JPH08155663A - Manufacture of duplex stainless steel welded tube - Google Patents

Abstract

PURPOSE: To provide the corrosion resistance of the same degree as that of the base metal to the weld zone by achieving the postheating treatment of a weld seam part after forming the duplex stainless steel strip in an open-tube shape, and achieving the laser beam welding of both edge parts under the prescribed condition. CONSTITUTION: The steel strip stuck made of duplex stainless steel is continuously formed in an open-tube shape, and each edge of the steel strip is butted to each other by a squeeze roll. This butted part is irradiated with the laser beam to achieve the tube-making welding under the condition expressed by the inequalities of P>=0.4Vt/exp(aT)>=20, where P is the laser beam output (KW), V is the welding speed (m/min), (t) is the thickness (mm) of the stock steel strip, T is the preheating temperature ( deg.C) at each edge part of the steel strip, and (a) is the constant (=0.0006), and T=0 deg.C in the case without preheating. After the laser beam welding, the weld seam part is postheated at 1000-1100 deg.C, and the ferritic ratio of the structure of the weld metal is adjusted so that the ferrite ratio as large as that base metal may be approximately 50%.

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 duplex stainless steel welded pipe, more specifically, a duplex stainless steel welded pipe having excellent corrosion resistance at the welded portion, which is suitable for use as a line pipe or an oil country tubular good. Regarding the method.

[0002]

2. Description of the Related Art Duplex stainless steel has a two-phase structure consisting of a ferrite phase and an austenite phase, is excellent in stress corrosion resistance, and has good toughness and weldability. Therefore, it is used as a material for oil wells or seawater. Widely used. The proper solution temperature of this duplex stainless steel is generally 1000.
The temperature is ˜1100 ° C., and the ferrite ratio (the abundance ratio of the ferrite phase) is usually 50% in this temperature range. The elements that contribute to the formation of the corrosion resistant film are Cr, Mo, and N. Cr and Mo are concentrated in the ferrite phase and N is concentrated in the austenite phase to maintain the balance of the corrosion resistance of both phases. ing.

On the other hand, in the case of welding the two-phase stainless steel, the weld metal reaches a temperature range higher than the proper solution temperature range and once becomes a ferrite single phase during welding.
The growth of the austenite phase is insufficient, and the ferrite ratio thereof is higher than the above-mentioned proper value of 50% of the base material. This deviation of the ferrite ratio from the base metal ferrite ratio deteriorates the corrosion resistance and mechanical properties of the weld metal due to improper element distribution, but in order to prevent this, the ferrite ratio of the weld metal is made close to that of the base metal. For this purpose, it is necessary to add an element such as N and Ni having an austenite forming action to the weld metal, or to subject the weld metal to a post thermal solution treatment.

Conventionally, duplex stainless steel welded pipes are usually
The material strip steel is formed into an open pipe shape, and the butted portion is heated by high frequency to perform electric resistance welding (hereinafter referred to as ERW method), or gas-tungsten-arc welding (hereinafter referred to as GTAW method) or submerged-arc. It is manufactured by performing pipe forming welding by an arc welding method such as welding (hereinafter referred to as SAW method). And, as a means for optimizing the ferrite ratio of the weld metal of the weld seam portion, usually, a method of subjecting the weld seam portion to post heat treatment is used in the ERW method, and a filler is used in arc welding methods such as the GTAW method and the SAW method. A method is used in which alloy elements such as Ni and N are added to molten metal during welding using a wire.

Further, in recent years, a laser welding method, which has a much higher welding speed than the above-mentioned conventional welding methods, has been under development. However, in this laser welding method, the filler wire is used because of facility restrictions. Since it is practically difficult to add alloying elements to the molten metal during welding, for example, in the case of duplex stainless steel, "INNOVATIONST
AINLESS STEL (1993) 3.251 ”, as well as austenitic stainless steels other than duplex stainless steel,
Examples of ferritic stainless steel and Mo-containing alloys include, for example, JP-A-63-278688.
63-278689 and 63-27869.
As disclosed in Japanese Patent No. 0, the post-heat treatment is applied to the weld seam portion after laser welding to recover the performance of the weld metal, as in the case of the ERW method.

However, the ERW method or GTAW
In the arc welding methods such as the SAW method and the SAW method, the cooling rate of the molten metal is slow, so the heat-affected zone (hereinafter, HA) is adjacent to the weld metal.
Z) occurs. Therefore, in the HAZ of the two-phase stainless steel welded as it is in the pipe forming welding by the ERW method or the arc welding method, the ferrite / austenite phase ratio shift occurs in the high temperature HAZ portion which is thermally affected by 1150 to 1250 ° C during welding. , An intermetallic compound precipitates in the low temperature HAZ part affected by heat of 700 to 950 ° C., and the corrosion resistance and mechanical properties of the welded part are significantly deteriorated. But HA
Since Z is a solid unlike the weld metal part which is molten metal, it is impossible to add alloy elements such as Ni and N using a filler wire. It has the drawback that deterioration of the product cannot be avoided.

As described above, it is possible to post-heat-treat the weld seam to recover the performance of the weld metal containing HAZ. However, in the ERW method and the arc welding method, the material is welded during welding. Evaporation of nitrogen in the steel from the molten metal near the surface reduces the nitrogen content in the weld metal. Therefore, even if the post heat treatment is performed, it is not possible to sufficiently recover the deviation of the ferrite / austenite phase ratio and the deterioration of the corrosion resistant film. Therefore, in these welding methods, the performance of the welded portion cannot be recovered to the same degree as that of the base metal portion even after the post heat treatment.

The conventional laser welding method is 5 to 10
This is a method using a low output laser oscillator of about kW, and when a duplex stainless steel welded pipe is pipe-welded using such a low output laser beam, nitrogen vaporization from the weld metal occurs, There is a drawback in that even if post-heat treatment is performed on the weld seam portion, the corrosion resistance of the weld metal cannot be restored to the same level as that of the base metal portion.

[0009]

SUMMARY OF THE INVENTION The object of the present invention was made in view of the above situation, and even if pipe welding is performed using a laser welding method capable of high-speed welding, it is equivalent to the base metal. It is an object of the present invention to provide a method for producing a duplex stainless steel welded pipe having a weld having corrosion resistance.

[0010]

The gist of the present invention is as follows.
There is a method for manufacturing a duplex stainless steel welded pipe.

A raw strip steel composed of duplex stainless steel is continuously formed into an open pipe shape by passing through a forming roll group.
Both edges of the steel strip are butted against each other with a squeeze roll, and at the time of pipe welding by irradiating the butted portion with a laser beam, the laser output is P (kW), the welding speed is V (m / min), and the raw strip steel is When the thickness is t (mm) and the preheating temperature of both edges of the steel strip is T (° C), the following formula (1)
A method for producing a duplex stainless steel welded pipe, which comprises performing a laser welding under the conditions that satisfy the above conditions and then performing a post heat treatment at a weld seam portion at 1000 to 1100 ° C.

P ≧ 0.4 Vt / exp (aT) ≧ 20 (1) where a: constant (= 0.006) T = 0 ° C. without preheating The inventors made the present invention by finding the following items (1) to (4) as a result of various experimental studies in forming and welding a duplex stainless steel welded pipe by using a laser welding method.

In order to completely penetrate the wall thickness of the material strip steel, a laser output P that satisfies the following formula (2) is required. In other words, when the laser output P of the laser oscillator is constant, the welding speed V and / or the preheating temperature T of both edge portions of the strip steel depends on the thickness t of the raw strip steel.
Must be set so as to satisfy the following expression (2).

P ≧ 0.4 Vt / exp (aT) (2) where a: constant (= 0.0006) without preheating, T = 0 ° C. On the other hand, the above ( As is clear from the relation of the equation (2), the welding speed V becomes slower as the material strip steel wall thickness t becomes thicker,
Alternatively, it is necessary to raise the preheating temperature T at both edges of the steel strip. However, if the welding speed V is slowed down or the preheating temperature T of both edges of the steel strip is increased, the cooling speed of the molten metal becomes slower, so that the cross-sectional shape of the welded portion becomes a wine cup shape as shown in FIG. The larger the contact area of molten metal with the atmosphere, and the thinner the thickness t of the material strip steel, the smaller the diffusion and migration distance of nitrogen in the molten metal, so that the evaporation of nitrogen from the molten metal becomes remarkable. Corrosion resistance of the welded part deteriorates. Therefore, in order to prevent the evaporation of nitrogen from the molten metal and prevent the deterioration of the corrosion resistance of the welded portion, the welding speed V is increased, the preheating temperature T of both edges of the steel strip is lowered, or the thickness t of the raw steel strip is reduced. Although it is necessary to increase the amount, it is necessary to perform laser welding under the conditions that satisfy the following formula (3) in order to prevent evaporation of nitrogen from the molten metal.

0.4 Vt / exp (aT) ≧ 20 (3) where a: constant (= 0.006) T = 0 ° C. without preheating

[0016]

The reason why the method of the present invention is limited as described above will be described in detail below.

The duplex stainless steel welded pipe produced by the pipe forming welding according to the method of the present invention must have excellent corrosion resistance of the base material, as a matter of course, regardless of its production history. It is necessary to use a material strip steel (hot rolled steel sheet) having excellent corrosion resistance, but its component composition is not particularly limited. However, as a material having excellent corrosion resistance, for example, SUS329J defined in JIS standard
It is preferable to use a standard material such as 3L, SUS329J4L, etc., and a material strip steel whose ferrite ratio is adjusted to about 50%.

[Laser welding conditions]
The pipe is continuously formed into an open pipe shape by passing it through a forming roll group by an ordinary method, both edges of the steel strip are butted against each other by a squeeze roll, and the butted portion is irradiated with a laser beam from above to perform pipe welding. Then, as is clear from the above formula (1), the laser output P is 20 k.
When a laser oscillator having a temperature of W or more is used and the material strip steel wall thickness is t (mm), the welding speed is V (m / min), and the preheating temperature of both edges of the strip steel is T (° C), By performing the laser welding under the condition satisfying the expression (3), a welded portion having a nitrogen content equivalent to that of the base metal portion and completely penetrating can be obtained even in a thick-walled tube having a thickness of 10 mm or more.

FIG. 2 shows [0.4 Vt / in the above equation (1).
exp (aT)] value and the relationship between the weld metal-to-base metal nitrogen content ratio of the weld metal and the pitting corrosion potential.
When the exp (aT)] value is less than 20, the nitrogen content of the weld metal is significantly lower than that of the base metal portion, and the pitting potential is lowered,
Corrosion resistance of welded part is inferior, but [0.4Vt / exp
When the value of (aT)] is 20 or more, the nitrogen content of the weld metal is almost the same as that of the base metal portion, the pitting corrosion potential is hardly reduced, and the corrosion resistance of the weld portion is excellent. From this, it is understood that it is necessary to satisfy the above expression (3) as a laser welding condition for eliminating the evaporation of nitrogen from the molten metal.

As described above, the above formula (2) is a condition for obtaining a weld metal that completely penetrates the wall thickness of the material strip steel, and the laser P is less than the value obtained by the above formula (2). If so, it becomes impossible to completely penetrate the wall thickness of the material strip steel. This is also clear from the results of the examples described later.

Therefore, in the present invention, the laser welding conditions are defined by the above equation (1).

It should be noted that a in each of the above formulas is a constant, and it has been found from a number of experimental results that it is appropriate to set it to "0.0006". Although not shown in the drawings, preheating of both edges of the steel strip is performed by arranging an annular induction heating coil or a contact tip, which is a high-frequency heating means used in the conventional ERW method, at a position preceding the squeeze roll and applying power to them. Is controlled to heat to a desired temperature.

[Post-Heat Treatment Conditions] Since the welded pipe obtained by pipe forming welding under the above laser welding condition does not have sufficient performance of the welded portion as it is, at least the welded seam portion is heated to weld metal. It is necessary to adjust the structure so that the ferrite rate equivalent to that of the base material is approximately 50%. However, if the heating temperature is less than 1000 ° C., the ferrite ratio equivalent to that of the base material cannot be completely recovered, and the corrosion resistance cannot be improved. In addition, intermetallic compounds such as χ phase and σ phase are precipitated and the corrosion resistance is improved. to degrade. Further, when the temperature exceeds 1100 ° C, the ferrite ratio becomes too high, and Cr, Ni, Mo
Corrosion resistance deteriorates due to improper distribution of elements such as. Therefore, the post heat treatment temperature of the welded portion was set to 1000 to 1100 ° C.

The holding time in the above temperature range is not particularly required to be fixed, but it is preferable to hold it for 30 seconds or more in order to recover the weld metal more uniformly. Further, the post-heat treatment of the weld seam portion may be performed by using an induction heating device capable of local heating used in the conventional ERW method, similarly to the preheating of both edge portions of the strip steel.

[0025]

Example A material strip steel (hot-rolled steel sheet) made of three types of duplex stainless steel having the composition shown in Table 1 was prepared.

[0026]

[Table 1]

These material strip steels are formed into an open pipe shape having each outer diameter shown in Table 2 by a conventional method, and when irradiating a laser beam from above to the abutting portions of both edges of the strip steels, Table 2 is shown. After performing laser welding under each condition, the weld seam portion was subjected to post heat treatment. A GTAW welded pipe was also prepared for comparison.

[0028]

[Table 2]

Welded portion of the obtained welded pipe (including HAZ)
Therefore, the length in the pipe axis direction is 10 mm, and the length in the pipe circumferential direction is 3
The test piece was sampled so that the weld seam was located at the center of the arc cross section at 0 mm. These test pieces are designated ASTM-D-
The corrosion resistance of the weld metal was evaluated by measuring the pitting potential in artificial seawater specified by 1141-86 using the test apparatus shown in FIG. 3 based on the method specified by JIS-G0577.

In the test piece, only 1 cm ² of the measurement cross section was exposed centering on the weld seam, and immediately before measurement, after polishing with No. 800 polishing paper, the applied potential to the test piece was changed from the natural electrode potential to the potential sweep rate of 20 mV / It was raised at min and the pitting corrosion potential was measured. Further, Ag / AgC is used as a reference electrode.
Using a 1-electrode, the test temperature was set to 60 ° C., Ar was degassed during the measurement, and pitting was generated at a potential at which the current density reached 100 μA / cm ² .

Further, the nitrogen content of the weld metal was measured by the inert gas carrier melting thermal conductivity method, and the evaporation degree was investigated by the ratio to the nitrogen content of the base metal part.

The test results are also shown in Table 2.

As is apparent from Table 2, the welded portion of the welded pipe produced by the method of the present invention has good corrosion resistance,
In the comparative examples that deviate from the conditions specified by the method of the present invention, the corrosion resistance of the welded portion is inferior. That is, Comparative Examples No. 1 to 9
Then, even though the [0.4Vt / exp (aT)] value is smaller than the laser output P, [0.4Vt / exp (aT)]
(AT)] value is less than 20, the evaporation amount of nitrogen is large and the corrosion resistance of the welded portion is poor. Further, the laser welding condition is within the range of the present invention, but the comparative example No. 10 in which the post heat treatment condition is outside the range of the present invention has a low post heat treatment temperature, so that an intermetallic compound (χ phase) precipitates, Since No. 11 has a high post-heat treatment temperature, the ferrite ratio is about 65%, which is far from 50% of the base metal value, and the element distribution is inadequate, and the corrosion resistance of the weld is poor. Furthermore, in the comparative example
No. 12 and No. 13 show [0.4 Vt / exp (a
T)] value was larger than the laser output P, so complete penetration welding could not be performed.

On the other hand, in the GTAW welded pipe of the conventional example, the corrosion resistance of the as-welded portion using the filler wire is HAZ.
Was inferior in corrosion resistance (see Nos. 20 to 22).

Further, after the post-heat treatment of the weld seam, the corrosion resistance of the weld was poor because the amount of nitrogen vaporized from the molten metal during GTAW welding was large (see Nos. 23 to 24).

[0036]

According to the method of the present invention, a duplex stainless steel welded pipe having a welded portion having a corrosion resistance equivalent to that of the base material can be produced with high efficiency even if the pipe has a relatively thick wall, Its industrial value is extremely high.

[Brief description of drawings]

FIG. 1 is a diagram showing a wine cup shape of a welded portion which occurs when the welding speed is slow.

FIG. 2 is a diagram showing the relationship between the [0.4 Vt / exp (aT)] value, the nitrogen content in the weld metal, and the pitting potential.

FIG. 3 is a diagram showing an outline of a test device for measuring a pitting potential.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication C21D 9/08 F 9/50 101 A C22C 38/00 302 H

Claims (1)

[Claims] 1. A raw strip steel consisting of duplex stainless steel is continuously formed into an open pipe shape by passing through a forming roll group.
Both edges of the steel strip are butted against each other with a squeeze roll, and at the time of pipe welding by irradiating the butted portion with a laser beam, the laser output is P (kW), the welding speed is V (m / min), and the raw strip steel is When the thickness is t (mm) and the preheating temperature of both edges of the steel strip is T (° C), laser welding is performed under the condition that satisfies the following formula (1), and then the welding seam part is back at 1000 to 1100 ° C. A method for producing a duplex stainless steel welded pipe, characterized by heat treatment. P ≧ 0.4 Vt / exp (aT) ≧ 20 (1) However, a: constant (= 0.0006) T = 0 ° C without preheating