JPH0847717A - Manufacture of welded steel pipe excellent in carbon dioxide gas corrosion resistance - Google Patents

Abstract

PURPOSE:To manufacture a welded steel pipe excellent in carbon dioxide gas corrosion resistance. CONSTITUTION:This is the manufacture (component and composition are expressed by wt.%.) of a welded steel pipe excellent in carbon dioxide gas corrosion resistance, which is equipped with the following processes. They are a process preparing a hot rolled plate containing 0.3 to 1% Cr in a carbon steel, a process forming the hot rolled plate continuously to an open pipe by multistage forming rolls, a process electrically heating both edge parts to be joined to the open pipe, and a process irradiating the both edge parts to be joined by a laser beam, controlling the quantity of upsetting by a squeeze roll, and being in press welding.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a welded steel pipe having excellent corrosion resistance in a humid environment containing carbon dioxide.

[0002]

2. Description of the Related Art The inner surface of a steel pipe for transporting petroleum containing carbon dioxide and natural gas may cause general corrosion, and a carbon steel material containing about 0.5% Cr is used for the purpose of suppressing this. (JP-A-4-341540). Seamless pipe and UO with excellent corrosion resistance by using this steel material
An arc welded steel pipe such as E can be obtained.

However, with respect to the electric resistance welded pipe, the above steel cannot be used in a wet carbon dioxide environment because selective corrosion of the electric resistance welded portion occurs. Steel pipes for line pipes are usually used properly according to size. Usually, UOE steel pipes are used for large diameter sizes, seamless pipes are used for small diameter and thick wall sizes, and electric resistance welded pipes are used for small diameter and thin wall sizes. These are based on the essential characteristics of each manufacturing process, and it is often difficult to use other steel pipes instead of ERW pipes.

[0004]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method for producing a small-diameter thin-walled welded pipe having excellent general corrosion resistance in a wet carbon dioxide environment and not causing selective corrosion of welded portions. .

[0005]

In order to solve the above-mentioned problems, according to the present invention, a hot-rolled steel sheet made of steel having a predetermined composition is continuously formed into an open pipe by a multi-stage forming roll, and both edge portions to be welded. Electrically heating the
A step of irradiating both edge portions to be joined with a laser beam, controlling the amount of upset by a squeeze roll, and performing pressure welding is adopted. By irradiating the laser beam, both edge portions to be joined can be heated to a higher temperature than before, so that the upset amount can be made smaller than before, and therefore a welded steel pipe excellent in carbon dioxide gas corrosion resistance can be manufactured.

(1) The invention of claim 1 is a method for producing a welded steel pipe excellent in carbon dioxide gas corrosion resistance, which comprises the following steps (component composition is wt%). (A) C: 0.01 to 0.2%, Si: 0.1 to 0.35%, Mn: 0.5 to 2
%, P: 0.03% or less, S: 0.03% or less, Cr: 0.3
A step of preparing a hot-rolled steel sheet containing 1 to 1%, (b) a step of continuously forming the hot-rolled steel sheet into an open pipe by a multi-stage forming roll, and (c) both of the open pipes to be joined. A step of electrically heating the edge portion, and (d) a step of irradiating the both edge portions to be joined with a laser beam, controlling the upset amount with a squeeze roll, and press-contacting.

(2) The invention of claim 2 is a method for producing a welded steel pipe excellent in carbon dioxide gas corrosion resistance (the composition of the composition is wt%), which comprises the following steps. (A) In addition to the components defined in claim 1 (a), Cu: 0.05-
0.5%, Ca: 0.001 to 0.006%, Ni: 0.5% or less, Mo:
0.5% or less, Nb: 0.001-0.1%, V: 0.001-0.1%, Ti: 0.
Of 001 to 0.1%, a step of preparing a hot-rolled steel sheet containing one or more kinds, and (b) a step of continuously forming the hot-rolled steel sheet into an open pipe with a multi-stage forming roll,
(C) electrically heating both edge portions of the open pipe to be joined, and (d) irradiating the both edge portions to be joined with a laser beam, pressing the squeeze roll to control the upset amount, and then performing pressure welding. Process.

(3) According to the invention of claim 3, the upset amount is controlled and the metal flow rising angle of the welded portion is set to 45.
The method for producing a welded steel pipe having excellent carbon dioxide gas corrosion resistance according to claim 1 or 2, characterized in that the temperature is not more than °.

[0009]

In the present invention, the composition of steel (wt) will be described below.
%) Is described above. C needs to be added in an amount of 0.01% or more to secure the strength of steel, but the upper limit is made 0.2% from the viewpoint of weldability and toughness.

Si is required to be added as a deoxidizing agent for steel in an amount of 0.1% or more, but excessive addition makes the steel brittle, so the upper limit is set.
0.35%

Mn must be added in an amount of 0.5% or more to secure the strength of the steel, but excessive addition deteriorates toughness, so the upper limit is made 2%.

Since P deteriorates the toughness of steel, the upper limit is 0.03.
%.

Since S deteriorates the toughness of steel, the upper limit is 0.03.
%.

The addition of 0.3% or more of Cr enhances the carbon dioxide corrosion resistance of the steel, but the addition of 1% or more deteriorates the weldability of the steel, so the upper limit is made 1%.

Cu may be added if necessary because the HIC resistance is improved by adding more than 0.05%, but excessive addition deteriorates the hot workability of steel, so the upper limit is 0.5%.
And

If Ca is added in an amount of 0.001% or more, the HIC resistance can be improved by controlling the morphology of inclusions, but Ca may be added as necessary, but excessive addition deteriorates the toughness of the steel, so the upper limit is 0.006. %.

Ni and Mo may be added as necessary in order to improve the HIC resistance of steel, but excessive addition of S and S
The upper limit is set to 0.5% to deteriorate the SCC characteristics.

Nb, V, and Ti improve strength when added in an amount of 0.001% or more, but excessive addition deteriorates toughness, so the upper limit is made 0.1%.

Next, the processing method will be described. A hot-rolled sheet having the above-mentioned composition is prepared and a processing step as described later is carried out. First, the reason for irradiating the laser beam will be described. In the electric resistance welded pipe, the electric resistance welded part is selectively corroded in a wet carbon dioxide environment. This is due to the potential difference between the electric resistance portion and the base metal portion,
The reason for this is explained as follows.

Compared with arc welding, electric resistance welding has a lower heat input, so that it is cooled very rapidly after joining. For this reason, MnS once solid-dissolved at the time of heating cannot be completely re-precipitated and a part is precipitated as FeS or MnS, but the rest remains in a supersaturated solid solution. As a result, the electric resistance welded portion has a higher concentration of solid solution S than the base material portion, so that the immersion potential becomes baser than the base material.

Further, observing the actual progress of selective corrosion of the electric resistance weld, the starting point is often inclusions, especially A-type inclusions such as MnS and FeS, and corrosion progresses along the inclusions in the initial stage. To do. In the base material portion, these inclusions extend in parallel to the rolling direction (L direction) and are not exposed on the plate surface. On the other hand, in the electric resistance welded portion, the surface corresponding to the C cross section of the plate is exposed on the surface by pressure contact, the inclusion extends in the thickness direction (Z direction) of the plate, and the tip is exposed on the surface. This is the starting point of corrosion and the path of progress.

Therefore, if the welding method combining the conventional electric sewing method and laser irradiation is adopted, there are the following advantages.
First, in the composite welding, since the heat input amount is large, it is not rapidly cooled as much as the electric resistance welding alone. Therefore, the amount of S dissolved in supersaturation is small and the electric potential of the electric resistance portion is not so base.

If the upset amount is further reduced, the inclusions are not exposed on the surface of the plate, so that the starting point of corrosion is reduced.
Here, the upset amount is defined as follows. Upset amount (mm) = coil width before pipe making (mm) -pipe circumference length
(mm)

Upset is a step of extruding oxides on the joint end surface, but in electric resistance welding, it is an essential step for maintaining the quality of the electric resistance welded portion. However, since the amount of heat input is large when the laser is used in combination, the oxide on the end face can be melted and dispersed, so that the weld quality can be maintained even if the upset is reduced or almost eliminated.

The welding method of the electric resistance welded pipe is classified into an induction system (by electromagnetic induction from an induction coil on the outer circumference of the pipe) and a resistance system (direct energization from an electrode in contact with the pipe) depending on the difference in the energization system. . However, there is no difference in the characteristics of the electric seams. Therefore, the present invention can be applied to any energization method.

Since the bead of the electric resistance welded portion is cut after welding, the larger the rising angle of the metal flow, the more the above-mentioned inclusions are exposed on the surface of the electric resistance welded portion. This exposed point serves as a starting point of corrosion and a traveling path. Therefore, it is desirable that the rising angle of the metal flow is low.
As a result of various experiments, the rising angle of the metal flow was set to 45.
If it is limited to ° or less, there are few exposed points on the surface of the electric seam,
It was found that the selective corrosion of the electric resistance welded portion was small in the wet carbon dioxide environment. Therefore, as described above, the rising angle of the metal flow is limited to 45 ° or less.

[0027]

EXAMPLE Steels having the chemical compositions shown in Table 1 were vacuum melted in a laboratory and cast into a 50 kg ingot. Heat this to 1200 ℃,
After rolling to a plate thickness of 50 mm, it was air-cooled. Steels A to E are steels of the present invention, and Steel F is a steel outside the present invention. 50 × from this steel plate
A 150 × 200 mm plate was cut out and rolled to a plate thickness of 6 mm at a heating temperature of 1200 ° C. and a rolling end temperature of 820 ° C. Immediately after the rolling was completed, it was cooled to 550 ° C. with a mist spray at a cooling rate of about 10 ° C./sec, and then inserted into an electric furnace heated to 550 ° C. to cool the furnace. The above is a simulation of hot rolling conditions.

[0028]

[Table 1]

6 × 35 × 1000 from steel sheet cooled to room temperature
mm test piece 1 was cut out and welded using a welding simulator. As shown in FIG. 1, this device feeds two steel plates to a multi-stage forming roll, resistance-heats the opposite steel plate edges with a high-frequency current supplied from a contact tip 2, and then press-contacts them with a squeeze roll 3. Further, it has a function of irradiating the edge joining portion with the carbon dioxide laser beam 4.

The welding conditions are a welding speed of 15 m / min, an input power of 200 kW from the contact tip 2, and an upset amount of 0.
It was changed in the range of ~ 4 mm. The laser output was 5 kW, and the beam diameter at the focal position was 0.5 mm, and irradiation was performed by focusing on the edge joining point from vertically above the steel sheet.

A sample 5 of 3.5 × 30 × 60 mm was cut out from this welded portion, the surface was wet-polished, and then a carbon dioxide corrosion test was carried out in the equipment shown in FIG. The test solution is artificial seawater saturated with carbon dioxide, the temperature of the solution is 80 ° C, the flow rate of the solution is 3 m / sec,
The test time is 300 hours.

By measuring the weight of the sample 5 before and after the corrosion test,
The corrosion weight loss C (mg / cm ² ) per unit area was determined. For use in a wet carbon dioxide environment, C ≦ 80 mg / cm ² is required. In addition, the test piece after the test was subjected to cross-sectional microscopy to examine the degree of selective corrosion of the electric resistance welded portion. The index α shown by the following equation was used for the evaluation of the selective corrosion of the electric resistance welded portion. If α = 1.0, there is no concern about selective corrosion of the electric resistance welded portion, but in practical use α ≤ 1.2
In that case, the trouble of selective corrosion of the electric resistance part hardly occurs.

Α = d1 / d2, d1: Corrosion depth of the seam (mm) d2: Corrosion depth of the base metal (mm)

Further, a welded pipe of 76.3φ (diameter) × 7.5t (thickness) mm was manufactured on an actual machine. Table 2 shows the chemical components and manufacturing conditions. A test piece was similarly cut out from the electric resistance welded portion after the production, and the same corrosion test was performed. The test results are shown in Table 2.
According to claims 1 and 2 of the present invention, it is understood that the amount of corrosion is small in a wet carbon dioxide gas environment and the selective corrosion of welded portions can be reduced as compared with the electric resistance welded pipe. Furthermore, according to claim 3, a welded pipe can be manufactured in which selective corrosion of welded portions does not occur at all.

[0035]

[Table 2]

[0036]

EFFECTS OF THE INVENTION According to the present invention, an electric resistance welded pipe for line pipe can be obtained which has a small amount of corrosion in a wet carbon dioxide environment and does not cause selective corrosion of the welded portion.

[0037]

[Brief description of drawings]

FIG. 1 is a perspective view showing an outline of a welding simulator used in an embodiment of the present invention.

FIG. 2 is a schematic sectional view showing a carbon dioxide corrosion test apparatus used in an example of the present invention.

[0038]

[Explanation of symbols]

 1 Test piece 2 Contact tip 3 Squeeze roll 4 Laser beam 5 Sample

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Reference number within the agency FI Technical indication C22C 38/00 301 F 38/18 (72) Inventor Yu Nagahama 1-chome, Marunouchi, Chiyoda-ku, Tokyo No. 2 Nippon Steel Tube Co., Ltd.

Claims (3)

[Claims] 1. A method for producing a welded steel pipe excellent in carbon dioxide gas corrosion resistance, which comprises the following steps (the composition of components is wt%): (A) Heat containing C: 0.01 to 0.2%, Si: 0.1 to 0.35%, Mn: 0.5 to 2%, P: 0.03% or less, S: 0.03% or less, Cr: 0.3 to 1% as main components. A step of preparing a rolled steel sheet, (b) a step of continuously forming the hot rolled steel sheet into an open pipe with a multi-stage forming roll, and (c)
A step of electrically heating both edge portions of the open pipe to be joined, and (d) a step of irradiating the both edge portions to be joined with a laser beam, controlling the upset amount with a squeeze roll, and press-contacting. 2. A method for producing a welded steel pipe excellent in carbon dioxide gas corrosion resistance, which comprises the following steps (the composition of components is wt%). (A) In addition to the components defined in claim 1 (a), Cu: 0.05 to 0.5%, Ca: 0.001 to 0.006%, Ni: 0.5% or less, Mo: 0.5% or less, Nb: 0.001 to 0.1%, V : 0.001-0.1% Ti: a step of preparing a hot-rolled steel sheet containing one or more of Ti: 0.001-0.1%, and (b) continuously opening the hot-rolled steel sheet with a multi-stage forming roll. The step of molding into
(C) electrically heating both edge portions of the open pipe to be joined, and (d) irradiating the both edge portions to be joined with a laser beam, and controlling the amount of upset by a squeeze roll to perform pressure welding. Process. 3. The welded steel pipe excellent in carbon dioxide corrosion resistance according to claim 1 or 2, wherein the upset amount is controlled so that the rising angle of the metal flow of the welded portion is 45 ° or less. Production method.