JP3201178B2 - Method for manufacturing duplex stainless steel welded pipe - Google Patents

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 having excellent corrosion resistance and suitable for use in a line pipe or an oil country tubular good.

[0002]

2. Description of the Related Art Duplex stainless steels have a two-phase structure of a ferrite phase and an austenitic phase, have excellent corrosion resistance such as stress corrosion cracking, and have good toughness and weldability. It is widely used as a material for oil well pipes of oil wells and gas wells containing a large amount of corrosive substances, line pipes for transporting crude oil, natural gas, and the like, or materials for seawater resistance. The appropriate solution treatment temperature of the duplex stainless steel is usually 1,050 to 1,100 ° C, and the ferrite ratio (ferrite phase abundance ratio) is usually 50% by heating and holding in this temperature range and then cooling with water. The alloy elements that contribute to the corrosion-resistant coating of the duplex stainless steel are Cr and M.
o and N, and Cr and Mo are concentrated in the ferrite phase, and N is concentrated in the austenite phase to maintain the balance of corrosion resistance of both phases.

[0003] When a duplex stainless steel is welded, the weld metal becomes a single ferrite phase once at a temperature higher than the above-mentioned proper solution treatment temperature at the time of welding, so that the growth of the austenite phase is insufficient and the ferrite ratio is low. It is more than 50% which is the above-mentioned appropriate value of the base material portion.

[0004] Further, since the critical solid solution amount of N in the ferrite phase is very small, N which cannot be dissolved completely precipitates as Cr nitride in the ferrite phase to deteriorate toughness.
Since a Cr-deficient layer is formed around the area where the Cr nitride is deposited, the corrosion resistance is also deteriorated, and the performance of the weld is significantly deteriorated.

Therefore, in order to prevent deterioration of the corrosion resistance and toughness of the weld, it is important that the ferrite rate of the weld be close to the ferrite rate of the base material. It is necessary to positively add elements to the molten metal or to perform post-thermal solution treatment on the welded portion.

Conventionally, not only duplex stainless steel but also a welded pipe is formed by continuously passing a raw steel strip through a group of forming rolls into a tubular shape, and using a squeeze roll to butt both edges of the steel strip. The butt portion is heated by high frequency and subjected to electric resistance welding (hereinafter, referred to as ERW) or arc welding such as gas-tungsten-arc welding (hereinafter, referred to as GTAW) or submerged-arc welding (hereinafter, referred to as SAW). Manufactured by pipe welding.

Therefore, when a duplex stainless steel welded pipe is manufactured by these welding methods, in order to optimize the ferrite ratio of the weld metal after welding, post-weld heat treatment is performed in electric resistance welding. In arc welding, it is necessary to actively add austenite-forming elements such as Ni and N into the molten metal by a filler wire. For example, “Duplex Stain-l
ess Steels '91 -p421, p461, p469 "contains the GTA
This is an example of an arc welding method such as W. When welding is performed by mixing nitrogen in a shielding gas when welding duplex stainless steel, N can be added to the molten metal in a solid solution, and the weld metal after welding It is known that the ferrite ratio can be optimized and the corrosion resistance of the weld metal can be improved.

In recent years, development of a laser welding method, which has a very high welding speed as compared with the above-mentioned conventional welding methods, has been promoted. However, in the laser welding method, welding is performed using a filler wire due to facility restrictions. Since it is difficult to add alloying elements such as N and Ni into gold, proper post-heat treatment is performed on the weld after laser welding to recover the performance of the weld metal. For example, although neither is intended for duplex stainless steel, austenitic stainless steel is disclosed in JP-A-63-278688, ferritic stainless steel is disclosed in JP-A-63-278689, and 63-278 is disclosed.
No. 690 discloses a Mo-containing low alloy steel,
A method is disclosed in which, after laser welding at a welding speed of a predetermined condition, a predetermined post-heat treatment is performed on a welded portion to recover the ductility of a weld metal.

On the other hand, as described above, when a duplex stainless steel welded pipe is welded by arc welding such as ERW or GTAW, the ferrite / austenite ratio is improved by adding nitrogen to the shielding gas. It is possible to plan. However, two of these welding methods
Cr nitrides remain in the ferrite phase in the weld metal of the weld portion of the duplex stainless steel welded pipe, and a Cr-deficient layer is formed around the ferrite phase. .

In a welding method such as ERW or arc welding in which the cooling rate of molten metal is low, a heat-affected zone (hereinafter, referred to as HAZ) is formed in the base metal portion adjacent to the weld metal. 1150-150
In the high temperature HAZ affected by the heat of 1250 ° C., a shift in the ferrite / austenite phase ratio occurs.
The low-temperature HAZ affected by heat at 00 to 950 ° C. has a disadvantage that the intermetallic compounds such as the χ phase and the σ phase are precipitated, so that the corrosion resistance and mechanical properties of the HAZ are extremely poor. However, unlike the weld metal part, HAZ is in a solid state at the time of welding, so that it is impossible to add an alloy element by a filler wire or to control the composition of a shielding gas.

Although it is possible to recover the corrosion resistance of the heat-affected zone to some extent by post-heat treatment after welding, the cooling rate of the molten metal is slow, so that during welding, the nitrogen partial pressure depends on the nitrogen partial pressure of the shielding gas. Evaporation or absorption takes place, and it is practically difficult to control the ferrite / austenite phase ratio of the weld metal by post-heat treatment, and the corrosion resistance cannot be restored to the same level as that of the base metal.

[0012]

SUMMARY OF THE INVENTION An object of the present invention has been made in view of the above situation, and has a welded portion having substantially the same corrosion resistance as a base material without post-heating treatment after welding.
It is an object of the present invention to provide a method for inexpensively manufacturing a duplex stainless steel welded pipe.

[0013]

The gist of the present invention is as follows.
Manufacturing method of duplex stainless steel welded pipe.

A material strip made of duplex stainless steel is continuously formed into an open pipe shape through a group of forming rolls, and both edges of the material strip are butted against each other by a squeeze roll, and a laser beam is applied to the butted portion. A method for producing a duplex stainless steel welded pipe, comprising performing laser welding in a shielding gas made of He or Ar containing nitrogen at a partial pressure of 0.01 to 0.2 atm for pipe welding.

In the method of the present invention, laser welding is performed under the conditions that the cooling rate V ₇₀₀ of the molten metal after laser welding at 700 ° C. obtained by the following equation (1) is in the range of 10 to 900 ° C./sec. Is more preferred.

V ₇₀₀ = 3.02 × 10 ⁻⁷ × (tV / P) ² × (700−T ₀ ) ³ (1) where t is the thickness of the steel strip of the material band (mm) V is the welding speed (m / min) P: laser output (kW) T _0: material steel strip both edges of the preheat temperature (℃) the present inventors have obtained a finding of ~ results of the following various experimental studies, this finding The present invention has been made based on the above.

As mentioned above, ERW or GTA
In arc welding of W, etc., the cooling rate of the molten metal is slow, so HA
Z is generated and the corrosion resistance is deteriorated. However, in the case of laser welding, since the cooling rate of the molten metal is much higher than that of the conventional arc welding method, the HAZ in which the corrosion resistance is deteriorated is narrow or
Or not at all.

In arc welding such as ERW or GTAW, nitrogen that cannot be dissolved completely in the ferrite phase of the weld metal when the welding is performed in a shielding gas atmosphere composed of He or Ar containing nitrogen contains Cr nitride. Despite the fact that the ferrite ratio recovers to the same level as that of the base material, the corrosion resistance is significantly deteriorated as compared with the base material. This is because the form of Cr nitride is mainly composed of Cr ₂ N. _Two
This is because a Cr-deficient layer is formed around N. On the other hand, when laser welding is performed in a shield gas made of He or Ar containing a predetermined concentration of nitrogen, the ferrite / austenite phase ratio in the weld metal is reduced. In addition to being able to improve the same as the base metal, the rapid cooling rate of the molten metal causes supersaturated solid solution of N in the weld metal, and the absolute amount of precipitation of Cr nitride decreases. The morphology changes to CrN, and the Cr-deficient layer formed around the CrN is reduced to improve the corrosion resistance.

The effect of the laser welding in the above shielding gas is as follows.
_When controlling ₇₀₀ to a predetermined range, it becomes more remarkable. That is, the value obtained by the above equation (1) is 10 to 9
The effect becomes remarkable if welding is performed at a temperature of 00 ° C./sec.

As far as the present inventors know, no detailed conventional knowledge on the structure and corrosion resistance of the weld metal when laser welding is performed in a shielding gas atmosphere made of He or Ar containing N (nitrogen) is found.

[0021]

The reasons for limiting the constituent elements of the present invention are as follows.

[Shielding gas] When the nitrogen content in the shielding gas is less than 0.01 atm in partial pressure, the amount of nitrogen absorbed in the molten metal is small, so that the ferrite /
The austenite phase ratio largely deviates from the ferrite / austenite phase ratio of the base material, and a large amount of Cr nitride mainly composed of Cr ₂ N precipitates, thereby significantly deteriorating the corrosion resistance of the weld. On the other hand, when the nitrogen content in the shielding gas is 0.
When the pressure exceeds 2 atm, nitrogen is originally an element that easily becomes a plasma, so that a large amount of nitrogen in the shielding gas is turned into plasma by irradiation with a laser beam. In addition to the decrease, the penetration welding cannot be performed. In addition, the amount of nitrogen absorbed into the molten metal decreases due to the decrease in the temperature of the molten metal due to the decrease in energy, and the effect of improving the corrosion resistance of the welded portion cannot be obtained. Therefore, the nitrogen content in the shielding gas was determined to be 0.01 to 0.2 atm in partial pressure.

In order to prevent the occurrence of welding defects due to oxides by preventing the oxidation of the material surface and to prevent the occurrence of welding defects due to oxides, and to minimize the reduction in laser energy absorption by laser irradiation, He gas or Ar gas, which is difficult to convert, was used.

[Cooling Rate of Weld Metal] The fact that the structure and corrosion resistance of a weld metal when a duplex stainless steel is welded by an arc welding method such as GTAW or the like is uniquely dependent on the cooling rate of the molten metal. Duplex Stainless Steels
'91 p347 'and' Duplex Stainless Steels '86 p15
5 "and the like. However, the effect of the cooling rate of the molten metal on the structure and corrosion resistance of the weld metal when laser welding two-phase stainless steel is not known. Therefore, as described above, the present inventors have conducted various experimental studies on the relationship, and assuming that the above expression (1) is satisfied, the effect of laser welding in He gas or Ar gas containing the above amount of nitrogen is considered. It was found to be more noticeable.

FIG. 1 is a graph showing the relationship between the cooling rate V ₇₀₀ of the molten metal at 700 ° C. and the potential for pitting corrosion of the weld metal, obtained by the above equation (1). As is apparent from FIG. 1, when laser welding is performed in a shielding gas atmosphere containing the above amount of nitrogen, if the cooling rate V ₇₀₀ of the weld metal at 700 ° C. is 10 to 900 ° C./sec, the welded portion of the welded metal is welded. It can be seen that the pitting corrosion resistance is further improved.

Therefore, it can be seen that it is desirable to perform laser welding under the condition that the cooling rate V ₇₀₀ of the weld metal at 700 ° C. is 10 to 900 ° C./sec. That is, if the cooling rate V ₇₀₀ is less than 10 ° C./sec, the cooling rate of the weld metal is too slow, so that precipitation of Cr nitride in the form of Cr ₂ N in the weld metal cannot be completely suppressed, On the other hand, if it exceeds 900 ° C./sec, the cooling rate is too high, so that the amount of nitrogen absorbed from the shielding gas into the weld metal becomes insufficient, and the corrosion resistance cannot be sufficiently improved.

In order to set the cooling rate V ₇₀₀ of the molten metal at 700 ° C. to a value within the range of 10 to 900 ° C./sec, the laser output P, the welding speed V, Any one or more of the preheating temperatures T ₀ of both edge portions may be appropriately adjusted.

The preheating of both edges of the steel strip is not shown, but an annular coil or a contact tip for high-frequency induction heating used in ERW is arranged in front of the squeeze roll, and the supplied electric power is adjusted and controlled. Heating to a predetermined temperature.

The cooling rate V of the molten metal at 700 ° C.
_The above equation (1) for obtaining ₇₀₀ is expressed as “Welding Journal 37 (1
958) 210S ", which is used to calculate the cooling rate of ADAMS molten metal.

[0030]

EXAMPLE A steel strip having the composition shown in Table 1 was prepared.

[0031]

[Table 1]

These steel strips were formed into an open pipe shape by conventional methods so as to obtain welded pipes having the respective outer diameters shown in Table 2, and when a laser beam was applied to the butted portion of both edges of the steel strip, a table was formed. Laser welding was performed under the respective conditions shown in FIG. For comparison, a tube welded with GTAW was also prepared.

[0033]

[Table 2]

The obtained welded portion of the welded pipe (HA)
Z), a test piece was sampled such that the welded seam portion was located at the center of an arc cross section having a pipe circumferential dimension of 30 mm and a pipe axial dimension of 10 mm. The corrosion resistance of the weld metal was determined by measuring the pitting potential of these test pieces in artificial seawater having the component composition shown in Table 3 using the test device shown in FIG. 2 based on the method specified in JIS-G0577. Was evaluated.

[0035]

[Table 3]

The test piece was exposed at 1 cm ² only at the measurement section centering on the welded seam portion. The test piece was polished with No. 800 abrasive paper immediately before measurement, and the potential applied to the test piece was changed from the natural electrode potential to a potential sweep rate of 20 mV /. go up at the speed of min,
The pitting potential was measured. In addition, Ag / Ag
Using a Cl electrode, the test temperature was set to 60 ° C., Ar degassing was performed during the measurement, and the occurrence of pitting corrosion was set to a potential at which the current density reached 100 μA / cm ² .

The results of measuring the pitting potential are also shown in Table 2.

As is clear from Table 2, in the present invention example, 5
A pitting corrosion potential of 21 mV or more is obtained, and the welded portion is excellent in corrosion resistance. In contrast, in the comparative example, the pitting potential was 5
Below 10 mV, the corrosion resistance of the weld is inferior. In the case of conventional pipe welding using GTAW, the pitting potential is 45%.
At 0 mV or less, the corrosion resistance of the welded part is inferior to the comparative example of the present invention.

[0039]

According to the method of the present invention, a duplex stainless steel pipe having excellent corrosion resistance at a welded portion can be provided at low cost by using it as it is, and its industrial value is great.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the cooling rate of molten metal at 700 ° C. and pitting potential.

FIG. 2 is a diagram schematically showing a test device for measuring pitting potential.

──────────────────────────────────────────────────の Continuation of the front page (51) Int.Cl. ⁷ identification code FI C22C 38/00 302 C22C 38/00 302H (58) Field surveyed (Int.Cl. ⁷ , DB name) B23K 26/00 B21C 37 / 08 B23K 26/12 B23K 26/14 C22C 38/00

Claims (2)

(57) [Claims] 1. A material strip made of duplex stainless steel is continuously formed into an open pipe shape through a group of forming rolls, and both edges of the material strip are butted with a squeeze roll. For welding pipes by irradiating with a laser beam, laser welding in a shielding gas consisting of He or Ar containing nitrogen at a partial pressure of 0.01 to 0.2 atm. . 2. The cooling rate V ₇₀₀ of the molten metal at 700 ° C. after laser welding, which is determined by the following equation, is 10 to 900 ° C./s.
The method for producing a duplex stainless steel welded pipe according to claim 1, wherein the laser welding is performed under a condition of ec. V ₇₀₀ = 3.02 × 10 ⁻⁷ × (tV / P) ² × (700
−T ₀ ) ³ , where t: material strip steel thickness (mm) V: welding speed (m / min) P: laser output (kW) T ₀ : preheating temperature of both edges of the material strip (° C.)