JPS609096B2 - Grooved corrosion-resistant ultra-low carbon ERW steel pipe - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a sewn steel pipe with excellent corrosion resistance, particularly resistance to groove corrosion at welded parts.

Electric steel pipes are widely used for piping, structures, and heat transfer, and steel types are selected that are suitable for each use.

Of these, piping for transporting industrial water, seawater, etc.
Thunder-sewn steel pipes made of carbon steel with a carbon content of approximately 0.15 to 0.20% are used, but in recent years, groove-like local corrosion (hereinafter referred to as groove corrosion) has occurred in the welded parts of such thread-sewn steel pipes. This is considered a problem as there have been cases where it has even led to leakage accidents. The cause of groove corrosion is that the natural potential of the lightning welded part is lower than that of the surrounding base material, so during use as water supply and drainage piping, the welded part becomes an anode and the area around it becomes a cathode, and the former becomes selective. This is thought to be due to intensive corrosion.

Therefore, in order to prevent groove corrosion, it is necessary to minimize the potential difference between the weld and the surrounding base metal.
There are various reasons why the natural potential of the welded part of the welded steel pipe becomes less noble than that of the surrounding base material, but one of the biggest reasons is the difference in the metallurgical structure of the two parts.

That is, since the welded part is locally heated and rapidly cooled during pipe manufacturing, the welded part has a hardened structure of martensite or bainite in a steel pipe with a normal C content. Since the part with such a hardened structure is base metal relative to the surrounding base metal part of the ferrite + pearlite structure, a local battery is formed during use and groove corrosion occurs in the welded part. One possible way to eliminate the microstructural non-uniformity between the weld zone and the base metal is to normalize the entire tube after welding the pipe to change the weld zone's structure from bainite and martensite to a ferrite + pearlite structure. It will be done.

However, such heat treatment not only incurs a large to medium cost increase in terms of equipment and manufacturing process, but also cannot be expected to completely homogenize the structure. This invention uses ultra-low carbon steel, which is completely different from conventionally used steel, for the wire that is the raw material for the welded steel pipe, and prevents the formation of hardened structures in the welded part even during welding and rapid cooling during pipe manufacturing. This was done based on the knowledge that by doing so, the above-mentioned local potential difference and the occurrence of groove corrosion caused by it can be prevented.

That is, the gist of this invention is CO. 03% or less “Sj
o. 35% or less, Mno. 10-1.00%, Cuo.
05 to 0.80%, and the welded portion is substantially a single-phase ferrite.

In the above-mentioned electric resistance welded steel pipe of the present invention, both the welded part and the base metal part are single-phase ferrite, and the crystal grains between the base metal part, the heat-affected zone, and the welded metal part, which are another cause of groove-like corrosion, are present in the welded part. The difference in size is also reduced, and it has sufficient groove corrosion resistance even when welded. Ferrite single-phase Kameso steel pipes are inferior in strength to ERW steel pipes with a ferrite + pearlite structure made from carbon steel, but are sufficient for piping applications that do not require exceptionally high strength. It can be used for many purposes, and conversely it has the advantage of being excellent in processability. but,
When strength is required depending on the application, an appropriate amount of components that strengthen the steel can be selectively added without affecting the structure of the welded part.

Desirable reinforcing elements are Nb, V,
It is B. The electric resistance welded steel pipe in which the balance of C, Si, Mn, and Cu is substantially iron.

'2-More Nbo. 01~0.15% "BO.001~
0.01%, VO. The electric resistance welded steel pipe contains one or more of 0.005 and 0.10%.

[3l Furthermore, Njo. 60% or less, rare earth elements 0.01
The above (2-) electric resistance welded steel pipe containing one or two of ~0.20%.

The reason for specifying the content of each component in this manner is as follows. Although C is a component that contributes to improving the strength of steel, it is an unfavorable component for corrosion resistance.

Particularly in lock-stitched steel pipes, as mentioned above, it is the main cause of generating a structure different from that of the base metal in the welded part, which intensifies the occurrence of groove corrosion, so it is better to have as little as possible. In this invention, 0.03% is the allowable upper limit that does not cause a hardened structure to occur in the welded part. Si is used as a deoxidizing agent, and 0.35% is the normal upper limit for Si-killed steel. 0.1% of Mn is necessary as a deoxidizing agent for steel and to improve strength. However, 1.0
%, the corrosion resistance of the weld will deteriorate. Cu is a component that improves groove corrosion resistance.

The dragon seam steel pipe of this invention already exhibits excellent groove corrosion resistance due to its ultra-low carbon content;
By further adding u, the groove corrosion resistance can be further improved. If Cu is less than 0.05%, there is little improvement in groove corrosion resistance, so the lower limit is set at 0.05%.
And so. Moreover, since hot workability deteriorates when added in excess of 0.80%, the upper limit was set at 0.8%. In this way C
By containing u, groove corrosion resistance is further improved,
Reliable groove corrosion resistance can be guaranteed even under severe usage conditions. The lock-sewn steel pipe of the present invention has extremely low C and low strength.

Therefore, when strength is required, it is desirable to add components that improve the strength of the steel without adversely affecting the groove corrosion resistance. B, V, and Nb are added for this purpose. If Nb is less than 0.01%, it is not effective in improving the strength, so the lower limit was set at 0.01%. Further, since adding more than 0.15% causes problems in hot workability, the upper limit was set at 0.15%. If less than 0.001% of B is added, no improvement in strength can be expected, and if more than 0.01% is added, hot workability deteriorates, so the lower limit was set to 0.001% and the upper limit was set to 0.01%. If V is less than 0.005%, there is no improvement in strength, and even if it is added in excess of 0.10%, the strength improvement effect remains unchanged, so the lower limit is set to 0.005% and the upper limit is set to 0.
It was set at 10%. Ni and rare earth elements are components that improve groove corrosion resistance.

These are also components added as necessary, and their contents are as follows. Ni is an element that increases the corrosion resistance of steel itself, and is also effective in improving the groove corrosion resistance of welded parts of open-seam steel pipes.

However, if it exceeds 0.60%, the hot workability of the steel will deteriorate. Rare elements have a stronger tendency to form sulfides than Mn,
This has the effect of preventing the formation of MnS, which has an adverse effect on the groove corrosion resistance of welded parts.

In both cases, this effect appears at 0.01% or more, and 0.20%
If it exceeds %, the pipe-manufacturability of the steel will deteriorate. In addition to the above components, there are impurities that are inevitably mixed into steel.

The main ones are P and S. P does not affect the corrosion resistance of the weld zone, but if it is present in a large amount, the steel will become vertical and weldability will deteriorate, so it should be kept at 0.040% or less. S deteriorates the corrosion resistance of the weld. Since this tendency becomes noticeable from around 0.030%, it is better to keep the upper limit to 0.030%. Examples Table 1 shows examples and comparative examples of the present invention.

Steel having the chemical composition shown in Table 1 was melted, rolled into a steel plate with a thickness of 2 willow, and electrical resistance welded to produce a steel pipe with a side length of 25.4 mm.

The electric resistance welded steel pipe manufactured by the above method is divided in half to a length of 50mm with the welded part as the center in the middle direction. We created a test piece of Vengeance and conducted a corrosion test in flowing artificial seawater, which is a harsh corrosive environment.

The conditions at that time are as follows. Artificial seawater (AST)
Adjusted by M-DI141) Temperature 5000, flow rate 2 skin/
sec, test period 15 days and 30 days After the test, cross-sectional observation was performed and groove corrosion depth was measured.

The results are also shown in Table 1. Table 1 Comparison of corrosion resistance between steel pipes of the present invention and comparative electrically conductive steel pipes (components are shown in wt%) Judgment of elbow groove corrosion (measurement of groove corrosion depth) × × >0.50, × 0.20 ~ Steels 1 to 8 with a value of 0.50, Δ of 0.05 to 0.20, and ◎ which do not cause any port corrosion are comparison materials, and in particular, the conventional steel L2 suffers from severe groove corrosion.

Nos. 3 to 6 are Cu steels.Although the groove corrosion resistance is slightly improved, the occurrence thereof cannot be prevented.

Nos. 7 and 8 have lower carbon content, but the structure of the welded part is not comparable to that of the base metal and is subject to groove corrosion.

9 or less is the implementation steel of this invention, and the carbon content is 0.03%
By doing the following, the difference in structure between the welded part and the base metal is substantially eliminated, so the groove corrosion resistance is significantly improved.

Claims (1)

[Claims] 1 C 0.03% or less, Si 0.35% or less, Mn 0.
10-1.00, containing Cu0.05-0.80%,
An as-welded groove-like corrosion-resistant ultra-low carbon electric resistance welded steel pipe made of steel in which the remainder is substantially iron, and the welded part is substantially single-phase ferrite. 2 C0.03% or less, Si0.35% or less, Mn0.
Contains 10-1.00%, Cu0.05-0.80%, further Nb0.01-0.15%, V0.005-0
．． 10%, one or two of B0.001-0.01%
An as-welded groove-shaped corrosion-resistant ultra-low carbon electric resistance welded steel pipe, the welded portion of which is made of steel containing at least 100% ferrite, the remainder of which is substantially iron, and whose welded portion is substantially single-phase ferrite. 3 C0.03% or less, Si0.35% or less, Mn0.
10-1.00%, Cu0.05-0.80%, further Nb0.01-0.15%, B0.001-0.01%
, one or more of V0.005-0.10%, Ni 0.60% or less, rare earth (Ce or La) 0.0
Containing one or two of 1 to 0.20%, the remainder is substantially iron, and the welded part is substantially single-phase ferrite. Extremely low groove corrosion resistance as welded. Carbon ERW steel pipe.