JP2699184B2 - ERW steel pipe with high resistance to selective corrosion of ERW - Google Patents

Description

The present invention relates to an electric resistance welded steel pipe having high resistance to selective corrosion of an electric resistance welded portion.

[Conventional technology]

Conventionally, ERW steel pipes are inexpensive and have good dimensional accuracy, so they have been used in large quantities as various pipes. Recently, the reliability of welded parts has improved dramatically, Are also diverse.

However, in the conventional use results, when using ERW steel pipes in a wet environment, the corrosion rate of the ERW part is higher than that of the base material part, selective corrosion (often called pitting or grooved corrosion) It has been reported that the problem may occur, and as one of the drawbacks of the ERW pipe, it is an obstacle to the expansion of the use of the ERW pipe. Therefore, there is a strong demand for an ERW steel pipe having high resistance to selective corrosion of the ERW part.

On the other hand, a technique for preventing selective corrosion of the electric resistance welded portion has been proposed. For example, Japanese Patent Publication No. 60-9096 discloses that the C content is reduced and Si, Mn, Cu or even Nb, V, B
An electric resistance welded steel pipe having improved pitting corrosion resistance by adjusting the content thereof is disclosed. In addition, Japanese Patent Publication No. 60-37173 discloses that the ERW part of an ERW steel pipe made from a steel strip with adjusted components such as C, Si, Mn, P, Al, and rare earth elements is heat treated to improve the pitting corrosion resistance. The disclosed ERW steel pipe is disclosed.

[Problems to be solved by the invention]

An object of the present invention is to provide an inexpensive ERW steel pipe having high resistance to selective corrosion of an ERW part.

[Means for solving the problem]

The present inventors have conducted a number of experiments and detailed studies from various viewpoints such as components and manufacturing processes in order to obtain an ERW steel pipe having high resistance to selective corrosion of the ERW part. By adding a large amount of C as compared with the above, the resistance to the selective corrosion of the electric resistance welded portion can be significantly improved, and at least one of Al and Cr is added, or at least one of Ti, Zr and Y is added. Resistance to selective corrosion of ERW by adding one or more of Cu, Ni, Sb, Sn, Mo, Nb, V, W, Ca and rare earth elements, or by combining them Was found to be able to be further improved.

[Action]

According to the study of the present inventors, the cause of the selective corrosion of the ERW part in the wet environment in the ERW steel pipe is that the ERW part (welded part) and the heat affected zone in the immediate vicinity thereof are compared with the surrounding base material part. Thus, a local anode having a low corrosion potential is formed, and a base material portion relatively functions as a cathode. In this case, since the area of the local anode (electrically welded portion) is considerably smaller than that of the cathode (base material), selective corrosion of the electrical welded portion is accelerated. Conventionally, it has been considered that the presence of a small amount (0.3% or less) of C in steel deteriorates the corrosion resistance of steel and further promotes the selective corrosion of the ERW part in ERW steel pipes. Therefore, as a means for improving the resistance of the electric resistance welded portion to the selective corrosion, it has been general to reduce the C content as much as possible.

On the other hand, the present inventors have analyzed in detail the influence of the base metal component on the selective corrosion of the electric resistance welded portion. As a result, contrary to the conventional wisdom, when the C content is increased, the C content becomes 0.65%. As described above, it has been found that the corrosion potential of the electric resistance welded portion becomes more noble than that of the base material, that is, the electric resistance welded portion serves as a local cathode and the base material portion serves as an anode. Further, since the area of the local cathode (electrically welded portion) is very small compared to the area of the anode (base material portion), the entire ERW steel pipe including the electrically welded portion is substantially uniformly corroded.
That is, it has been found that selective corrosion of the electric resistance welded portion can be suppressed. This is an epoch-making knowledge that has never been known before, and is a new means for suppressing the selective corrosion of the electric resistance welded portion. It should be noted that, in the present invention, the selective corrosion of the electric resistance welded portion can be suppressed without extremely reducing the S in the steel, which is conventionally considered to significantly promote the selective corrosion of the electric resistance welded portion. It is sufficient that S is controlled to a level that does not hinder the production of the ERW steel pipe from the viewpoint of ERW weldability and workability as described later. In conventional erosion-resistant ERW steel pipes, the S content
Although there were not a few of them which were reduced to 0.005% or less, it was quite costly to reduce the S content, whereas C used in the present invention was an extremely low-cost element and special Does not require a sophisticated refining process.

Furthermore, the present inventors further added at least one of Al, Cr, at least one of Ti, Zr, Y to a material having a C content increased to 0.65% or more, and added Cu, N
i, Sb, Sn, Mo, Nb, V, W, Ca, at least one of rare earth elements
It has been found that the resistance to the selective corrosion of the ERW part can be further improved by adding a seed or by combining them.

 Hereinafter, the reasons for limiting each component in the present invention will be described in detail.

C is the most important addition element in the present invention. As described above, if the addition amount is less than 0.65%, the effect of suppressing the selective corrosion of the electric resistance welded portion is not sufficient. Since the effect of suppressing the selective corrosion of the sewn portion not only saturates but only reduces the weldability and workability unnecessarily, the range of C should be 0.65 to 1.25%.

Si is an element effective for deoxidation, but if it exceeds 0.5%, the resistance to electric resistance welding deteriorates, so the upper limit of the addition amount is set to 0.5%.

Mn is an element that improves strength and toughness, but its effect is not sufficient if the addition amount is less than 0.20%, and if it exceeds 2.0%, the resistance to electric resistance welding decreases, so the range of Mn is 0.20 to 2.
0%.

Since P is an impurity element that lowers the resistance to electric resistance welding, it is desirable that P is low, and it should be 0.035% or less.

If S exceeds 0.030%, the resistance to electric resistance welding and workability deteriorate, so the upper limit of the additive amount is made 0.030%.

Although the above is the basic component system of the steel of the present invention, in the present invention, in addition to the purpose of further increasing the resistance to selective corrosion of the electric resistance welded portion and other purposes according to the respective applications,
(A) one or more of Al, (b) Cr, (c) Ti, Zr, Y, (d) C
u, Ni, Sb, Sn, Mo, Nb, V, W, Ca, at least one of rare earth elements,
Or a combination of any two or more of the above (a) to (d).

Al is an extremely useful element for deoxidizing steel,
Even if added in excess of 3%, the effect is saturated and causes problems during casting, so the upper limit content is 0.
3%.

Cr is an element that improves the corrosion resistance of the base material and the electric resistance welded part together with the improvement of the strength. However, even if added over 3%, the effect is saturated and the cost is only increased unnecessarily. Is 3%.

Ti, Zr, and Y are elements that improve the resistance of the ERW part to selective corrosion. However, if the added amount is less than 0.005% in total, the effect is not sufficient. Since the effect is saturated and the cost is unnecessarily increased, the addition range is set to 0.005 to 0.3% in total of one type or two or more types.

Cu is an element that improves the corrosion resistance of the base metal and improves the resistance of the electric resistance welded portion to selective corrosion. However, if added in excess of 1%, the hot workability is significantly impaired, so the upper limit content is set to 1%.

Ni is an element that improves the resistance of the electric resistance welded portion to selective corrosion, but its effect is saturated even if it is added in excess of 1%, and the cost is only increased unnecessarily, so the upper limit content is 1%. And

Sb and Sn are elements that improve the resistance of the electric resistance welded portion to selective corrosion. However, even if added over 0.5%, the effect is saturated and the hot workability is significantly impaired. 0.5%.

Mo, Nb, V, and W are effective elements for improving strength and improving resistance to selective corrosion of ERW, but their effects are saturated even if they are added in excess of 1%, which unnecessarily increases costs. Therefore, the upper limit content is set to 1%.

Ca and rare earth elements (REM) are elements that improve the resistance of the ERW part to selective corrosion. However, even if added over 0.3%, their effects are saturated and they cause coarse inclusions. , The upper limit content is 0.3%. In addition,
In the present invention, the rare earth element refers to an element having an atomic number of 57 to 71 or 89 to 103.

Cu, Ni, Sb, Sn, Mo, N for the purpose of suppressing selective corrosion of ERW
When b, V, W, Ca, and rare earth elements are added, the effect is not increased even if the total of each element exceeds 1%.
When two or more of these elements are added in combination, the upper limit of the total addition amount is 1%.

 Next, examples of the present invention will be described.

ERW steel pipes 1 to 34 (wall thickness 12 mm,
An outer diameter of 250 mm) was manufactured by a normal electric resistance welding process, and a test piece having a width of 40 mm and a length of 100 mm was sampled from these steel pipes and subjected to a corrosion test. At the time of collecting the test piece, the test piece was collected so that the electric resistance welded portion was located at the center in the width direction. In the corrosion test, a rotary immersion type corrosion test was performed for 2 months under a severe condition in which the test solution was rotated at 1.5 m / s in a 3% aqueous NaCl solution constantly saturated by bubbling air. The results of the corrosion test are also shown in Table 1.

In addition, the evaluation of the corrosion resistance of the electric resistance welded part is as follows: when R is the electric resistance of the electric resistance welded part relative to the average overall corrosion rate, xx is R ≧
3, × is 3> R ≧ 1.3, Δ is 1.3> R ≧ 1.1, ○ is 1.1> R ≧
1.05 and ◎ are 1.05> R.

As shown in Examples, Examples 1 to 28 which are steel pipes of the present invention
It is clear that the resistance to the selective corrosion of the electric resistance welded part is much better than those of Comparative Examples 29 to 34 which are comparative steel pipes.

[Effect of the Invention] As described above, the resistance of the steel pipe of the present invention to the selective corrosion of the electric resistance welded part is extremely excellent, and the effect is remarkable when used in various pipes such as a humid environment, which contributes to industrial development. Very large.

Claims (5)

(57) [Claims] 1. The composition contains, by weight%, C 0.65 to 1.25%, Si 0.5% or less, Mn 0.20 to 2.0%, P 0.035% or less, and S 0.030% or less, with the balance substantially consisting of Fe and inevitable impurities. An electric resistance welded steel pipe having high resistance to selective corrosion of an electric resistance welded part. 2. The electric resistance welded steel pipe according to claim 1, further comprising:
ERW steel pipe with high resistance to selective corrosion of ERW containing 0.3% Al or less. 3. The ERW steel pipe according to claim 1, wherein the ERW part further contains 3% or less of Cr and has a high resistance to selective corrosion. 4. The resistance to selective corrosion of an electric resistance welded portion of the electric resistance welded steel pipe according to any one of claims 1 to 3, further containing 0.005 to 0.3% of one or more of Ti, Zr, and Y. Great ERW steel pipe. 5. The electric resistance welded steel pipe according to claim 1, further comprising one or more of Cu, Ni, Sb, Sn, Mo, Nb, V, W, Ca, and a rare earth element. 1% or less in total, provided that the content of each element is 1% or less for Cu and Ni and 0% for Sb and Sn.
5% or less, Mo, Nb, V, W 1% or less, Ca, rare earth element 0.3%
An ERW steel pipe having the following resistance to selective corrosion of the ERW part.