JP7553883B1 - Duplex Stainless Steel Pipe - Google Patents

Abstract

The present disclosure provides a duplex stainless steel pipe that provides high strength and excellent corrosion resistance. The duplex stainless steel pipe according to the present disclosure has, by mass%, C: 0.030% or less, Si: 0.20 to 1.00%, Mn: 0.5 to 7.0%, P: 0.040% or less, S: 0.020% or less, Al: 0.100% or less, Ni: 4.0 to 9.0%, Cr: 20.0 to 30.0%, Mo: 0.5 to 2.0%, Cu: 1.5 to 3.0%, N: 0.15 to 0.30%, V: 0.01 to 0.50%, Nb: 0.03 The steel sheet contains 0 to 0.300%, Co: 0.10 to 0.50%, and Sn: 0.001 to 0.050%, and the microstructure is composed of 35.0 to 65.0% by volume of ferrite, 0 to less than 1.0% of a σ phase, and the balance being austenite, the undissolved Nb content is 0.008% or more by mass, the ratio of the undissolved Nb content to the undissolved Al content is 1.0 or more, and the yield strength is 655 MPa or more.

Description

This disclosure relates to steel pipes, and more particularly to duplex stainless steel pipes.

Oil wells and gas wells (hereinafter, oil wells and gas wells will be collectively referred to simply as "oil wells") may be in a corrosive environment containing corrosive gases. Here, corrosive gas means carbon dioxide gas and/or hydrogen sulfide gas. Therefore, the steel materials used in oil wells are required to have excellent corrosion resistance in corrosive environments.

To date, a method for improving the corrosion resistance of steel materials has been known in which the chromium (Cr) content is increased and a passive film mainly composed of Cr oxide is formed on the steel material surface. Therefore, in environments where excellent corrosion resistance is required, duplex stainless steel materials with an increased Cr content are sometimes used.

In recent years, the development of deep wells below sea level has become even more active. The steel used in these deep wells must have high strength. Therefore, duplex stainless steel, which combines high strength with excellent corrosion resistance, is in demand as the steel used in oil wells.

JP 2018-193591 A (Patent Document 1) and WO 2012/121232 (Patent Document 2) propose a duplex stainless steel material having high strength and excellent corrosion resistance.

The duplex stainless steel material disclosed in Patent Document 1 has the following composition, in mass %, C: 0.005 to 0.04%, Si: 0.2 to 1.0%, Mn: 0.1 to 2.0%, P: 0.040% or less, S: 0.010% or less, Ni: 3 to 7%, Cr: 23 to 28%, Mo: 0.5 to 1.5%, Cu: 2 to 4%, N: 0.10 to 0.35%, Al: 0.001 to 0.04%, W: 0. The alloy contains Cr: 0-1.0%, Co: 0-1.0%, V: 0-1.0%, Nb: 0-0.2%, Ti: 0-0.2%, Ca: 0-0.02%, Mg: 0-0.02%, B: 0-0.02%, and rare earth elements (REM): 0-0.2%, with the balance being Fe and impurities, and has a chemical composition that satisfies formulas (1) to (3), and has a yield strength YS of 655 MPa or more. Here, formulas (1) to (3) are as follows.
YS/150≦Ni+Mo+0.5W+Cu-Mn≦YS/75 (1)
Cr+3.3×(Mo+0.5W)+16N≧30.0 (2)
Mo+0.5W+Ni≦7.50 (3)
Patent Document 1 describes that in this duplex stainless steel material, high strength and excellent corrosion resistance can be obtained by adjusting the element contents in the chemical composition and the yield strength so as to satisfy formulas (1) to (3).

The duplex stainless steel material disclosed in Patent Document 2 contains, in mass%, C: 0.03% or less, Si: 0.3% or less, Mn: 3.0% or less, P: 0.040% or less, S: 0.008% or less, Cu: 0.2 to 2.0%, Ni: 5.0 to 6.5%, Cr: 23.0 to 27.0%, Mo: 2.5 to 3.5%, W: 1.5 to 4.0%, and N: 0.24 to 0.40%. The chemical composition contains σ-phase susceptibility index X (=2.2Si+0.5Cu+2.0Ni+Cr+4.2Mo+0.2W) of 52.0 or less, strength index Y (=Cr+1.5Mo+10N+3.5W) of 40.5 or more, and pitting corrosion resistance index PREW (=Cr+3.3(Mo+0.5W)+16N) of 40 or more. The structure of the steel is such that, when a straight line parallel to the thickness direction from the surface layer to a depth of 1 mm is drawn in a thickness direction cross section parallel to the rolling direction, the number of boundaries between ferrite phase and austenite phase that intersect the straight line is 160 or more. Patent Document 2 states that this duplex stainless steel can be strengthened without impairing corrosion resistance.

JP 2018-193591 A International Publication No. 2012/121232

The duplex stainless steel materials disclosed in Patent Documents 1 and 2 provide high strength and excellent corrosion resistance. However, high strength and excellent corrosion resistance may be provided by means other than those disclosed in Patent Documents 1 and 2.

The object of the present disclosure is to provide a duplex stainless steel pipe that has high strength and excellent corrosion resistance.

The duplex stainless steel pipe according to the present disclosure has, by mass%, C: 0.030% or less, Si: 0.20-1.00%, Mn: 0.5-7.0%, P: 0.040% or less, S: 0.020% or less, Al: 0.100% or less, Ni: 4.0-9.0%, Cr: 20.0-30.0%, Mo: 0.5-2.0%, Cu: 1.5-3.0%, N: 0.15-0.30%, V: 0.01-0.50%, Nb: 0.030-0.300%, Co: 0.10-0.50%, Sn: 0.001-0.050%, Ta: 0-0.100%, Ti: 0-0.100%, Zr: 0. 0.020% or less, and the balance being Fe and impurities; the microstructure is composed of 35.0 to 65.0% by volume of ferrite, 0 to less than 1.0% by volume of σ phase, and the balance being austenite; the undissolved Nb content is 0.008% or more by mass; the ratio of the undissolved Nb content to the undissolved Al content is 1.0 or more; and the yield strength is 655 MPa or more.

The duplex stainless steel pipes disclosed herein provide high strength and excellent corrosion resistance.

FIG. 1 is a schematic diagram of an example of a straightener. FIG. 2 is a front view of the straightener shown in FIG.

The inventors have investigated duplex stainless steel pipes that provide high strength and excellent corrosion resistance from the viewpoint of chemical composition. As a result, the inventors have determined that the following components, by mass%, are C: 0.030% or less, Si: 0.20-1.00%, Mn: 0.5-7.0%, P: 0.040% or less, S: 0.020% or less, Al: 0.100% or less, Ni: 4.0-9.0%, Cr: 20.0-30.0%, Mo: 0.5-2.0%, Cu: 1.5-3.0%, N: 0.15-0.30%, V: 0.01-0.50%, Nb: 0.030-0.300%, Co: 0.10-0.50%. It was believed that a duplex stainless steel pipe consisting of the following components would provide high strength and excellent corrosion resistance: 0%, Sn: 0.001-0.050%, Ta: 0-0.100%, Ti: 0-0.100%, Zr: 0-0.100%, Hf: 0-0.100%, W: 0-0.200%, Sb: 0-0.100%, Ca: 0-0.020%, Mg: 0-0.020%, B: 0-0.020%, rare earth elements: 0-0.200%, and the balance being Fe and impurities.

Therefore, the present inventors further investigated means for obtaining even higher strength from viewpoints other than chemical composition in a duplex stainless steel pipe satisfying the above-mentioned chemical composition. The present inventors first investigated the microstructure of the duplex stainless steel pipe. The microstructure of a duplex stainless steel pipe having the above-mentioned chemical composition is mainly composed of ferrite and austenite. The present inventors discovered that strength and corrosion resistance can be stably increased by configuring the microstructure of a duplex stainless steel pipe having the above-mentioned chemical composition so that the ferrite volume fraction is 35.0 to 65.0%, with the remainder being substantially composed of austenite.

The present inventors further considered that the strength of the duplex stainless steel pipe could be further increased by precipitating Nb carbonitrides in the duplex stainless steel pipe. Nb carbonitrides are fine precipitates. Therefore, the strength of the duplex stainless steel pipe is increased by precipitation strengthening by Nb carbonitrides. However, in the temperature range in which Nb carbonitrides are formed during the manufacturing process of the duplex stainless steel pipe, σ phase is also likely to be formed. The σ phase reduces the corrosion resistance of the duplex stainless steel pipe. Therefore, the present inventors conducted a study on the relationship between the amount of Nb carbonitrides formed, the amount of σ phase formed, and the strength and corrosion resistance. The amount of Nb carbonitrides formed correlates with the non-solid-solubilized Nb content. In a duplex stainless steel pipe satisfying the above-mentioned chemical composition, if the non-solid-solubilized Nb content is 0.008% or more by mass%, Nb carbonitrides are sufficiently formed to increase the strength. Furthermore, even if the undissolved Nb content is 0.008% or more by mass percent, excellent corrosion resistance can be maintained by making the microstructure into a volume fraction of 35.0 to 65.0% ferrite, 0 to less than 1.0% σ phase, and the remainder austenite.

However, it was found that even duplex stainless steel pipes that satisfy the above-mentioned characteristics may still not have sufficient strength. Therefore, the inventors conducted further studies. As a result, it was newly found that Al nitrides are a factor that reduces the strength of duplex stainless steel pipes with the above-mentioned chemical composition.

In the above chemical composition, the N content is higher than the C content in order to obtain high strength through solute N. In such a chemical composition, the N content in Nb carbonitride is considered to be higher than the C content. In other words, in the above chemical composition, N not only contributes to solid solution strengthening as solute N, but also contributes to precipitation strengthening by Nb carbonitride.

On the other hand, N also combines with Al to form Al nitrides. Al nitrides are coarser precipitates than Nb carbonitrides. Therefore, Al nitrides contribute very little to precipitation strengthening. Therefore, in a duplex stainless steel pipe with the above-mentioned chemical composition, if the amount of Al nitride produced is relatively greater than the amount of Nb carbonitride produced, there may be a shortage of solute N and Nb nitrides. In this case, the strength of the duplex stainless steel pipe cannot be sufficiently increased.

Al nitrides correlate with the undissolved Al content. Therefore, the inventors further investigated the relationship between the undissolved Nb content and the undissolved Al content and the strength and corrosion resistance. As a result, the inventors found that if the undissolved Nb/Al ratio, which is the ratio of the undissolved Nb content to the undissolved Al content, is 1.0 or more, a sufficient amount of Nb nitrides and dissolved N can be secured, and as a result, high strength and excellent corrosion resistance can be achieved at the same time.

It is possible that a duplex stainless steel pipe satisfying the above-mentioned chemical composition and microstructure can achieve both a yield strength of 655 MPa or more and excellent corrosion resistance by setting the undissolved Nb content to 0.008% or more by mass% and setting the undissolved Nb/Al ratio to 1.0 or more through a mechanism other than the above. However, it is proven by the examples described below that a duplex stainless steel pipe having the above-mentioned chemical composition and microstructure can achieve both a high yield strength of 655 MPa or more and excellent corrosion resistance by setting the undissolved Nb content to 0.008% or more by mass% and setting the undissolved Nb/Al ratio to 1.0 or more.

The duplex stainless steel pipe of this embodiment, completed based on the above findings, has the following configuration.

The chemical composition of the duplex stainless steel pipe of the first configuration is, in mass%, C: 0.030% or less, Si: 0.20 to 1.00%, Mn: 0.5 to 7.0%, P: 0.040% or less, S: 0.020% or less, Al: 0.100% or less, Ni: 4.0 to 9.0%, Cr: 20.0 to 30.0%, Mo: 0.5 to 2.0%, Cu: 1.5 to 3.0%, N: 0.15 to 0.30%, V: 0.01 to 0.50%, Nb: 0. The microstructure of the duplex stainless steel pipe is composed of 35.0 to 65.0% by volume of ferrite, 0 to less than 0.02% by volume of σ phase, and the remainder is composed of austenite. The duplex stainless steel pipe further has a non-dissolved Nb content of 0.008% by mass or more, a ratio of the non-dissolved Nb content to the non-dissolved Al content of 1.0 or more, and a yield strength of 655 MPa or more.

The duplex stainless steel pipe of the second configuration is a duplex stainless steel pipe of the first configuration, and has a chemical composition containing one or more elements selected from the group consisting of Ta: 0.001-0.100%, Ti: 0.001-0.100%, Zr: 0.001-0.100%, Hf: 0.001-0.100%, W: 0.001-0.200%, Sb: 0.001-0.100%, Ca: 0.001-0.020%, Mg: 0.001-0.020%, B: 0.001-0.020%, and rare earth elements: 0.001-0.200%.

The duplex stainless steel pipe according to this embodiment will be described in detail below. In the following description, the duplex stainless steel pipe will also be referred to simply as "steel pipe." In addition, "%" for elements means mass % unless otherwise specified.

[Features of the duplex stainless steel pipe according to this embodiment]
The duplex stainless steel pipe of this embodiment satisfies the following features 1 to 4.
(Feature 1)
The chemical composition is, in mass%, C: 0.030% or less, Si: 0.20-1.00%, Mn: 0.5-7.0%, P: 0.040% or less, S: 0.020% or less, Al: 0.100% or less, Ni: 4.0-9.0%, Cr: 20.0-30.0%, Mo: 0.5-2.0%, Cu: 1.5-3.0%, N: 0.15-0.30%, V: 0.01-0.50%, Nb: 0.030-0.30%. 0%, Co: 0.10-0.50%, Sn: 0.001-0.050%, Ta: 0-0.100%, Ti: 0-0.100%, Zr: 0-0.100%, Hf: 0-0.100%, W: 0-0.200%, Sb: 0-0.100%, Ca: 0-0.020%, Mg: 0-0.020%, B: 0-0.020%, rare earth elements: 0-0.200%, and the balance is Fe and impurities.
(Feature 2)
The microstructure is composed of, by volume, 35.0 to 65.0% ferrite, 0 to less than 1.0% σ phase, and the balance austenite.
(Feature 3)
The undissolved Nb content is 0.008% or more by mass percent, and the ratio of the undissolved Nb content to the undissolved Al content is 1.0 or more.
(Feature 4)
The yield strength is 655 MPa or more.
Features 1 to 4 will be explained below.

[Feature 1: Chemical composition]
The chemical composition of the duplex stainless steel pipe according to this embodiment contains the following elements.

C: 0.030% or less Carbon (C) is inevitably contained. That is, the lower limit of the C content is more than 0%. C forms Cr carbides at the grain boundaries and increases the corrosion sensitivity at the grain boundaries. Therefore, if the C content exceeds 0.030%, the corrosion resistance of the steel pipe decreases even if the contents of other elements are within the range of this embodiment. Therefore, the C content is 0.030% or less.
The C content is preferably as low as possible. However, excessive reduction in the C content significantly increases the production cost. Therefore, in consideration of industrial production, the preferred lower limit of the C content is 0.001%, more preferably 0.002%, and even more preferably 0.005%.
The upper limit of the C content is preferably 0.029%, more preferably 0.028%, and further preferably 0.027%.

Si: 0.20-1.00%
Silicon (Si) deoxidizes steel. If the Si content is less than 0.20%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment.
On the other hand, if the Si content exceeds 1.00%, the toughness and hot workability of the steel pipe decrease even if the contents of other elements are within the ranges of this embodiment.
Therefore, the Si content is 0.20 to 1.00%.
The lower limit of the Si content is preferably 0.21%, more preferably 0.22%, further preferably 0.25%, and further preferably 0.30%.
The upper limit of the Si content is preferably 0.95%, more preferably 0.92%, further preferably 0.91%, and further preferably 0.90%.

Mn: 0.5-7.0%
Manganese (Mn) deoxidizes and desulfurizes the steel. Mn also improves the hot workability of the steel pipe. If the Mn content is less than 0.5%, the other element contents are in accordance with the present invention. Even if the form is within the range, the above-mentioned effects cannot be obtained sufficiently.
On the other hand, Mn segregates at grain boundaries together with impurities such as P and S. Therefore, if the Mn content exceeds 7.0%, even if the contents of other elements are within the range of this embodiment, the steel sheet may be easily oxidized at high temperatures. The corrosion resistance of steel pipes in the environment decreases.
Therefore, the Mn content is 0.5 to 7.0%.
The lower limit of the Mn content is preferably 0.6%, more preferably 0.8%, further preferably 1.0%, and further preferably 1.2%.
The upper limit of the Mn content is preferably 6.8%, more preferably 6.5%, still more preferably 6.3%, still more preferably 6.2%, and still more preferably 6.0%. %.

P: 0.040% or less Phosphorus (P) is inevitably contained. In other words, the lower limit of the P content is more than 0%. P segregates at grain boundaries. Therefore, if the P content exceeds 0.040%, the corrosion resistance of the steel pipe decreases even if the contents of other elements are within the range of this embodiment. Therefore, the P content is 0.040% or less.
The P content is preferably as low as possible. However, excessive reduction in the P content significantly increases the production cost. Therefore, in consideration of industrial production, the lower limit of the P content is preferably 0.001%, more preferably 0.002%, and even more preferably 0.003%.
The upper limit of the P content is preferably 0.038%, more preferably 0.036%, further preferably 0.035%, and further preferably 0.030%.

S: 0.020% or less Sulfur (S) is inevitably contained. That is, the lower limit of the S content is more than 0%. S segregates at grain boundaries. Therefore, if the S content exceeds 0.020%, the toughness and hot workability of the steel pipe will decrease even if the contents of other elements are within the range of this embodiment. Therefore, the S content is 0.020% or less.
The S content is preferably as low as possible. However, excessive reduction in the S content significantly increases the manufacturing cost. Therefore, in consideration of industrial production, the preferable lower limit of the S content is 0.001%, more preferably 0.002%, more preferably 0.003%, more preferably 0.004%, and even more preferably 0.005%.
The upper limit of the S content is preferably 0.018%, more preferably 0.016%, and further preferably 0.014%.

Al: 0.100% or less Aluminum (Al) is inevitably contained. That is, the lower limit of the Al content is more than 0%. Al deoxidizes steel. On the other hand, if the Al content exceeds 0.100%, coarse oxide-based inclusions are generated. Therefore, even if the contents of other elements are within the range of this embodiment, the toughness of the steel pipe decreases. Therefore, the Al content is 0.100% or less.
The lower limit of the Al content is preferably 0.001%, more preferably 0.005%, further preferably 0.007%, and further preferably 0.010%.
The upper limit of the Al content is preferably 0.095%, more preferably 0.092%, further preferably 0.090%, and further preferably 0.085%.
The Al content in the chemical composition of the duplex stainless steel pipe of this embodiment means the content of "acid-soluble Al", that is, sol. Al.

Ni: 4.0-9.0%
Nickel (Ni) stabilizes the austenite structure of steel pipes. In other words, Ni stabilizes the two-phase structure of ferrite and austenite. Ni also increases the corrosion resistance of steel pipes. When the Ni content is less than 4.0%, If such elements are present, the above-described effects cannot be sufficiently obtained even if the contents of the other elements are within the ranges of this embodiment.
On the other hand, if the Ni content exceeds 9.0%, the volume fraction of austenite becomes too high, and in this case, the strength of the steel pipe decreases even if the contents of other elements are within the ranges of this embodiment.
Therefore, the Ni content is 4.0 to 9.0%.
The lower limit of the Ni content is preferably 4.1%, more preferably 4.3%, and further preferably 4.5%.
The upper limit of the Ni content is preferably 8.8%, more preferably 8.6%, still more preferably 8.4%, still more preferably 8.2%, and still more preferably 8.0%. %.

Cr:20.0~30.0%
Chromium (Cr) forms a passive film as an oxide on the surface of steel pipes, improving the corrosion resistance of the steel pipes. Cr also increases the volume fraction of the ferrite structure of the steel pipes. By obtaining a sufficient ferrite structure, the steel pipes If the Cr content is less than 20.0%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment.
On the other hand, if the Cr content exceeds 30.0%, the hot workability of the steel pipe decreases even if the contents of other elements are within the ranges of this embodiment.
Therefore, the Cr content is 20.0 to 30.0%.
The lower limit of the Cr content is preferably 20.2%, more preferably 20.5%, further preferably 21.0%, and further preferably 21.5%.
The upper limit of the Cr content is preferably 29.8%, more preferably 29.6%, even more preferably 29.5%, even more preferably 29.0%, and even more preferably 28.5%. %.

Mo: 0.5-2.0%
Molybdenum (Mo) enhances the corrosion resistance of steel pipes. Mo also dissolves in steel to enhance the strength of steel pipes. If the Mo content is less than 0.5%, the contents of other elements are the same as in this embodiment. Even if the amount is within this range, the above-mentioned effects cannot be obtained sufficiently.
On the other hand, if the Mo content exceeds 2.0%, the hot workability of the steel pipe decreases even if the contents of other elements are within the ranges of this embodiment.
Therefore, the Mo content is 0.5 to 2.0%.
The lower limit of the Mo content is preferably 0.6%, more preferably 0.7%, and further preferably 0.8%.
The upper limit of the Mo content is preferably 1.9%, more preferably 1.8%, still more preferably 1.7%, still more preferably 1.6%, and still more preferably 1.5%. %.

Cu: 1.5-3.0%
Copper (Cu) precipitates in the steel pipe to increase the strength of the steel pipe. If the Cu content is less than 1.5%, the above effect can be obtained even if the contents of other elements are within the range of this embodiment. is not being obtained sufficiently.
On the other hand, if the Cu content exceeds 3.0%, the hot workability of the steel pipe decreases even if the contents of other elements are within the ranges of this embodiment.
Therefore, the Cu content is 1.5 to 3.0%.
The lower limit of the Cu content is preferably 1.6%, more preferably 1.8%, and further preferably 2.0%.
The upper limit of the Cu content is preferably 2.9%, more preferably 2.8%, and further preferably 2.7%.

N: 0.15-0.30%
Nitrogen (N) dissolves in steel pipes to increase their strength. N also combines with Nb to form Nb carbonitrides, which increase the strength of steel pipes through precipitation strengthening. If the N content is less than 0.15%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment.
On the other hand, if the N content exceeds 0.30%, the toughness and hot workability of the steel pipe decrease even if the contents of other elements are within the ranges of this embodiment.
Therefore, the N content is 0.15 to 0.30%.
The lower limit of the N content is preferably 0.16%, more preferably 0.18%, and further preferably 0.20%.
The upper limit of the N content is preferably 0.29%, more preferably 0.28%, even more preferably 0.27%, even more preferably 0.26%, and even more preferably 0. It is 25%.

V:0.01~0.50%
Vanadium (V) enhances the strength of a steel pipe. If the V content is less than 0.01%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment.
On the other hand, if the V content exceeds 0.50%, even if the contents of other elements are within the range of this embodiment, the strength of the steel pipe becomes too high, and the toughness and hot workability of the steel pipe deteriorate. .
Therefore, the V content is 0.01 to 0.50%.
The lower limit of the V content is preferably 0.02%, more preferably 0.03%, still more preferably 0.05%, still more preferably 0.07%, and still more preferably 0.10%. %.
The upper limit of the V content is preferably 0.48%, more preferably 0.47%, still more preferably 0.45%, still more preferably 0.42%, and still more preferably 0.40%. %.

Nb: 0.030-0.300%
Niobium (Nb) forms carbonitrides and enhances the strength of steel pipes. If the Nb content is less than 0.030%, the above effect can be obtained even if the contents of other elements are within the range of this embodiment. is not being obtained sufficiently.
On the other hand, if the Nb content exceeds 0.300%, even if the contents of other elements are within the ranges of this embodiment, the strength of the steel pipe becomes too high and the toughness of the steel pipe decreases.
Therefore, the Nb content is 0.030 to 0.300%.
The lower limit of the Nb content is preferably 0.031%, more preferably 0.033%, still more preferably 0.035%, 0.037%, and still more preferably 0.040%. .
The upper limit of the Nb content is preferably 0.294%, more preferably 0.290%, further preferably 0.280%, and further preferably 0.250%.

Co:0.10~0.50%
Cobalt (Co) forms a coating on the surface of the steel pipe, improving the corrosion resistance of the steel pipe. Co also improves the hardenability of the steel pipe and stabilizes its strength. If the Co content is less than 0.10%, For example, even if the contents of other elements are within the ranges of this embodiment, the above effects cannot be sufficiently obtained.
On the other hand, if the Co content exceeds 0.50%, the production cost increases significantly even if the contents of other elements are within the ranges of this embodiment.
Therefore, the Co content is 0.10 to 0.50%.
The lower limit of the Co content is preferably 0.11%, more preferably 0.13%, and further preferably 0.15%.
The upper limit of the Co content is preferably 0.48%, more preferably 0.45%, further preferably 0.40%, and further preferably 0.35%.

Sn: 0.001-0.050%
Tin (Sn) enhances the corrosion resistance of steel pipes. If the Sn content is less than 0.001%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment.
On the other hand, if the Sn content exceeds 0.050%, even if the contents of other elements are within the range of this embodiment, liquation embrittlement cracking occurs at the grain boundaries, and the hot workability of the steel pipe is reduced. do.
Therefore, the Sn content is 0.001 to 0.050%.
The lower limit of the Sn content is preferably 0.002%, more preferably 0.003%, still more preferably 0.005%, still more preferably 0.006%, and still more preferably 0.008%. %, and more preferably 0.010%.
The upper limit of the Sn content is preferably 0.048%, more preferably 0.045%, further preferably 0.043%, and further preferably 0.040%.

The remainder of the chemical composition of the duplex stainless steel pipe according to this embodiment is composed of Fe and impurities. Here, impurities in the chemical composition refer to substances that are mixed in from raw materials such as ore, scrap, or the manufacturing environment when industrially manufacturing duplex stainless steel pipe, and are acceptable to the extent that they do not adversely affect the duplex stainless steel pipe according to this embodiment.

[Optional elements]
The chemical composition of the above-mentioned duplex stainless steel pipe may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Ta: 0-0.100%, Ti: 0-0.100%, Zr: 0-0.100%, Hf: 0-0.100%, W: 0-0.200%, Sb: 0-0.100%, Ca: 0-0.020%, Mg: 0-0.020%, B: 0-0.020%, and rare earth elements: 0-0.200%.
These optional elements will now be described.

[First group (Ta, Ti, Zr, Hf, and W)]
The duplex stainless steel pipe of the present embodiment may contain, in place of a portion of Fe, one or more elements selected from the group consisting of Ta, Ti, Zr, Hf, and W. All of these elements are optional elements, and increase the strength of the steel pipe.

Ta: 0~0.100%
Tantalum (Ta) is an optional element and may not be contained, that is, the Ta content may be 0%.
When contained, that is, when the Ta content exceeds 0%, Ta forms carbonitrides and increases the strength of the steel pipe. Even if even a small amount of Ta is contained, the above effect can be obtained to some extent.
However, if the Ta content exceeds 0.100%, even if the contents of other elements are within the ranges of this embodiment, the strength of the steel pipe becomes too high and the toughness of the steel pipe decreases.
Therefore, the Ta content is 0 to 0.100%.
The lower limit of the Ta content is preferably 0.001%, more preferably 0.002%, still more preferably 0.003%, still more preferably 0.005%, and still more preferably 0.010%. %, and more preferably 0.015%.
The upper limit of the Ta content is preferably 0.080%, more preferably 0.070%, still more preferably 0.060%, still more preferably 0.050%, and still more preferably 0.040%. %.

Ti: 0~0.100%
Titanium (Ti) is an optional element and may not be contained, that is, the Ti content may be 0%.
When contained, that is, when the Ti content exceeds 0%, Ti forms carbonitrides and increases the strength of the steel pipe. Even if even a small amount of Ti is contained, the above effect can be obtained to some extent.
However, if the Ti content exceeds 0.100%, even if the contents of other elements are within the ranges of this embodiment, the strength of the steel pipe becomes too high and the toughness of the steel pipe decreases.
Therefore, the Ti content is 0 to 0.100%.
The lower limit of the Ti content is preferably 0.001%, more preferably 0.002%, still more preferably 0.003%, still more preferably 0.005%, and still more preferably 0.010%. %, and more preferably 0.015%.
The upper limit of the Ti content is preferably 0.098%, more preferably 0.095%, still more preferably 0.090%, still more preferably 0.085%, and still more preferably 0.080%. %, more preferably 0.075%, and even more preferably 0.070%.

Zr: 0-0.100%
Zirconium (Zr) is an optional element and may not be contained, that is, the Zr content may be 0%.
When contained, that is, when the Zr content is more than 0%, Zr forms carbonitrides and increases the strength of the steel pipe. Even if even a small amount of Zr is contained, the above effect can be obtained to some extent.
However, if the Zr content exceeds 0.100%, even if the contents of other elements are within the ranges of this embodiment, the strength of the steel pipe becomes too high and the toughness of the steel pipe decreases.
Therefore, the Zr content is 0 to 0.100%.
The lower limit of the Zr content is preferably 0.001%, more preferably 0.002%, still more preferably 0.003%, still more preferably 0.005%, and still more preferably 0.010%. %, and more preferably 0.015%.
The upper limit of the Zr content is preferably 0.090%, more preferably 0.080%, still more preferably 0.070%, still more preferably 0.060%, and still more preferably 0.050%. %.

Hf: 0-0.100%
Hafnium (Hf) is an optional element and may not be contained, that is, the Hf content may be 0%.
When contained, that is, when the Hf content is more than 0%, Hf forms carbonitrides and increases the strength of the steel pipe. Even if even a small amount of Hf is contained, the above effect can be obtained to some extent.
However, if the Hf content exceeds 0.100%, even if the contents of other elements are within the ranges of this embodiment, the strength of the steel pipe becomes too high and the toughness of the steel pipe decreases.
Therefore, the Hf content is 0 to 0.100%.
The lower limit of the Hf content is preferably 0.001%, more preferably 0.002%, still more preferably 0.003%, still more preferably 0.005%, and still more preferably 0.010%. %, and more preferably 0.015%.
The upper limit of the Hf content is preferably 0.095%, more preferably 0.090%, still more preferably 0.085%, still more preferably 0.080%, and still more preferably 0.070%. %, more preferably 0.060%, and even more preferably 0.050%.

W: 0-0.200%
Tungsten (W) is an optional element and may not be contained, that is, the W content may be 0%.
When W is contained, that is, when the W content exceeds 0%, W forms carbonitrides and increases the strength of the steel pipe. Even if even a small amount of W is contained, the above effect can be obtained to a certain extent. However, If the W content exceeds 0.200%, even if the contents of other elements are within the ranges of this embodiment, the strength of the steel pipe becomes too high and the toughness of the steel pipe decreases.
Therefore, the W content is 0 to 0.200%.
The lower limit of the W content is preferably 0.001%, more preferably 0.003%, still more preferably 0.005%, still more preferably 0.010%, and still more preferably 0.015%. %.
The upper limit of the W content is preferably 0.180%, more preferably 0.150%, still more preferably 0.130%, still more preferably 0.100%, and still more preferably 0.080%. %, and more preferably 0.050%.

[Second group: Sb]
The chemical composition of the duplex stainless steel pipe of this embodiment may further contain Sb instead of a portion of Fe.

Sb: 0-0.100%
Antimony (Sb) is an optional element and may not be contained, that is, the Sb content may be 0%.
When contained, that is, when the Sb content is more than 0%, Sb enhances the corrosion resistance of the steel pipe. Even if even a small amount of Sb is contained, the above effect can be obtained to some extent.
However, if the Sb content exceeds 0.100%, even if the contents of other elements are within the range of this embodiment, the ductility of the steel pipe at high temperatures decreases, and the hot workability of the steel pipe decreases. do.
Therefore, the Sb content is 0 to 0.100%.
The lower limit of the Sb content is preferably 0.001%, more preferably 0.003%, still more preferably 0.005%, still more preferably 0.010%, and still more preferably 0.015%. %.
The upper limit of the Sb content is preferably 0.090%, more preferably 0.085%, even more preferably 0.080%, even more preferably 0.070%, and even more preferably 0.060%. %, and more preferably 0.050%.

[Third group (Ca, Mg, B, and rare earth elements)]
The chemical composition of the duplex stainless steel pipe of this embodiment may further contain one or more elements selected from the group consisting of Ca, Mg, B, and rare earth elements in place of a portion of Fe. All of these elements are optional elements, and improve the hot workability of the steel pipe.

Ca: 0-0.020%
Calcium (Ca) is an optional element and may not be contained, that is, the Ca content may be 0%.
When contained, that is, when the Ca content exceeds 0%, Ca fixes S in the steel pipe as sulfides, thereby improving the hot workability of the steel pipe. The above effects can be obtained to some extent.
However, if the Ca content exceeds 0.020%, the oxides in the steel pipe become coarse, and therefore the toughness of the steel pipe decreases even if the contents of other elements are within the ranges of this embodiment.
Therefore, the Ca content is 0 to 0.020%.
The lower limit of the Ca content is preferably 0.001%, more preferably 0.002%, further preferably 0.003%, and further preferably 0.005%.
The upper limit of the Ca content is preferably 0.018%, more preferably 0.016%, still more preferably 0.014%, still more preferably 0.012%, and still more preferably 0.010%. %.

Mg: 0-0.020%
Magnesium (Mg) is an optional element and may not be contained, that is, the Mg content may be 0%.
When Mg is contained, that is, when the Mg content is more than 0%, Mg fixes S in the steel pipe as sulfides, thereby improving the hot workability of the steel pipe. The above effects can be obtained to some extent.
However, if the Mg content exceeds 0.020%, the oxides in the steel material become coarse, and therefore the toughness of the steel pipe decreases even if the contents of other elements are within the ranges of this embodiment.
Therefore, the Mg content is 0 to 0.020%.
The lower limit of the Mg content is preferably 0.001%, more preferably 0.002%, still more preferably 0.003%, still more preferably 0.005%, and still more preferably 0.006%. %.
The upper limit of the Mg content is preferably 0.018%, more preferably 0.016%, and further preferably 0.015%.

B: 0-0.020%
Boron (B) is an optional element and may not be contained. In other words, the B content may be 0%.
When B is contained, that is, when the B content is more than 0%, B suppresses the segregation of S to the grain boundaries in the steel pipe, and improves the hot workability of the steel pipe. If so, the above effects can be obtained to some extent.
However, if the B content exceeds 0.020%, boron nitride (BN) is formed, and therefore the toughness of the steel pipe decreases even if the contents of other elements are within the ranges of this embodiment.
Therefore, the B content is 0 to 0.020%.
The lower limit of the B content is preferably 0.001%, more preferably 0.002%, further preferably 0.003%, and further preferably 0.005%.
The upper limit of the B content is preferably 0.018%, more preferably 0.016%, still more preferably 0.014%, still more preferably 0.012%, and still more preferably 0.010%. %.

Rare earth elements: 0-0.200%
The rare earth elements (REM) are optional elements and may not be contained, i.e., the REM content may be 0%.
When REM is contained, that is, when the REM content is more than 0%, REM fixes S in the steel pipe as sulfides, thereby improving the hot workability of the steel pipe. The effect is achieved to some extent.
However, if the REM content exceeds 0.200%, the oxides in the steel pipe become coarse, and therefore the toughness of the steel pipe decreases even if the contents of other elements are within the ranges of this embodiment.
Therefore, the REM content is 0 to 0.200%.
The lower limit of the REM content is preferably 0.001%, more preferably 0.005%, still more preferably 0.008%, still more preferably 0.010%, and still more preferably 0.020%. %.
The upper limit of the REM content is preferably 0.180%, more preferably 0.160%, even more preferably 0.140%, even more preferably 0.120%, and even more preferably 0.100%. %, more preferably 0.080%, even more preferably 0.060%, and even more preferably 0.050%.

In this specification, REM refers to one or more elements selected from the group consisting of scandium (Sc), atomic number 21; yttrium (Y), atomic number 39; and the lanthanides lanthanum (La), atomic number 57, to lutetium (Lu), atomic number 71. In this specification, the REM content refers to the total content of these elements.

[Feature 2: Microstructure]
The microstructure of the duplex stainless steel pipe of this embodiment is composed of 35.0 to 65.0% by volume of ferrite, 0 to less than 1.0% of σ phase, and the remainder being austenite. The amount of other structures in the microstructure other than ferrite, austenite, and σ phase is negligibly small. Specifically, the microstructure of the duplex stainless steel pipe of this embodiment may contain minute amounts of precipitates, inclusions, etc. in addition to ferrite and austenite. However, in the chemical composition of the duplex stainless steel pipe of this embodiment, the volume fraction of precipitates, inclusions, etc. is negligibly small compared to the volume fractions of ferrite, austenite, and σ phase.

In the microstructure of the duplex stainless steel pipe of this embodiment, the volume fraction of ferrite is 35.0 to 65.0%. If the volume fraction of ferrite is too low, the yield strength and/or corrosion resistance of the steel pipe may decrease. On the other hand, if the volume fraction of ferrite is too high, the toughness and/or hot workability of the steel pipe may decrease.
Therefore, in the microstructure of the duplex stainless steel pipe of this embodiment, the volume fraction of ferrite is 35.0 to 65.0%.
The lower limit of the volume fraction of ferrite is preferably 36.0%, and more preferably 37.0%.
The upper limit of the volume fraction of ferrite is preferably 64.0%, and more preferably 63.0%.

In the microstructure of the duplex stainless steel pipe of this embodiment, the σ phase reduces the corrosion resistance. Therefore, it is preferable that the volume fraction of the σ phase is small. If the volume fraction of the σ phase is 1.0% or more, the corrosion resistance of the duplex stainless steel pipe decreases. Therefore, the volume fraction of the σ phase is 0 to less than 1.0%.
The lower the volume fraction of the σ phase, the more preferable, and the most preferable volume fraction of the σ phase is 0%. However, if the volume fraction of the σ phase is reduced too much, the manufacturing cost will increase significantly. Therefore, the preferable lower limit of the volume fraction of the σ phase is more than 0%, and more preferably 0.1%.

The remainder of the microstructure is made up of austenite. When the microstructure is made up of 35.0 to 65.0% by volume ferrite, 0 to less than 1.0% σ phase, and the remainder austenite, high strength and excellent corrosion resistance can be obtained, provided that the other characteristics 1, 3, and 4 are satisfied.

[Method for measuring ferrite volume fraction and σ phase volume fraction]
The volume fraction of ferrite in a duplex stainless steel pipe can be determined by a method in accordance with ASTM E562 (2019).
Specifically, a test piece for microstructure observation having an observation surface of 5 mm in the axial direction and 5 mm in the circumferential direction is taken from the center of the wall thickness of a duplex stainless steel pipe. In this specification, the circumferential direction of the steel pipe means a direction perpendicular to the axial direction and radial direction of the pipe. Note that the size of the test piece is not particularly limited as long as the above observation surface can be obtained.

The observation surface of the test piece is mirror-polished. The mirror-polished observation surface is electrolytically etched in a 7% potassium hydroxide etching solution to reveal the structure. The observation surface with the revealed structure is observed in 10 fields of view using an optical microscope. The area of each field of view is 1.00 mm ² (magnification: 100 times). In each field of view, ferrite and austenite are identified from the contrast. When electrolytically etched in a 7% potassium hydroxide etching solution, the low-brightness area corresponds to ferrite, and the high-brightness area corresponds to austenite. Therefore, a person skilled in the art can easily identify ferrite and austenite from the contrast.
The area ratio of the identified ferrite is measured by the point counting method in accordance with ASTM E562 (2019). The arithmetic average value of the area ratio of ferrite obtained in each field of view (total of 10 pieces) is defined as the volume ratio (%) of ferrite. The volume ratio (%) of ferrite is the value obtained by rounding off the first decimal place of the obtained value.

The volume fraction of the σ phase in a duplex stainless steel pipe is determined by the following method.
The observation surface on which the above-mentioned structure appears is observed in five fields of view using an optical microscope. The area of each field of view is 0.0625 mm ² (magnification 400 times, 250 μm × 250 μm). In each field of view, the σ phase is identified from the contrast. In the observation surface that has been electrolytically corroded in a 7% potassium hydroxide corrosive solution, the σ phase can be identified as a black area with a lower brightness compared to other structures. In addition, element concentration analysis (EDS analysis) may be performed in each field of view to identify the σ phase. When element concentration analysis is performed, the σ phase is identified by the following method. In each field of view, particles are identified based on the contrast. EDS analysis is performed on the identified particles. In the EDS analysis, the acceleration voltage is set to 20 kV, and the target elements are quantified as N, Mo, Al, Si, P, S, Ca, Ti, Cr, Mn, Fe, Cu, and Nb. Based on the EDS analysis results of each particle, when the Cr content in the particle is 35.0% or more and the Mo content is 3.0% or more, in mass%, the particle is identified as a σ phase.

The area of the identified σ phase is determined. The area ratio (%) of the σ phase is determined based on the total area of the σ phase in the five visual fields and the total area of the five visual fields. The determined area ratio (%) of the σ phase is regarded as the volume ratio (%) of the σ phase. In this embodiment, the volume ratio (%) of the σ phase is the value obtained by rounding the obtained value to one decimal place.

[(Feature 3) Regarding the content of undissolved Nb and the undissolved Nb/Al ratio]
In the duplex stainless steel pipe of this embodiment, the undissolved Nb content is 0.008% or more by mass percent, and the undissolved Nb/Al ratio, which is the ratio of the undissolved Nb content to the undissolved Al content, is 1.0 or more. These matters will be described below.

[Regarding non-solid-solubilized Nb content]
The non-soluble Nb is Nb that is not dissolved in the base material and is contained in precipitates. In the duplex stainless steel pipe of this embodiment, a high yield strength of 655 MPa or more can be obtained by sufficiently generating Nb carbonitrides in the steel pipe. If the non-soluble Nb content is less than 0.008% by mass, the Nb carbonitrides are not sufficiently generated in the steel pipe. Therefore, sufficient yield strength cannot be obtained. Therefore, the non-soluble Nb content is 0.008% by mass or more.

The lower limit of the undissolved Nb content is preferably 0.009% by mass, more preferably 0.010%, more preferably 0.012%, more preferably 0.015%, more preferably 0.020%, and even more preferably 0.025%.
The upper limit of the undissolved Nb content is not particularly limited. In the case of a chemical composition that satisfies the characteristic 1, the upper limit of the undissolved Nb content is, for example, 0.300%, for example, 0.250%.

[Regarding the non-solid-solubilized Nb/Al ratio]
The undissolved Nb/Al ratio, which is the ratio of the undissolved Nb content to the undissolved Al content, can be defined by the following formula.
Non-solid solution Nb/Al ratio = non-solid solution Nb content (mass%) / non-solid solution Al content (mass%)

The non-dissolved Nb/Al ratio is an index of the ratio of the amount of Nb carbonitrides produced in the steel pipe to the amount of Al nitrides produced in the steel pipe. As described above, the Nb carbonitrides in the steel pipe increase the strength of the steel pipe by precipitation strengthening. On the other hand, the Al nitrides are coarser than the Nb carbonitrides and do not substantially contribute to precipitation strengthening. The Al nitrides further reduce the amount of dissolved N in the steel pipe and also reduce the amount of dissolved N used to produce Nb carbonitrides. Therefore, even if a certain amount of Nb carbonitrides is produced, if the amount of Al nitrides is excessive relative to the amount of Nb carbonitrides produced, the amount of dissolved N in the steel pipe and the amount of Nb carbonitrides produced cannot be obtained sufficiently. As a result, sufficient strength may not be obtained in the duplex stainless steel pipe.

In a duplex stainless steel pipe that satisfies Features 1 and 2, if the undissolved Nb content is 0.008% or more and the undissolved Nb/Al ratio is 1.0 or more, a sufficient amount of Nb carbonitrides and a sufficient amount of dissolved N can be ensured in the steel pipe. As a result, the yield strength of the duplex stainless steel pipe can be increased to 655 MPa or more.

The lower limit of the undissolved Nb/Al ratio is preferably 1.1, more preferably 1.2, more preferably 1.5, more preferably 2.0, more preferably 2.5, and even more preferably 3.0.
The upper limit of the undissolved Nb/Al ratio is not particularly limited. In the case of a chemical composition that satisfies the characteristic 1, the upper limit of the undissolved Nb/Al ratio is, for example, 70.0, for example, 65.0.

[Method of measuring undissolved Nb content and undissolved Nb/Al ratio]
The undissolved Nb content and the undissolved Nb/Al ratio are determined by the following method.
A cylindrical test piece having a diameter of 8 mm and a length of 50 mm is taken from the duplex stainless steel pipe. Specifically, the cylindrical test piece is prepared with the central axis at the center of the wall thickness of the steel pipe. The axial direction of the cylindrical test piece is the pipe axial direction of the steel pipe.

Constant-current electrolysis is performed on the cylindrical test specimens using a 10% AA-based solution (a solution containing, by volume, 10% acetylacetone, 1% tetramethylammonium chloride, and 89% methanol solution).

First, preliminary electrolysis is performed to remove deposits (surface scale and impurities) from the surface of the cylindrical test piece. In preliminary electrolysis, an area from the surface of the scale to a depth of approximately 100 μm is electrolyzed at room temperature (25°C) with a current of 1000 mA. After preliminary electrolysis, the cylindrical test piece is immersed in an alcohol solution. The cylindrical test piece immersed in the alcohol solution is subjected to ultrasonic cleaning to remove deposits from the surface of the cylindrical test piece. The mass of the cylindrical test piece from which the deposits have been removed, i.e., the mass of the cylindrical test piece before constant current electrolysis, is measured.

Next, constant current electrolysis is performed on the cylindrical test piece. Specifically, a new 10% AA-based solution is prepared. Then, electrolysis is performed using the new 10% AA-based solution at room temperature with a current density maintained at 20 mA/ ^cm2 . After the constant current electrolysis, the cylindrical test piece is immersed in an alcohol solution, and then ultrasonic cleaning is performed on the cylindrical test piece to remove deposits on the cylindrical test piece surface. The mass of the cylindrical test piece from which the deposits have been removed is measured, and this is taken as the mass of the cylindrical test piece after constant current electrolysis.

The 10% AA solution used for constant current electrolysis and the alcohol solution used for subsequent ultrasonic cleaning are suction filtered through a filter with a mesh size of 0.2 μm to extract the residue.

Chemical elemental analysis is performed on the extracted residue. Specifically, the residue is dissolved in acid to obtain a solution. Chemical elemental analysis is performed on the solution using ICP-AES to quantitatively analyze Nb and Al. The Nb content (mass %) in the residue and the Al content (mass %) in the residue are determined based on the Nb mass and Al mass obtained by quantitative analysis and the mass difference of the cylindrical test piece before and after constant current electrolysis. The Nb content in the obtained residue is defined as the undissolved Nb content (mass %). The Al content in the obtained residue is defined as the undissolved Al content (mass %). The undissolved Nb/Al ratio is determined based on the undissolved Nb content and the undissolved Al content.

[Feature 4: Yield strength]
The duplex stainless steel pipe according to this embodiment has a yield strength of 655 MPa or more (95 ksi or more). The duplex stainless steel pipe according to this embodiment satisfies Features 1 to 3. As a result, excellent corrosion resistance is obtained, and a high yield strength of 655 MPa or more is also obtained.

The lower limit of the yield strength of the duplex stainless steel pipe according to this embodiment is preferably 660 MPa or more, more preferably 665 MPa, even more preferably 670 MPa, and even more preferably 675 MPa.
The upper limit of the yield strength of the duplex stainless steel pipe according to this embodiment is not particularly limited, but is, for example, 800 MPa.

[Method of measuring yield strength]
The yield strength of the duplex stainless steel pipe of this embodiment is determined by conducting a tensile test in accordance with a method in accordance with ASTM E8/E8M (2022).
Specifically, a circular-arc test piece is taken from the duplex stainless steel pipe of this embodiment. The circular-arc test piece has, for example, the same thickness as the wall thickness of the steel pipe, a width of 25.4 mm, and a gauge length of 50.8 mm. The longitudinal direction of the circular-arc test piece is parallel to the axial direction of the steel pipe.

A tensile test is performed in air at room temperature (25°C) using a circular arc test piece. In this embodiment, the 0.2% offset yield strength obtained from the tensile test is defined as the yield strength (MPa). In this embodiment, the yield strength (MPa) is an integer value obtained by rounding off the first decimal place of the obtained numerical value.

[Effects of the duplex stainless steel pipe of this embodiment]
The duplex stainless steel pipe of this embodiment satisfies Features 1 to 4. Therefore, the duplex stainless steel pipe of this embodiment has a high yield strength of 655 MPa or more (95 ksi or more), and further has excellent corrosion resistance.

[Corrosion resistance]
In this embodiment, the corrosion resistance of the duplex stainless steel pipe is evaluated by the following method.

[Corrosion resistance evaluation method]
A test piece for a four-point bending test is taken from the duplex stainless steel pipe of this embodiment. The size of the test piece is, for example, 2 mm in thickness, 10 mm in width, and 75 mm in length. The test piece is prepared from the center of the wall thickness of the steel pipe. In this case, the length direction of the test piece is parallel to the axial direction of the steel pipe.

A 20% by weight aqueous sodium chloride solution adjusted to pH = 4.0 is used as the test solution. In accordance with ASTM G39-99 (2021), a stress equivalent to 90% of the actual yield stress is applied to the test piece by four-point bending. The test piece to which the stress has been applied is sealed in an autoclave together with the test jig. The test solution is injected into the autoclave, leaving the gas phase, to form a test bath. After degassing the test bath, a mixed gas of 0.2 bar H ₂ S gas and 30 bar CO ₂ gas is pressurized and sealed in the autoclave, and the test bath is stirred to saturate the mixed gas. After sealing the autoclave, the test bath is stirred at 90 ° C for 720 hours.

In this embodiment, after 720 hours in the above-mentioned test environment, the specimen is observed with a 10x magnifying glass to check for the presence or absence of cracks. If the occurrence of cracks is suspected based on the observation with the magnifying glass, the specimen is further observed with an optical microscope at a magnification of 100x to check for the presence or absence of cracks. If no cracks are found, the specimen is evaluated as having "excellent corrosion resistance."

[Shapes of duplex stainless steel pipes]
The duplex stainless steel pipe of the present embodiment may be a welded pipe or a seamless steel pipe, and is preferably a seamless steel pipe.

[Production method]
An example of a method for manufacturing the duplex stainless steel pipe of this embodiment having the above-mentioned configuration will be described below. Note that the method for manufacturing the duplex stainless steel pipe of this embodiment is not limited to the manufacturing method described below.

The method for manufacturing a duplex stainless steel pipe of this embodiment includes the following steps.
(Step 1) Material preparation step (Step 2) Hot working step (Step 3) Solution treatment step (Step 4) Straightening step (Step 5) Aging heat treatment step Each step will be explained below.

[(Process 1) Material preparation process]
In the material preparation step, a material having a chemical composition that satisfies Feature 1 is prepared. The material may be prepared by manufacturing or by purchasing from a third party. That is, the method of preparing the material is not particularly limited.

When manufacturing raw materials, they are manufactured, for example, by the following method. Molten steel having the above-mentioned chemical composition is manufactured. The molten steel is used to manufacture cast pieces (slabs, blooms, or billets) by continuous casting. The molten steel may be used to manufacture steel ingots by ingot casting. If necessary, the slabs, blooms, or ingots may be rolled to manufacture billets. The raw materials are manufactured by the above-mentioned steps.

[(Step 2) Hot working step]
In the hot working step, the raw material prepared in the raw material preparation step is hot worked to produce a mother pipe. The hot working may be hot forging, hot extrusion, or hot rolling. The method of hot working is not particularly limited and may be a well-known method.

As a hot working method, for example, hot extrusion such as the Eugène-Séjournet method or the Erhardt push bench method may be performed, or piercing rolling by the Mannesmann method, which is a type of hot rolling, may be performed. Note that hot working may be performed only once or multiple times. For example, the above-mentioned hot extrusion may be performed after the above-mentioned piercing rolling is performed on the material. For example, the above-mentioned piercing rolling may be performed on the material, and then elongation rolling, which is a type of hot rolling, may be performed. That is, in the hot working process, hot working is performed by a well-known method to manufacture a blank pipe. Note that the heating temperature during hot working is, for example, 1000 to 1280°C.

[(Step 3) Solution treatment step]
In the solution treatment process, the mother pipe after the hot working process is subjected to solution treatment. Specifically, the mother pipe is loaded into a heat treatment furnace and heated. The mother pipe is then held at a desired temperature (solution temperature) and then rapidly cooled. The solution treatment satisfies the following conditions:
(Condition 1)
The average temperature rise rate HR1 during heating of the blank tube from 700 to 900° C. is set to 0.25° C./sec or more.
(Condition 2)
The solution temperature T1 is set to 980 to 1100°C.
Each condition will be explained below.

(Regarding condition 1)
During heating for solution treatment, Al nitrides are likely to form in the temperature range of 700 to 900°C. By shortening the residence time in the temperature range of 700 to 900°C as much as possible, the formation of Al nitrides in the mother tube is suppressed, and as a result, the undissolved Nb/Al ratio in the produced mother tube is increased. If the average heating rate HR1 in the range of 700 to 900°C is 0.25°C/sec or more, the undissolved Nb/Al ratio in the produced mother tube can be sufficiently increased. The upper limit of the average heating rate HR1 is, for example, 0.60°C/sec.

(Regarding condition 2)
The solution treatment temperature T1 affects the ferrite volume fraction in the microstructure of the steel pipe. If the solution treatment temperature T1 is too low, the ferrite volume fraction of the duplex stainless steel pipe will be less than 35.0%, and the strength and/or corrosion resistance of the manufactured duplex stainless steel pipe may decrease. On the other hand, if the solution treatment temperature T1 is too high, the ferrite volume fraction of the duplex stainless steel pipe after solution treatment will be 65.0% or more, and the corrosion resistance of the steel pipe may decrease. If the solution treatment temperature T1 is 980 to 1100°C, the ferrite volume fraction of the duplex stainless steel pipe will be in an appropriate range.

The holding time t1 at the solution temperature T1 is, for example, 10 to 180 minutes. Here, the solution temperature T1 means the temperature (°C) of the heat treatment furnace used to perform the solution treatment. The holding time t1 at the solution temperature T1 means the time (minutes) the blank tube is held at the solution temperature.

[(Step 4) Straightening step]
In the straightening process, the mother pipe that has been subjected to the above-mentioned solution treatment process is straightened at room temperature. By applying strain to the mother pipe by the straightening, a sufficient amount of strain is obtained in the next aging heat treatment process. Nb carbonitride is produced.

Figure 1 is a schematic diagram of a rotary straightener, which is an example of a straightening machine. Referring to Figure 1, the rotary straightener has multiple stands ST1 to STn (n is a natural number equal to or greater than 3). In Figure 1, the rotary straightener has four stands. However, the number of stands is not particularly limited as long as it is three or more. For example, straightening may be performed by three stands, or by five or more stands. Each stand has a pair of inclined rolls. Each stand is arranged in a row along a pass line PL through which the raw tube passes. Of the multiple stands, the inclined rolls of the stands other than stand ST2 are arranged on the pass line PL, and the inclined roll of stand ST2 is offset from the pass line PL.

Figure 2 is a front view of the straightening machine of Figure 1. The view on the left of Figure 2 is a cross-sectional view perpendicular to the axial direction of the mother tube before straightening is performed. The view on the right of Figure 2 is a front view of the stand with the smallest roll gap DB. In the straightening of this embodiment, crash straightening is performed. Crash straightening refers to straightening in which a mother tube is pressed down and deformed into an elliptical shape. With reference to Figure 2, the amount of reduction applied to the mother tube in the stand with the smallest roll gap DB is defined as the crush amount δc (mm). The crush amount δc can be calculated by subtracting the roll gap DB in the stand with the smallest roll gap DB from the outer diameter DA of the mother tube before straightening.

The straightening process satisfies the following conditions:
(Condition 3)
The crush amount δc is set to 3 mm or more.

(Regarding condition 3)
If the crush amount δc in the straightening is too small, sufficient strain is not introduced into the mother pipe before the aging heat treatment. Therefore, a sufficient amount of Nb carbonitride is not generated in the next aging heat treatment process. If the crush amount δc is 3 mm or more, sufficient strain is introduced into the mother pipe. As a result, a sufficient amount of Nb carbonitride is generated in the duplex stainless steel pipe after the aging heat treatment process, and a sufficient amount of undissolved Nb is obtained. The upper limit of the crush amount is, for example, 8 mm.

[(Step 5) Aging heat treatment step]
In the aging heat treatment step, the mother pipe is subjected to aging heat treatment. In the aging heat treatment step of the present embodiment, when the aging heat treatment is performed, a sufficient amount of Nb carbonitrides is generated while suppressing the generation of the σ phase. The aging heat treatment satisfies the following conditions.
(Condition 4)
The aging heat treatment temperature T2 satisfies the following formula (A).
T2<700-(0.5Cr+0.3Mn+3Mo+1.5Si+8Nb-Ni-0.6Cu-4Co-10Sn) ² (A)
Here, the content by mass % of the corresponding element in the duplex stainless steel pipe is substituted for each element symbol in formula (A).
(Condition 5)
The holding time t2 at the aging heat treatment temperature T2 satisfies formula (B).
t2>(1/60)×10 ^{(3200/(T2+273.15)-3Nb)} (B)
Here, the aging heat treatment temperature T2 (°C) is substituted for T2 in formula (B), and the Nb content in mass% of the duplex stainless steel pipe is substituted for Nb in formula (B).
Each condition will be explained below.

(Regarding condition 4)
FnA is defined as follows:
FnA=700-(0.5Cr+0.3Mn+3Mo+1.5Si+8Nb-Ni-0.6Cu-4Co-10Sn) ²
FnA means the lower limit of the temperature (°C) at which the formation of the σ phase is promoted. Cr, Mn, Mo, Si, and Nb in FnA are elements that promote the formation of the σ phase. On the other hand, Ni, Cu, Co, and Sn are elements that suppress the formation of the σ phase. If the aging heat treatment temperature T2 is equal to or higher than FnA, the formation of the σ phase is promoted in the mother tube during the aging heat treatment. As a result, the volume fraction of the σ phase is excessively high in the manufactured duplex stainless steel pipe. If the aging heat treatment temperature T2 is lower than FnA, the formation of the σ phase is sufficiently suppressed in the mother tube during the aging heat treatment. As a result, the volume fraction of the σ phase is sufficiently reduced in the manufactured duplex stainless steel pipe.

(Regarding condition 5)
FnB is defined as follows:
FnB=(1/60)×10 ^{(3200/(T2+273.15)-3Nb)}
FnB means the lower limit of the holding time (minutes) required to generate a sufficient amount of Nb carbonitrides. Since the Nb content in the steel pipe has a large effect on the generation of Nb carbonitrides, Nb is included in FnB. If the holding time t2 at the aging heat treatment temperature T2 is equal to or less than FnB, a sufficient amount of Nb carbonitrides is not generated in the manufactured duplex stainless steel pipe. Therefore, a sufficient amount of non-dissolved Nb is not obtained. If the holding time t2 is longer than FnB, a sufficient amount of Nb carbonitrides is generated in the manufactured duplex stainless steel pipe. Therefore, a sufficient amount of non-dissolved Nb is obtained.

[Other steps]
The manufacturing method of this embodiment may include manufacturing steps other than those described above. For example, the duplex stainless steel pipe after the aging heat treatment step may be subjected to a pickling treatment step. In this case, the pickling treatment step may be performed by a well-known method, and is not particularly limited. Note that, in the manufacturing method of this embodiment, it is not necessary to perform a cold working step after the hot working step and before the solution treatment step. Even if the cold working step is omitted, a duplex stainless steel pipe with sufficient strength can be obtained.

The above steps allow the duplex stainless steel pipe of this embodiment to be manufactured. Note that the above-mentioned method for manufacturing the duplex stainless steel pipe is only one example, and the duplex stainless steel pipe of this embodiment may be manufactured by other methods. Below, the duplex stainless steel pipe of this embodiment will be described in more detail using examples.

A seamless duplex stainless steel pipe having the chemical composition shown in Tables 1A and 1B was manufactured.

Specifically, molten steel was melted using a 50 kg vacuum melting furnace. The molten steel was used to manufacture steel ingots by ingot casting. Note that "-" in Table 1B means that the content of the corresponding element was at the impurity level. For example, the Ta content, Ti content, Zr content, Hf content, W content, Sb content, Ca content, Mg content, B content, and REM content of steel code A were 0% when rounded off to the fourth decimal place.

The ingots were subjected to hot working (hot extrusion) to produce blank tubes. The heating temperature during hot working was 1000-1280°C. Solution treatment was performed on the blank tubes of each test number that had been hot worked. The average heating rate HR1 (°C/sec) from 700-900°C during the solution treatment, the solution temperature T1 (°C), and the holding time t1 (min) at the solution temperature T1 were as shown in Table 2.

Straightening was carried out on the solution-treated blank pipe using a three-stand rotary straightener. The crush amount δc (mm) during straightening was as shown in Table 2. Aging heat treatment was carried out on the blank pipe after straightening. The aging heat treatment temperature T2 (°C) and holding time t2 (min) during the aging heat treatment were as shown in Table 2. Table 2 also shows FnA (°C) and FnB (min) for each test number. Using the above manufacturing process, duplex stainless steel pipes of each test number were manufactured.

[Evaluation test]
The following evaluation tests were carried out on the duplex stainless steel pipes with each test number.
(Test 1) Ferrite volume fraction and σ phase volume fraction measurement test (Test 2) Undissolved Nb content and undissolved Nb/Al ratio measurement test (Test 3) Yield strength measurement test (Test 4) Corrosion resistance test Each test will be described below.

[(Test 1) Ferrite volume fraction and σ phase volume fraction measurement test]
The ferrite volume fraction (%) and the σ-phase volume fraction (%) of the duplex stainless steel pipe of each test number were determined according to the method described in the above [Method of measuring ferrite volume fraction and σ-phase volume fraction]. The obtained ferrite volume fraction (%) and σ-phase volume fraction (%) are shown in Table 3.

[(Test 2) Measurement of non-solid-solubilized Nb content and non-solid-solubilized Nb/Al ratio]
The undissolved Nb content (mass%) and the undissolved Nb/Al ratio were determined for the duplex stainless steel pipe of each test number according to the method described in [Method of measuring undissolved Nb content and undissolved Nb/Al ratio] above. The undissolved Nb content (mass%), undissolved Al content (mass%), and undissolved Nb/Al ratio obtained are shown in Table 3.

[(Test 3) Yield Strength Measurement Test]
The yield strength (MPa) of the duplex stainless steel pipe of each test number was determined according to the method described in the above-mentioned [Method of measuring yield strength]. The arc-shaped test piece had the same thickness as the wall thickness of the steel pipe, a width of 25.4 mm, and a gauge length of 50.8 mm. The obtained yield strength (MPa) is shown in Table 3 as "YS (MPa)".

[(Test 4) Corrosion resistance test]
The corrosion resistance of the duplex stainless steel pipes of each test number was evaluated according to the method described in the above [Corrosion Resistance Evaluation Method]. The size of the test specimen was 2 mm thick, 10 mm wide, and 75 mm long. It was determined that excellent corrosion resistance was obtained for the test specimens in which no cracks were confirmed after 720 hours. When excellent corrosion resistance was obtained, it was indicated as "NO SSC" in the "Corrosion Resistance" column of Table 3. On the other hand, it was determined that excellent corrosion resistance was not obtained for the test specimens in which cracks were confirmed after 720 hours. When excellent corrosion resistance was not obtained, it was indicated as "SSC" in the "Corrosion Resistance" column of Table 3.

[Evaluation Results]
With reference to Tables 1A, 1B, 2, and 3, the duplex stainless steel pipes of Test Nos. 1 to 18 satisfied Features 1 to 4. Therefore, these seamless steel pipes had a high yield strength of 655 MPa or more. Furthermore, they had excellent corrosion resistance. That is, the duplex stainless steel pipes of Test Nos. 1 to 18 had both a high yield strength of 655 MPa or more and excellent corrosion resistance.

On the other hand, in test number 19, the Nb content was too high. As a result, excellent corrosion resistance was not obtained.

In test number 20, the Nb content was too low. Therefore, the non-dissolved Nb content was too low. As a result, the yield strength was low at less than 655 MPa.

In test numbers 21 and 22, the aging heat treatment temperature T2 was FnA or higher during the aging heat treatment, and formula (A) was not satisfied. Therefore, the σ-phase volume fraction was high, at 1.0% or higher. As a result, excellent corrosion resistance was not obtained.

In test numbers 23 and 24, the holding time t2 at the aging heat treatment temperature T2 was shorter than FnB in the aging heat treatment, and formula (B) was not satisfied. Therefore, the non-dissolved Nb content was too low. As a result, the yield strength was low at less than 655 MPa.

In test numbers 25 and 26, the average heating rate HR1 at 700-900°C during solution treatment was too slow. This resulted in a low undissolved Nb/Al ratio. As a result, the yield strength was low at less than 655 MPa.

In test numbers 27 and 28, the crush amount δc was too small during straightening. Therefore, the non-solubilized Nb/Al ratio was low. As a result, the yield strength was low at less than 655 MPa.

The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by appropriately modifying the above-described embodiments within the scope of the present disclosure.

Claims (2)

The chemical composition, in mass%, is
C: 0.030% or less,
Si: 0.20-1.00%,
Mn: 0.5-7.0%,
P: 0.040% or less,
S: 0.020% or less,
Al: 0.100% or less,
Ni: 4.0-9.0%,
Cr: 20.0-30.0%,
Mo: 0.5-2.0%,
Cu: 1.5-3.0%,
N: 0.15-0.30%,
V: 0.01-0.50%,
Nb: 0.030-0.300%,
Co: 0.10-0.50%,
Sn: 0.001 to 0.050%,
Ta: 0-0.100%,
Ti: 0 to 0.100%,
Zr: 0 to 0.100%,
Hf: 0-0.100%,
W: 0-0.200%,
Sb: 0 to 0.100%,
Ca: 0-0.020%,
Mg: 0 to 0.020%,
B: 0 to 0.020%,
Rare earth elements: 0 to 0.200%, and
The balance is Fe and impurities,
The microstructure is composed of, by volume, 35.0 to 65.0% ferrite, 0 to less than 1.0% σ phase, and the remainder austenite;
The undissolved Nb content is 0.008% or more by mass%, and the ratio of the undissolved Nb content to the undissolved Al content is 1.0 or more;
The yield strength is 655 MPa or more.
Duplex stainless steel pipe. 2. The duplex stainless steel pipe of claim 1,
The chemical composition is
Ta: 0.001 to 0.100%,
Ti: 0.001 to 0.100%,
Zr: 0.001 to 0.100%,
Hf: 0.001-0.100%,
W: 0.001-0.200%,
Sb: 0.001 to 0.100%,
Ca: 0.001-0.020%,
Mg: 0.001-0.020%,
B: 0.001 to 0.020%, and
Rare earth elements: 0.001 to 0.200%; containing one or more elements selected from the group consisting of:
Duplex stainless steel pipe.

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