JPS5914536B2 - ERW steel pipe - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is characterized by the addition of Ti tZr to steel which has a significantly lower content of Al and S components than conventional steel and to which no Cu component has been added.
The present invention relates to an inexpensive electric resistance welded steel pipe whose electric resistance welded portion has high selective corrosion resistance to industrial water, seawater, etc. to which one or more components of ty are added, and a method for manufacturing the same.

Conventional electric resistance welded steel pipes are inexpensive and have good dimensional accuracy, so their use as piping for industrial water or seawater or piping for heat exchangers has been increasing year by year.

However, although the corrosion resistance of the base metal is sufficient according to past usage results, there is a drawback that only the center of the welded part with a width of about 100 microns of solder is selectively corroded ten times more than the base metal. As a result, leakage accidents often occur in ERW steel pipes.

In order to prevent the above-mentioned corrosion of ERW steel pipes, complete annealing of the entire ERW steel pipe and excessive improvement of material quality have been considered, but these measures increase costs and are not effective in preventing corrosion. However, it is not enough.

In view of the above, the present inventors have conducted extensive research on the mechanism of selective corrosion in the welded parts of ERW steel pipes, and have found that they have lower AA and S contents and Cu
The present invention was completed by using a steel material in which one or more of TitZr and tY is added to steel that is not added.According to the research of the present inventors, The cause of selective corrosion of the ERW parts of ERW steel pipes in industrial water or seawater is that the weld center of the ERW part has a lower natural potential than the surrounding base material, so the center of the weld part may be damaged due to local corrosion. The anode and its surroundings become the cathode, and corrosion progresses at the anode.

The reason why the natural potential at the center of the weld becomes extremely low and the center of the weld becomes the anode of the local battery is because (a) the crystal grains around the weld are fine or have a hardened structure. .

(MnS and FeS, which are easily eluted into water, tend to concentrate in the center of the weld.

(The morphology of 1Mns and FeS becomes linear along the weld line.

It is possible that

The present invention improves the above (a) and H by making the steel composition low A7, making it low sulfur, reducing MnS and FeS, and further adding Zr sTi tY to make water insoluble (7) Z
The purpose is to improve (b) by forming a sulfide of r tT i tY, and to significantly increase the selective corrosion resistance of the welded portion of the electric resistance welded steel pipe against industrial water in the as-welded state.

Furthermore, the welding part of the electric resistance welded steel pipe made of the above components was adjusted to 850.
Selective corrosion can be almost completely prevented by local heat treatment at 950° C. for 30 minutes or less.

Next, the reason for limiting the components of the electric resistance welded steel pipe of the present invention will be explained in detail below.

C is necessary to obtain strength depending on the purpose of the ERW steel pipe.
．． Less than 30% are eligible.

Si is normally necessary for deoxidation, but too much Si deteriorates workability and weldability, so it is limited to 0.35% or less.

Mn is necessary for desulfurization and improving hot workability in the steelmaking process, but if it is less than 0.25, no effect can be obtained, and if it exceeds 1, it is not preferable for making the structure of the weld zone uniform. .

Since P is an impurity element, it is desirable that it be as low as possible.
．． 035% or less.

Since S becomes a point of corrosion at the center of the weld and generates FeS and MnS, it is desirable that the content is low;
．． 035% or less.

When AA is present in an amount of about 0.03% of the amount present in ordinary steel, crystal grains near the weld are refined, and local dissolution at the center of the weld is promoted.

Furthermore, Al exists in the form of Al2O3 in the center of the weld, and serves as a generation nucleus for linear sulfides such as MnS and FeS.

For this reason, the basic point of the steel of the present invention is that by using low S and low Al, the generation of potential difference is prevented by refining the crystal grains near the weld, and the generated sulfide is also small and has a spherical shape instead of a linear shape. As a result, resistance to selective corrosion of welds increases significantly.

For this purpose, the amount of Al should be 0.01% or less.

T itZ r t Y is necessary to further improve the resistance to selective corrosion of ERW steel pipes, and is a water-insoluble T 1 t that replaces S in the steel under conditions of a low AI content of 0.01% or less.
In order to make Z r tY sulfide, T i tZr
, any one component or two or more components of Y from 0.05 to 0.3
Add 0 coefficient.

Regarding the conditions of local heating, if it is lower than 850°C, the effect of improving corrosion resistance will be small, and if it is higher than 950°C, brain growth will occur, resulting in a decrease in the strength of the welded part and deterioration of workability, which is not preferable.

On the other hand, even if the heating time exceeds 30 minutes, it is not very effective in improving corrosion resistance.

Next, the present invention will be explained in detail with reference to Examples.

Example 1 Table 1 shows the chemical composition of the steel of the present invention used in Example 1 along with the comparative steel.

Table 2 shows 25ψ x 2t x 1001 (industrial water
300p pm Cl, flow rate 2F7L/see, temperature 5
These are the results of a test conducted at 0°C for one month.

From Table 2, the AI content of ordinary steel (A-D
) and low S steel (E) are both severely affected by selective corrosion.

Also, steel added with sulfide-forming elements (TitZr tY) (
If F 2 , G 2 , H) have a normal amount of Al, resistance to selective corrosion of welds is not sufficient.

However, normal S steel I with low Al and sulfide-forming elements added
, and M improves to the point where there is almost no selective corrosion even when welded.

Furthermore, when TitZr tY is added singly or as two components at low S (J, L, N, O, P, Q), selective corrosion can be completely prevented even in the as-welded state or, of course, by local heating. I understand.

Claims (1)

[Claims] I C0.3% or less, Si0.35% or less, M n
0.25-1.0%, Po, 035 or less, So, 03
5 inches or less, Ai, 01% or less and Ti tZr tY
2C0, an electric resistance welded steel pipe with high resistance to selective corrosion of the electric resistance welded part, characterized by containing 0.05 to 0.3% of one or more of the following, and the remainder being substantially Fe; ，
30% or less, Si 0.35'1 or less, Mn 0.25-1
．． 0%, Po, 035% or less, 80.035 Chi or less, A
10.01% or less and any one of Ti tZr tY
After welding a steel plate containing 0.05 to 0.30% of a species or two or more species and the remainder substantially consisting of Fe to make an electric resistance welded steel pipe, only the weld bead portion is heated at 850 ° C to 950 ° C for 30 minutes. The following is a method for manufacturing an electric resistance welded steel pipe having high resistance to selective corrosion of the electric resistance welded portion, which comprises performing local heat treatment.