JPH06256843A - Production of two-phase stainless welded steel pipe having excellent strength, toughness and corrosion resistance - Google Patents

Abstract

PURPOSE:To produce the austenitic-ferritic two-phase stainless welded steel pipe having excellent strength, toughness and corrosion resistance by subjecting a welded steel pipe made of a hot rolled sheet of an Ni-Cr stainless steel as a blank material to a soln. heat treatment under specific conditions. CONSTITUTION:The hot rolled sheet of the stainless steel contg., by weight 5, <0.05% C, <1.5% Si, <2.0% Mn, 3.0 to 5.0% Ni, 21 to 25% Cr and <0.35% N and having >=400 2%-yield point index sigma0.2 expressed by equation 1 is heated to a temp. region between 900 deg.C and temp. T deg.C expressed by equation 2 and is then cooled. The seam welded steel pipe is produced from this sheet material as the blank material and is subjected to the soln. heat treatment by heating to 900 to 1150 deg.C, then cooling at >=1 deg.C/sec cooling rate in a 500 to 850 deg.C temp. region. The ferritic-austenitic two-phase stainless welded steel pipe having 0.40 to 0.60 volumetric rate of the ferrite phase of the steel pipe base material and >=23.5 of both pitting corrosion resistance indices alphaP1, gammaP1 respectively of the ferrite phase and austenite phase expressed by equations 3, 4 is produced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention provides an economical austenitic / ferrite-based duplex stainless steel welded steel pipe excellent in strength, toughness and corrosion resistance, which is suitable for use in a chloride-containing environment or a C0 ₂ corrosive environment. The present invention relates to a method for manufacturing.

[0002]

_2. Description of the Related Art In recent years, due to exhaustion of energy resources, C0 ₂
Also, petroleum or natural gas containing corrosive substances such as chlorides is being utilized, and the use of highly corrosion-resistant steel pipe is increasing as a steel pipe for transporting such oil or natural gas at high pressure. . As a highly corrosion-resistant steel pipe, 13Cr steel has the characteristics of high strength and low cost, but its poor weldability has not been used as a welded steel pipe.

Further, since 13Cr steel is inferior in corrosion resistance in a high temperature environment exceeding 100 ° C., it is unsuitable for use in such a high temperature environment. Conventionally, in a high temperature environment, 18Cr-8Ni type SUS304 is used. , Austenitic stainless steel such as 16Cr-11Ni-2Mo SUS316, or SUS3
Austenitic / ferritic duplex stainless steels such as 29J3L and SUS329J4L are used.

However, the welded steel pipe made of austenitic stainless steel such as SUS304 and SUS316 has lower strength (0.2% proof stress of about 250 MPa) than the welded steel pipe made of 13Cr steel, and chloride. There is a problem that stress corrosion cracking resistance is poor in an environment containing a lot of materials. On the other hand, a welded steel pipe made of a duplex stainless steel is excellent in all properties such as strength, toughness and stress corrosion cracking resistance. However, the conventional duplex SUS329J3L steel and SUS329J4L steel duplex stainless welded steel pipe is
Since it contains a large amount of molybdenum and nickel,
There is a problem that the manufacturing cost is increased and the material is expensive as a material used in an environment containing almost no hydrogen sulfide.

Therefore, the development of a duplex stainless steel which is excellent in strength, toughness and stress corrosion cracking resistance and is inexpensive is under way. For example, JP-A-1-165750 and JP-A-1-165750.
Japanese Patent No. 1-201446 and Japanese Patent Publication No. 4-42464 disclose steels containing a small amount of molybdenum and nickel (hereinafter referred to as Prior Art 1).

On the other hand, in order to improve pitting corrosion resistance, which is important as corrosion resistance in an environment containing chloride, it is effective to increase the contents of chromium, molybdenum and nitrogen in stainless steel. In Kaihei 1-165750 and Japanese Patent Laid-Open No. 3-82740, as an index of pitting corrosion resistance of duplex stainless steel, PI = Cr (%) + 3 × Mo (similar to that of austenitic stainless steel. %) + 16 × N (%) (hereinafter referred to as Prior Art 2) is disclosed.

[0007]

The above-mentioned prior art 1 has the following problems. That is, for example, Japanese Patent Publication No.
As disclosed in Japanese Patent No. 3 etc., the toughness of duplex stainless steel deteriorates as the volume fraction (α _f ) of the ferrite phase increases. Therefore, the volume ratio (α _f ) of the ferrite phase is
Usually designed around 0.5. However, in the case of a duplex stainless steel having an extremely low nickel content, its toughness deteriorates even _if the volume ratio (α _f ) of the ferrite phase is about 0.5.

Prior art 2 has the following problems.
That is, in the duplex stainless steel, since the content of each component of the ferrite phase and the austenite phase is different, in the PI using the average composition as an index of the pitting corrosion resistance as in the prior art 2, the pitting corrosion resistance is overestimated. There are cases where Further, even if the steel contains a considerable amount of chromium, molybdenum and nitrogen, if the nickel content is extremely low, the pitting corrosion resistance deteriorates.

Therefore, an object of the present invention is to solve the above-mentioned problems, have 0.2% proof stress (400 MPa or more) equivalent to 13Cr steel, and have excellent toughness, and in an environment containing chloride or C0 ₂ . It is an object of the present invention to provide a method for economically producing a welded steel pipe made of a duplex stainless steel, which has overall corrosion resistance, pitting corrosion resistance and stress corrosion cracking resistance higher than that of SUS316 steel.

[0010]

A method for manufacturing a duplex stainless steel welded steel pipe according to the present invention is characterized by the following. Carbon (C): 0.05 wt.% Or less, Silicon (Si): 1.5 wt.% Or less, Manganese (Mn): 2.0 wt.% Or less, Nickel (Ni): 3.0 to 5.0 wt.%, Chromium (Cr): 21.0 to 25.0 wt.%, Nitrogen (N): 0.25 wt.% Or less, and the rest: Fe and inevitable impurities. The following (1) formula σ _0.2 = 20 x Cr (%) + 11 x Mn (%) - Ni (%) + 133 × N (%) - 38 ───── (1) index (sigma _0.2) showing a 0.2% proof stress obtained by 400
A steel ingot or a steel slab having the chemical composition described above is prepared, a steel plate is prepared by hot rolling the steel ingot or the steel slab, and then 900 ° C. or more with respect to the steel plate, the following (2) formula The original plate is prepared by annealing, which consists of heating at a temperature in the range of T (° C) or less, which is calculated by the following, and then cooling, T = 71 x Ni (%) + 6 x Mn (%) -36 x Cr (%) −42 × Si (%) ＋ 1037 × C (%) + 1113 × N (%) + 1608 ────── (2) Then, the raw plate is molded to prepare a raw pipe and the seam portion of the raw pipe. Welded metal of the seam, carbon (C): 0.05 wt.% Or less, silicon (Si): 1.5 wt.% Or less, manganese (Mn): 2.0 wt.% Or less, nickel (Ni): 3.0 to 5.0 wt.%, Chromium (Cr): 21.0 to 25.0 wt.%, Nitrogen (N): 0.25 wt.% Or less, oxygen (O): 0.035 wt.% Or less, and the rest: Fe and inevitable impurities, And 0.
2% proof stress (σ _0.2 ) an index indicating a welded steel pipe having a chemical composition that is 400 or more, then, with respect to the welded steel pipe, heating at a temperature in the range of 900 ~ 1150 ℃, 500
A solution heat treatment consisting of cooling at a rate of 1 ° C / sec or more in a temperature range of up to 850 ° C, and thus,
The volume ratio α _f of the ferrite phase of the base material of the welded steel pipe is 0.40.
Within the range of ~ 0.60, and the following (3) of the base material.
The pitting corrosion resistance index of the ferrite phase (α
_P1 ) and the austenite phase pitting corrosion resistance index (γ _P1 ) determined by the following equation (4) are both 23.5 or more, and α _P1 = 23 × Cr (%) / (3 × α _f + 20) ── ────────────── (3) γ _P1 = 20 × Cr (%) / (3 × α _f + 20) − 16 × N (%) / (α _f -1) ─ ─── (4) And, in the seam weld metal of the welded steel pipe, the volume ratio (α _f ) of the ferrite phase is in the range of 0.30 to 0.50, and the pitting corrosion resistance of the ferrite phase of the weld metal Index (α _P1 ) and pitting corrosion index of austenite phase (γ
_P1 ) is greater than either α _P1 or γ _P1 of the base material, whichever is lower, to produce a duplex stainless welded steel pipe.

The method for producing a duplex stainless steel welded steel pipe according to the second aspect of the present invention is characterized by the following: Carbon (C): 0.05 wt.% Or less, Silicon (Si): 1.5 wt.% Or less, Manganese (Mn): 2.0 wt.% Or less, Nickel (Ni): 3.0 to 5.0 wt.%, Chromium (Cr): 21.0 to 25.0 wt.%, Nitrogen (N): 0.25 wt.% Or less, and the rest: Fe and unavoidable impurities, consisting of the following (1) formula σ _0.2 = 20 x Cr (%) + 11 x Mn (%) - Ni (%) + 133 × N (%) - 38────── (1) index (sigma _0.2) showing a 0.2% proof stress obtained by 400
A steel ingot or a steel slab having the above chemical composition is prepared, and the steel ingot or the steel slab is heated at a temperature of T (° C.) or less obtained by the following formula (2), and 900 ° C. The original plate is prepared by performing hot rolling consisting of finishing at the above temperature, and T = 71 × Ni (%) + 6 × Mn (%) − 36 × Cr (%) − 42 × Si (%) + 1037. × C (%) + 1113 × N (%) + 1608 ─────── (2) Next, the raw plate is molded to prepare a raw pipe, and the seam portion of the raw pipe is welded. , Carbon (C): 0.05 wt.% Or less, silicon (Si): 1.5 wt.% Or less, manganese (Mn): 2.0 wt.% Or less, nickel (Ni): 3.0 to 5.0 wt.%, Chromium (Cr) : 21.0 to 25.0 wt.%, Nitrogen (N): 0.25 wt.% Or less, oxygen (O): 0.035 wt.% Or less, and the rest: Fe and inevitable impurities, and the above-mentioned 0.
An index showing 2% proof stress (σ _0.2 ) is prepared a welded steel pipe having a chemical composition that is 400 or more, and then, with respect to the welded steel pipe, heating at a temperature in the range of 900 to 1150 ° C., 500
A solution heat treatment consisting of cooling at a rate of 1 ° C / sec or more in a temperature range of up to 850 ° C, and thus,
The volume ratio α _f of the ferrite phase of the base material of the welded steel pipe is 0.40.
Within the range of ~ 0.60, and the following (3) of the base material.
The pitting corrosion resistance index of the ferrite phase (α
_P1 ) and the austenite phase pitting corrosion resistance index (γ _P1 ) determined by the following equation (4) are both 23.5 or more, and α _P1 = 23 × Cr (%) / (3 × α _f + 20) ── ────────────── (3) γ _P1 = 20 × Cr (%) / (3 × α _f + 20) − 16 × N (%) / (α _f -1) ─ ─── (4) And, in the seam weld metal of the welded steel pipe, the volume ratio (α _f ) of the ferrite phase is in the range of 0.30 to 0.50, and the pitting corrosion resistance of the ferrite phase of the weld metal Index (α _P1 ) and pitting corrosion index of austenite phase (γ
_P1 ) is greater than either α _P1 or γ _P1 of the base material, whichever is lower, to produce a duplex stainless welded steel pipe.

[0012]

The chemical composition of the base metal plate and the weld metal and the volume fraction of the ferrite phase in the method of the present invention will be described below.
The reason why (α _f ) is limited to the above range will be described. (1) Carbon (C): Carbon is an austenite forming element. However, if the carbon content increases beyond 0.05 wt.%, Carbides are generated in the steel and the corrosion resistance deteriorates. Therefore, the carbon content of the original plate and the weld metal should be limited to 0.05 wt.% Or less.

(2) Silicon (Si): Silicon is an element useful as a deoxidizer. However, when the silicon content exceeds 1.5 wt.%, The susceptibility to cracking at the time of welding increases, and the intermetallic compound is generated in the steel to deteriorate the hot workability. Therefore, the silicon content of the original plate and the weld metal should be limited to 1.5 wt.% Or less.

(3) Manganese (Mn): Manganese is an austenite forming element and has a deoxidizing action. However, if the manganese content exceeds 2.0 wt.% And increases, the pitting corrosion resistance in an environment containing chloride deteriorates.
Therefore, the manganese content of the original plate and the weld metal is 2.0w.
It should be limited to t.% or less.

(4) Nickel (Ni): Nickel is a strong austenite-forming element, and is an essential element for obtaining the volume fraction (α _f ) of the ferrite phase described later.
However, when the nickel content is less than 3.0 wt.%, The ductility, toughness and pitting corrosion resistance are significantly deteriorated even _if the volume ratio (α _f ) of the ferrite phase is adjusted to an appropriate value described later. On the other hand, if the nickel content exceeds 5.0 wt.%, Not only the cost will increase, but also the content of nitrogen, which is an austenite forming element, in order to adjust the volume ratio (α _f ) of the ferrite phase to an appropriate value. As a result, it may be disadvantageous from the viewpoint of pitting corrosion resistance. Therefore, the nickel content of the original plate and the weld metal should be limited to the range of 3.0 to 5.0 wt.%.

(5) Chromium (Cr): Chromium is a strong ferrite-forming element, and has the effect of enhancing general corrosion resistance and pitting corrosion resistance. However, if the chromium content is less than 21.0 wt.%, The volume ratio (α _f ) of the ferrite phase cannot be adjusted to an appropriate value described later without generating martensite. On the other hand, the chromium content is 25.0
When it exceeds wt.%, the toughness decreases and the σ phase precipitates, so that the corrosion resistance and hot workability deteriorate. Therefore,
The chromium content of the original plate and the weld metal should be limited within the range of 21.0 to 25.0 wt.%.

(6) Nitrogen (N): Nitrogen is a strong austenite forming element, and is an effective element that imparts pitting corrosion resistance to steel. However, the nitrogen content is 0.25 wt.%
If it exceeds, the deformation resistance at the time of hot rolling increases, so that the original plate is cracked and blows at the time of welding. Therefore, the nitrogen content of the original plate and the weld metal is 0.
It should be limited to 25 wt.% Or less.

(7) Oxygen (O): When the oxygen content increases, the toughness and pitting corrosion resistance deteriorate. Therefore, the oxygen content of the original stainless steel is 0.01% by ordinary steelmaking technology.
wt.% or less. However, the seam portion of a stainless welded steel pipe is often welded by a submerged arc using a low basicity flux from the viewpoint of improving efficiency. Therefore, an increase in the oxygen content of the weld metal is unavoidable, but from the viewpoint of preventing deterioration of the toughness and pitting corrosion resistance of the weld metal, the oxygen content of the weld metal is 0.03
It should be limited to 5 wt.% Or less.

[0019] (8) The volume ratio of the ferrite phase (alpha _f): In the two-phase stainless steel, the volume ratio of the ferrite phase (alpha _f)
Has a great influence on various properties of steel. That is, when the volume ratio (α _f ) of the ferrite phase is less than 0.40, the hot rolling property deteriorates significantly, and when α _f exceeds 0.60, the ductility and toughness of the steel deteriorate. Therefore, the volume ratio (α _f ) of the ferrite phase of the original plate should be limited to the range of 0.40 to 0.60. On the other hand, since the crystal grains of the weld metal are significantly coarser than the original plate, the volume fraction of the ferrite phase (α _f )
If it exceeds 0.50, the ductility and toughness of the weld metal deteriorate. If the volume ratio (α _f ) of the ferrite phase is less than 0.30, the stress corrosion cracking resistance decreases. Therefore, the volume fraction (α _f ) of the ferrite phase of the weld metal should be limited to the range of 0.30 to 0.50.

The present inventors have found that molybdenum-free 2
In duplex stainless steel, the effect of the components on the strength was investigated in detail. As a result, it was found that the index (σ _0.2 ) indicating the 0.2% proof stress of the duplex stainless steel containing no molybdenum is represented by the following formula (1). Therefore, in this invention, the same level of
In order to provide 0.2% proof stress, the contents of chromium, manganese, nickel and nitrogen in the original plate and the weld metal should be limited so that the value obtained by the following formula (1) is 400 or more. σ _0.2 = 20 × Cr (%) ＋11 × Mn (%) − Ni (%) + 133 × N (%) −38 ──── (1)

On the other hand, as a result of repeated studies on the pitting corrosion resistance in the environment containing chloride, the pitting corrosion resistance is also different in the duplex stainless steel because the contents of the respective components of the ferrite phase and the austenite phase are different. It was found that, unlike the ferrite phase and the austenite phase, pitting corrosion occurs early in the phase with poor pitting resistance.

The index of pitting corrosion resistance in the ferrite phase (α _P1 ) and the index of pitting corrosion resistance in the austenite phase (γ _P1 ) of the two-phase stainless steel containing no molybdenum are as follows. It depends on the volume fraction of the phase (α _f ). Therefore,
In order to impart pitting corrosion resistance equivalent to that of SUS316 steel to duplex stainless steel, the pitting corrosion resistance index (α _P1 ) in the ferrite phase obtained by the following equation (3) and the following
It is necessary that the index (γ _P1 ) of pitting corrosion resistance in the austenite phase calculated by the equation (4) is 23.5 or more. α _P1 = 23 × Cr (%) / (3 × α _f + 20) ──────────────── (3) γ _P1 = 20 × Cr (%) / (3 × α _f + 20) -16 × N (%) / (α _f -1) ──── (4)

From the above, the content of chromium and nitrogen in the original plate is adjusted so that the index of pitting corrosion resistance in the ferrite phase (α _P1 ) and the index of pitting corrosion resistance in the austenite phase (γ _P1 ) are both 23.5 or more. The amount and volume fraction of the ferrite phase (α _f ) should be limited. If the pitting corrosion resistance of the weld metal is lower than the pitting corrosion resistance of the original plate,
Since pitting corrosion occurs preferentially in the weld metal, the index of pitting corrosion resistance in the ferrite phase of the weld metal (α _P1 ) and the index of pitting corrosion resistance in the austenite phase (γ _P1 ) are
The chromium and nitrogen contents of the weld metal and the volume fraction (α _f ) of the ferritic phase should be limited so that they are higher than the lower one of the α _P1 and the γ _P1 of the original plate.

In the method of the first embodiment of the present invention, a steel ingot or a steel slab having the above-described chemical composition is hot-rolled to prepare a steel sheet, and then the steel sheet is heated to 900 ° C. or higher and the following ( 2) An original plate is prepared by annealing, which consists of heating at a temperature within the range of T (° C) or less obtained by the formula and then cooling, and T = 71 × Ni (%) + 6 × Mn (%) −36. × Cr (%) −42 × Si (%) ＋ 1037 × C (%) + 1113 × N (%) + 1608 ────── (2) Next, the original plate is molded to prepare a raw pipe, and the seam of the raw pipe is prepared. Weld the parts to prepare a welded steel pipe. Next, the welded steel pipe thus prepared is characterized by undergoing solution heat treatment under predetermined conditions.

In the above process, during annealing,
If the heating temperature for the steel sheet is less than 900 ° C, the original sheet will not be sufficiently softened, and it will be difficult to form the tubular body thereafter. On the other hand, when the heating temperature during annealing is high, the volume ratio (α _f ) of the ferrite phase increases, and when α _f exceeds 0.60,
Not only the ductility is lowered, but also significant wrinkling may occur on the original plate during the tube forming. Therefore, as a result of a detailed study on the relationship between the volume fraction of the ferrite phase (α _f ), the heating temperature and the chemical composition, the present inventors have found that the volume fraction of the ferrite phase (α _f ) is 0.60 or less. Temperature T (℃)
It has been found that can be expressed by the above equation (2). Therefore, the heating temperature for the steel sheet during annealing should be limited to the range of 900 ° C. or higher and T (° C.) or lower determined by the above equation (2).

In the method of the second embodiment of the present invention, a steel ingot or a steel slab having the above-mentioned chemical composition is heated at a temperature of T (° C.) or less obtained by the above equation (2), and 900 A raw plate is prepared by performing hot rolling consisting of finishing at a temperature of ℃ or more, and then, similarly to the method of the first embodiment, the raw plate is molded to prepare a raw pipe, and the seam portion of the raw pipe is welded. It is characterized in that a welded steel pipe such as a UOE steel pipe is prepared, and then a solution heat treatment is applied to the welded steel pipe thus prepared under predetermined conditions. According to the method of the second embodiment, it is possible to form a tubular body without performing softening annealing on a hot-rolled steel sheet, so that the manufacturing cost can be further reduced.

The inventors of the present invention have made repeated studies on the conditions under which tube forming can be performed on a hot-rolled steel sheet without softening and annealing. As a result, the following was found.
That is, in the steel of the present invention, the volume ratio (α _f ) of the ferrite phase of the hot rolled steel sheet is almost the same as the volume ratio (α _f ) of the ferrite phase during heating of the steel ingot or the billet.

Therefore, in order to prevent the generation of wrinkles during the forming of the tubular body, the heating temperature for the steel ingot or the steel slab during hot rolling is set so that the volume fraction of ferrite phase (α _f ) is 0.60.
It should be limited to the temperature below T ° C. calculated by the above equation (2). Also, sufficient dynamic recrystallization or recovery must occur during rolling in order to omit softening annealing. Therefore, the finishing temperature for hot rolling should be limited to 900 ° C or higher.

By welding the seam portion of the raw pipe prepared by molding the original plate into a tubular body, the weld metal of the duplex stainless steel and the heat-affected zone of the base material are precipitated with carbonitride and ferrite phase. The volume fraction (α _f ) of is significantly increased. Therefore, in order to eliminate the precipitated carbonitrides and optimize the volume ratio (α _f ) of the ferrite phase, it is necessary to subject the welded steel pipe welded at the seam portion to solution heat treatment.

The above-mentioned solution heat treatment must be carried out under the conditions of heating at a temperature in the range of 900 to 1150 ° C. and cooling at a rate of 1 ° C./sec or more in the temperature range of 500 to 850 ° C. There is. That is, the heating temperature is 900 ℃
When the amount is less than the above, the carbonitride does not form a solid solution, so that the corrosion resistance deteriorates. On the other hand, when the heating temperature exceeds 1150 ° C., the volume ratio (α _f ) of the ferrite phase increases and the crystal grains become coarse, so that the ductility and toughness deteriorate.

Chromium carbonitrides are generally at 500-850 ° C.
It precipitates in the temperature range within the range. However, in the solution heat treatment, when the cooling rate in the temperature range within the range of 500 to 850 ° C for the welded steel pipe heated under the above-mentioned conditions is less than 1 ° C / sec, the grains of chromium carbonitride are accompanied by precipitation. Intercalation becomes significant. Therefore, in solution heat treatment, 50
The cooling rate in the temperature range of 0 to 850 ° C should be limited to 1 ° C / sec or more.

[0032]

EXAMPLES Next, the present invention will be described by way of Examples in comparison with Comparative Examples. Example 1 As shown in Table 1, steel ingots having a chemical composition within the scope of the present invention and having a weight of 50 kg of steels a to i, at least one of which has a chemical composition outside the scope of the present invention for comparison. Trial steel a '
A steel ingot having a weight of ˜1 ′ and a weight of 50 kg was prepared, and a commercially available SUS316 steel sheet and a commercially available 13Cr steel sheet were prepared as conventional steels.

Then, the steel ingots of the test steels a to i and the comparative test steels a ′ to l ′ were heated at a temperature of 1150 ° C., and then 900
It was hot-rolled at a finishing temperature of ℃, and then air-cooled in a temperature range of 500 to 850 ℃ at a cooling rate of 0.7 ℃ / sec to prepare a steel plate having a thickness of 15 mm. For the steel sheet prepared in this way, heating at a temperature of 1050 ° C
Solution heat treatment was performed, which consisted of cooling at a rate of 30 ° C / sec in the temperature range of 850 ° C. Thus, the volume ratio (α _f ) of the ferrite phase within the scope of the present invention shown in Table 1, the index (σ _0.2 ) showing the 0.2% proof stress obtained by the equation (1), and the equation (3) are obtained. Specimens of the present invention Nos. 1 to 9 and comparative specimens as original plates having the pitting corrosion resistance index (α _P1 ) of the ferrite phase and the pitting corrosion resistance index (γ _P1 ) of the austenite phase obtained by the equation (4) N
o.1-12 were prepared.

[0034]

[Table 1]

The above-mentioned sample No. 1 of the present invention as an original plate
~ 9, each of the comparative specimen No. 1'to 12 'and the conventional steel specimen, a microstructure observation test piece, a tensile test piece,
2mmV notched Charpy impact test piece, general corrosion test piece, stress corrosion cracking test piece and test piece for measuring pitting corrosion potential were sampled and 0.2% proof stress, tensile strength, elongation, 0 ° C and -50
The states of absorbed energy at ℃, general corrosion resistance, pitting potential, stress corrosion cracking resistance (σ _th ) and end face cracking of the steel sheet were examined, and the results are shown in Table 2.

[0036]

[Table 2]

The volume ratio (α _f ) of the ferrite phase is
The measurement was performed by subjecting the micro sample to 20% NaOH electrolytic etching. General corrosion resistance is 3% NaCl-10 at a temperature of 200 ° C.
In AtmC0 ₂ conditions, it was assessed by measuring the corrosion rate when the strip-shaped test piece having a thickness of 3mm × width 20 mm × length 30mm was immersed for 720 hours. Stress corrosion cracking resistance is 45% MgC
l ₂ Perform constant load tensile test for up to 500 hours in boiling solution,
Maximum stress that does not break even after 500 hours has passed
It was evaluated as (σ _th ). The pitting potential was set to a potential at which the current density was 100 μA / cm ^{2 according} to JIS GO577. Then, the degree of the end surface crack of the steel sheet was visually determined.

As is clear from Tables 1 and 2, 0.2%
The index indicating the yield strength (σ _0.2 ), the pitting corrosion resistance index of the ferrite phase (α _P1 ) and the pitting corrosion resistance index of the austenite phase (γ _P1 ) are all outside the range of the present invention and are low. And No. 2 were inferior in 0.2% proof stress and pitting potential. The comparative test samples No. 3 and No. 5, which had a small pitting corrosion resistance index (α _P1 ) of the ferrite phase outside the range of the present invention, were inferior in pitting potential. The volume ratio (α _f ) of the ferrite phase is less than the range of the present invention, and comparative sample Nos. 4 and N
In o.6, the end face cracking of the steel plate is large, and in particular, the comparative specimen No. 4 in which the volume ratio (α _f ) of the ferrite phase is less than 0.30 is
The resistance to stress corrosion cracking (σ _th ) was also poor.

Comparative samples No. 7 and No. 8 having a nickel content outside the range of the present invention and having a small content were inferior in absorbed energy and pitting potential. Comparative specimens No. 9 and No. 12 having a low austenite phase pitting corrosion resistance index (γ _P1 ) outside the range of the present invention were inferior in pitting potential. The comparative specimens No. 10 and No. 11 in which the volume ratio (α _f ) of the ferrite phase was out of the range of the present invention were inferior in elongation and absorbed energy.

The conventional SUS316 steel sheet was inferior in 0.2% proof stress and stress corrosion cracking resistance (σ _th ). The 13Cr steel sheet was inferior in elongation, absorbed energy, general corrosion resistance and pitting corrosion resistance.

On the other hand, in the specimens Nos. 1 to 9 of the present invention, the 0.2% proof stress is well corresponding to σ _0.2 obtained by the equation (1), and all are 400 MPa or more, and are excellent in ductility and toughness. Was there. In addition, the general corrosion resistance in a CO ₂ corrosive environment and the pitting corrosion resistance in an environment containing chloride are both SUS 3
It was 16 or more steel, and the stress corrosion cracking resistance was significantly superior to SUS316 steel.

FIG. 1 is a graph showing the relationship between the nickel content and the pitting potential and the absorbed energies at 0 ° C. and -50 ° C., showing the samples No. 2, 3, 4 of the present invention and the nickel content. Is out of the scope of the present invention is less comparative specimen No. 7 and
8 shows the pitting potential and the absorbed energies at 0 ° C and -50 ° C. As is clear from FIG. 1, the pitting corrosion potentials of the comparative specimens No. 7 and 8 having a nickel content of less than 3 wt.
Its pitting resistance index of the ferrite phase (alpha _P1) and the austenitic phase pitting resistance index (gamma _P1) is low even within the scope of the present invention, pitting corrosion resistance was poor. The absorbed energies at 0 ° C. and -50 ° C. of the comparative specimens Nos. 7 and 8 were low even though the ferrite phase volume ratio was within the range of the present invention, and the toughness was poor.

FIG. 2 shows the volume ratio (α _f ) of the ferrite phase.
Is a graph showing the relationship between the absorbed energy at 0 ° C and -50 ° C, the elongation, and the end surface cracking of the steel sheet, which are the present invention sample Nos. 1 to 9 and the comparative sample Nos. 4 and 6. No.10,11
It shows absorbed energy at 0 ℃ and -50 ℃, elongation, and end face cracking of steel sheet. As is clear from FIG.
Cracking of the steel plate end surface during rolling of the comparative specimen Nos. 4 and 6 in which the volume ratio (α _f ) of the ferrite phase was less than 0.40 was large,
The comparative sample Nos. 10 and 11 in which the volume ratio (α _f ) of the ferrite phase exceeded 0.60 had low absorbed energy and elongation at 0 ° C and -50 ° C, and was poor in toughness and ductility.

Example 2 A steel ingot having a chemical composition within the scope of the present invention shown in Table 1 and having a weight of 50 kg of test steels e, f, and g was tested within the scope of the present invention shown in Table 3. Hot rolling at heating temperature and finishing temperature, or hot rolling of the above steel ingot, followed by annealing at a temperature within the range of the present invention, the specimen N of the present invention having a thickness of 15 mm shown in Table 3
o.10-15 were prepared. On the other hand, the weight of the test steels e, f, and g is 5
A 0 Kg steel ingot is hot-rolled at a heating temperature or finishing temperature outside the range of the present invention shown in Table 3 together, or after the steel ingot is hot-rolled, annealed at a temperature outside the range of the present invention. Table 3
Samples Nos. 13 to 18 for comparison having a thickness of 15 mm shown in were prepared. The cooling after hot rolling or after annealing was air cooling.

[0045]

[Table 3]

Specimen N of the present invention prepared as described above
Samples for micro observation were taken from o.10 to 15 and comparative samples No.13 to 18, and the volume ratio of the ferrite phase (α
_f ) and hardness (HV: 10Kgf) were measured, and the measurement results are also shown in Table 3.

As is clear from Table 3, the comparative specimens No. 13 and No. 1 which are high in annealing temperature after hot rolling and outside the range of the present invention are obtained.
In No. 6, the volume ratio (α _f ) of the ferrite phase was out of the range of the present invention, and there was a high possibility that wrinkles would occur during the tube formation. Comparative sample No. 18, which has a low annealing temperature outside the range of the present invention, has a high hardness (HV: 10 Kgf), and thus it was considered difficult to form a tubular body by UOE.

Comparative sample No. 14 having a low finishing temperature during hot rolling outside the range of the present invention, which was not annealed after hot rolling, had a hardness (HV: 10Kgf). Therefore, it was difficult to form a tubular body by UOE. And
The heating temperature during hot rolling is outside the range of the present invention and is high. Comparative Samples No. 15 and No. 17 have a large volume fraction (α _f ) of the ferrite phase outside the range of the present invention, Wrinkles were more likely to occur during formation.

On the other hand, the annealing temperature after annealing after hot rolling and the heating temperature and finishing temperature during hot rolling after annealing after hot rolling are Inventive specimen No. 1 which is within the scope of the present invention
0 to 15 have a volume ratio (α _f ) of the ferrite phase within the range of the present invention and have an appropriate hardness (HV: 10 Kgf),
Therefore, it seems that the tube formation by UOE can be favorably performed.

Example 3 A steel ingot of sample steel j having a chemical composition within the scope of the present invention shown in Table 4 was hot rolled into a steel plate having a thickness of 20 mm in a hot rolling plant. Butt the obtained two original plates,
The seam portion was welded under the conditions shown in Table 5 by two-electrode submerged arc welding using a wire having a chemical composition shown in Table 4 and a flux having a chemical composition shown in Table 6. Thus, two types of welded steel plates, in which the chemical composition of the original plate and the weld metal of the seam are within the scope of the present invention,
Also, two types of welded steel plates were produced in which the oxygen content of the weld metal was out of the range of the present invention. The groove shape is
The inner depth was 5.5 mm, the outer depth was 7 mm and the bevel angle was 45 °.

The above-mentioned two kinds of welded steel plates in which the chemical composition of the original plate and the weld metal are within the scope of the present invention, and the two kinds of welded steel sheets in which the oxygen content of the weld metal is out of the scope of the present invention. In contrast, heating at a temperature of 1050 ° C,
A solution heat treatment consisting of cooling at a rate of 20 ° C / sec in a temperature range within the range of 500 to 850 ° C is performed, and thus,
The original plate and the weld metal have chemical composition within the scope of the present invention, volume fraction of ferrite phase (α _f ), index indicating 0.2% proof stress (σ _0.2 ), pitting corrosion index of ferrite phase (α _P1 ), and austenite. Has a phase pitting corrosion resistance index (γ _P1 ),
Test samples No. 16 and No. 17 of the present invention shown in Table 7 and comparative test samples No. 19 and No. 20 having a large oxygen content of the weld metal outside the range of the present invention were manufactured.

[0052]

[Table 4]

[0053]

[Table 5]

[0054]

[Table 6]

[0055]

[Table 7]

[0056]

[Table 7]

From the specimens No. 16 and No. 17 of the present invention and the specimens No. 19 and No. 20 for comparison as described above, samples for micro observation, Charpy impact test pieces, intergranular corrosion test pieces (thickness 3
mm × width 20 mm × length 30 mm) and pitting test piece (thickness 3 mm ×
40 mm wide x 40 mm long) was collected.

In order to study the influence of the solution heat treatment conditions on the welded steel sheet, the sample No. 16 of the present invention was heated to a temperature in the range of 850 to 1200 ° C. and then heated to a temperature of 500 to 85 ° C.
Solution heat treatment consisting of cooling at a rate of 0.3 to 30 ° C./sec in a temperature range within 0 ° C. was performed, and each of the above-mentioned test pieces was sampled.

The Charpy impact test piece was sampled so that the center of the notch was at the center of the weld metal, and the intergranular corrosion test piece and the pitting corrosion test piece were at the center of the weld metal. Collected in. Also, the intergranular corrosion resistance is JI
65% nitric acid corrosion test according to S G0573 (hereinafter referred to as "Huiy test"), pitting corrosion resistance according to JIS G0578 ferric chloride test at a temperature of 20 ℃ (hereinafter referred to as "pitting test") It was

FIG. 3 shows the relationship between the oxygen content of the weld metal, the absorbed energy of the weld metal at −20 ° C., and the corrosion rate of the welded portion by the pitting corrosion test. As is apparent from FIG. 3, Comparative Samples No. 19 and No. 20, in which the oxygen content of the weld metal is higher than the range of the present invention, deteriorated the toughness and pitting corrosion resistance of the weld metal.

FIG. 4 shows the heating temperature during solution heat treatment, the corrosion rate in the Huey test of the weld, the absorbed energy at -20 ° C. of the weld metal, and the volume fraction of the ferrite phase (α
_f )). As is clear from FIG. 4, when the heating temperature during the solution heat treatment was outside the range of the present invention and was low, the intergranular corrosion resistance and toughness of the weld metal deteriorated. Further, when the heating temperature during solution heat treatment is higher than the range of the present invention, the volume fraction (α _f ) of the ferrite phase of the weld metal increases outside the range of the present invention, and the toughness deteriorates significantly. .

FIG. 5 shows the relationship between the cooling rate in the temperature range of 500 to 850 ° C. and the corrosion rate in the Huey test of the weld, that is, the intergranular corrosion resistance. As is clear from FIG. 5, when the cooling rate in the temperature range of 500 to 850 ℃ during solution heat treatment is less than 1 ℃ / sec, even if the heating temperature is 1050 ℃, intergranular corrosion resistance of the weld metal Sex significantly decreased.

Example 4 The steel ingots of the test steels j and k having the chemical composition within the scope of the present invention shown in Table 8 are shown in Table 9 in the hot rolling plant by the symbols ~ Hot rolling at a heating temperature and finishing temperature in the range of, or after hot rolling the steel ingot, the present invention specimen consisting of a steel plate with a thickness of 20 mm, annealed at a temperature within the range of the present invention No .18-20 were produced. For comparison, comparative specimens Nos. 21 to 24 made of steel plates having a thickness of 20 mm, in which the heating temperature or the finishing temperature during hot rolling are out of the range of the present invention indicated by the symbols in Table 9, are shown. Manufactured.

[0064]

[Table 8]

[0065]

[Table 9]

The above-mentioned specimens No. 18 to 20 of the present invention and specimens No. 21 to 24 for comparison were subjected to UOE processing to form a tubular body similar to the case of producing a welded steel pipe having an outer diameter of 20 inches. The tube formability at that time is also shown in Table 9. As is clear from Table 9, the comparative test sample No. 21, which has a high annealing temperature outside the range of the present invention when it is subjected to the annealing treatment after the hot rolling, has wrinkles at the time of forming the tubular body. Occurred. Then, a comparative specimen whose annealing temperature is outside the range of the present invention and is low
No. 24 was unable to form a tubular body.

Comparative sample No. 23, in which the annealing temperature after hot rolling was not applied and the heating temperature during hot rolling was outside the range of the present invention and was high, wrinkles were generated during tube forming. did.
Then, the comparative sample No. 22 having a finishing temperature outside the range of the present invention and having a low finishing temperature could not be formed into a tubular body. On the other hand, after hot rolling, the annealing temperature when subjected to the annealing treatment, and the heating temperature and the finishing temperature during the hot rolling when not subjected to the annealing treatment after the hot rolling are both the same. The sample moldability of the test samples Nos. 18 to 20 of the present invention, which are within the scope of the invention, were extremely good.

Example 5 The steel ingots of the test steels j and k having the chemical composition within the scope of the present invention shown in Table 8 were prepared under the conditions within the scope of the present invention.
Hot rolling or hot rolling followed by annealing was performed, and the obtained original plate having a thickness of 20 mm was formed into a tubular body by UOE processing to prepare a raw tube. The seam portion of this raw pipe was welded by two-electrode submerged arc welding using the wire C or E shown in Table 8 and the flux a shown in Table 6. Thus, the chemical composition of the weld metal, volume fraction of ferrite phase (α _f ), index indicating 0.2% proof stress (σ _0.2 ), pitting corrosion resistance index of ferrite phase (α _P1 ) and pitting corrosion resistance index of austenite phase ( γ _P1 ) is within the scope of the present invention.
Samples Nos. 21 to 23 of the present invention shown in Table 10 and made of welded steel pipe having an outer diameter of inch were prepared.

For comparison, welding was performed using the flux a shown in Table 6 and the wires A to D shown in Table 8 to the original plate having the chemical composition and manufacturing conditions within the scope of the present invention, Volume fraction of ferrite phase of weld metal (α _f ),
One of the index indicating 0.2% proof stress (σ _0.2 ), the pitting corrosion resistance index of the ferrite phase (α _P1 ) and the pitting corrosion resistance index of the austenite phase (γ _P1 ) is outside the range of the present invention, Comparative specimens No. 25 to 28 shown in Table 10 were prepared, which consisted of welded steel pipes having an outer diameter.

[0070]

[Table 10]

The test pieces No. 21 to 23 of the present invention and the test pieces No. 25 to 28 for comparison were heated at a temperature of 1050 ° C.
Solution heat treatment was performed by cooling at a rate of 20 ℃ / sec in the temperature range of 0 〜 850 ℃. From each specimen subjected to such solution heat treatment, a tensile test piece,
Charpy impact test piece, general corrosion test piece (thickness 3 mm x width
20mm x length 30mm), pitting test piece (thickness 3mm x width 40mm x length 40mm) and stress corrosion cracking test piece (thickness 2mm x width 10mm)
X length of 115 mm) was taken so that the center of each test piece would be the center of the weld metal. Further, the same test piece as above was sampled from the base material of each specimen.

Tensile strength,-
The absorbed energy at 20 ° C., general corrosion resistance, pitting corrosion resistance and stress corrosion cracking resistance were measured, and the measurement results are shown in Table 11. The stress corrosion cracking resistance was measured as follows. That is, as shown in the schematic front view of FIG. 6, the two glass restraints 2 and 2 for restraining both end portions in the length direction of the test piece A from the upper surface thereof and the middle portion in the length direction of the test piece A for the lower surface thereof. A four-point bending jig 1 composed of two glass push-up tools 3 and 3 that are pushed up from the glass plate at predetermined intervals was used. 4 above
The test piece A is bent at four points by the point bending jig 1 to obtain the test piece A.
0.5% strain was added to the surface of the test piece A in that state.
And when immersed for 720 hours under the conditions of 0.99 ° C. of 20% NaCl -10atmCO _2, presence or absence of stress corrosion cracking occurring on the test piece A was evaluated by (hereinafter referred to as 4-point bending test).

[0073]

[Table 11]

As is clear from Tables 10 and 11,
The pitting corrosion resistance index (α _P1 ) of the ferrite phase of the weld metal or the pitting corrosion resistance index (γ _P1 ) of the austenite phase is α _{P1 of the} base metal.
And a comparative specimen N with a lower value of γ _P1
In o.25 and No.28, pitting corrosion occurred in the weld metal and the pitting resistance was poor. Volume fraction of ferrite phase of weld metal (α
_f ) is less than the range of the present invention, and the number of comparative specimen No. 26 is small.
In the 4-point bending test, the weld metal cracked and the stress corrosion cracking resistance was poor. The comparative sample No. 27, in which the volume ratio (α _f ) of the ferrite phase of the weld metal was out of the range of the present invention, was inferior in absorbed energy at −20 ° C.

On the other hand, the specimens Nos. 21 to 23 of the present invention are
Tensile strength, absorbed energy at -20 ℃, general corrosion resistance,
It was excellent in both pitting corrosion resistance and stress corrosion cracking resistance.

[0076] As described above, according to the method of the present invention, excellent have and toughness 13Cr steel equivalent 0.2% proof stress (or 400 MPa), and, in an environment containing chloride or C0 ₂ SUS316 Industrially useful effects can be obtained in which a duplex stainless steel welded pipe having the general corrosion resistance, pitting corrosion resistance and stress corrosion cracking resistance equal to or higher than that of steel can be economically produced.

[Brief description of drawings]

[Fig.1] Nickel content, pitting potential and 0 ℃, -50
It is a graph which shows the relationship with the absorbed energy of ° C.

[Fig. 2] Volume ratio of ferrite phase (α _f ) and 0 ℃, -50
It is a graph which shows the relationship with the absorbed energy of ° C, elongation, and the end face crack of a steel plate.

FIG. 3 is a graph showing the relationship between the oxygen content of the weld metal, the absorbed energy of the weld metal at −20 ° C., and the corrosion rate of the welded portion by the pitting corrosion test.

FIG. 4 is a graph showing the relationship between the heating temperature during solution heat treatment, the corrosion rate in the Huey test of the weld, the absorbed energy of the weld metal at −20 ° C., and the volume fraction (α _f ) of the ferrite phase.

FIG. 5 is a graph showing the relationship between the cooling rate in the temperature range of 500 to 850 ° C. and the corrosion rate of weld metal in the Huey test, that is, the intergranular corrosion resistance.

FIG. 6 is a schematic front view of a 4-point bending jig used for measuring stress corrosion cracking resistance.

[Explanation of symbols]

 1 4-point bending jig, 2 retainer, 3 push-up tool, A test piece.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Internal reference number FI technical display location C22C 38/40 38/58

Claims (2)

[Claims] 1. Carbon (C): 0.05 wt.% Or less, Silicon (Si): 1.5 wt.% Or less, Manganese (Mn): 2.0 wt.% Or less, Nickel (Ni): 3.0 to 5.0 wt.%, Chromium (Cr): 21.0 to 25.0 wt.%, Nitrogen (N): 0.25 wt.% Or less, and the balance: Fe and unavoidable impurities, consisting of the following equation (1) σ _0.2 = 20 × Cr (%) +11 × Mn (%) - Ni ( %) + 133 × N (%) - 38 ───── (1) index (sigma _0.2) showing a 0.2% proof stress obtained by 400
A steel ingot or a steel slab having the chemical composition described above is prepared, a steel plate is prepared by hot rolling the steel ingot or the steel slab, and then 900 ° C. or more with respect to the steel plate, the following (2) formula The original plate is prepared by annealing, which consists of heating at a temperature in the range of T (° C) or less, which is calculated by the following, and then cooling, T = 71 x Ni (%) + 6 x Mn (%) -36 x Cr (%) −42 × Si (%) ＋ 1037 × C (%) + 1113 × N (%) + 1608 ────── (2) Then, the raw plate is formed into a tube to prepare a raw tube and Weld the seam part of, and the weld metal of the seam part is carbon (C): 0.05 wt.% Or less, silicon (Si): 1.5 wt.% Or less, manganese (Mn): 2.0 wt.% Or less, nickel ( Ni): 3.0 to 5.0 wt.%, Chromium (Cr): 21.0 to 25.0 wt.%, Nitrogen (N): 0.25 wt.% Or less, oxygen (O): 0.035 wt.% Or less, and the rest: Fe and Inevitable impurities, and 0.
2% proof stress (σ _0.2 ) to prepare a welded steel pipe having a chemical composition of which the index is 400 or more, then, to the welded steel pipe, heating at a temperature in the range of 900 ~ 1150 ℃, 500 ~ 850 In the temperature range of ℃, the solution heat treatment consisting of cooling at a rate of 1 ℃ / sec or more is performed, and thus, the volume ratio α _f of the ferrite phase of the base material of the welded steel pipe is within the range of 0.40 to 0.60. And the pitting corrosion resistance index (α _P1 ) of the ferrite phase determined by the following formula (3) and the pitting corrosion resistance index (γ _P1 ) of the austenite phase determined by the following formula (4) Also 23.5
Above, α _P1 = 23 × Cr (%) / (3 × α _f + 20) ──────────────── (3) γ _P1 = 20 × Cr (%) / (3 × α _f +20) −16 × N (%) / (α _f −1) ───── (4) Then, the volume ratio of the ferrite phase (α of the weld metal of the welded steel pipe) _f ) is in the range of 0.30 to 0.50, and the pitting corrosion resistance index (α _P1 ) of the ferrite phase and the pitting corrosion resistance index (γ) of the austenite phase of the weld metal are
Excellent in strength, toughness and corrosion resistance, characterized by producing a two-phase stainless welded steel pipe in which _P1 ) is greater than the lower one of the α _P1 and the γ _P1 of the base material. Method for manufacturing duplex stainless steel welded pipe. 2. Carbon (C): 0.05 wt.% Or less, Silicon (Si): 1.5 wt.% Or less, Manganese (Mn): 2.0 wt.% Or less, Nickel (Ni): 3.0 to 5.0 wt.%, Chromium (Cr): 21.0 to 25.0 wt.%, Nitrogen (N): 0.25 wt.% Or less, and the balance: Fe and unavoidable impurities, consisting of the following equation (1) σ _0.2 = 20 × Cr (%) +11 × Mn (%) - Ni ( %) + 133 × N (%) - 38────── (1) index (sigma _0.2) showing a 0.2% proof stress obtained by 400
A steel ingot or a steel slab having the chemical composition as described above is prepared, and the steel ingot or the steel slab is heated at a temperature of T (° C.) or less obtained by the following equation (2), and 900 ° C. The original plate is prepared by performing hot rolling consisting of finishing at the above temperature, and T = 71 × Ni (%) + 6 × Mn (%) − 36 × Cr (%) − 42 × Si (%) + 1037. × C (%) + 1113 × N (%) + 1608 ─────── (2) Next, the raw plate is molded to prepare a raw pipe, and the seam portion of the raw pipe is welded. , Carbon (C): 0.05 wt.% Or less, silicon (Si): 1.5 wt.% Or less, manganese (Mn): 2.0 wt.% Or less, nickel (Ni): 3.0 to 5.0 wt.%, Chromium (Cr) : 21.0 to 25.0 wt.%, Nitrogen (N): 0.25 wt.% Or less, oxygen (O): 0.035 wt.% Or less, and the rest: Fe and inevitable impurities, and the above-mentioned 0.
2% yield strength index (σ _0.2 ) to prepare a welded steel pipe having a chemical composition that is 400 or more, then, to the welded steel pipe, heating at a temperature in the range of 900 ~ 1150 ℃, 500 ~ 850 In the temperature range of ℃, the solution heat treatment consisting of cooling at a rate of 1 ℃ / sec or more is performed, and thus, the volume ratio α _f of the ferrite phase of the base material of the welded steel pipe is within the range of 0.40 to 0.60. And the pitting corrosion resistance index (α _P1 ) of the ferrite phase determined by the following formula (3) and the pitting corrosion resistance index (γ _P1 ) of the austenite phase determined by the following formula (4) Also 23.5
Above, α _P1 = 23 × Cr (%) / (3 × α _f + 20) ──────────────── (3) γ _P1 = 20 × Cr (%) / (3 × α _f +20) −16 × N (%) / (α _f −1) ───── (4) Then, the volume ratio of the ferrite phase (α of the weld metal of the welded steel pipe) _f ) is in the range of 0.30 to 0.50, and the pitting corrosion resistance index (α _P1 ) of the ferrite phase and the pitting corrosion resistance index (γ) of the austenite phase of the weld metal are
Excellent in strength, toughness and corrosion resistance, characterized by producing a two-phase stainless welded steel pipe in which _P1 ) is greater than the lower one of the α _P1 and the γ _P1 of the base material. Method for manufacturing duplex stainless steel welded pipe.