JP3535716B2 - Painted steel with excellent durability - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel material used for a steel structure in which maintenance work including repainting of bridges or steel towers is difficult to carry out on a daily basis. As a preventive measure, it belongs to a steel structure suitable for a salt damage environment such as a cold region where chlorides may be sprayed on roads.

[0002]

[Prior Art] JIS weather resistant hot rolled steel for welding structures (SMA: JIS G 3114) and high weather resistant rolled steel as a weather resistant steel with appropriate amounts of chemical components such as Cr, Cu, Ni and P added to steel. (SPA: JIS
G 3125) is defined, and a weather resistant steel material is disclosed in the patent publication mentioned later. Weather-resistant steel is a steel in which the progress of permanent corrosion is blocked by a dense stable rust layer formed on the surface of the steel material, and it has been used in inland regions.

[0003]

However, it takes about 10 years or longer for a weather resistant steel to form a stable rust layer, and in practice, initial corrosion and the accompanying outflow of red rust occur. It's a problem. This tendency is particularly strong in Japan, which has a warm and humid climate. Rust stabilization treatment is generally carried out for the purpose of preventing contamination of surrounding structures by rust juice until rust stabilization when using weatherproof steel naked. However, this method also has a problem that the expected effect cannot be obtained because the formation of a dense rust layer is hindered in an environment where a large amount of salt comes in, as in the case of naked use, only by preventing rust.

On the other hand, means for solving the above-mentioned problems of weather resistant steel have also been conventionally proposed. Japanese Patent Publication Sho 53-22530
JP-B, JP-B-56-33991, JP-B-58-39915, JP-B-58-17833, JP-A-02-133480,
Japanese Examined Patent Publication No. 06-21273 proposes a method of coating a resin on the surface of weather-resistant steel to prevent invasion of flying salt from the external environment and promote the formation of stable rust. For example, in Japanese Patent Laid-Open No. 02-133480, Fe having a scaly crystal structure is disclosed.
₃ O ₄ , phosphoric acid, butyral resin and the surface treatment liquid that promotes the formation of stable rust with the balance being a solvent is disclosed in Japanese Examined Patent Publication No. 06-21273, in which P, Cu, Cr, Ni, Si and Mo compounds A rust-stabilized surface treatment method is disclosed in which one or more kinds of Fe ₂ O ₃ + Fe ₃ O ₄ , phosphoric acid, a bisphenol-based epoxy resin, and the balance being a solvent and a coating solution containing a coating auxiliary agent are applied. However, none of these methods is an improvement of the weather-resistant steel material itself and has a problem in promoting the formation of stable rust. That is, the resin coating usually has minute defects,
The effect of the coating film cannot be expected at the defective portion. Furthermore, the progress of corrosion at the coating film defect portion causes crevice corrosion at the coating film-substrate interface, which may lead to peeling or dropping of the coating film itself before the formation of the stable rust layer. Therefore, the use of weather-resistant steel in an environment where the salt content is inevitable is restricted, which is a big problem.

Further, P and Cr are elements that greatly deteriorate the weldability, and the range of application of the weather resistant steel is to be narrowed in the structure where steel materials are often welded on site, especially in structures such as bridges. .

The present invention has been made in order to solve the above problems, and provides a coated steel material having both the durability and the good weldability of the coated steel material used in an environment in which salt scattering is inevitable. The purpose is to

[0007]

[Means for Solving the Problems] The gist thereof is mass%,
C: 0.20% or less, Si: 0.10 to 1.0%, Mn: 2.5% or less, Cu:
0.05 to 3.0%, Ni: 0.05 to 6.0%, S: 0.02% or less, P:
0.05% or less (excluding 0.05%) , Cr: 0.20% or less, Ti: 0.
A coated steel material containing 03 to 1.0% with the balance being Fe and unavoidable impurities with excellent durability.

Further, as a chemical component, Al: 0.05-0.50%
It is a coated steel material containing the above and having excellent durability.

Further, as a chemical component, Ca: 0.0001 to 0.01
%, La: 0.0001 to 0.05%, Ce: 0.0001 to 0.05%, and the coated steel material having excellent durability as described above.

Further, as a chemical component, Al: 0.05 to 0.50%
Contains Ca: 0.0001-0.01%, La: 0.0001-0.05%,
The above-mentioned coated steel material having excellent durability and containing at least one of Ce: 0.0001 to 0.05%.

Further, it is a basic component in consideration of weldability.
It is a coated steel material with a Cr content of 0.05% or less and excellent durability.

When a dense and stable rust layer is formed on the steel surface, corrosion-promoting factors such as water, oxygen or chlorine ions existing in the environment are hard to reach the steel which is the base material by physical or electrochemical action. It is known that the corrosion reaction of steel is delayed and the corrosion rate decreases to a negligible level without special anticorrosion treatment. Weather-resistant steel is a steel that positively utilizes the self-corrosion action due to such dense rust.

Specifically, a weather resistant steel can be obtained by adding a trace amount of an element such as Cr, Cu, Ni or P, which accelerates the densification of generated rust. That is, the weather resistant steel is a steel type that can exhibit its action and effect when used as a bare material. However, as described in the problems to be solved by the invention, the accelerating action of stable rust formation that the weather-resistant steel has is not sufficiently exerted in a salt damage environment in which flying salt content cannot be ignored. Therefore, various countermeasures have been devised, such as applying a thin resin coating film on the steel surface in order to prevent the flying salt from reaching the steel until stable rust is formed. The actual situation is that it is not an effective countermeasure due to the problem of defects.

As a result of extensive studies on the corrosion mechanism in the coating film defective portion, the present inventors have found that Cr contained as a steel component has an influence as a corrosion factor. That is, when the steel starts a corrosion reaction in a coating film defect, Cr ions dissolved in a trace amount together with iron atoms also add the action of Cl ions to cause a pH decrease in the defect, thereby increasing the oxidizability of aggregated water in the defect. It has been found that the acceleration has the effect of inducing crevice corrosion at the coating-substrate interface.

Therefore, it can be understood that the following three viewpoints are important as an idea of the component design for improving the durability of the resin coated weather resistant steel in the salt damage area by deduction from the above corrosion mechanism.

(1) Minimize the Cr addition amount to reduce the corrosion promoting factor in the defective portion of the coating film. (2) Search for and add a stable rust formation promoting element that is an alternative to Cr addition. When there is a resin coating film, it is difficult for salt to reach the steel due to the shielding effect of the coating film in the healthy part of the coating film, and also in the defective part of the coating film, if the defect (scratch) width is sufficiently small, The thickness of the film acts as a physical barrier, making it difficult for the incoming salt to reach the substrate. Therefore, if it is possible to control the remarkable decrease in pH inside the coating film defects and control the constituent elements, it is considered possible to provide a coated steel material having a long life even in a salt damage environment. (3) Searching for and adding an element having an action of buffering the pH decrease in the coating film defect. In other words, add an element that has the effect of raising the pH to the alkaline side by dissolving a trace amount.

If the above conditions are satisfied, the steel can form stable rust under the coating film defect, but the covering material covering the surface is the most common organic resin type because of economical efficiency, workability and simplicity. It is recommended to apply the paint. Polyester system,
Any resin can be applied as long as it can cover the steel surface such as epoxy type and urethane type, but in our experiments, it is tough, flexible, has high impact strength, and has excellent adhesion to metal. Butyral resin is the most excellent resin.

The present invention has been derived from the above viewpoints, and the effect of each added component and the reason for limiting the addition range will be described below.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION First, it is an essential element for weathering steel.
P and Cr will be described. P and Cr are stable rust layers
Weather resistance mainly for conventional naked use to promote generation
An element that is indispensable in steelIt However, P melts
It is an element that significantly deteriorates the contact property and is used for field welding of steel materials.
Weather resistance, especially in structures such as bridges
The scope of application of steel is to be narrowed. Because of this,
Considering the weldability of weatherable steel, P was set to 0.05% as the upper limit.
(Claim 1).Also, Cr is, as described above,
Causes a decrease in pH, and the oxidizability of aggregated moisture in the defects
Inducing crevice corrosion at the coating-substrate interface by promoting
Since it causes the action to occur, the upper limit of Cr is 0.2%.
(Claim 1).

However, Cr, like P, has a large poor weldability.
Since it is an element that makes the weathering steel more difficult to weld.
When setting a point , Cr is set to 0.05% as an upper limit (Claim 5). When importance is attached to weldability, the carbon equivalent (Ceq) may be taken into consideration within the component range of the present invention.

Ti is an essential additive element which is an alternative to the addition of Cr selected in the present invention from the idea of the above (2), and Cr, Cu,
Like Ni, it has a beneficial effect of densifying the generated rust and promoting the formation of a stable rust layer, and also has very excellent corrosion resistance. It also has the advantage of cleaning steel. Such an effect is obtained by adding 0.01% or more, but if added over 0.03%, the effect remarkably increases, and addition by 0.05% or more is more desirable. However, even if it is added excessively, the effect tends to be saturated and it is economically unfavorable, so the upper limit of Ti is 1.0%. Therefore, the Ti content is in the range of 0.01 to 1.0%.

Al, like Ti, is an additive element that is an alternative to the addition of Cr selected in the present invention from the idea of the above (2), and C
Similar to r, Cu, and Ni, it has the effect of densifying the generated rust and promoting the formation of a stable rust layer. Such an effect is obtained by adding 0.05% or more, and the effect is increased by the combined addition with Ti. However, even if excessive addition is performed, not only the effect shows a saturation tendency, but also deterioration of weldability is feared. Therefore, the upper limit of Al is 0.50%. Therefore, the Al content is
The range is 0.05 to 0.50%.

Ca, La and Ce are additive elements having a function of buffering the pH decrease in the coating film defects selected in the present invention from the idea of the above (3), and these elements are under coating film corrosion progress. In the process, it is an element that exhibits alkalinity due to a slight amount of dissolution associated with the corrosion reaction of iron (solution pH buffering effect at the anode dissolution tip), and has the effect of suppressing crevice corrosion at coating film defects.
That is, these elements raise the pH during dissolution and suppress crevice corrosion. As described above, Cr lowers the pH during dissolution and promotes crevice corrosion, but Cr increases the pH during dissolution (C
a, La, Ce) suppress the progress of crevice corrosion,
Moreover, if there is no element (Cr) that lowers the pH by dissolution, the progress of crevice corrosion can be prevented. Due to these synergistic effects, the coated steel material of the present invention suppresses the adverse effect of the chloride ion concentration due to salt from the outside in the coating film defect portion and exhibits excellent weather resistance. These elements exert their effects even when only one kind is added or when they are combined.

These elements exhibit the above effect when added in an amount of 0.0001% or more, but the effect is saturated even if the addition amount is excessively increased. Further, addition of a large amount of Ca has a risk of causing damage to the furnace wall of the refining furnace in steelmaking. Therefore, considering the upper limit of each addition amount, Ca content is in the range of 0.0001-0.01%, La content is 0.0001-0.05%.
%, And the Ce content is 0.0001 to 0.05%.

Next, the reasons for limiting C, Si, Mn, Cu, Ni and S will be described. C is an essential element for securing the strength of steel, but if the addition amount increases, the weldability and corrosion resistance deteriorate. Therefore, the C content is set to 0.20% or less. In order to secure sufficient weldability and corrosion resistance, C
The content is preferably 0.15% or less.

Si is a solid solution strengthening element, and it is necessary to add 0.10% or more to obtain its effect. Further, Si also promotes the formation of stable rust and has the effect of improving corrosion resistance. But,
Addition of a large amount causes a decrease in weldability. Therefore, Si
The content should be in the range of 0.10 to 1.0%. The desirable addition range is 0.15 to 0.65%.

Mn is effective as an element for securing strength instead of C. However, if it is present in the steel in a large amount, workability and toughness may be deteriorated and corrosion resistance may be deteriorated due to accelerated formation of MnS. Therefore, the Mn content is set to 2.5% or less.

Cu is an element that is electrochemically nobler than iron, and is an essential element that has the action of densifying the formed rust and promoting stable rust formation. Although such an effect is exhibited by the addition of 0.05% or more, even if it is added in an amount of more than 1.0%, no further effect is obtained, and rather the material may be embrittled during hot rolling. Therefore, the Cu content is 0.05-1.
The range is 0%. From the viewpoint of preventing hot brittleness,
The upper limit of the amount of Cu added is preferably 0.5%. However, if the content of S, which is a hot embrittlement promoting element, is 0.02% or less, Cu can be added up to 3.0%. In this case, S is involved in the formation of MnS or FeS, which is the starting point of corrosion, so the S content should be 0 to improve durability.
It is effective to limit it to 02% or less, and the effect is the upper limit of Cu.
It is remarkably demonstrated by adding up to 3%. The S content is preferably 0.01% or less, and more preferably 0.005% or less.

Ni is an element having the same corrosion resistance improving action as Cu, and it is necessary to add 0.05% or more to obtain the effect. Furthermore, Ni also has the effect of suppressing hot embrittlement, which is a concern when adding Cu. However, even if added over 3.0%, the effect of improving corrosion resistance is saturated. Therefore, Ni
The content should be 0.05 to 3.0%. However, if the content of S, which is an element that promotes hot embrittlement, is 0.02% or less, Ni can be added up to an upper limit of 6.0%. Also in this case, MnS or FeS becomes the starting point of corrosion as described above, so it is effective to limit the S content to 0.02% or less for improving durability, and the Ni addition amount is up to 6%. Increasing the amount inside is effective for improving durability.

As described above, as a similar method of adding an alkali metal, there is Japanese Patent No. 2572447, but this patent is not intended for coated steel materials, and as a result, it is a factor that accelerates corrosion in coating film defects. The technical idea of the present invention is fundamentally different from the technical idea of the present invention in that it makes no mention of a clear statement of reducing Cr or the effect of Ti peculiar to the present invention.

Further, it is possible to reduce the corrosion accelerating factor in the coating film defect portion (reduction of Cr), to add a stable rust formation accelerating element (combination addition of Ti and Al) as an alternative to Cr addition, Addition of elements (Ca, La,
The combined addition of Ce) and the reduction of elements that adversely affect the weldability (reduction of P and Cr) do not adversely affect each other, but have mutual effects. These elements can also be added in combination, since it may lead to the improvement of

[0032]

EXAMPLES Hereinafter, examples will be described. A test material was manufactured by combining a steel plate with a chemical composition shown in Table 1, Table 2 and Table 3 and a resin, and as shown in FIG. 1, a cross-cut type artificial coating film defect on the surface of the test material with a cutter knife. And the long-term durability of the test material was evaluated by an acceleration test and an atmospheric exposure test. With regard to the coating treatment on the steel sheet, in all paint systems, after sandblasting as a base treatment, spray coating was applied to a thickness of 10 μm. In the table, paint B means butyral resin, P means polyester resin, E means epoxy resin, U means urethane resin, and F means fluororesin.

The accelerated test is the combined cycle test shown in FIG. 2 (carbon arc lamp irradiation → salt water immersion (0.1% and 0.5%
And 3.0%)) → constant temperature and humidity), and after 60 cycles, the appearance and extent of undercoat corrosion from the crosscut were evaluated. Air exposure test, facing south, horizontal
1 year exposure at 30 ° tilt (once weekly 0.1% salt water spray)
did. In the evaluation of the atmospheric exposure test, the appearance and the spread width of undercoat corrosion from the crosscut were evaluated, as in the accelerated test. The spread width of the under-coating corrosion from the cross cut was evaluated by measuring the spread width at 8 points and averaging the measured values. Also, the rating number (RN) for appearance evaluation is 10 when the appearance is the best and decreases in order as the damage increases, and when the damage is the greatest (general corrosion)
I set it to 1. Relative to each comprehensive evaluation ◎, ○, △,
Displayed as x. The results are shown in Table 4, Table 5 and Table 6.

Further, FIG. 3 shows the relationship between the amount of Ti added and the weather resistance performed by the present inventors, apart from the above-mentioned examples.
The test material is a material according to claim 1 of the present invention, and the experimental method is the combined cycle test shown in FIG. The evaluation is a comprehensive evaluation based on the surface properties (rating number) and the blister width in the coating film defect portion. The weather resistance index in FIG. 3 is represented by 1-average blister width (mm), and the larger the index, the better the weather resistance. Although the weather resistance increases with an increase in the amount of Ti added, it can be seen that the effect is considerably high if added in an amount of 0.05% or more.

As is clear from Table 4, Table 5 and Table 6, the difference between superiority and inferiority between the coated steel material according to the present invention and the comparative steel is clear. The comparative steels will be explained individually. No. 1 is ordinary steel, No. 2 and No. 3 are so-called weathering steels, but Cr is
Since it contains, the pH is lowered and the spread of undercoat corrosion is large. No. 4 contains too much C, so No. 5 does not contain elements that promote the formation of stable rust that substitutes for Cr and elements that buffer the pH drop. It is considered that the results shown in Table 3 are obtained because the corrosion resistance is insufficient and the results are shown in Table 3. Also, No. 32 and N
In O.33, the amount of S was large, so hot cracking occurred in the steel sheet manufacturing process, so accelerated tests and atmospheric exposure tests could not be performed. The results shown in Table 4, Table 5 and Table 6 prove the sufficient usefulness of the present invention.

[0036]

[Table 1]

[0037]

[Table 2]

[0038]

[Table 3]

[0039]

[Table 4]

[0040]

[Table 5]

[0041]

[Table 6]

[0042]

As is clear from the above description, the coated steel material according to the present invention has excellent long-term durability even in a salt damage environment in which the influence of flying salt is not negligible, and the steel structure can be maintained. As a steel material that can minimize management work, it has excellent properties that it can be used in a wide range of areas from the inland area including the cold district where chloride is scattered on the road to the coastal suburbs, and the Cr and P contents are reduced. Therefore, it has good weldability that conventional weather-resistant steel does not have.

[Brief description of drawings]

FIG. 1 is a diagram showing the shapes of test materials used in an acceleration test and an atmospheric exposure test.

FIG. 2 is an explanatory diagram of combined cycle test conditions for accelerated tests.

FIG. 3 is a diagram showing the relationship between the Ti addition amount and weather resistance.

Front page continuation (72) Fumio Yuse 1-5-5 Takatsukadai, Nishi-ku, Kobe-shi, Hyogo Prefecture Kobe Steel Works, Kobe Research Institute (72) Inventor Toshiaki Suga 1 Kanazawa-machi, Kakogawa-shi, Hyogo Kobe Steel Co., Ltd. In Kakogawa Steel Works (56) Reference JP-A-8-157969 (JP, A) JP-A-3-253541 (JP, A) JP-A-9-67619 (JP, A) JP-A-57 -2865 (JP, A) JP 61-257456 (JP, A) JP 7-62446 (JP, A) JP 11-71632 (JP, A) JP 62-243738 (JP, A) ) (58) Fields surveyed (Int.Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (8)

(57) [Claims] 1. In mass%, C: 0.20% or less, Si: 0.10-1.
0%, Mn: 2.5% or less, Cu: 0.05 to 3.0%, Ni: 0.05 to 6.
0%, S: 0.02% or less, P: 0.05% or less (excluding 0.05%) ,
Cr: 0.20% or less, Ti: 0.03 to 1.0%, balance Fe
And coated steel material with excellent durability, which is characterized by comprising unavoidable impurities. 2. A coated steel material having excellent durability according to claim 1, further containing Al: 0.05 to 0.50% by mass . 3. Further, in mass% , Ca: 0.0001 to 0.01
%, La: 0.0001 to 0.05%, and Ce: 0.0001 to 0.05%, the coated steel material having excellent durability according to claim 1 or 2 . 4. Further, in mass% , Al: 0.05-0.50% is contained, Ca: 0.0001-0.01%, La: 0.0001-0.05%, C
The coated steel material with excellent durability according to any one of claims 1 to 3, which contains one or more of e: 0.0001 to 0.05%. 5. In mass%, C: 0.20% or less, Si: 0.10-1.
0%, Mn: 2.5% or less, Cu: 0.05 to 3.0%, Ni: 0.05 to 6.
0%, S: 0.02% or less, P: 0.05% or less (excluding 0.05%) ,
Cr: 0.05% or less, Ti: 0.03 to 1.0%, balance F
A coated steel material with excellent durability, which is characterized by comprising e and inevitable impurities. 6. The coated steel material with excellent durability according to claim 5, which further contains Al: 0.05 to 0.50% by mass . 7. Further, in mass% , Ca: 0.0001 to 0.01
%, La: 0.0001 to 0.05%, Ce: 0.0001 to 0.05%, and the coated steel material having excellent durability according to claim 5 or 6 , containing at least one of them. 8. Further, by mass% , Al: 0.05-0.50% is contained, Ca: 0.0001-0.01%, La: 0.0001-0.05%, C
The coated steel material having excellent durability according to any one of claims 5 to 7, which contains any one or more of e: 0.0001 to 0.05%.