more than 1.00%, Nb: not less than 0.005% and not more than 0.200%, and / or Sn: not less than 0.005% and not more than 0.200%

W is an important requirement in the present invention, and coexists with Nb and / or Sn, thereby remarkably improving the weatherability of a steel material in a high air salinity environment. In addition, WO ₄ ^{2 -} elutes with the anode reaction of the steel, and is distributed as WO ₄ ^{2 -} in the green layer, thereby electrostatically preventing the chloride ions of the corrosion promoting factor from penetrating the green layer to reach the substrate . Further, by precipitating a compound containing W on the surface of the steel, the anode reaction of the steel is suppressed. In order to sufficiently obtain these effects, it is necessary to contain not less than 0.05% of W. On the other hand, if it exceeds 1.00%, the increase in the amount of W added leads to an increase in cost. Therefore, the W content is set to 0.05% or more and 1.00% or less. It is preferably not less than 0.10% and not more than 0.70%.

Nb is an important requirement in the present invention and coexists with W, so that Nb has an effect of remarkably improving the weatherability of a steel in a high air salinity environment. Further, in the anode portion, it is concentrated in the vicinity of the interface between the green layer and the base iron to suppress the anode reaction and the cathode reaction. In order to sufficiently obtain these effects, Nb must be contained in an amount of 0.005% or more. On the other hand, if it exceeds 0.200%, the toughness is lowered. Therefore, the content of Nb is 0.005% or more and 0.200% or less. Preferably, it is 0.010% or more and 0.050% or less.

Sn is an important requirement in the present invention and has the effect of significantly improving the weatherability of a steel material in a high air saline environment by coexisting with W. Further, an oxide film containing Sn is formed on the surface of the steel to suppress the anode reaction and the cathode reaction of the steel, thereby improving the weatherability of the structural steel. In order to sufficiently obtain these effects, Sn must be contained in an amount of 0.005% or more. On the other hand, if it exceeds 0.200%, the ductility and toughness of the steel are deteriorated. Therefore, the Sn content should be 0.005% or more and 0.200% or less. Preferably, it is 0.010% or more and 0.050% or less.

Nb and Sn can exhibit the effect of the present invention by containing at least any one of them. However, if both Nb and Sn are contained, the effect of improving weather resistance is more remarkable. The reason for the effect of containing both Nb and Sn is not clear. However, in an environment in which the drying process and the wetting process are repeated, the conditions under which Nb and Sn are remarkably effective (for example, temperature, relative humidity, Or environmental conditions such as the concentration of salinity in rust), it is considered that Nb and Sn improve the weatherability more remarkably by complementing the respective effects.

In addition, in securing the mechanical properties and weldability of the steel, there is an advantage that it is possible to reduce the addition amounts of Nb and Sn without deteriorating the weather resistance. For this reason, it is a preferable invention to include both Nb and Sn.

The remainder is Fe and inevitable impurities. Herein, N: not more than 0.010%, O: not more than 0.010%, and Ca: not more than 0.0010% can be allowed as inevitable impurities. Further, Ca contained as an inevitable impurity deteriorates the toughness of the weld heat affected zone if it is present in a large amount in the steel, so that Ca is preferably 0.0010% or less.

In addition to the above-mentioned component elements, the following alloying elements may be added as required.

Cr: more than 0.1% and not more than 1.0%

Cr is an element for forming a dense green layer by making the green particles fine and improving weather resistance. In order to sufficiently obtain this effect, Cr must be contained in an amount exceeding 0.1%. On the other hand, if it exceeds 1.0%, the weldability is lowered. Therefore, if contained, the Cr content is more than 0.1% and not more than 1.0%, preferably not less than 0.2% and not more than 0.7%.

In the present invention, one or more selected from Co, Mo, Sb and REM may be included for the following reasons.

Co: 0.01% or more and 1.00% or less

Co is distributed throughout the green layer, and fine green particles are formed by forming fine green particles, thereby improving the weatherability of the structural steel. In order to sufficiently obtain such an effect, it is necessary to contain 0.01% or more. On the other hand, exceeding 1.00% results in an increase in cost accompanied by an increase in Co consumption. Therefore, when contained, the Co content is 0.01% or more and 1.00% or less, preferably 0.10% or more and 0.50% or less.

Mo: 0.005% or more and 1.000% or less

Mo prevents the chloride ions of the corrosion promoting factor from penetrating through the green layer and reaching the substrate by distributing MoO ₄ ^{2 -} in the green layer due to the elution of MoO ₄ ^{2 -} with the anode reaction of the steel. Further, by precipitating a compound containing Mo on the surface of the steel, the anode reaction of the steel is suppressed. In order to sufficiently obtain these effects, it is necessary to contain 0.005% or more. On the other hand, exceeding 1.000% results in an increase in cost accompanying an increase in Mo consumption. Therefore, when contained, the Mo content is 0.005% or more and 1.000% or less, preferably 0.100% or more and 0.500% or less.

Sb: 0.005% or more and 0.200% or less

Sb is an element which suppresses the anode reaction of the steel and suppresses the hydrogen generation reaction as the cathode reaction, thereby improving the weatherability of the structural steel. In order to sufficiently obtain such an effect, it is necessary to contain 0.005% or more. On the other hand, if it exceeds 0.200%, the toughness deteriorates. Therefore, when contained, the Sb content is 0.005% or more and 0.200% or less, preferably 0.020% or more and 0.060% or less.

REM: 0.0001% or more and 0.1000% or less

The REM is distributed throughout the green layer, and it has the effect of improving the weatherability of the structural steel by forming a dense green layer by making the green particles fine. In order to obtain this effect sufficiently, it is necessary to contain 0.0001% or more. On the other hand, if it exceeds 0.1000%, the effect becomes saturated. Therefore, when contained, the REM content is 0.0001% or more and 0.1000% or less, preferably 0.0010% or more and 0.0350% or less.

In the present invention, at least one selected from Ti, V, Zr, B and Mg may be included for the following reasons.

Ti: not less than 0.005% and not more than 0.200%

Ti is an element necessary for increasing the strength. In order to obtain this effect sufficiently, it is necessary to contain 0.005% or more. On the other hand, if it exceeds 0.200%, the toughness deteriorates. Therefore, when contained, the Ti content is 0.005% or more and 0.200% or less, preferably 0.010% or more and 0.100% or less.

V: not less than 0.005% and not more than 0.200%

V is an element necessary for increasing the strength. In order to obtain this effect sufficiently, it is necessary to contain 0.005% or more. On the other hand, if it exceeds 0.200%, the effect becomes saturated. Therefore, when contained, the V content is 0.005% or more and 0.200% or less, preferably 0.010% or more and 0.100% or less.

Zr: 0.005% or more and 0.200% or less

Zr is an element necessary for increasing the strength. In order to obtain this effect sufficiently, it is necessary to contain 0.005% or more. On the other hand, if it exceeds 0.200%, the effect becomes saturated. Therefore, when contained, the Zr content is 0.005% or more and 0.200% or less, preferably 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. In order to obtain this effect sufficiently, it is necessary to contain 0.0001% or more. On the other hand, if it exceeds 0.0050%, the toughness deteriorates. Therefore, when contained, the B content is 0.0001% or more and 0.0050% or less, preferably 0.0010% or more and 0.0050% 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. In order to obtain this effect sufficiently, it is necessary to contain 0.0001% or more. On the other hand, if the content exceeds 0.0100%, the amount of inclusions in the steel increases, resulting in deterioration of toughness. Therefore, when contained, the Mg content is 0.0001% or more and 0.0100% or less, preferably 0.0020% or more and 0.0050% or less.

Further, in order to sufficiently obtain the effect of improving the weatherability in a high salt environment, it is necessary that the R value defined below satisfies a predetermined range.

R value ≥ 2.0

only,

R value = (R1 value + R2 value) ^0.5

R1 value = [Cu] × 0.2 + [Ni] × 3 + [W] × 4 + [Nb] × 20 + [Sn] × 8.5

R 2 value = (Log _{1 .5} ([W] ² +0.002) +16 (Log ₂ ([Nb] x0.1 + 0.001) +10) +

(Log ₂ ([Sn] x 0.1 + 0.001) +10) x 0.75) x 0.5

In the above, [M] represents the content (mass%) of the element M, and when it does not contain the element M, it is set to zero.

The R value is an index indicating the weatherability of the steel material. In order to obtain excellent weatherability in a high salt environment, the R value is set to 2.0 or more.

The structural steel material having excellent weather resistance of the present invention can be produced by a method in which the steel having the above composition is dissolved by a conventional method using a solvent such as a steel converter or an electric furnace and is subjected to ordinary casting, Steel sheet, steel sheet, bar steel or the like by hot rolling the slab obtained by the above method. The heating and rolling conditions may be appropriately determined depending on the required material and combinations such as controlled rolling, accelerated cooling, or reheating heat treatment may be used.

Further, by using the structural steel obtained in the above manner as a structural member of a steel structure, it is possible to obtain a steel structure having excellent weatherability in an environment having a large amount of air in the air such as near the shore.

[Example]

A steel sheet having the chemical composition shown in Table 1 was melted and heated to 1150 占 폚, hot-rolled, and air-cooled to room temperature to prepare a steel sheet having a thickness of 6 mm. Then, a test specimen of 35 mm x 35 mm x 5 mm was taken from the obtained steel sheet. The surface of the test piece was subjected to grinding processing such that the surface roughness Ra was 1.6 탆 or less and the edge face and the back side were sealed with a tape , And the surface area of the exposed area was 25 mm x 25 mm.

As a method for evaluating the weather resistance in a 0.4 m dd environment, the test piece obtained by the above was subjected to dry and wet repetitive corrosion test, and the amount of plate thickness reduction was measured. The dry test repetitive corrosion test method and the plate thickness reduction amount measurement method are shown below.

Dry Repeat Corrosion Test

As shown in Fig. 1, the above-mentioned test piece was subjected to a temperature and humidity cycle in a state in which salt was attached to one side surface. In the temperature and humidity cycle, the drying process at a temperature of 40 DEG C and a relative humidity of 40% RH was performed for 11 hours and a transfer time was set to 1 hour. Next, the wet process for a temperature of 25 DEG C and a relative humidity of 95% The time was set to 1 hour, and the cycle was changed from 24 hours to 1 cycle for 12 weeks (84 cycles). The attachment of the salt was performed 12 times in total before the start of the temperature and humidity cycles and every 7 cycles of the temperature and humidity cycles (before the drying process of the temperature and humidity cycles). At this time, a predetermined amount of the adjusted artificial seawater solution was applied to the surface of the test piece so that the amount of salt adhering to the surface of the steel sheet was 1.4 mg / dm 2.

Measuring plate thickness reduction

After repeating the wet and dry repeated corrosion test, distilled water was added to 3 mL of 37% hydrochloric acid, 3.5 g of hexamethylene tetramine and 3 mL of Hibiron (inhibitor) In addition, the test piece was immersed in a rust removing solution of 1 L (liters), and after demolding, the weight was measured. In addition, the measurement of the weight was based on the method described in the 145th Corrosion Method Symposium "Accuracy of Corrosion Assessment Method". The difference between the obtained weight and the initial weight was obtained and divided by the area of the test surface of the test piece to calculate the average plate thickness reduction amount of the test piece single surface.

The results of the corrosion test obtained above are shown in Tables 1-1 to 1-6 together with the component composition and the R value.

It can be seen from Tables 1-1 to 1-6 that in the case of the present invention in which the R value is 2.0 or more, the sheet thickness reduction amount is 11.8 to 13.9 占 퐉 and has excellent weather resistance.

On the other hand, in the comparative example deviating from the range defined in the present invention, the R value is less than 2.0 and the sheet thickness reduction amount is 14.3 to 17.7 탆, which is poor in weather resistance as compared with the case of the present invention.

(Table 1-1)

(Table 1-2)

(Table 1-3)

(Table 1-4)

(Table 1-5)

(Table 1-6)

Claims (5)

Wherein the steel sheet contains at least one selected from the group consisting of Al: 0.060% or more and less than 0.140%, Si: 0.05% to 2.00%, Mn: 0.20% to 2.00%, P: 0.005% to 0.030% : 0.001 to 0.100%, Cu: 0.10 to 0.35%, Ni: 0.18 to 0.65%, W: 0.05 to 0.70% Sn: 0.005% or more and 0.200% or less, the balance being iron and inevitable impurities, and satisfying the following formula (1).
R value > Equation (1)
R value = (R1 value + R2 value) ^0.5
R1 value = [Cu] × 0.2 + [Ni] × 3 + [W] × 4 + [Nb] × 20 + [Sn] × 8.5
R 2 value = (Log _{1 .5} ([W] ² +0.002) +16 (Log ₂ ([Nb] x0.1 + 0.001) +10) +
(Log ₂ ([Sn] x 0.1 + 0.001) +10) x 0.75) x 0.5
In the above, [M] represents the content (mass%) of the element M, and when it does not contain the element M, it is set to zero. The method according to claim 1,
Further, a structural steel containing, by mass%, not less than 0.1% but not more than 1.0% of Cr. 3. The method according to claim 1 or 2,
And at least one selected from the group consisting of Co: 0.01 to 1.00%, Mo: 0.005 to 1.000%, Sb: 0.005 to 0.200%, REM: 0.0001 to 0.1000% Structural steel. 3. The method according to claim 1 or 2,
0.005% or more and 0.200% or less; Z: 0.005% or more and 0.200% or less; B: 0.0001% or more and 0.0050% or less; Mg: 0.0001% or more and 0.0100% or less; And at least one selected from the following. The method of claim 3,
0.005% or more and 0.200% or less; Z: 0.005% or more and 0.200% or less; B: 0.0001% or more and 0.0050% or less; Mg: 0.0001% or more and 0.0100% or less; And at least one selected from the following.

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