JP5446278B2 - Structural steel with excellent weather resistance - Google Patents

Description

  The present invention relates to a structural steel material, particularly a welded structure in a corrosive environment in which chloride is involved, such as a bridge or a steel tower constructed in an area where there is a lot of incoming chloride, such as the vicinity of the coast or an area where antifreezing agents are dispersed. It is related with what is suitable for using.

  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.

  In recent years, repairs are becoming difficult due to soaring labor costs and a decrease in the number of painters, and an example in which weathering steel is applied to steel structures such as bridges is increasing.

  Weather-resistant steel is a steel material whose corrosion rate is remarkably reduced by being covered with a highly corrosion-resistant rust layer enriched with alloy elements such as copper, phosphorus, and chromium 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.

  However, in coastal areas where there is a lot of incoming salt, the protection of the rust layer of weathering steel that is intended for use in atmospheric exposure environments is low. To improve the weather resistance in these environments, copper, phosphorus, chromium In addition, a weather-resistant steel to which a large amount of an effective element such as tungsten is added has been developed (for example, Patent Document 1, Patent Document 2, Patent Document 3, and Patent Document 4).

On the other hand, the Ministry of Construction, based on the results of exposure corrosion tests on weathering steel in various places in Japan, the area where the weathering steel can be used without coating is 0.05 mdd (mg / dm ² / day). The guideline is limited to the following areas.

  In such an environment, the weather resistant steels described in Patent Documents 1 to 4 do not have sufficient weather resistance to be used without coating. Therefore, in order to improve the weather resistance in coastal areas, chromium or nickel Patent Document 5, Patent Document 6 and the like disclose steel having excellent weather resistance in an environment in which a large amount of elements such as these are added and the amount of incoming salt is relatively high.

  Further, in a salty environment, chromium has been found to promote perforation corrosion, and Ni-Mo based weathering steel containing no chromium is disclosed in Patent Document 7 and Patent Document 8.

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 Japanese Patent Laid-Open No. 7-207340 Japanese Patent Laid-Open No. 7-242993 JP-A-8-134487 Japanese Patent Laid-Open No. 11-21622

  However, steels containing a large amount of chromium described in Patent Document 5 and Patent Document 6 are prone to weld defects such as low-temperature cracks, and require measures to prevent weld 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.

  In addition, because it contains a large amount of nickel, hardenability increases, and by normal hot rolling, a bainite structure is precipitated, toughness is insufficient, and steel with practical weldability and toughness is produced. Difficult to do.

  The weather resistant steels described in Patent Document 7 and Patent Document 8 contain a large amount of Ni and Mo, which are Ni—Mo-based components and expensive alloy elements, and thus the steel material cost is significantly increased.

  As described above, it was impossible to produce structural steel having practical weather resistance and weldability with an inexpensive component composition in an area where the amount of incoming salt was 0.05 mdd or more.

  Therefore, the present invention is an inexpensive component composition that does not use a large amount of expensive elements such as Ni, Cr, and Mo, and has excellent weather resistance in an environment where a salt content of 0.05 to less than 0.5 mdd is flying. The purpose is to provide steel.

  The present inventors have studied earnestly about the component composition of steel from the viewpoint of flow rust suppression, which is important in the evaluation of weather resistance, and that it is effective that an element that improves weather resistance is contained in a certain amount or more in the ratio with C. I found out.

The present invention has been further studied on the basis of the above knowledge, that is, the present invention,
1. In mass%, C: 0.15% or less, Si: 0.7% or less, Mn: 0.2 to 1.5%, P: less than 0.03%, S: 0.02% or less, Cu: 0 0.05-1.0%, Ni: 0.05-1.0%, Cr: 0.03-1.0%, Al: 0.01-0.1%, Zr: 0.01-1.0 % And one or more of V, Mo, W, Ti, Nb, and Ta are contained in an amount of 0.03 to 1.0%, satisfying the following formulas (1) and (2), and the balance being substantially Structural steel with excellent weather resistance, made of Fe.
Σ = (V / 4 + Mo / 8 + W / 15 + Ti / 4 + Nb / 8 + Ta / 15 + Zr / 8) / C> 1 (1)
Pcm (%) = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B <0.25 (0.25)
However, in the formulas (1) and (2), each element is a content, and an element not contained is 0.
2. The structural steel material having excellent weather resistance according to 1, further comprising Sb and / or Sn having a total composition of 0.1% or less by mass.

  According to the present invention, it is possible to obtain a steel that has good weather resistance even in an environment where the amount of incoming salt exceeds 0.05 mdd and that can be used in large quantities as a structural steel at low cost, and is extremely useful in industry. .

The figure which shows the influence of parameter formula: (SIGMA) = (V / 4 + Mo / 8 + W / 15 + Ti / 4 + Nb / 8 + Ta / 15 + Zr / 8) / C which affects on board thickness reduction.

  [Ingredient composition] In the description, “%” means “mass%”.

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

Si
Si is added as a deoxidizing agent and a strength improving element at the time of steel making, but if it is added excessively exceeding 0.7%, the toughness is remarkably lowered. Therefore, the Si content is set to 0.7% or less.

Mn
Mn needs to be added by 0.2% or more in order to ensure a predetermined strength. However, if it is added in excess of 1.5%, a bainite structure is likely to be generated, and mechanical properties, particularly toughness, are deteriorated. Therefore, the Mn content is in the range of 0.2 to 1.5%.

P
In the stage where the rust layer in the initial stage of use is thin, P is easily dissolved out in water as phosphate ions and goes out of the rust layer. Phosphate ions that have come out of the rust layer tend to precipitate iron divalent ions as rust and cause the piers to become dirty. Therefore, the P content is limited to 0.03%, preferably 0.02% or less, from the viewpoint of suppressing flow rust.

S
S is an element harmful to weldability and toughness. Therefore, the S content is set to 0.02% or less.

Cu
Since Cu has an effect of improving the weather resistance, 0.05% or more is added. On the other hand, if it exceeds 1.0%, it is economically disadvantageous, so 0.05 to 1.0%.

Ni
Ni has the effect of improving the weather resistance in a salty environment by coexistence with Mo. Moreover, since it has the effect of suppressing flow rust by contributing to the early stabilization of rust, 0.05% or more is added. On the other hand, steel containing a large amount of nickel is disadvantageous in terms of economy, and a bainite structure is likely to occur, and mechanical properties, particularly toughness, are deteriorated.

Cr
Since Cr has an effect of improving weather resistance, 0.03% or more is added. However, when a large amount of Cr is contained, perforation corrosion is promoted in a salty environment which is the object of the steel of the present invention. 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 1.0% or less, preferably 0.5% or less.

Al
Al is added in an amount of 0.01% or more as a deoxidizer at the time of steelmaking. However, if excessively added, inclusions that become the starting point of corrosion are likely to be generated, so the content was made 0.1% or less.

Zr
Zr has the effect of improving the weather resistance, so 0.01% or more is added. However, if a large amount of Zr is contained, the hardness of the welded portion is increased to promote cold cracking, and the weldability is significantly deteriorated. Therefore, the Zr content is set to 0.01 to 1.0%. It is important to regulate to 0.5% or less.

One or more of V, Mo, W, Ti, Nb and Ta are added, or one or more of V, Mo, W, Ti, Nb and Ta are added. If each addition amount is less than 0.03%, the effect of improving weather resistance is hardly seen, so the content is made 0.03% or more. On the other hand, since the effect will be saturated if it exceeds 1.0%, when adding, it is made 0.03-1.0%.

Sb and / or Sn
In the present invention, in order to further improve the weather resistance, it is possible to contain Sb and / or Sn whose total amount is 0.1% or less by mass%. Since Sb and / or Sn deteriorates the base metal toughness and the HAZ part toughness, the total amount is 0.1% or less by mass%.

  Σ = (V / 4 + Mo / 8 + W / 15 + Ti / 4 + Nb / 8 + Ta / 15 + Zr / 8) / C, where each element is a content (mass%), and 0 is not included.

  This parameter formula (Σ) is to make the steel in the above component range excellent in weather resistance, and Σ = (V / 4 + Mo / 8 + W / 15 + Ti / 4 + Nb / 8 + Ta / 15 + Zr / 8) / C> 1. .

  FIG. 1 shows the effect of this parameter equation (Σ) on the thickness reduction amount. Details of the effect on the weather resistance of V, Mo, W, Ti, Nb, and Ta are not clear, but a certain relationship with the C content is recognized, and V / 4 + Mo / 8 + W / 15 + Ti / 4 + Nb / 8 + Ta / 15 + Zr. When / 8 exceeds the C content, Σ> 1. FIG. 1 is a graph showing the relationship between Σ and the plate thickness reduction amount (mm) for inventive examples and comparative examples in the examples described later. From this result, it can be seen that Σ> 1 is essential for the thickness reduction amount to be 0.03 mm or less.

Pcm (%) = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B <0.25
Pcm is set to 0.25 or less so that the preheating temperature at the time of welding is 50 ° C. or less so that it can be easily used as structural steel. However, the content of each element (mass%) is set to 0 if not included.

  In addition, the balance being substantially made of Fe means that unavoidable impurities and other elements may be contained as long as the effects of the present invention are not impaired.

  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.

  Steel ingots having the composition shown in Table 1 are melted and heated to 1200 ° C. to start hot rolling, rolling is finished at 850 ° C. at a cumulative reduction of 30% at 950 ° C. or lower, and air-cooled to room temperature. Thus, a steel plate having a thickness of 25 mm was manufactured.

  Each obtained steel plate was processed into a test piece having a surface of 150 mm × 70 mm × 5 mm and a surface ΔΔΔ finish, and exposed to a coast having an annual salt content of 0.1 mdd for one year. After completion of the exposure, 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 determined and converted into an average reduction in thickness.

Moreover, the y-type weld cracking test was implemented as weldability evaluation. The test was conducted with CO ₂ welding as the test bead in an atmosphere of 30 ° -80%, and the preheating temperature for preventing weld cracking was set to room temperature.

  When weatherproof steel is applied to a bridge without painting, it is required that the average thickness reduction amount for 50 years including the inner girder where the corrosive environment is severe is 0.3 mm or less. The average annual thickness reduction is equivalent to 0.030 mm or less). The range of 0.03 mm or less was defined as the scope of the present invention.

  Table 2 shows the results of the exposure test and the weld cracking prevention temperature. Test No. Inventive steels A to O of 1 to 8 all have an average thickness reduction of 0.03 mm or less, a weld crack prevention preheating temperature is room temperature, and have excellent weather resistance and good weldability. Was confirmed.

  On the other hand, Test No. whose component composition is outside the scope of the present invention. Comparative steels I to W of Nos. 9 to 23 have an average thickness reduction amount exceeding 0.03 mm. The comparative steels J, M, O, R, and T to W of 10, 13, 15, 18, 20 to 23 had a high weld cracking prevention preheating temperature of 100 ° C. and poor weldability.

Steel ingots having the component compositions shown in Table 3 were melted, and a steel plate having a thickness of 25 mm was produced according to the production conditions of Example 1. In addition, each molten steel was made into the component composition in the range of this invention of Claim 2. About the obtained steel plate, according to Example 1, the average thickness reduction amount and the welding crack prevention preheating temperature were calculated | required.

  Table 4 shows the exposure test results and the weld cracking prevention temperature. Test No. In all of 1 to 6, it was confirmed that the average thickness reduction amount was 0.03 mm or less, the weld crack prevention preheating temperature was room temperature, and excellent weather resistance and good weldability were obtained.

Claims (2)

In mass%, C: 0.15% or less, Si: 0.7% or less, Mn: 0.2 to 1.5%, P: less than 0.03%, S: 0.02% or less, Cu: 0 0.05-1.0%, Ni: 0.05-1.0%, Cr: 0.03-1.0%, Al: 0.01-0.1%, Zr: 0.01-1.0 % And one or more of V, Mo, W, Ti, Nb, and Ta are contained in an amount of 0.03 to 1.0%, satisfying the following formulas (1) and (2), and the balance being substantially Structural steel with excellent weather resistance, made of Fe.
Σ = (V / 4 + Mo / 8 + W / 15 + Ti / 4 + Nb / 8 + Ta / 15 + Zr / 8) / C> 1 (1)
Pcm (%) = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B <0.25 (0.25)
However, in the formulas (1) and (2), each element is a content, and an element not contained is 0.   The structural steel material having excellent weather resistance according to claim 1, further comprising Sb and / or Sn having a total amount of 0.1% or less in terms of component composition.

Images