KR20150132566A - Steel material having excellent atmospheric corrosion resistance - Google Patents

Abstract

It is mainly used in bridges and outdoors, and provides a weather-resistant steel suitable as a structural steel under an environment in which there is a dry period with a small amount of rain, such as a high temperature and a dry season.
It is necessary to satisfy at least one of the following expressions (1) and (2) and satisfy the expression (3), and furthermore, a necessary amount of C, Si, Mn, P, S, Al, Cu, At least one of Ni, Cr, Mo, W, Co, Sb, Ti, V, Zr, B, REM, Ca and Mg and the balance of iron and inevitable impurities. (Cu-0.01) x (Sn-0.005) x (Nb-0.005) x 10 ^4? 0.08 (1) , (Cu + 10 x Sn) / (2 x Ni + 0.5 x Si) <10 (3) where each symbol represents the content (% by mass)

Description

STEEL MATERIAL HAVING EXCELLENT ATMOSPHERIC CORROSION RESISTANCE [0002]

The present invention relates to a steel material excellent in atmospheric corrosion resistance, which is mainly used for outdoors such as bridges, and particularly, a steel material which is required to have weatherability under an environment in which a high- steel material.

Conventionally, weathering steel is used for steel structures used for outdoor use such as bridges. The weathering steel is a highly protective rust layer that is concentrated in an atmospheric exposure environment and contains alloying elements such as Cu, P, Cr and Ni, it is known that bridges using weather-resistant steel are able to withstand decades of public use while being paintless.

The weathering steel was developed in the United States in the 1930s and applied to architecture. After that, weather resistant steel developed in each blast furnace maker has been introduced to Japan and applied to bridges and the like. BRICS and countries with remarkable economic development in East Asia are currently in the process of repairing the infrastructure. However, non-painted bridges using weatherproof steel are required to be applied because they can reduce the maintenance cost compared with the coating bridges. Further, even in the painted bridges, when the coating life is prolonged, the frequency of repainting is reduced and the maintenance cost is suppressed. Therefore, a steel material capable of extending the coating life is desired.

In these countries, in particular, since a high temperature environment is included in many countries, it is important that weather resistance steel is applied in such an environment, and furthermore, it is low cost.

As a requirement for a low-cost steel material, it is also necessary that the alloy is easy to manufacture from the viewpoint of delivery and production efficiency in addition to low alloy cost.

In BRICS and many East Asian countries, it is expected that the demand for performance improvement for weather resistant steel which can be used in a paintless state at low cost and which has strength as structural steel will be strengthened in the future.

Various methods for improving the weatherability of steel have been proposed in the past. For example, Patent Document 1 discloses a high-weatherability steel containing Cu and at least 1% by mass of Ni added thereto. Patent Document 2 discloses a steel material excellent in weather resistance to which 1 mass% or more of Ni and Mo is added.

In addition, Patent Document 3 discloses a steel material excellent in weather resistance added with Cu and Ti in addition to Ni. Patent Document 4 discloses a steel material for welding structure containing a large amount of Ni and further containing Cu, Mo, Sn, Sb and P, and Patent Document 5 discloses a steel material excellent in corrosion resistance added with Sn.

Japanese Laid-Open Patent Publication No. 11-172370 Japanese Laid-Open Patent Publication No. 2002-309340 Japanese Patent Application Laid-Open No. 11-71632 Japanese Patent Application Laid-Open No. 10-251797 Japanese Laid-Open Patent Publication No. 2012-255184

However, when the content of Ni is increased as in Patent Documents 1 to 3, or when the content of Ni and P is increased as in Patent Document 4 and the steel containing Cu, Mo, Sn, Sb, The price of the steel increases. In the case of Patent Document 4, since the content of P is high, the weldability is lowered. In Patent Document 5, since the content of Sn is increased, surface hot shortness is promoted.

It is therefore an object of the present invention to provide a steel material having a low content of expensive alloying elements such as Ni and Mo, excellent manufacturability, low price, excellent strength as structural steel, and excellent weather resistance.

In order to solve the above problems, the inventors of the present invention have studied extensively and have found that by containing a certain amount of Cu in combination with a small amount of Nb and Sn in the component composition, the steel material without paint coating, and durability of paint are improved. The reasons for the excellent weatherability and durability of coating are not known but are estimated as follows.

1. Since Cu cools rust particles to dense the green layer, a corrosion-accelerating factor such as oxygen or chloride ions penetrates the green layer to form the base material (steel (steel). Further, since Cu is concentrated in the vicinity of the interface of the steel, the anode reaction of the steel is suppressed.

2. Nb is concentrated in the vicinity of the interface of the substrate (steel), thereby suppressing the anode reaction and the cathode reaction of the steel.

3. Since Sn is concentrated near the interface of the substrate (steel) like Nb, the anode reaction and the cathode reaction of the steel are suppressed.

4. When Cu, Nb and Sn are contained in combination, the corrosion resistance is remarkably improved as compared with the case where these alloying elements are contained alone.

The present invention has been made based on the obtained recognition, and furthermore,

(1) A composition comprising, in% by mass,

C: not less than 0.030%, not more than 0.200%

Si: not less than 0.10%, not more than 1.00%

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

P: not less than 0.003%, not more than 0.030%

S: not less than 0.0001%, not more than 0.020%

Al: 0.001% or more, 0.100% or less,

Cu: not less than 0.01%, not more than 0.50%

Nb: 0.005% or more, 0.200% or less,

Sn: not less than 0.005%, not more than 0.200%

A steel material excellent in weatherability, which satisfies at least any one of the expressions (1) and (2) and satisfies the expression (3), and the balance of iron and inevitable impurities.

(Cu-0.01) x (Sn-0.005) x (Nb-0.005) x 10 ^4? 0.08 (1)

(Cu-0.01) x (Ni-0.01) x (Cr-0.01) x 50 0.08 (2)

(Cu + 10 x Sn) / (2 x Ni + 0.5 x Si) &lt; 10 (3)

Here, the symbol of each element represents the content (% by mass), and the element which does not contain the element is 0.

(2) In addition, in terms of mass%

Ni: 0.01% or more, 1.00% or less,

Cr: 0.01% or more, 1.00% or less,

Mo: 0.005% or more, 1.000% or less,

W: 0.010% or more, 1.000% or less,

Co: 0.010% or more, 0.500% or less,

Sb: 0.005% or more, 0.200% or less,

(1), characterized by containing at least one element selected from the group consisting of iron and iron.

(3) In addition, in terms of% by mass,

Ti: 0.005% or more, 0.200% or less,

V: 0.005% or more, 0.200% or less,

Zr: 0.005% or more, 0.200% or less,

B: 0.0001% or more, 0.0050% or less,

(1) or (2), characterized by containing at least one element selected from the group consisting of iron and iron.

(4) In addition, in terms of% by mass,

REM: 0.0001% or more, 0.0100% or less,

Ca: not less than 0.0001%, not more than 0.0100%

Mg: not less than 0.0001%, not more than 0.0100%

(1) to (3), characterized by containing at least one element selected from the group consisting of iron and iron.

(5) In addition, by mass%

Ta: 0.001% or more, 0.100% or less,

Bi: 0.001% or more, 0.100% or less,

Se: 0.001% or more, 0.100% or less,

Hf: not less than 0.001%, not more than 0.100%

(1) to (4), characterized by containing at least one member selected from the group consisting of iron oxide and iron oxide.

(6) A steel material excellent in weatherability, which has a coating film on the surface of a steel material having the composition described in any one of (1) to (5).

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to obtain a structural steel having excellent manufacturability, low cost of alloy, and excellent in weatherability at low cost, and is industrially useful.

(Mode for carrying out the invention)

Hereinafter, the present invention will be described in detail. In the description,% is expressed in mass%.

C: not less than 0.030%, not more than 0.200%

C is an element which improves the strength of the steel, and in order to secure a predetermined strength as a structural steel, it is necessary to contain 0.030% or more. On the other hand, the weldability and toughness are lowered at 0.200% or more. Therefore, the C content should be 0.030% or more and less than 0.200%.

Si: not less than 0.10%, not more than 1.00%

Si is an important element in the present invention. Si forms a dense green layer by making the green particles of the entire green layer finer and thereby improves the weatherability of the steel. Further, it has an effect of preventing the surface of the steel material from cracking during hot rolling. In order to obtain these effects, it is necessary to contain 0.10% or more. On the other hand, if it exceeds 1.00%, the toughness and weldability are remarkably lowered. Therefore, the Si content is set to 0.10% or more and 1.00% or less. , Preferably 0.20% or more, 0.80% or less, more preferably 0.40% or more, and 0.60% or less.

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

Mn is an element which improves the strength of the steel, and it is necessary to contain 0.20% or more of Mn in order to secure a predetermined strength as a structural steel. On the other hand, if it exceeds 2.00%, the toughness and weldability are deteriorated. Therefore, the Mn content should be 0.20% or more and 2.00% or less.

P: not less than 0.003%, not more than 0.030%

P is an element that improves the weatherability of the steel. In order to obtain such an effect, it is necessary to contain 0.003% or more. On the other hand, if it exceeds 0.030%, the weldability is lowered. Therefore, the P content should be 0.003% or more and 0.030% or less.

S: not less than 0.0001%, not more than 0.020%

If S is contained in excess of 0.020%, the weldability and toughness are lowered. 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 0.020% or less.

Al: not less than 0.001%, not more than 0.100%

Al is an element necessary for deoxidation during steelmaking. In order to obtain such an effect, it is necessary to contain 0.001% or more. On the other hand, if it exceeds 0.100%, weldability is adversely affected. Therefore, the Al content is set to 0.001% or more and 0.100% or less.

Cu: not less than 0.01%, not more than 0.50%

Cu is an important element in the present invention. Nb and Sn, thereby remarkably improving the weatherability of the steel. Cu is enriched near the interface between the green layer and the substrate (hereinafter, simply referred to as a green layer-base (steel) interface or a steel interface), thereby suppressing the anode reaction of the steel. Further, it has an effect of forming a dense green layer by making the green particles of the green layer finer and suppressing the permeation of chloride ions as a corrosion promoting factor into the base (steel). This effect is obtained at a content of 0.01% or more. On the other hand, if it exceeds 0.50%, the alloy cost accompanied by the increase of the Cu content is increased. Therefore, the Cu content is set to 0.01% or more and 0.50% or less. , Preferably not less than 0.05%, not more than 0.40%, more preferably not less than 0.07% and not more than 0.30%.

Nb: not less than 0.005%, not more than 0.200%

Nb is an important element in the present invention. Cu and Sn, thereby remarkably improving the weatherability of the steel. Nb is concentrated in the vicinity of the interface between the green layer and the base material (steel) in the anode portion, thereby suppressing the anode reaction and the cathode reaction. In order to obtain such an effect sufficiently, it is necessary to contain 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 not less than 0.008% and not more than 0.100%, and more preferably not less than 0.010% and not more than 0.030%.

Sn: not less than 0.005%, not more than 0.200%

Sn is an important element in the present invention. Coexisting with Cu and Nb has the effect of remarkably improving the weatherability of the steel material. Sn forms an oxide film containing Sn at the steel interface and suppresses the anode reaction and the cathode reaction of the steel to improve the weatherability of the steel material. 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 ductility and toughness of the steel are lowered. Therefore, the Sn content should be 0.005% or more and 0.200% or less. , Preferably not less than 0.010%, not more than 0.100%, more preferably not less than 0.020%, and not more than 0.050%.

(Cu-0.01) x (Sn-0.005) x (Nb-0.005) x 10 ^4? 0.08 (1)

(Cu-0.01) x (Ni-0.01) x (Cr-0.01) x 50 0.08 (2)

(1) is defined to improve the corrosion resistance by suppressing the corrosion reaction by concentrating alloying elements in the vicinity of the green layer-base (steel) interface. The effect of improving the corrosion resistance in the green layer-base (steel) interface recognized in the present invention is effective not mainly in the whole green layer but in the vicinity of the interface. Therefore, the amount of Nb or Sn added to the substrate (steel) . However, Nb, Sn is contained so as to satisfy, since the effect is manifested by the coexistence with Cu, Cu, Sn, Nb is, (Cu-0.01) × ( Sn-0.005) × (Nb-0.005) × 10 4 ≥0.08 .

(2) is defined to improve the corrosion resistance by densifying the whole green layer and suppressing permeation of corrosion promoting factors. The weathering steel improves the corrosion resistance by suppressing permeation of oxygen or chloride ions, which are corrosion promoting factors, by dense green layers in which elements such as Cu, Cr and Ni are concentrated. Cu, Ni and Cr are contained so as to satisfy (Cu-0.01) x (Ni-0.01) x (Cr-0.01) x 50? 0.08 in order to obtain an effect of improving the corrosion resistance in an atmospheric environment.

In the present invention, the composition of the components is adjusted so as to satisfy at least one of the expressions (1) and (2). As described later, in the present invention, Ni and Cr are optional elements. When Ni and Cr are not contained, Ni and Cr in the formula (2) are respectively 0. In the present invention, the left side of the formulas (1) and (2) is referred to as a corrosion resistance index.

(Cu + 10 x Sn) / (2 x Ni + 0.5 x Si) &lt; 10 (3)

Since the steel material according to the present invention contains Cu and Sn at the same time, surface cracks during hot rolling may be a problem. Since Ni and Si have the effect of suppressing such surface cracking, the composition of the components is adjusted so as to satisfy the expression (3) in order to prevent surface cracking. As described later, in the present invention, Ni is a selective element. When Ni is not contained, Ni in the formula (3) is set to zero. In the present invention, the left side of the expression (3) is referred to as a surface hot shortness index.

In the formulas (1), (2) and (3), each element is a content (% by mass)

The above is the basic composition of the present invention, and the balance is Fe and inevitable impurities. Herein, as the inevitable impurities, 0.010% or less of N and 0.010% or less of O can be allowed. Further, when Ca is contained as an unavoidable impurity, the presence of a large amount of Ca in the steel reduces the toughness of the weld heat affected zone, so that it is preferably 0.0010% or less.

Further, in the case of improving the characteristics, at least one element selected from Ni, Cr, Mo, W, Co, and Sb may be added as the selective element.

Ni: not less than 0.01%, not more than 1.00%

Ni has the effect of forming a dense green layer by making the green particles fine and improving the weatherability of the structural steel. In order to sufficiently obtain this effect, it is necessary to contain not less than 0.01%. On the other hand, if it exceeds 1.00%, the alloy cost is increased. Therefore, when Ni is contained, the content is set to 0.01% or more and 1.00% or less. , Preferably not less than 0.01%, less than 0.20%, more preferably, not less than 0.01% and less than 0.10%.

Cr: not less than 0.01%, not more than 1.00%

Cr is an element that forms a dense green layer by making the green particles fine and improves weather resistance. In order to obtain such an effect, it is necessary to contain 0.01% or more. On the other hand, when Cr is added excessively, the weldability is deteriorated. Therefore, when Cr is contained, the content is set to 0.01% or more and 1.00% or less.

Mo: not less than 0.005%, not more than 1.000%

In the case of Mo, MoO ₄ ^{2 -} is eluted with the anode reaction of the steel, and MoO ₄ ^{2 -} is distributed in the green layer, thereby preventing the chloride ions of the corrosion promoting factor from penetrating through the green layer to reach the substrate (steel). Further, the precipitation of the Mo-containing compound on the steel interface suppresses the anode reaction of the 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 1.000%, the increase in the consumption of Mo leads to an increase in cost. Therefore, when contained, the Mo content is set to 0.005% or more and 1.000% or less.

W: not less than 0.010%, not more than 1.000%

W is, WO ₄ ² along with the lecture anode ^reaction is the dissolution, WO ₄ ² during nokcheung ^- being distributed as, chloride ions of the corrosion promoting factor transmitted through the nokcheung prevents entirely forward to reach the substrate (steel) . Further, by precipitating a compound containing W at the steel interface, the anode reaction of the steel is suppressed. In order to sufficiently obtain such an effect, it is necessary to contain 0.010% or more. On the other hand, if it exceeds 1.000%, the cost associated with the increase of W consumption is caused. Therefore, when contained, the W content is set to 0.010% or more and 1.000% or less.

Co: 0.010% or more, 0.500% or less

Co is distributed throughout the green layer, and it has the effect of improving the weatherability of the steel material by forming a dense green layer by making the green particles fine. In order to sufficiently obtain such an effect, it is necessary to contain 0.01% or more. On the other hand, exceeding 0.500% results in an increase in cost accompanied by an increase in Co consumption. Therefore, when contained, the Co content is 0.010% or more and 0.500% or less.

Sb: not less than 0.005%, not more than 0.200%

Sb is an element that suppresses the anode reaction of the steel and improves the weatherability of the steel material by suppressing the hydrogen generation reaction as the cathode reaction. In order to sufficiently obtain such an effect, it is necessary to contain 0.005% or more. On the other hand, if Sb is contained excessively, the toughness is lowered. Therefore, when Sb is contained, the content is set to 0.005% or more and 0.200% or less.

Further, in the case of improving the characteristics, at least one selected from Ti, V, Zr and B may be added as the selective element.

Ti: not less than 0.005%, 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 is lowered. Therefore, when contained, the Ti content is set to 0.005% or more and 0.200% or less.

V: not less than 0.005%, 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 should be 0.005% or more and 0.200% or less.

Zr: not less than 0.005%, not more than 0.200%

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 should be 0.005% or more and 0.200% or less.

B: not less than 0.0001%, not more than 0.0050%

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 is lowered. Therefore, when contained, the B content is 0.0001% or more and 0.0050% or less.

Further, in the case of improving the characteristics, at least one selected from REM, Ca, and Mg may be added as the selective element.

REM: not less than 0.0001%, not more than 0.0100%

The REM is distributed throughout the green layer, and the green particles are finely formed to form a dense green layer, thereby improving the weatherability of the steel. In order to obtain this effect sufficiently, it is necessary to contain 0.0001% or more. On the other hand, if it exceeds 0.0100%, the effect becomes saturated. Therefore, when contained, the REM content is 0.0001% or more and 0.0100% or less.

Ca: not less than 0.0001%, not more than 0.0100%

Ca is an element effective for improving the toughness of the weld heat-affected zone by fixing S in the steel. In order to obtain this effect sufficiently, it is necessary to contain 0.0001% or more. On the other hand, when the content exceeds 0.0100%, the amount of inclusions in the steel increases, resulting in deterioration of toughness. Therefore, when contained, the content of Ca should be 0.0001% or more and 0.0100% or less.

Mg: not less than 0.0001%, not more than 0.0100%

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 it exceeds 0.0100%, the amount of inclusions in the steel increases, resulting in lowering of toughness. Therefore, when contained, the Mg content is 0.0001% or more and 0.0100% or less.

Further, in the case of improving the characteristics, at least one selected from Ta, Bi, Se and Hf may be added as the selective element.

Ta: not less than 0.001%, not more than 0.100%

Ta is distributed throughout the green layer, and the green particles are finely formed to form a dense green layer, thereby improving the weatherability of the steel. In order to obtain this effect sufficiently, it is necessary to contain 0.001% or more. On the other hand, if it exceeds 0.100%, the effect becomes saturated. Therefore, when contained, the Ta content is set to 0.001% or more and 0.100% or less.

Bi: not less than 0.001%, not more than 0.100%

Bi is distributed throughout the green layer, and fine green particles are formed to form a dense green layer, thereby improving the weatherability of the steel. In order to obtain this effect sufficiently, it is necessary to contain 0.001% or more. On the other hand, if it exceeds 0.100%, the effect becomes saturated. Therefore, when contained, the Bi content is set to 0.001% or more and 0.100% or less.

Se: not less than 0.001%, not more than 0.100%

Se dissolves oxygen ions in response to the anode reaction of the steel and is distributed in the green layer to prevent the chloride ions of the corrosion promoting factor from penetrating through the green layer to reach the base steel. In order to obtain this effect sufficiently, it is necessary to contain 0.001% or more. On the other hand, if it exceeds 0.100%, the increase in the consumption of Se causes a cost increase. Therefore, when contained, the content of Se is set to 0.001% or more and 0.100% or less.

Hf: not less than 0.001%, not more than 0.100%

Hf is distributed throughout the green layer, and has the effect of improving the weatherability of the steel material by forming a dense green layer by making the green particles fine. In order to obtain this effect sufficiently, it is necessary to contain 0.001% or more. On the other hand, if it exceeds 0.100%, the effect becomes saturated. Therefore, when contained, the Hf content is set to 0.001% or more and 0.100% or less.

The steel material having excellent weather resistance of the present invention is obtained by hot-rolling a steel having the above-mentioned composition composition by a conventional continuous casting or blooming to obtain a steel plate, a shaped steel, Steel sheet, bar steel and the like.

The heating and rolling conditions may be appropriately determined according to the required material, and combinations such as controlled rolling, accelerated cooling, or reheating heat treatment are also possible.

The content of each element can be determined by spark discharge-emission spectrochemical analysis, X-ray fluorescence analysis, ICP (atomic emission spectrometry) emission spectrometry, and ICP mass spectrometry ), A combustion method, and the like.

Example

The steel having the composition shown in Table 1 was melted and heated to 1150 占 폚 and then subjected to hot rolling to air-cool to room temperature to start a steel sheet having a thickness of 12 mm. Then, a test piece having a size of 35 mm x 35 mm x 5 mm was obtained from the obtained steel sheet. The surface of the test piece was subjected to grinding so that the surface had a surface roughness Ra of 1.6 탆 or less. The end surface and the rear surface were tape-sealed and the surface was also tape-sealed so that the surface exposed portion had an area of 25 mm x 25 mm.

The test pieces thus obtained were subjected to a wet and dry cyclic corrosion test, and the weather resistance at the time of use was evaluated. As an evaluation test for weatherability, a corrosion test simulating the environment inside a girder where there is no place exposed to rain, which is considered to be the most severe environment in a structure such as an actual bridge, was performed. In this evaluation test, a high temperature environment was assumed. The conditions of the corrosion test are as follows. The drying step at a temperature of 50 占 폚 and a relative humidity of 35% RH for 15 hours and then a transition time of 1 hour and then a wetting step at 30 占 폚 and a relative humidity of 95% RH for 7 hours, The time-shifting time was taken, and a total of 24 hours was made as one cycle. The artificial seawater solution adjusted so that the amount of salt adhering to the surface of the test piece was 0.05 mdd was applied once a week to the surface of the test piece during the drying process so that the salt amount adhered to the surface of the test piece was 5.6 mg in terms of Cl The diluted artificial seawater solution is added to the surface of the specimen with a pipette once a week during the drying process). Under these conditions, 84 cycles of 12 weeks were carried out.

After completion of the corrosion test, the test piece was immersed in an aqueous solution containing hydrochloric acid and hexamethylenetetramine to remove rust, and then the weight thereof was measured. The difference between the weight and the initial weight was determined to obtain an average plate thickness reduction amount of one surface. When the average plate thickness reduction amount was 14 μm or less, it was evaluated that the weather resistance was excellent.

Further, a high temperature tensile test was conducted to evaluate the surface hot shortness of the steel material. Tensile test specimens having a diameter of 6 mm and a distance between marks of 16 mm were taken from a steel sheet having a thickness of 12 mm, which was started by the method described previously, and subjected to a high temperature tensile test. Tensile test specimens were taken so that the longitudinal direction and the rolling direction of the tensile test specimen were the same. The test was carried out under the conditions of a deformation rate of 5 / s and a displacement of 6 mm after heating at 1030 캜 for one hour in the atmosphere. After the test, the parallel portion of the test piece was cut in the major axis direction so as to pass through the center of the test piece, and the cross section was observed. The depth of the crack was observed by observing the 10-field field at a magnification of 50 times. When the maximum depth of the crack was 150 μm or less, it was judged that there was no problem in surface hot shortness.

Table 2 shows the corrosion amount (average plate thickness reduction 탆) and the maximum crack depth in the high-temperature tensile test. It can be seen from Tables 1 and 2 that the corrosion resistance index is 0.08 or more and the value of the surface hot shortness index is less than 10. 1 to 22, 26 and 27 have excellent weather resistance because of an average plate thickness reduction of 14 μm or less and a maximum crack depth in a high temperature tensile test of 150 μm or less and also have a problem in surface cracking during hot rolling It can be seen that there is not.

On the other hand, the comparative steel No. having a corrosion resistance index of less than 0.08. 23 shows the average sheet thickness reduction amount of 16.8 占 퐉; 24 shows the average plate thickness reduction amount of 14.5 占 퐉; 25 has an average plate thickness reduction of 14.9 mu m, which is inferior to the steel of the present invention in weatherability. Further, 25 has a surface hot shortness index of 16.42 which exceeds 10 and a surface hot shortness is remarkable in that the maximum crack depth exceeds 150 탆. In Table 1, the corrosion resistance index has a larger value of the left side of the expressions (1) and (2).

Next, in order to investigate the durability of the coating, 1, 2, 7, 12, 15, 17, 18, 20, and 23 were tested in the following manner.

A test piece having a size of 150 mm x 70 mm x 5 mm was taken from the obtained steel sheet. The test piece was subjected to shot blasting so that the surface had a surface roughness of ISO Sa 2.5, ultrasonic degreasing was performed for 5 minutes in acetone, and the test piece was air-dried to obtain a test piece. One side was coated with a solvent-based epoxy resin paint, and the other side and the other side were coated with a silicone-based silicone sealant. The painting was performed with the specifications shown in Table 3. After coating, a cross cut having a width of 1 mm and a length of 70 mm was put in the center of the test piece to form an initial defect. Then, a corrosion test was carried out under the following conditions. A cycle of 0.5 hours (5% NaCl), a relative humidity of 95% for 1.5 hours, a hot air drying at 50 占 폚 for 2 hours, and a hot air drying at 30 占 폚 for 2 hours was carried out in accordance with JIS K 5621. The paint system 1 was 50 cycles, the paint system 2 was 150 cycles, and the paint system 3 was 220 cycles. After completion of the corrosion test, coating film blisters from crosscuts were measured at ten points, and an average value was obtained to obtain an evaluation index of the durability of the coating. The coating film swelling was swollen on both sides with the cut as a center, and the width of one side was determined as the coating film swelling width. It was determined that the durability of the coating film was excellent when the average value of the coat film bulging width was 10 mm or less in the coating system 1 and 5 mm or less in the coating system 2 and the coating system 3.

Table 4 shows the average value of the film swelling width. From Table 4, it can be seen that the average value of the coating film swelling width in the inventive steel is 10 mm or less in the coating system 1, 5 mm or less in the coating system 2 and the coating system 3, and the durability of the coating film is excellent. On the other hand, in the comparative steels, the average value of the coating film bulging width exceeded 10 mm in the coating system 1, and 5 mm in the coating system 2 and the coating system 3, respectively, indicating that the durability of the coating film is poor.

Claims (6)

The composition of matter, in% by mass,
C: not less than 0.030%, not more than 0.200%
Si: not less than 0.10%, not more than 1.00%
Mn: not less than 0.20%, not more than 2.00%
P: not less than 0.003%, not more than 0.030%
S: not less than 0.0001%, not more than 0.020%
Al: 0.001% or more, 0.100% or less,
Cu: not less than 0.01%, not more than 0.50%
Nb: 0.005% or more, 0.200% or less,
Sn: not less than 0.005%, not more than 0.200%
A steel material excellent in weatherability, which satisfies at least any one of the expressions (1) and (2) and satisfies the expression (3), and the balance of iron and inevitable impurities.
(Cu-0.01) x (Sn-0.005) x (Nb-0.005) x 10 ^4? 0.08 (1)
(Cu-0.01) x (Ni-0.01) x (Cr-0.01) x 50 0.08 (2)
(Cu + 10 x Sn) / (2 x Ni + 0.5 x Si) &lt; 10 (3)
Here, the symbol of each element represents the content (% by mass), and the element which does not contain the element is 0. The method according to claim 1,
Further, in terms of% by mass,
Ni: 0.01% or more, 1.00% or less,
Cr: 0.01% or more, 1.00% or less,
Mo: 0.005% or more, 1.000% or less,
W: 0.010% or more, 1.000% or less,
Co: 0.010% or more, 0.500% or less,
Sb: 0.005% or more, 0.200% or less,
By weight of at least one member selected from the group consisting of iron and iron. 3. The method according to claim 1 or 2,
Further, in terms of% by mass,
Ti: 0.005% or more, 0.200% or less,
V: 0.005% or more, 0.200% or less,
Zr: 0.005% or more, 0.200% or less,
B: 0.0001% or more, 0.0050% or less,
By weight of at least one member selected from the group consisting of iron and iron. 4. The method according to any one of claims 1 to 3,
Further, in terms of% by mass,
REM: 0.0001% or more, 0.0100% or less,
Ca: not less than 0.0001%, not more than 0.0100%
Mg: not less than 0.0001%, not more than 0.0100%
By weight of at least one member selected from the group consisting of iron and iron. 5. The method according to any one of claims 1 to 4,
Further, in terms of% by mass,
Ta: 0.001% or more, 0.100% or less,
Bi: 0.001% or more, 0.100% or less,
Se: 0.001% or more, 0.100% or less,
Hf: not less than 0.001%, not more than 0.100%
By weight of at least one member selected from the group consisting of iron and iron. A steel material excellent in weatherability, characterized by having a coated film on a surface of a steel material having the composition according to any one of claims 1 to 5.