JP3846218B2 - Structural steel with excellent weather resistance - Google Patents

Description

【００４１】
暴露試験結果と溶接割れ防止温度を表２に示す。耐候性鋼を無塗装で橋梁に適用する場合、腐食環境が厳しい内桁も含めて５０年間の板厚減少量が０．３ｍｍ以下であるためには、実施例の暴露形式では１年間の板厚減少量が０．０３０ｍｍ以下でなければならない。また１年間の流れ錆量が０．１５ｇ／ｍ²を超えると、構造物の汚損が目立つようになる。
【００４２】
【表２】

【００４３】
本発明鋼Ａ〜Ｆは、すべて板厚減少量が０．０２５ｍｍ以下であり、１年間の流れさび量が０．１５ｇ／ｍ^２以下である。従って、耐候性は十分優れており、流れさび量も十分小さい。また、溶接割れ防止予熱温度は室温であり、良好な溶接性も有している。
【００４４】
比較鋼Ｈ、Ｉについては、板厚減少量は小さいが、Ｐ含有量が本発明の範囲を超えて高すぎたために流れ錆量が多すぎた。
【００４５】
比較鋼Ｊについては、Ｎｉ含有量が本発明の範囲以下で低すぎたために、板厚減少量が大きすぎ、流れ錆量も多すぎた。
【００４６】
比較鋼Ｋについては、Ｍｏ含有量が本発明の範囲以下で低すぎたために、板厚減少量が大きすぎ、流れ錆量も多すぎた。
【００４７】
比較鋼Ｌについては、Ｎｉ、Ｍｏ含有量は本発明の範囲内であるが、Ｎｉ＋３Ｍｏ量が本発明の範囲以下のため、板厚減少量が大きすぎ、流れ錆量も多すぎた。
【００４８】
比較鋼Ｍについては、Ｎｉ、Ｍｏ含有量がいずれも発明の範囲以下で低すぎたために、板厚減少量が大きすぎ、流れ錆量も多すぎた。
【００４９】
比較鋼Ｎについては、Ｃｒ含有量が本発明の範囲を超えて高すぎたために穴あき腐食が生じた結果、板厚減少量が大きすぎ、流れ錆量も多すぎた。
【００５０】
比較鋼Ｏ、Ｐについては、Ｐｃｍが０．２５を超えて大きいために、溶接割れ防止予熱温度が１００℃と高く、溶接性に劣っていた。
【００５１】
【発明の効果】
以上説明したように、この発明によれば、飛来塩分量が０．０５ｍｄｄを超える環境においても良好な耐候性を有し、且つ初期の流れ錆発生の少ない耐候性鋼を提供でき、また、溶接性、靭性にも優れるため、溶接構造用鋼としても充分な性能を有し、産業上有用な効果をもたらすことが可能である。
【図面の簡単な説明】
【図１】流れ錆発生に及ぼすＰ含有量の影響を示すグラフである。[0001]
BACKGROUND OF THE INVENTION
This invention relates to a structural steel material, particularly a welded structure in a corrosive environment involving chlorides, such as bridges and steel towers constructed in areas where there is a lot of incoming chloride, 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 materials.
[0002]
[Prior art]
Steel structures such as bridges have a practical period of several decades, and many of them need to be subjected to anticorrosion treatment such as painting. Painting is a means having a high anticorrosion effect, but the coating film deteriorates over time and the anticorrosion function is reduced, so that periodic repair is required.
[0003]
However, particularly in recent years, there has been a problem that the repair becomes difficult due to a rise in labor costs and a decrease in the number of painters. In order to avoid this problem, an example in which weathering steel is applied to steel structures such as bridges is increasing.
[0004]
Weather-resistant steel is a steel material whose corrosion rate is remarkably reduced by being covered with a highly corrosion-resistant rust layer in which alloy elements such as copper, phosphorus and chromium are concentrated in an air exposure environment. Because of its high weather resistance, it is known that bridges using weathering steel can withstand service for decades, often unpainted.
[0005]
However, it is known that in an environment where there is a lot of incoming salt, such as the coastal area, the protection of the rust layer of the weather resistant steel is low, and it is difficult to obtain corrosion resistance that can withstand practical use. In order to produce a steel material that can be practically used in such an environment, methods such as adding a large amount of effective elements such as copper, phosphorus, chromium, tungsten, etc. are disclosed in Japanese Patent Publication Nos. 51-28048 and 57-10941, It is disclosed in Japanese Laid-Open Patent Publication No. 3-158436, Japanese Patent Laid-Open No. 4-6245, and the like.
[0006]
In recent years, the Ministry of Construction has conducted exposure corrosion tests on weathering steel at various locations in Japan, and as a region where weathering steel can be used without coating, the amount of salt that comes in is 0.05 mdd (mg / dm ² / day). The guideline is limited to the following areas.
[0007]
Therefore, the weathering steel produced in Japanese Patent Publication No. 51-28048, Japanese Patent Publication No. 57-10941, Japanese Patent Laid-Open No. 3-158436, Japanese Patent Laid-Open No. 4-6245, etc. is sufficient for use without coating. Does not have good weather resistance.
[0008]
Furthermore, flowing rust is often regarded as a problem of soiling structures when using weather-resistant steel without painting. This is because divalent ions generated by the dissolution of iron in the anode flow out of the rust layer formed on the steel surface, and then are dried by piers to become rust, which stains the piers brown. The amount of flowing rust is large within one year from the start of use. This is because the rust layer is not yet sufficiently densified at this time and the protective property is low.
[0009]
On the other hand, steel manufacturing technology with improved weather resistance in coastal areas is characterized by the method of improving the weather resistance of steel in environments with relatively high levels of incoming salt by adding a large amount of elements such as chromium and nickel. It is disclosed in, for example, Kaihei 7-207340 and Japanese Patent Laid-Open No. 7-242993.
[0010]
However, steel containing a large amount of chromium is liable to cause welding defects such as low-temperature cracking, and requires measures to prevent welding defects such as preheating. In the case of outdoor structures such as bridges, on-site work such as preheating and inspection of welding defects is difficult, resulting in 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 and the toughness becomes insufficient by ordinary hot rolling. That is, with these techniques, it is difficult to produce steel having practical weldability and toughness.
[0012]
In contrast, JP-A-8-134487 and JP-A-11-21622 disclose Ni-Mo based weathering steel that does not contain chromium. It has been found that, in a salty environment, chromium promotes perforation corrosion and is reported to exhibit excellent weather resistance.
[0013]
However, these weather resistances are evaluated by the maximum perforation depth, and the initial flow rust that impairs the appearance has not been studied.
[0014]
[Problems to be solved by the invention]
As described above, according to the above-described conventional technology, in a region where the amount of incoming salt is 0.05 mdd or more, a welded structural steel having practical weather resistance and weldability, and less flow rust and good toughness is manufactured. It was impossible to do.
[0015]
Accordingly, 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 content of 0.05 mdd or more and less than 0.5 mdd is flying.
[0016]
[Means for Solving the Problems]
In order to achieve the above-mentioned object, the present inventors diligently studied the component composition of the steel material from the viewpoint of suppressing flow rust in the Ni-Mo component system. As a result, the occurrence of flow rust was correlated with the P content, and it was found that the reduction of the P content was effective. In addition, it has been found that the lower limit of the amount of Ni is 1 mass% and the addition amount is increased from the conventional level, whereby the occurrence of flow rust is further suppressed, thereby achieving both high weather resistance and weldability and less flow rust. It was found that it is possible to produce steel materials with good toughness.
[0017]
The present invention has been made on the basis of the above findings, and is characterized by the following.
[0018]
The invention according to claim 1 is: 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: 0.30 to 1.5%, Al: 0.01 to 0.1%, P: less than 0.03%, Cr: 0.1% or less (more than, mass%), Ni and Mo satisfy Ni + 3Mo ≧ 1.5 mass%, Pcm: 0.25 mass% or less (provided that Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15), and the balance is Fe and inevitable impurities. It has the characteristics.
[0019]
The invention described in claim 2 is characterized in that, in the invention described in claim 1, it contains 0.25 mass% or less of Cu.
[0020]
DETAILED DESCRIPTION OF THE INVENTION
First, the reason why the component composition is limited to the above range in the present invention will be described.
[0021]
C:
C is added to ensure a predetermined strength, but if it exceeds 0.15 mass%, the weldability and toughness deteriorate, so the C content is set to 0.15 mass% or less.
[0022]
Si:
Si is added as a deoxidizing agent and a strength improving element at the time of steelmaking, but if it is added excessively exceeding 0.7 mass%, the toughness is remarkably lowered. Therefore, the Si content is set to 0.7 mass% or less.
[0023]
Mn:
Mn needs to be added in an amount of 0.2 mass% or more in order to ensure a predetermined strength. However, when it exceeds 1.5 mass% and it adds excessively, it will become easy to produce a bainite structure, and mechanical characteristics, especially toughness will deteriorate. Therefore, the Mn content is in the range of 0.2 to 1.5 mass%.
[0024]
P:
P is easily dissolved out in water as phosphate ions when the rust layer at the initial stage of use is thin, and easily comes out of the rust layer. Phosphate ions that have come out of the rust layer tend to precipitate iron divalent ions as rust, and may cause the piers to be soiled.
[0025]
FIG. 1 shows the effect of the amount of P on flow rust generation in a component system of 1.5Ni-0.3Mo. This is exposed to a coastal zone with a salt content of 0.4 mdd for one year, a wide-mouthed container is placed under the test piece, rust is collected with rainwater, etc., and the amount of Fe ions in the aqueous solution is determined by atomic absorption spectrometry. It is the result of obtaining the flow rust amount for one year.
[0026]
It can be seen from FIG. 1 that when the amount of P exceeds 0.03 mass%, the amount of Fe ions flowing out increases rapidly. Conventionally, P has been added in a certain amount because it has an anticorrosive effect in long-term use, but this can be compensated for by increasing the Ni content and the Mo content. Therefore, the P content is limited to 0.03 mass%, preferably 0.02 mass% or less from the viewpoint of suppressing flow rust.
[0027]
S:
Since S is an element harmful to weldability and toughness, it was set to 0.02 mass% or less.
[0028]
Al:
Al is added in an amount of 0.01 mass% or more as a deoxidizer during steelmaking. However, if excessively added, inclusions that become the starting point of corrosion tend to occur, so the content was made 0.1 mass% or less.
[0029]
Cr:
Since Cr has an effect of improving the weather resistance in an environment with a low salt content, Cr has been actively added to conventional steels. However, in the salty environment targeted by the steel of the present invention, there is an effect of promoting perforated corrosion. In addition, the hardness of the welded portion is increased to promote cold cracking, and the weldability is remarkably deteriorated. Therefore, it is important to regulate to 0.1 mass% or less, preferably 0.05 mass% or less.
[0030]
Ni:
Ni has the effect of improving the corrosion resistance in a salty environment due to the coexistence with Mo. Moreover, it contributes to the early stabilization of rust and has the effect of suppressing flow rust. In order to suppress flow rust, addition of 1 mass% or more is necessary to supplement the rust stabilization effect associated with the lowering of P. On the other hand, when the amount exceeds 4 mass%, the effect is saturated and disadvantageous in terms of economy, and a bainite structure is likely to occur, and mechanical properties, particularly toughness, are deteriorated. Therefore, the Ni content is in the range of 1 to 4 mass%.
[0031]
Mo:
Mo has the effect of improving the corrosion resistance in a salty environment due to the coexistence with Ni. Addition of less than 0.30 mass% is ineffective, while addition over 1.5 mass% is disadvantageous in terms of economy, and a bainite structure is likely to occur, and mechanical properties, particularly toughness, deteriorate. Therefore, the Mo content is set in the range of 0.30 to 1.5 mass%.
[0032]
Ni + 3Mo:
By adding Mo with an appropriate amount of Ni, the weather resistance is significantly improved. The details of the effect of Ni and Mo on the weather resistance are not clear, but are considered as follows.
[0033]
Mo is considered to have an effect of enhancing the tightness of rust and preventing corrosion factors such as moisture and salt from contacting 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 property of rust which is easily broken and making it difficult to cause defects such as cracks. Since the synergistic effect by these two different properties is exhibited, it is considered that the weather resistance is remarkably improved by adding Mo together with an appropriate amount of Ni. In order to compensate for the decrease in rust stabilization due to the reduction in the P content, it is necessary to add Ni and Mo that satisfy Ni + 3Mo ≧ 1.5 mass%.
[0034]
Cu:
Cu has the effect of improving the corrosion resistance, and can be added as necessary. However, if it exceeds 0.25 mass%, the effect is saturated and disadvantageous in terms of economy . It was made into 25 mass% or less.
[0035]
In addition, in order to prevent cold cracking in welding and make the preheating temperature at the time of welding work 50 degrees C or less, there is no problem in actual operation, Pcm defined by the following formula is 0.25 mass% or less Is preferred.
[0036]
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15
[0037]
The balance is Fe and inevitable impurities.
[0038]
In this invention, it manufactures to steel materials, such as a thick plate and a shape steel, by hot-rolling the slab obtained by the normal continuous casting and the block method. The heating and rolling conditions may be appropriately determined according to the required material, and a combination of controlled rolling, accelerated cooling, reheating heat treatment, or the like is also possible.
[0039]
【Example】
Steel ingots having the composition shown in Table 1 were melted and heated to 1200 ° C. to start hot rolling, rolling was completed at 850 ° C. at a cumulative reduction of 30% at 950 ° C. or less, and the thickness was 25 mm. A steel plate was prepared. After rolling, the product was air cooled to room temperature. The obtained steel plate was processed into 150 mm × 70 mm × 5 mm, surface ΔΔΔ finished test pieces and exposed to the coast with an annual salt content of 0.4 mdd for one year. During the exposure period, a wide-mouthed container was placed under the test piece, and the rust was collected with rain water and collected once a month. The amount of Fe ions in the collected aqueous solution was quantified by atomic absorption spectrometry, and the amount of flowing rust for one year was determined. After the exposure was completed, the test piece was immersed in an aqueous solution of hexamethylenetetramine added to hydrochloric acid and derusted, and then the weight was measured. The difference from the initial weight was obtained and converted into a reduction in thickness. Moreover, the y-type weld cracking test was implemented as weldability evaluation.
[0040]
[Table 1]

[0041]
Table 2 shows the results of the exposure test and the weld cracking prevention temperature. When weatherproof steel is applied to a bridge without painting, the reduction in thickness in 50 years, including the inner girder where the corrosive environment is severe, is 0.3 mm or less. The thickness reduction amount must be 0.030 mm or less. Moreover, when the flow rust amount for one year exceeds 0.15 g / m < ² >, the structure becomes conspicuous.
[0042]
[Table 2]

[0043]
The steels A to F of the present invention all have a plate thickness reduction amount of 0.025 mm or less and a flow rust amount of 0.15 g / m ² or less for one year. Therefore, the weather resistance is sufficiently excellent, and the flow rust amount is sufficiently small. Further, the preheating temperature for preventing weld cracking is room temperature, and it has good weldability.
[0044]
Regarding the comparative steels H and I, although the thickness reduction amount was small, the P content was too high exceeding the range of the present invention, so that the flow rust amount was too much.
[0045]
Regarding the comparative steel J, since the Ni content was too low below the range of the present invention, the thickness reduction amount was too large and the flow rust amount was too large.
[0046]
About comparative steel K, since Mo content was too low below the range of the present invention, the amount of board thickness reduction was too large, and the amount of flow rust was too much.
[0047]
Regarding the comparative steel L, the contents of Ni and Mo are within the range of the present invention, but since the amount of Ni + 3Mo is less than the range of the present invention, the thickness reduction amount is too large and the flow rust amount is too large.
[0048]
For the comparative steel M, the Ni and Mo contents were both too low within the scope of the invention, so that the reduction in plate thickness was too large and the flow rust amount was too large.
[0049]
As for the comparative steel N, the Cr content was too high exceeding the range of the present invention, and as a result of perforation corrosion, the thickness reduction amount was too large and the flow rust amount was too large.
[0050]
About comparative steel O and P, since Pcm was large exceeding 0.25, the welding crack prevention preheating temperature was as high as 100 degreeC, and it was inferior to weldability.
[0051]
【The invention's effect】
As described above, according to the present invention, it is possible to provide a weather resistant steel having good weather resistance even in an environment where the amount of incoming salt exceeds 0.05 mdd, and with less initial flow rust generation, and welding. Since it has excellent properties and toughness, it has sufficient performance as a welded structural steel and can bring about industrially useful effects.
[Brief description of the drawings]
FIG. 1 is a graph showing the effect of P content on flow rust generation.

Claims (2)

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: 0.30 to 1.5%,
Al: 0.01 to 0.1%
Containing
P: less than 0.03%,
Cr: 0.1% or less (above, mass%)
And Ni and Mo are Ni + 3Mo ≧ 1.5 mass%,
Pcm: 0.25 mass% or less,
However, Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15
The structural steel material which satisfy | fills and is excellent in the weather resistance which consists of remainder Fe and inevitable impurities .   The structural steel material excellent in weather resistance according to claim 1, wherein Cu is contained in an amount of 0.25 mass% or less.

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