JPH0762214B2 - Electric resistance welded steel pipe having high resistance to selective corrosion of the electric resistance welded part and method for manufacturing the same - Google Patents

Description

TECHNICAL FIELD The present invention relates to an electric resistance welded steel pipe having a large resistance to selective corrosion of the electric resistance welded portion and a method for manufacturing the same.

[Conventional technology]

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

However, in the past usage record, when ERW steel pipe is used in a humid environment, selective corrosion (often referred to as groove corrosion or groove corrosion), in which the corrosion rate of the ERW portion is higher than that of the base material, occurs. In some cases, it has been reported that one of the drawbacks of ERW pipes is the obstacle to the widespread use of ERW pipes. Therefore, there is a strong demand for an electric resistance welded steel pipe having a high resistance to selective corrosion of the electric resistance welded portion.

On the other hand, a technique for preventing selective corrosion of the electric resistance portion has been proposed so far. For example, in Japanese Examined Patent Publication No. 60-37173, the electric resistance welded portion of an electric resistance welded steel pipe made from a strip steel adjusted with components such as C, Si, Mn, P, Al and rare earth elements is heat-treated to form a groove corrosion resistance. An improved electric resistance welded steel pipe is disclosed. Furthermore,
In Japanese Examined Patent Publication No. 59-14536, there is an electric resistance welded steel pipe with reduced Al and at the same time the content of Ti, Zr, Y, etc. is adjusted to improve groove corrosion resistance. Reduced and at the same time Cu
A copper-containing electric resistance welded steel pipe in which the contents of Ti, Zr, Y, etc. are adjusted and the electric resistance welded portion is heat treated to improve the groove corrosion resistance, and C is 0.03 in JP-B-60-9096. %, And at the same time, copper-containing electric resistance welded steel pipes having reduced Si are disclosed.

[Problems to be Solved by the Invention]

An object of the present invention is to inexpensively provide an electric resistance welded steel pipe having high resistance to selective corrosion of the electric resistance welded portion.

[Means for Solving the Problems]

 The gist of the present invention is as follows.

(1) C: 0.04 to 0.30%, Mn: 2.0% or less, P: 0.035% or less, S: 0.030% or less, Si: 0.1 to 0.35%, Al: 0.007% or less, and Ni: 1.0 in addition to these. % Or less, Nb: 0.0
05-0.1%, V: 0.005-0.1% of 1 type or 2 types or more, and the balance consists essentially of Fe and unavoidable impurities. Great ERW steel pipe.

(2) C: 0.04 to 0.30%, Mn: 2.0% or less, P: 0.035% or less, S: 0.030% or less, Si: 0.1 to 0.35%, Al: 0.007% or less, and Ni: 1.0 in addition to these. % Or less, Nb: 0.0
05-0.1%, V: 0.005-0.1% of 1 type or 2 types or more, and the balance consists of Fe and unavoidable impurities. A steel plate is welded to form an electric resistance welded steel pipe, and then a weld bead. Part
A method for producing an electric resistance welded steel pipe having a large resistance to selective corrosion of the electric resistance welded portion, which is characterized by performing local heating at 750 to 1050 ° C for 20 minutes or less.

In order to obtain an electric resistance welded steel pipe having high resistance to selective corrosion of the electric resistance welded part, the present inventors have conducted many experiments and detailed studies from many viewpoints such as components and manufacturing processes. In comparison, the resistance to selective corrosion of the electric resistance seam can be significantly improved by reducing the content of Al. Further, Ni is added, or at least one of Nb and V is added, or It has been found that the resistance to selective corrosion of the electric resistance welded portion can be further improved by combining the above.

[Work]

According to the research conducted by the present inventors, the cause of selective corrosion of the electric resistance welded pipe in the electric resistance welded part in the wet environment is that the electric resistance welded part (welded part) and the heat-affected zone in the vicinity corrode compared with the surrounding base metal part. The local anode has a low electric potential, and the base metal portion serves as a cathode relatively. In this case, since the area of the local anode (electrically sewn portion) is considerably smaller than that of the cathode (base metal portion), selective corrosion of the electric sewn portion is accelerated.

It has been conventionally considered that when a small amount (about 0.03%) of S in steel is present, it deteriorates the corrosion resistance of steel and further promotes selective corrosion of the electric resistance welded portion in the electric resistance welded steel pipe. Therefore, as a means for improving the resistance to selective corrosion of the electric resistance welded portion, it has been common to reduce the S content as much as possible or add an element that renders S harmless. Due to the progress of refining technology in recent years,
Although the reduction of S is relatively easy, according to the research by the present inventors, the effect of suppressing groove corrosion by only reducing S is remarkable only when S is lower than about 0.003%. However, since further reduction is required for practical use, it is not realistic to suppress the groove corrosion by reducing S, because the cost is still significantly increased.

On the other hand, as a result of detailed analysis of the influence of the base metal component on the selective corrosion of the electric resistance welded portion, the present inventors, unlike conventional common sense, reduce S as compared with the chemical component of the conventional electric resistance welded steel pipe. It has been found that, if Al is reduced without using it, the corrosion potential of the electric resistance portion becomes equal to or more noble than the base material portion, that is, the electric resistance portion becomes the local cathode and the base material portion becomes the anode. Furthermore, the area of the local cathode (electrically welded part) is much smaller than the area of the anode (base metal part), so that the entire electric resistance welded steel pipe including the electric resistance welded part is substantially uniformly corroded, that is, the electric resistance welded part. It was found that the selective corrosion of can be suppressed. This is a finding that has never been known before, and is a new means for suppressing selective corrosion of the electric resistance welded portion. That is, in the present invention, it is possible to suppress the selective corrosion of the electric resistance welded portion without significantly reducing S in steel, which is conventionally considered to significantly promote the selective corrosion of the electric resistance welded portion. From the viewpoint of electric resistance weldability and workability, it is sufficient to control S to a level that does not hinder the production of electric resistance welded steel pipe, as described later.
In conventional groove-corrosion resistant ERW steel pipe, S content is 0.005%
Although there have been many reductions to the following, reduction to such S content requires a considerable cost, whereas the present invention does not require a special refining process, and the cost increase is minimal.

Furthermore, the present inventors have found that by adding Ni to a material with reduced Al, adding at least one of Nb and V, or combining these, the resistance to selective corrosion of the electric resistance welded portion can be further improved. It was

The reasons for limiting each component in the present invention will be described in detail below.

C is an element necessary to obtain the strength according to the application of the electric resistance welded steel pipe, and if it is less than 0.04%, it becomes difficult to secure the strength. Meanwhile 0.
If it exceeds 30%, the weldability will be impaired. Therefore, the range of C is suitably 0.04 to 0.30%.

Si is usually an effective element for deoxidation, but if it is less than 0.1%, the effect of deoxidation is insufficient, and 0.3% from the viewpoint of workability and weldability.
Since it is common to add about 0.1 to 0.1
It was set to 0.35%.

Mn is an element that improves strength and toughness, but if it is added in an amount exceeding 2.0%, the electric resistance weldability deteriorates, so the range of Mn is made 2.0% or less.

Since P is an impurity element that deteriorates the electric resistance weldability, its lower content is desirable, and it should be 0.035% or less.

If S exceeds 0.030%, the electric resistance weldability and workability deteriorate, so the upper limit of addition is made 0.030%.

Al is usually an effective element for deoxidation, and it is general to add about 0.03%, but if the content exceeds 0.007%, the effect of suppressing selective corrosion of the electric resistance welded part cannot be obtained, and Al content is 0.
007% or less.

Although the above is the basic composition system of the present invention steel, in the present invention, depending on the respective application for the purpose of further increasing the resistance to selective corrosion of other electric resistance welded portions and other purposes, N
Any of i, Nb, V, or and can be contained.

Ni is an element that improves the corrosion resistance of the base metal part as well as the resistance to selective corrosion of the electric resistance welded part, but if it is added in excess of 1.0%, it will unnecessarily increase the cost, so the upper limit content is 1.0 %.

Nb and V are elements that improve the strength and resistance to selective corrosion of the electric resistance welded portion, but if less than 0.005%, there is no effect, and if added over 0.1%, the effect is saturated, and it becomes a prank. Since the cost is only increased, the content of each is set to 0.005 to 0.1%. Further, the local heating conditions of the weld beet portion are not preferable at a temperature lower than 750 ° C., since no significant effect is observed in improving the corrosion resistance, while when the temperature exceeds 1050 ° C., the mechanical strength in the vicinity of the welded portion is lowered.
Furthermore, even if the heating time exceeds 20 minutes, not only the effect is saturated but also the productivity is unnecessarily reduced, so that the heating time is 20 minutes at maximum. Therefore, the heating temperature is 750 to 1050 ° C and the heating time is 20 minutes or less.

(Example) Next, the Example of this invention is described.

ERW steel pipes 1 to 10 whose composition is shown in Table 1 (wall thickness 10 mm, outer diameter
380 mm) was manufactured by a normal electric resistance welding process, and test pieces with a width of 40 mm and a length of 100 mm were taken from these steel pipes and subjected to a corrosion test. When collecting the test piece, the test piece was sampled so that the electric resistance portion was located at the center in the width direction. For the corrosion test, a rotary immersion corrosion test was conducted for two months under severe conditions in which the test solution was flowed at 1.5 m / s in a saturated 3 wt% NaCl aqueous solution which was saturated by bubbling air. The corrosion test results are also shown in Table 1.

Note that the corrosion resistance of the electric resistance welded portion is evaluated when R is the ratio of the corrosion rate of the electric resistance welded portion to the average corrosion rate of the base metal portion, xx is R ≧ 3, x is 3> R ≧ 1.3, and Δ is 1.3> R ≧ 1.1, ○ is 1.1
> R ≧ 1.05, and ⊚ is 1.05> R.

As shown in Examples, Examples 1 to 7 which are copper pipes of the present invention
It is obvious that the resistance to selective corrosion of the electric resistance welded portion is very excellent as compared with the comparative steel pipes of Examples 8 to 10.

[Effects of the Invention] As described above, the resistance of the steel pipe of the present invention to selective corrosion of the electric resistance welded portion is extremely excellent, and the effect is remarkable when used in various pipes such as a wet environment, and contributes to the development of industry. It is extremely large.

Claims (2)

[Claims] 1. C: 0.04 to 0.30%, Mn: 2.0% or less, P: 0.035
% Or less, S: 0.030% or less, Si: 0.1 to 0.35%, Al: 0.007%
Below, and in addition to these, Ni: 1.0% or less, Nb:
0.005 to 0.1%, V: 0.005 to 0.1%, containing one or more of them, and the balance consisting essentially of Fe and unavoidable impurities. Great ERW steel pipe. 2. C: 0.04 to 0.30%, Mn: 2.0% or less, P: 0.035
% Or less, S: 0.030% or less, Si: 0.1 to 0.35%, Al: 0.007%
Below, and in addition to these, Ni: 1.0% or less, Nb:
A steel plate containing 0.005 to 0.1% and V: 0.005 to 0.1% of 1 or 2 or more, and the balance being substantially Fe and inevitable insoluble materials, to form an electric resistance welded steel pipe, and then welding. Bead part
A method for producing an electric resistance welded steel pipe having a large resistance to selective corrosion of the electric resistance welded portion, which is characterized by performing local heating at 750 to 1050 ° C for 20 minutes or less.