JP2002309340A - Structural steel having excellent weather resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide structural steel which has excellent weather and flowing- rust resistances in an environment where salt of 0.05 to <0.5 mdd flies, and has good toughness. SOLUTION: The structural steel has a composition containing, by mass, <=0.15% C, <=0.7% Si, 0.2 to 1.5% Mn, <=0.02% S, 1 to 4% Ni, 0.1 to 1.5% Mo and 0.01 to 0.1% Al, and in which the contents of P are controlled to <0.03%, and Cr to <=0.1%, and Ni and Mo satisfy Ni+3Mo>=1.5 mass%, and the balance substantially Fe.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to structural steel materials, particularly chlorides such as bridges and steel towers constructed in areas with a large amount of flying chlorides, such as near the coast or in areas where antifreeze is sprayed. The present invention relates to a weather-resistant steel having high corrosion resistance, high weldability and good toughness suitable for a welded structure material in a corrosive environment.

[0002]

2. Description of the Related Art Steel structures such as bridges have a practical use period of several tens of years, and therefore, many of them require anticorrosion treatment such as painting. Painting is a means having a high anticorrosion effect, but it requires periodic repair because the coating deteriorates with time and the anticorrosion function is reduced.

[0003] In recent years, however, there has been a problem that repairs become difficult due to a rise in labor costs and a decrease in the number of painters. To avoid this problem, weather resistant steel is increasingly applied to steel structures such as bridges.

[0004] In weather-resistant steel, in an atmospheric exposure environment,
A steel material whose corrosion rate is significantly reduced by being covered with a highly corrosion-resistant rust layer in which alloying elements such as copper, phosphorus and chromium are concentrated. Due to its high weather resistance, bridges made of weather resistant steel are known to withstand decades of service, often unpainted.

[0005] However, it is known that in an environment with a high amount of flying salt, such as in a coastal area, the protection of the rust layer of weather-resistant steel is low, and it is difficult to obtain corrosion resistance sufficient for practical use. To produce steel materials that can be used in such environments, copper, phosphorus, chromium,
Methods such as adding a large amount of an effective element such as tungsten are disclosed in JP-B-51-28048 and JP-B-57-1.
No. 0941, JP-A-3-158436, JP-A-4-6245 and the like.

In recent years, the Ministry of Construction has conducted an exposure corrosion test on weather-resistant steel at various places in Japan, and found that the weather-resistant steel could be used as an unpainted area and the amount of incoming salt was 0.05 mdd.
(Mg / dm ² / day).

Accordingly, Japanese Patent Publication No. Sho 51-28048,
JP-B-57-10941, JP-A-3-15843
No. 6, JP-A-4-6245, etc., does not have sufficient weather resistance for use without coating.

[0008] Furthermore, when using weather-resistant steel without painting,
Flow rust is often regarded as a problem that stains a structure. This is due to the anodic dissolution of iron 2
After the valence ions flow out of the rust layer generated on the surface of the steel material, they are dried and become rust on the pier, etc., and stain the pier, etc. brown. The amount of flow rust generated is large within one year from the start of use. This is because the rust layer has not been sufficiently densified at this time, and the protection property is low.

On the other hand, as a technique for producing steel having improved weatherability in coastal areas, a large amount of elements such as chromium and nickel are added to improve the weatherability of steel in an environment where the amount of flying salt is relatively high. The method is disclosed in
No. 7,340, JP-A-7-242993 and the like.

[0010] However, steel containing a large amount of chromium tends to cause welding defects such as low-temperature cracking, and requires measures such as preheating to prevent welding defects. In the case of an outdoor structure such as a bridge, it is difficult to perform on-site work such as preheating and inspection for welding defects, which causes adverse effects such as an increase in construction costs.

[0011] Further, steel containing a large amount of nickel has increased hardenability, and bainite structure is precipitated by ordinary hot rolling, resulting in insufficient toughness. That is, with these techniques, it is difficult to produce steel having practical weldability and toughness.

On the other hand, JP-A-8-134587 and JP-A-11-21622 disclose weather-resistant steels containing a chromium-free Ni-Mo component.
This has been found that chromium promotes perforated corrosion in a salty environment, and is reported to exhibit excellent weatherability.

[0013] However, these weather resistances are evaluated at the maximum perforation depth, and initial flow rust that impairs the appearance is not studied.

[0014]

As described above, according to the above-mentioned prior art, in regions where the amount of flying salt is 0.05 mdd or more, practical weatherability and weldability are obtained, flow rust is reduced, and toughness is reduced. It was not possible to produce a welded structural steel of good quality.

Therefore, the object of the present invention is to provide
An object of the present invention is to provide a structural steel having excellent weather resistance, low flow rust, and good toughness in an environment where a salt of d or more and less than 0.5 mdd is flying.

[0016]

Means for Solving the Problems In order to achieve the above object, the present inventors have intensively studied the composition of a steel material in a Ni-Mo component system from the viewpoint of suppressing flow rust. As a result, it was found that the occurrence of the flow rust had a correlation with the P content, and that the reduction of the P content was effective. Further, it has been found that the generation of flow rust is further suppressed by increasing the lower limit of the Ni content to 1 mass%, which is higher than in the past, thereby achieving both high weather resistance and weldability, and reducing flow rust. It has been found that it is possible to produce a steel material having good toughness.

The present invention has been made based on the above findings, and has the following features.

The invention according to claim 1 is characterized in that: C: 0.15% or less, Si: 0.7% or less, Mn: 0.2 to 1.5%,
S: 0.02% or less, Ni: 1 to 4%, Mo: 0.1
To 1.5%, Al: 0.01 to 0.1%,
P: less than 0.03%, Cr: 0.1% or less (or more, ma
ss%) and Ni and Mo are Ni + 3Mo ≧
It is characterized in that it satisfies 1.5 mass% and the balance substantially consists of Fe.

A second aspect of the present invention is characterized in that, in the first aspect of the present invention, Cu is contained in an amount of 0.4 mass% or less.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First, the reason why the composition of the present invention is limited to the above range will be described.

C: C is added in order to secure a predetermined strength, but if it exceeds 0.15 mass%, the weldability and toughness deteriorate, so the C content was made 0.15 mass% or less.

Si: Si is added as a deoxidizing agent and a strength improving element at the time of steel making, but when added in excess of 0.7 mass%, the toughness is significantly reduced. Therefore, Si
The content was set to 0.7 mass% or less.

Mn: Mn needs to be added in an amount of 0.2 mass% or more in order to secure a predetermined strength. But,
If it is added in excess of 1.5 mass%, a bainite structure is likely to be generated, and mechanical properties, particularly, toughness are deteriorated. Therefore, the Mn content is between 0.2 and 1.5 mas.
s%.

P: At the initial stage of use, when the rust layer is thin, P easily dissolves in water as phosphate ions and easily exits the rust layer. Phosphate ions that have come out of the rust layer can easily precipitate iron divalent ions as rust, which can cause soiling of the pier.

FIG. 1 shows the effect of the amount of P on the generation of flowing rust in a 1.5Ni-0.3Mo component system. It was exposed to a coastal area with a flying salt content of 0.4 mdd for one year, placed a wide-mouthed container under the test piece, collected rust flowing with rainwater, etc., and quantified the amount of Fe ions in the aqueous solution by atomic absorption spectrometry. It is the result of calculating the flow rust amount for one year.

FIG. 1 shows that when the P content exceeds 0.03 mass%, the outflow of Fe ions sharply increases. Conventionally, P has been added in a certain amount because it has an anticorrosive effect in long-term use.
This can be compensated for by increasing the o content. Therefore, the P content is 0.0% from the viewpoint of suppressing flow rust.
It is limited to 3 mass%, preferably 0.02 mass% or less.

S: Since S is an element harmful to weldability and toughness, S is set to 0.02 mass% or less.

Al: Al is used as a deoxidizing agent in steelmaking at 0.1%.
Although 0.1 mass% or more is added, if it is excessively added, inclusions serving as corrosion starting points are easily generated.
ass% or less.

Cr: Since Cr has the effect of improving weather resistance in an environment with a low salt content, it has been positively added to conventional steels. However, in an environment with a large amount of salt, which is a target of the steel of the present invention, there is an effect of promoting perforation corrosion. In addition, the hardness of the weld is increased to promote low-temperature cracking, thereby significantly deteriorating weldability. Therefore, 0.1ma
It is important to regulate the content to ss% or less, preferably 0.05 mass% or less.

Ni: Ni has an effect of improving corrosion resistance in an environment with a high salt content due to coexistence with Mo. It also has the effect of contributing to early stabilization of rust and suppressing flow rust. In order to suppress the flow rust and further supplement the rust stabilizing effect accompanying the lower P, it is necessary to add 1 mass% or more. On the other hand, if the content exceeds 4 mass%, the effect is saturated and the economy is disadvantageous, and a bainite structure is easily formed, and mechanical properties, particularly toughness, are deteriorated.
Therefore, the Ni content is in the range of 1 to 4 mass%.

Mo: Mo has the effect of improving the corrosion resistance in an environment with a high salt content by coexistence with Ni. 0.
Addition of less than 1 mass% has no effect, and 1.5mass
Addition of more than s% is disadvantageous in terms of economy, and
A bainite structure is likely to occur, and mechanical properties, particularly toughness, deteriorate. Therefore, the Mo content ranges from 0.1 to 1.
It was within the range of 5 mass%.

Ni + 3Mo: By adding Mo together with an appropriate amount of Ni, the weather resistance is significantly improved. N
The details of the effects of i and Mo on the weather resistance are not clear, but are considered as follows.

Mo is considered to have the effect of increasing the rusting of the rust and preventing corrosion factors such as moisture and salt from coming into contact with the steel surface. On the other hand, Mo has the property of making rust brittle, and defects such as cracks are likely to occur. Ni
Has the property of improving the properties of fragile rust and making defects such as cracks less likely to occur. Since a synergistic effect is exhibited by these two different properties, Mo is added to an appropriate amount of N
It is considered that the weather resistance is remarkably improved by the addition with i. In order to compensate for the decrease in rust stabilization due to the decrease in the P content, it is necessary to add Ni and Mo satisfying Ni + 3Mo ≧ 1.5 mass%.

Cu: Cu has an effect of improving corrosion resistance and can be added as needed.
If the addition exceeds 3, the effect is saturated, and it is disadvantageous in terms of economy. Therefore, the addition is set to 0.4 mass% or less.

In order to prevent low-temperature cracking during welding and to make the preheating temperature at the time of welding less than 50 ° C. which is not a problem in actual operation, Pcm defined by the following equation is 0.25.
It is preferably at most mass%.

Pcm = C + Si / 30 + Mn / 20 + C
u / 20 + Ni / 60 + Cr / 20 + Mo / 15

It should be noted that the balance substantially consists of Fe,
This means that unavoidable impurities and other elements may be contained as long as the effects of the present invention are not impaired.

In the present invention, a slab obtained by ordinary continuous casting or sizing is hot-rolled to produce a slab such as a thick plate or a section steel. The heating and rolling conditions may be appropriately determined according to the required material, and a combination of controlled rolling, accelerated cooling, or reheating heat treatment is also possible.

[0039]

EXAMPLE A steel ingot having the composition shown in Table 1 was melted,
Start hot rolling by heating to 0 ° C.
At 850 ° C. with a cumulative reduction of 25% and a thickness of 25%.
mm steel plate was prepared. After the completion of the rolling, it was air-cooled to room temperature. The obtained steel plate was processed into a test piece having a surface of 150 mm x 70 mm x 5 mm and a surface of?
Exposure to a 4 mdd shore for one year. During the exposure period, a wide-mouthed container was placed under the test piece to collect rust flowing with rainwater and collected once a month. The amount of Fe ions in the collected aqueous solution was quantified by an atomic absorption method, and the flow rust amount for one year was determined. After the end of the exposure, the test piece was immersed in an aqueous solution of hydrochloric acid and hexamethylenetetramine to remove rust, and the weight was measured. In addition, a y-type weld crack test was performed as an evaluation of weldability.

[0040]

[Table 1]

Table 2 shows the results of the exposure test and the welding crack prevention temperature. When weather-resistant steel is applied to bridges without painting, the reduction in sheet thickness for 50 years, including the inner girder where the corrosive environment is severe, is 0.3
In order to be below mm, the amount of reduction in thickness per year must be below 0.030 mm in the exposure type of the embodiment. Further, if the flow rust amount per year exceeds 0.15 g / m ² , the soiling of the structure becomes noticeable.

[0042]

[Table 2]

The steels A to G of the present invention all have a reduction in thickness of 0.025 mm or less and a flow rust amount of 0.1 year per year.
5 g / m ² or less. Therefore, the weather resistance is sufficiently excellent and the amount of flowing rust is sufficiently small. In addition, the welding crack prevention preheating temperature is room temperature, and it has good weldability.

For Comparative Steels H and I, the amount of reduction in thickness was small, but the amount of flowing rust was too large because the P content was too high beyond the range of the present invention.

In Comparative Steel J, since the Ni content was too low below the range of the present invention, the reduction in sheet thickness was too large.
The amount of flowing rust was too large.

For the comparative steel K, the Mo content was too low below the range of the present invention, so that the reduction in the thickness was too large,
The amount of flowing rust was too large.

For the comparative steel L, the contents of Ni and Mo were within the range of the present invention, but the amount of Ni + 3Mo was less than the range of the present invention, so the reduction in sheet thickness was too large and the amount of flowing rust was too large. .

For the comparative steel M, the Ni and Mo contents were both below the range of the present invention and were too low, so that the reduction in sheet thickness was too large and the amount of flowing rust was too large.

For Comparative Steel N, the Cr content was too high beyond the range of the present invention, and as a result, pitting corrosion occurred. As a result, the reduction in sheet thickness was too large, and the amount of flowing rust was too large.

For the comparative steels O and P, Pcm is 0.2
Since it is larger than 5, the welding crack prevention preheating temperature is 10
The temperature was as high as 0 ° C., and the weldability was poor.

[0051]

As described above, according to the present invention, there is provided a weather-resistant steel having good weather resistance even in an environment where the amount of flying salt exceeds 0.05 mdd, and with little initial flow rust. In addition, since it has excellent weldability and toughness, it has sufficient performance as a welded structural steel, and can bring about an industrially useful effect.

[Brief description of the drawings]

FIG. 1 is a graph showing the effect of P content on flow rust generation.

Claims (2)

[Claims] C: 0.15% or less, Si: 0.7% or less, Mn: 0.2 to 1.5%, S: 0.02% or less, Ni: 1 to 4%, Mo: 0 0.1 to 1.5%, Al: 0.01 to 0.1%, P: less than 0.03%, Cr: 0.1% or less (more than mass%), and Ni And Mo satisfy Ni + 3Mo ≧ 1.5 mass%, and the balance is substantially Fe, and is a structural steel material excellent in weather resistance. 2. The structural steel material having excellent weather resistance according to claim 1, wherein the steel material contains 0.4 mass% or less of Cu.