STAINLESS STEEL SEAMLESS PIPE AND METHOD FOR MANUFACTURING SAME

Abstract

A stainless steel seamless pipe has a component composition containing, in terms of mass%, C: 0.06% or less, Si: 1.0% or less, Mn: 0.01% or more and 0.90% or less, P: 0.05% or less, S: 0.005% or less, Cr: 15.70% or more and 18.00% or less, Mo: 1.60% or more and 3.80% or less, Cu: 1.10% or more and 4.00% or less, Ni: 3.0% or more and 6.0% or less, AI: 0.10% or less, N: 0.10% or less, O: 0.010% or less, V: 0.120% or more and 1.000% or less, C, Si, Mn, Cr, Ni, Mo, Cu, and N satisfying a predetermined formula, and a remainder consisting of Fe and unavoidable impurities, and has a microstructure including 30% or more of a martensitic phase, 60% or less of a ferritic phase, and 40% or less of a retained austenitic phase, and a yield strength is 758 MPa or higher.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This is the U.S. National Phase application of PCT/JP2021/022014, filed Jun. 10, 2021 which claims priority to Japanese Patent Application No. 2020-116091, filed Jul. 6, 2020, the disclosures of these applications being incorporated herein by reference in their entireties for all purposes.

FIELD OF THE INVENTION

The present invention relates to a stainless steel seamless pipe suitable for use in oil wells and gas wells (hereinafter, simply referred to as oil wells). The present invention relates particularly to a stainless steel seamless pipe having improved corrosion resistance under severely corrosive high-temperature environments containing carbon dioxide (CO₂) or chlorine ion (Cl^-), environments containing hydrogen sulfide (H₂S), and the like and having an improved strength at high temperatures.

BACKGROUND OF THE INVENTION

In recent years, from the viewpoint of the depletion of energy resources, which is expected in the near future, development of oil wells under severely corrosive environments such as environments including a deep oil field or containing carbon dioxide and environment containing hydrogen sulfide, which are referred to as sour environments, which have not been addressed in the related art, is actively underway. Steel pipes for an oil well that are used under such environments are required to have a high strength and high corrosion resistance.

In the related art, 13 Cr martensitic stainless steel pipes have been ordinarily used as steel pipes for an oil well that are used for mining in oil fields and gas fields under environments containing CO₂, Cl^-, and the like. However, recently, development of oil wells that are used at higher temperatures (high temperatures of up to 200° C.) has been underway, and there is a case where 13 Cr martensitic stainless steel pipes lack in corrosion resistance. There is a demand for steel pipes for an oil well having high corrosion resistance that can be used even under such environments.

In response to such a demand, there are, for example, techniques listed in Patent Literature 1 to Patent Literature 5. Patent Literature 1 describes a stainless steel for an oil well having a composition containing, in terms of mass%, C: 0.05% or less, Si: 1. 0 % or less, Mn: 0.01 to 1.0 %, P: 0.05% or less, S: less than 0.002%, Cr: 16 to 18%, Mo: 1.8 to 3%, Cu: 1.0 to 3.5%, Ni: 3.0 to 5.5%, Co: 0.01 to 1.0 %, Al: 0.001 to 0.1%, O: 0.05% or less, and N: 0.05% or less, in which Cr, Ni, Mo, and Cu satisfy a specific relationship.

In addition, Patent Literature 2 describes a high strength stainless steel seamless pipe for an oil well having a composition containing, in terms of mass%, C: 0.05% or less, Si: 1.0% or less, Mn: 0.1 to 0.5%, P: 0.05% or less, S: less than 0.005%, Cr: more than 15.0% and 19.0% or less, Mo: more than 2.0% and 3.0% or less, Cu: 0.3 to 3.5%, Ni: 3.0% or more and less than 5.0%, W: 0.1 to 3.0%, Nb: 0.07 to 0.5%, V: 0.01 to 0.5%, Al: 0.001 to 0.1%, N : 0.010 to 0.100%, and O: 0.01% or less, in which Nb, Ta, C, N, and Cu satisfy a specific relationship, and having a microstructure including, in terms of volume fraction, 45% or more of a tempered martensitic phase, 20 to 40% of a ferritic phase, and more than 10% and 25% or less of a retained austenitic phase. This is considered to make it possible to obtain a high strength stainless steel seamless pipe for an oil well exhibiting a strength of 862 MPa or higher in terms of yield strength (YS) and sufficient corrosion resistance even under severely corrosive high-temperature environments containing CO₂, Cl^-, and H₂S.

In addition, Patent Literature 3 describes a high strength stainless steel seamless pipe for an oil well having a composition containing, in terms of mass%, C: 0.005 to 0.05%, Si: 0.05 to 0.50%, Mn: 0.20 to 1.80%, P: 0.030% or less, S: 0.005% or less, Cr: 12.0 to 17.0%, Ni: 4.0 to 7.0%, Mo: 0.5 to 3.0%, Al: 0.005 to 0.10%, V: 0.005 to 0.20%, Co: 0.01 to 1.0%, N: 0.005 to 0.15%, and O: 0.010% or less, in which Cr, Ni, Mo, Cu, C, Si, Mn, and N satisfy a specific relationship.

In addition, Patent Literature 4 describes a high strength stainless steel seamless pipe for an oil well having a composition containing, in terms of mass%, C: 0.05% or less, Si: 0.5% or less, Mn: 0.15 to 1.0%, P: 0.030% or less, S: 0.005% or less, Cr: 14.5 to 17.5%, Ni: 3.0 to 6.0%, Mo: 2.7 to 5.0%, Cu: 0.3 to 4.0%, W: 0.1 to 2.5%, V: 0.02 to 0.20%, Al: 0.10% or less, and N: 0.15% or less, in which C, Si, Mn, Cr, Ni, Mo, Cu, N, and W satisfy a specific relationship, and having a microstructure containing, in terms of volume fraction, more than 45% of a martensitic phase as the main phase, 10 to 45% of a ferritic phase as the second phase, and 30% or less of a retained austenitic phase. This is considered to make it possible to obtain a high strength stainless steel seamless pipe for an oil well exhibiting a strength of 862 MPa or higher in terms of yield strength (YS) and sufficient corrosion resistance even under severely corrosive high-temperature environments containing CO₂, Cl^-, and H₂S.

In addition, Patent Literature 5 describes a high strength stainless steel seamless pipe for an oil well having a composition containing, in terms of mass%, C: 0.05% or less, Si: 0.5% or less, Mn: 0.15 to 1.0%, P: 0.030% or less, S: 0.005% or less, Cr: 14.5 to 17.5%, Ni: 3.0 to 6.0%, Mo: 2.7 to 5.0%, Cu: 0.3 to 4.0%, W: 0.1 to 2.5%, V: 0.02 to 0.20%, Al: 0.10% or less, N: 0.15% or less, and B: 0.0005 to 0.0100%, in which C, Si, Mn, Cr, Ni, Mo, Cu, N, and W satisfy a specific relationship, and having a microstructure containing, in terms of volume fraction, more than 45% of a martensitic phase as the main phase, 10 to 45% of a ferritic phase as the second phase, and 30% or less of a retained austenitic phase. This is considered to make it possible to obtain a high strength stainless steel seamless pipe for an oil well exhibiting a strength of 862 MPa or higher in terms of yield strength (YS) and sufficient corrosion resistance even under severely corrosive high-temperature environments containing CO₂, Cl^-, and H₂S.

Patent Literature

[PTL 1] Pamphlet of International Publication No. WO 2013/146046

[PTL 2] Pamphlet of International Publication No. WO 2017/138050

[PTL 3] Pamphlet of International Publication No. WO 2017/168874

[PTL 4] Pamphlet of International Publication No. WO 2018/020886

[PTL 5] Pamphlet of International Publication No. WO 2018/155041

SUMMARY OF THE INVENTION

As described above, since development of oil wells that are used at higher temperatures has been underway, it is considered that, for steel pipes for an oil well, there is a demand for having all of a high strength, excellent carbon dioxide corrosion resistance under severely corrosive high-temperature environments containing CO₂ and Cl^-, and furthermore, excellent sulfide stress cracking resistance (SSC resistance).

In addition, in the case of being used at high temperatures, there is a case where steel pipes for an oil well are required to also have a strength at high temperatures (elevated temperature strength). Specifically, there is a case where the ratio of the yield stress (0.2% proof stress) at 200° C. to the yield stress (0.2% proof stress) at room temperature is required to be 0.85 or more.

Patent Literature 1 to Patent Literature 5 disclose stainless steels having improved corrosion resistance, but there is a case where the stainless steels are not satisfactory in terms of having all of corrosion resistance at high temperatures, high sulfide stress cracking resistance, and a high elevated temperature strength.

Aspects of the present invention intend to solve such a problem of the related art, and an object according to aspects of the present invention is to provide a stainless steel seamless pipe having a high strength of 758 MPa (110 ksi) or higher in terms of yield strength, excellent corrosion resistance, and an excellent elevated temperature strength and a method for manufacturing the same.

The expression “excellent corrosion resistance” mentioned herein refers to a case where the stainless steel seamless pipe has “excellent carbon dioxide corrosion resistance” and “excellent sulfide stress cracking resistance”.

The expression “excellent carbon dioxide corrosion resistance” mentioned herein refers to a case where the corrosion rate is 0.127 mm/y or slower in a test in which a test piece is immersed in a test solution held in an autoclave: 20 mass% NaCl aqueous solution (liquid temperature: 200° C., CO₂ gas atmosphere of 30 atm) for an immersion time set to 336 hours.

In addition, the expression “excellent sulfide stress cracking resistance” mentioned herein refers to a case where a test piece does not break or crack after a test that is carried out by immersing the test piece in an aqueous solution having a pH adjusted to 3.0 by adding acetic acid and sodium acetate to a test solution held in an autoclave: 0.165 mass% NaCl aqueous solution (liquid temperature: 25° C., CO₂ gas of 0.99 atm, H₂S atmosphere of 0.01 atm) and exposing the test piece for 720 hours in a state in which 90% of the yield stress is applied to the test piece.

In addition, the expression “excellent elevated temperature strength” mentioned herein refers to a case where the ratio of the yield stress (0.2% proof stress) at 200° C. to the yield stress (0.2% proof stress) at room temperature is 0.85 or more after carrying out a tensile test based on the specification of JIS Z 2241 and an elevated temperature tensile test based on the specification of JIS G 0567.

The method of each test described above is also described in detail in examples described below.

In order to achieve the above-described object, the present inventors carried out intensive studies regarding a variety of factors affecting the elevated temperature strength and corrosion resistance of stainless steel. As a result, it was possible to obtain an excellent elevated temperature strength by containing a predetermined amount or more of V. In addition, it was possible to obtain excellent corrosion resistance (excellent carbon dioxide corrosion resistance and excellent sulfide stress cracking resistance) by containing a predetermined amount or more of Cr, Mo, and Cu and, additionally, setting the Mn content to a certain amount or less.

Aspects of the present invention have been completed by carrying out additional studies based on such findings, and are as follows.

A stainless steel seamless pipe having a component composition containing, in terms of mass%:

• C: 0.06% or less;
• Si: 1.0% or less;
• Mn: 0.01% or more and 0.90% or less;
• P: 0.05% or less;
• S: 0.005% or less;
• Cr: 15.70% or more and 18.00% or less;
• Mo: 1.60% or more and 3.80% or less;
• Cu: 1.10% or more and 4.00% or less;
• Ni: 3.0% or more and 6.0% or less;
• Al: 0.10% or less;
• N: 0.10% or less;
• 0: 0.010% or less;
• V: 0.120% or more and 1.000% or less,
• C, Si, Mn, Cr, Ni, Mo, Cu, and N satisfying Formula (1) below; and
• a remainder consisting of Fe and unavoidable impurities,
• in which the stainless steel seamless pipe has a microstructure including, in terms of volume fraction, 30% or more of a martensitic phase, 60% or less of a ferritic phase, and 40% or less of a retained austenitic phase, and
• a yield strength is 758 MPa or higher.
• $\begin{array}{cc}\begin{array}{l}13.0\le -5.9\times (7.82+27C-0.91Si+0.21Mn-0.9Cr+\\ Ni-1.1Mo+0.2Cu+11N)\le 50.0\end{array}& \text{­­­(1)}\end{array}$

Here, each of C, Si, Mn, Cr, Ni, Mo, Cu, and N is the content (mass%) of each element and is regarded as zero in a case of being not contained.

The stainless steel seamless pipe according to [1], in which the volume fraction of the martensitic phase is 40% or more, the volume fraction of the retained austenitic phase is 30% or less, and the yield strength is 862 MPa or higher.

The stainless steel seamless pipe according to [1] or [2], further contains, in addition to the component composition, in terms of mass%, one group or two or more groups selected from Group A to Group D below:

• Group A: W: 3.0% or less,
• Group B: Nb: less than 0.10%,
• Group C: one or more selected from B: 0.010% or less, Ta: 0.3% or less, Co: 1.5% or less, Ti: 0.3% or less, and Zr: 0.3% or less, and
• Group D: one or more selected from Ca: 0.01% or less, REM: 0.3% or less, Mg: 0.01% or less, Sn: 1.0% or less, and Sb: 1.0% or less.

A method for manufacturing the stainless steel seamless pipe according to any one of [1] to [3], the method including:

• heating a steel pipe material having the component composition at a temperature within a heating temperature range of 1100° C. to 1350° C. to make the steel pipe material into a seamless steel pipe by hot-working;
• subsequently, carrying out a quenching treatment by reheating the seamless steel pipe to a temperature within a heating temperature range of 850° C. to 1150° C. and cooling the seamless steel pipe to a cooling stop temperature of 50° C. or lower at a cooling rate for air cooling or faster; and
• thereafter, carrying out a tempering treatment by heating the seamless steel pipe to a temperature within a tempering temperature range of 500° C. to 650° C.

According to aspects of the present invention, it is possible to obtain a stainless steel seamless pipe having a high strength of 758 MPa (110 ksi) or higher in terms of yield strength, excellent corrosion resistance, and an excellent elevated temperature strength and a method for manufacturing the same.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Hereinafter, embodiments of the present invention will be described in detail.

A stainless steel seamless pipe according to aspects of the present invention has a component composition containing, in terms of mass%, C: 0.06% or less, XSi: 1.0% or less, Mn: 0.01% or more and 0.90% or less, P: 0.05% or less, S: 0.005% or less, Cr: 15.70% or more and 18.00% or less, Mo: 1.60% or more and 3.80% or less, Cu: 1.10% or more and 4.00% or less, Ni: 3.0% or more and 6.0% or less, Al: 0.10% or less, N: 0.10% or less, O: 0.010% or less, V: 0.120% or more and 1.000% or less, C, Si, Mn, Cr, Ni, Mo, Cu, and N satisfying Formula (1) below, a remainder consisting of Fe and unavoidable impurities, in which the stainless steel seamless pipe has a microstructure including, in terms of volume fraction, 30% or more of a martensitic phase, 60% or less of a ferritic phase, and 40% or less of a retained austenitic phase, a yield strength is 758 MPa or higher.

$\begin{array}{cc}\begin{array}{l}13.0\le -5.9\times (7.82+27\text{C}-0.91\text{Si}+0.21\text{Mn}-0.9\text{Cr}+\\ \text{Ni}-1.1\text{Mo}+0.2\text{Cu}+11\text{N}\le 50.0\end{array}& \text{­­­(1)}\end{array}$

Here, each of C, Si, Mn, Cr, Ni, Mo, Cu, and N is the content (mass%) of each element and is regarded as zero in a case of being not contained.

First, reasons for limiting the component composition of the stainless steel seamless pipe according to aspects of the present invention will be described. Hereinafter, unless particularly otherwise described, “mass%” will be simply expressed as “%”.

C: 0.06% or Less

C is an element that is unavoidably contained in steelmaking processes. When more than 0.06% of C is contained, the corrosion resistance deteriorates. Therefore, the C content is set to 0.06% or less. The C content is preferably 0.05% or less, more preferably 0.04% or less, and still more preferably 0.03% or less. When the decarburization cost is taken into account, the lower limit of the C content is preferably 0.002% and more preferably 0.003% or more.

Si: 1.0% or Less

Si is an element that acts as a deoxidizing agent. However, when more than 1.0% of Si is contained, the hot workability and the corrosion resistance deteriorate. Therefore, the Si content is set to 1.0% or less. The Si content is preferably 0.7% or less, more preferably 0.5% or less, and still more preferably 0.4% or less. The lower limit is not particularly provided as long as a deoxidizing effect can be obtained. However, for the purpose of obtaining a sufficient deoxidizing effect, the Si content is preferably 0.03% or more and more preferably 0.05% or more.

Mn: 0.01% or More and 0.90% or Less

Mn is an element that acts as a deoxidizing material and a desulfurizing material and improves hot workability. In order to obtain the deoxidation and desulfurization effects and also to improve the strength, the Mn content is set to 0.01% or more. On the other hand, when more than 0.90% of Mn is contained, the sulfide stress cracking resistance deteriorates. Therefore, the Mn content is set to 0.01% or more and 0.90% or less. The Mn content is preferably 0.03% or more and more preferably 0.05% or more. In addition, the Mn content is preferably 0.7% or less, more preferably 0.5% or less, and still more preferably 0.4% or less.

P: 0.05% or Less

P is an element that degrades the carbon dioxide corrosion resistance and the sulfide stress cracking resistance and is preferably reduced as much as possible in accordance with aspects of the present invention, but 0.05% or less of P is acceptable. Therefore, the P content is set to 0.05% or less. The P content is preferably 0.04% or less, more preferably 0.03% or less, and still more preferably 0.02% or less.

S: 0.005% or Less

S is an element that significantly degrades the hot workability and impairs stable operation in hot pipe making processes. In addition, S is present as a sulfide-based inclusion in steel and degrades the sulfide stress cracking resistance. Therefore, S is preferably reduced as much as possible, but 0.005% or less of S is acceptable. Therefore, the S content is set to 0.005% or less. The S content is preferably 0.004% or less, more preferably 0.003% or less, and still more preferably 0.002% or less.

Cr: 15.70% or More and 18.00% or Less

Cr is an element that forms a protective film on the surface of the steel pipe and contributes to improvement in the corrosion resistance. When the Cr content is less than 15.70%, it is not possible to secure desired carbon dioxide corrosion resistance and desired sulfide stress cracking resistance. Therefore, it is necessary to contain 15.70% or more of Cr. On the other hand, when more than 18.00% of Cr is contained, the ferrite fraction excessively increases, which makes it impossible to secure a desired strength. Therefore, the Cr content is set to 15.70% or more and 18.00% or less. The Cr content is preferably 16.00% or more and more preferably 16.30% or more. In addition, the Cr content is preferably 17.50% or less and more preferably 17.00% or less.

Mo: 1.60% or More and 3.80% or Less

Mo stabilizes the protective film on the surface of the steel pipe, increases resistance to pitting corrosion caused by Cl^- or a low pH, and enhances the carbon dioxide corrosion resistance and the sulfide stress cracking resistance. In order to obtain a desired corrosion resistance, it is necessary to contain 1.60% or more of Mo. On the other hand, when more than 3.80% of Mo is added, the ferrite fraction excessively increases, which makes it impossible to secure a desired strength. Therefore, the Mo content is set to 1.60% or more and 3.80% or less. The Mo content is preferably 1.80% or more and more preferably 2.00% or more. In addition, the Mo content is preferably 3.5% or less, more preferably 3.0% or less, and still more preferably 2.8% or less.

Cu: 1.10% or More and 4.00% or Less

Cu has effects of strengthening the protective film on the surface of the steel pipe and enhancing the carbon dioxide corrosion resistance and the sulfide stress cracking resistance. In order to obtain a desired strength and desired corrosion resistance, particularly, desired carbon dioxide corrosion resistance, it is necessary to contain 1.10% or more of Cu. On the other hand, when the content excessively increases, since the hot workability of steel deteriorates, the Cu content is set to 4.00% or less. Therefore, the Cu content is set to 1.10% or more and 4.00% or less. The Cu content is preferably 1.80% or more and more preferably 2.00% or more. In addition, the Cu content is preferably 3.20% or less, more preferably 3.00% or less, and still more preferably 2.7% or less.

Ni: 3.0% or More and 6.0% or Less

Ni increases the strength of steel by solid solution strengthening and improves the toughness of steel. In order to secure toughness that is required for oil well pipes, it is necessary to contain 3. 0% or more of Ni. On the other hand, when more than 6.0% of Ni is contained, the stability of a martensitic phase deteriorates, and the strength deteriorates. Therefore, the Ni content is set to 3.0% or more and 6.0% or less. The Ni content is preferably 3.5% or more, more preferably 4.0% or more, and still more preferably 4.5% or more. In addition, the Ni content is preferably 5.5% or less and more preferably 5.2% or less.

Al: 0.10% or Less

Al is an element that acts as a deoxidizing agent. However, when more than 0.10% of Al is contained, the corrosion resistance deteriorates. Therefore, the Al content is set to 0.10% or less. The Al content is preferably 0.07% or less, more preferably 0.05% or less, and still more preferably 0.04% or less. The lower limit is not particularly provided as long as a deoxidizing effect can be obtained. However, for the purpose of obtaining a sufficient deoxidizing effect, the Al content is preferably 0.005% or more and more preferably 0.01% or more.

N: 0.10% or Less

N is an element that is unavoidably contained in steelmaking processes, but also an element that increases the strength of steel. However, when more than 0.10% of N is contained, a nitride is formed to degrade the corrosion resistance. Therefore, the N content is set to 0.10% or less. The N content is preferably 0.08% or less, more preferably 0.05% or less, and still more preferably 0.03% or less. The lower limit value of the N content is not particularly provided, but the excessive reduction of the N content causes an increase in the steelmaking cost. Therefore, the N content is preferably 0.002% or more and more preferably 0.003% or more.

O: 0.010% or Less

Oxygen (O) is present as an oxide in steel and thus adversely affects a variety of characteristics. Therefore, in accordance with aspects of the present invention, oxygen is desirably reduced as much as possible. In particular, when O content is more than 0.010%, the hot workability and the corrosion resistance deteriorate. Therefore, the O content is set to 0.010% or less. The O content is preferably 0.005% or less.

V: 0.120% or More and 1.000% or Less,

V is an important element in accordance with aspects of the present invention that improves the elevated temperature strength. V forms a carbonitride and enables a high strength to be obtained not only at room temperature but also at high temperatures by precipitation strengthening. In order to obtain a desired elevated temperature strength, 0.120% or more of V is contained. On the other hand, even when more than 1.000% of V is contained, the effect is saturated. Therefore, in accordance with aspects of the present invention, the V content is set to 0.120% or more and 1.000% or less. In addition, the V content is preferably 0.180% or more, more preferably 0.250% or more, and still more preferably 0.300% or more. In addition, the V content is preferably 0.500% or less, more preferably 0.400% or less, and still more preferably 0.300% or less.

In accordance with aspects of the present invention, the stainless steel seamless pipe contains the elements such that the above-described component composition is satisfied and, furthermore, C, Si, Mn, Cr, Ni, Mo, Cu, and N satisfy Formula (1) below.

$\begin{array}{cc}\begin{array}{l}13.0\le -5.9\times (7.82+27C-0.91Si+0.21Mn-0.9Cr+\\ Ni-1.1Mo+0.2Cu+11N)\le 50.0\end{array}& \text{­­­(1)}\end{array}$

Here, each of C, Si, Mn, Cr, Ni, Mo, Cu, and N is the content (mass%) of each element and is regarded as zero in a case of being not contained.

The expression “-5.9 × (7.82 + 27C - 0.91Si + 0.21Mn -0.9Cr + Ni - 1.1Mo + 0.2Cu + 11N)” in Formula (1) (hereinafter, also simply referred to as “the central polynomial expression of Formula (1)” or “central value”) is obtained as an index that indicates the formation tendency of a ferritic phase. When the alloy element shown in Formula (1) is contained in an adjusted manner so as to satisfy Formula (1), it is possible to stably realize a multi-phase microstructure including a martensitic phase and a ferritic phase or a martensitic phase, a ferritic phase, and a retained austenitic phase. In a case where an alloy element shown in Formula (1) is not contained, the content of the corresponding element is treated as 0% in the value of the central polynomial expression of Formula (1)

When the value of the central polynomial expression of Formula (1) is less than 13.0, a ferritic phase decreases in a hot working temperature range, and the yield at the time of manufacturing the stainless steel seamless pipe is decreased. On the other hand, when the value of the central polynomial expression of Formula (1) exceeds 50.0, the ferritic phase exceeds 60% in terms of the volume fraction, and it becomes impossible to secure a desired strength. Therefore, in Formula (1) specified in accordance with aspects of the present invention, the value of the left side, which serves as the lower limit, is set to 13.0, and the value of the right side, which serves as the upper limit, is set to 50.0. The value of the left side, which serves as the lower limit, in Formula (1) specified in accordance with aspects of the present invention is preferably 15.0 and more preferably 20.0. In addition, the value of the right side is preferably 45.0 and more preferably 40.0. That is, the value of the central polynomial expression in Formula (1) is set to 13.0 or more and set to 50.0 or less. The value is preferably set to 15.0 or more and set to 45.0 or less. The value is more preferably set to 20.0 or more and set to 40.0 or less.

In accordance with aspects of the present invention, the remainder other than the above-described component composition includes Fe and unavoidable impurities.

The above-described essential elements make it possible for the stainless steel seamless pipe according to aspects of the present invention to obtain target characteristics. In accordance with aspects of the present invention, for the purpose of further improving the characteristics, in addition to the above-described basic component composition, one or more of the following selective elements (W, Nb, B, Ta, Co, Ti, Zr, Ca, REM, Mg, Sn, and Sb) may be contained as necessary.

Specifically, in accordance with aspects of the present invention, it is possible to contain, in addition to the above-described component composition, W: 3.0% or less.

In addition, in accordance with aspects of the present invention, it is possible to contain, in addition to the above-described component composition, Nb: less than 0.10%.

Furthermore, in accordance with aspects of the present invention, it is possible to contain, in addition to the above-described component composition, one or more selected from B: 0.010% or less, Ta: 0.3% or less, Co: 1.5% or less, Ti: 0.3% or less, and Zr: 0.3% or less.

Furthermore, in accordance with aspects of the present invention, it is possible to contain, in addition to the above-described component composition, one or more selected from Ca: 0.01% or less, REM: 0.3% or less, Mg: 0.01% or less, Sn: 1.0% or less, and Sb: 1.0% or less.

W: 3.0% or Less

W is an element that contributes to improvement in the strength of steel and is capable of enhancing the carbon dioxide corrosion resistance and the sulfide stress cracking resistance by stabilizing the protective film on the surface of the steel pipe. W significantly improves, particularly, the corrosion resistance when contained in combination with Mo. W can be contained as necessary in order to obtain the above-described effects. On the other hand, even when more than 3.0% of W is contained, the effects are saturated. Therefore, in the case of containing W, the W content is preferably set to 3.0% or less. The W content is more preferably less than 1.5% and still more preferably 1.0% or less. In addition, in the case of containing W, the W content is more preferably 0.05% or more and still more preferably 0.10% or more.

Nb: Less Than 0.10%

Nb is an element that increases the strength and also an element that improves the corrosion resistance and can be contained as necessary. On the other hand, when 0.10% or more of Nb is contained, there is a case where a desired elevated temperature strength cannot be obtained. Therefore, in the case of containing Nb, the Nb content is preferably set to less than 0.10%. The Nb content is more preferably 0.05% or less and still more preferably 0.03% or less. In addition, the Nb content is more preferably 0.005% or more and still more preferably 0.010% or more.

B: 0.010% or Less

B is an element that increases the strength and can be contained as necessary. In addition, B also contributes to improvement in the hot workability and also has an effect of suppressing the occurrence of fissuring or cracking in pipe making processes. On the other hand, even when more than 0.010% of B is contained, the hot workability improvement effect rarely appears, and the low-temperature toughness deteriorates. Therefore, in the case of containing B, the B content is preferably set to 0.010% or less. The B content is more preferably 0.008% or less and still more preferably 0.007% or less. In addition, the B content is more preferably 0.0005% or more and still more preferably 0.0010% or more.

Ta: 0.3% or Less

Ta is an element that increases the strength and also an element that improves the corrosion resistance and can be contained as necessary. In order to obtain such an effect, 0.001% or more of Ta is preferably contained. On the other hand, even when more than 0.3% of Ta is contained, the effect is saturated. Therefore, in the case of containing Ta, the Ta content is preferably limited to 0.3% or less. The Ta content is more preferably 0.25% or less, still more preferably 0.06% or less, more preferably 0.050% or less, and still more preferably 0.025% or less. The Ta content is more preferably 0.005% or more.

Co: 1.5% or Less

Co is an element that increases the strength and can be contained as necessary. In addition to the above-described effect, Co also has an effect of improving the corrosion resistance. In order to obtain such an effect, 0.0005% or more of Co is preferably contained. The Co content is more preferably 0.005% or more and still more preferably 0.010% or more. On the other hand, even when more than 1.5% of Co is contained, the effect is saturated. Therefore, in the case of containing Co, the Co content is preferably limited to 1.5% or less. The Co content is more preferably less than 0.150%.

Ti: 0.3% or Less

Ti is an element that increases the strength and can be contained as necessary. In order to obtain such an effect, 0.0005% or more of Ti is preferably contained. On the other hand, when more than 0.3% of Ti is contained, the toughness deteriorates. Therefore, in the case of containing Ti, the Ti content is preferably limited to 0.3% or less.

Zr: 0.3% or Less

Zr is an element that increases the strength and can be contained as necessary. In addition to the above-described effect, Zr also has an effect of improving the sulfide stress cracking resistance. In order to obtain such an effect, 0.0005% or more of Zr is preferably contained. On the other hand, even when more than 0.3% of Zr is contained, the effect is saturated. Therefore, in the case of containing Zr, the Zr content is preferably limited to 0.3% or less.

Ca: 0.01% or Less

Ca is an element that contributes to improvement in the sulfide stress cracking resistance by the control of the form of a sulfide and can be contained as necessary. In order to obtain such an effect, 0.0005% or more of Ca is preferably contained. On the other hand, even when more than 0.01% of Ca is contained, the effect is saturated, and it becomes impossible to expect an effect corresponding to the content. Therefore, in the case of containing Ca, the Ca content is preferably limited to 0.01% or less.

REM: 0.3% or Less

REM (rare earth metal) is an element that contributes to improvement in the sulfide stress cracking resistance by the control of the form of a sulfide and can be contained as necessary. In order to obtain such an effect, 0.0005% or more of REM is preferably contained. On the other hand, even when more than 0.3% of REM is contained, the effect is saturated, and it becomes impossible to expect an effect corresponding to the content. Therefore, in the case of containing REM, the REM content is preferably limited to 0.3% or less.

The REM mentioned in accordance with aspects of the present invention refers to lanthanoid elements of scandium (Sc) (atomic number: 21) and yttrium (Y) (atomic number: 39), and lanthanum (La) (atomic number: 57) to lutetium (Lu) (atomic number: 71). The REM concentration in accordance with aspects of the present invention is the total content of one or more elements selected from the above-described REMs.

Mg: 0.01% or Less

Mg is an element that improves the corrosion resistance and can be contained as necessary. In order to obtain such an effect, 0.0005% or more of Mg is preferably contained. On the other hand, even when more than 0.01% of Mg is contained, the effect is saturated, and it becomes impossible to expect an effect corresponding to the content. Therefore, in the case of containing Mg, the Mg content is preferably limited to 0.01% or less.

Sn: 1.0% or Less

Sn is an element that improves the corrosion resistance and can be contained as necessary. In order to obtain such an effect, 0.001% or more of Sn is preferably contained. On the other hand, even when more than 1.0% of Sn is contained, the effect is saturated, and it becomes impossible to expect an effect corresponding to the content. Therefore, in the case of containing Sn, the Sn content is preferably limited to 1.0% or less.

Sb: 1.0% or Less

Sb is an element that improves the corrosion resistance and can be contained as necessary. In order to obtain such an effect, 0.001% or more of Sb is preferably contained. On the other hand, even when more than 1.0% of Sb is contained, the effect is saturated, and it becomes impossible to expect an effect corresponding to the content. Therefore, in the case of containing Sb, the Sb content is preferably limited to 1.0% or less.

Next, reasons for limiting the microstructure of the stainless steel seamless pipe according to aspects of the present invention will be described.

The stainless steel seamless pipe according to aspects of the present invention has the above-described component composition and has a microstructure including, in terms of volume fraction, 30% or more of a martensitic phase, 60% or less of a ferritic phase, and 40% or less of a retained austenitic phase.

In the stainless steel seamless pipe according to aspects of the present invention, in order to secure a desired strength, a martensitic phase is set to 30% or more in terms of the volume fraction. The martensitic phase is preferably set to 40% or more. The martensitic phase is preferably set to 70% or less and more preferably set to 65% or less.

In addition, in accordance with aspects of the present invention, the volume fraction of ferritic phase included is 60% or less. When a ferritic phase is included, it is possible to suppress the propagation of sulfide stress cracking, and excellent corrosion resistance can be obtained. On the other hand, when the volume fraction exceeds 60%, and a large amount of a ferritic phase is precipitated, there is a case where it becomes impossible to secure a desired strength. The ferritic phase is preferably 5% or more in terms of the volume fraction. The ferritic phase is more preferably 10% or more. In addition, the ferritic phase is preferably 50% or less in terms of the volume fraction. The ferritic phase is more preferably 45% or less.

Furthermore, in accordance with aspects of the present invention, in addition to the martensitic phase and the ferritic phase, 40% or less of an austenitic phase (retained austenitic phase) is included in terms of the volume fraction. The presence of the retained austenitic phase improves the ductility and the toughness. On the other hand, when a large amount of an austenitic phase is precipitated such that the volume fraction exceeds 40%, it becomes impossible to secure a desired strength. Therefore, the retained austenitic phase is set to 40% or less in terms of the volume fraction. The retained austenitic phase is preferably 5% or more in terms of the volume fraction. In addition, the retained austenitic phase is 35% or less in terms of the volume fraction. The retained austenitic phase is more preferably 30% or less in terms of the volume fraction.

Here, the above-described microstructure of the stainless steel seamless pipe according to aspects of the present invention can be measured by the following method. First, a test piece for microstructure observation is corroded with a Vilella’s reagent (a reagent obtained by mixing picric acid, hydrochloric acid, and ethanol in fractions of 2 g, 10 ml, and 100 ml, respectively), an image of the microstructure is captured with a scanning electron microscope (magnification: 1000 times), and the microstructural fraction (area ratio (%)) of the ferritic phase is calculated using an image analyzer. This area ratio is defined as the volume fraction (%) of the ferritic phase.

In addition, a test piece for X-ray diffraction is ground and polished such that a cross section orthogonal to the pipe axial direction (C cross section) becomes a measurement surface, and the structural fraction of the retained austenitic (γ) phase is measured using the X-ray diffraction method. The microstructural fraction of the retained austenitic phase is obtained by measuring the diffraction X-ray integrated intensities of the (220) plane of γ and the (211) plane of α (ferrite) and converting the diffraction X-ray integrated intensities using the following equation.

$\text{γ}\left(\text{volume fraction}\right)=100/\left(1+\left(\text{I}\text{α}\text{R}\text{γ}/\text{I}\text{γ}\text{R}\text{α}\right)\right)$

Here, Iα indicates the integrated intensity of α, Rα indicates the crystallographic theoretical calculation value of α, Iγ indicates the integrated intensity of γ, and Rγ indicates the crystallographic theoretical calculation value of γ.

Further, the remainder other than the ferritic phase and the retained γ phase obtained by the above-described measurement method is defined as the fraction of the martensitic phase.

Hereinafter, a preferable manufacturing method for the stainless steel seamless pipe according to aspects of the present invention will be described.

It is preferable that molten steel having the above-described component composition is melted by a common melting method such as a converter and made into a steel pipe material such as a billet by an ordinary method such as a continuous casting method or an ingot-blooming method. The heating temperature of the steel pipe material before hot working is preferably 1100° C. to 1350° C. In such a case, it is possible to satisfy both hot workability during pipe making and the low-temperature toughness of a final product.

Next, the obtained steel pipe material is made into a pipe by hot working using a Mannesmann plug mill-type or Mannesmann mandrel mill-type step, which is an ordinary well-known pipe-making method, thereby producing a seamless steel pipe having desired dimensions and the above-described composition. After the hot working, a cooling treatment may be carried out. There is no particular need to limit this cooling treatment (cooling step). After the hot working, it is preferable to cool the pipe to room temperature at approximately a cooling rate for air cooling as long as the pipe remains within the above-described component composition range according to aspects of the present invention.

In accordance with aspects of the present invention, a heat treatment including a quenching treatment and a tempering treatment is further carried out on the obtained seamless steel pipe.

The quenching treatment is a treatment in which the seamless steel pipe is reheated to a temperature within a heating temperature range of 850° C. to 1150° C. and then cooled at a cooling rate for air cooling or faster. The cooling stop temperature at this time is 50° C. or lower in terms of the surface temperature of the seamless steel pipe.

When the heating temperature is lower than 850° C., reverse transformation from martensite to austenite does not occur, transformation from austenite to martensite does not occur during cooling, and a desired strength cannot be secured. On the other hand, when the heating temperature exceeds 1150° C., and the seamless steel pipe reaches a high temperature, crystal grains become coarse. Therefore, the heating temperature of the quenching treatment is set to a temperature within a range of 850° C. to 1150° C. The heating temperature of the quenching treatment is preferably 900° C. or higher. The heating temperature of the quenching treatment is preferably 1100° C. or lower. In addition, when the cooling stop temperature exceeds 50° C., transformation from austenite to martensite does not sufficiently occur, and the retained austenite fraction becomes excessive. Therefore, in accordance with aspects of the present invention, the cooling stop temperature during the cooling in the quenching treatment is set to 50° C. or lower. Here, the expression “cooling rate for air cooling or faster” is 0.01° C./s or faster.

In addition, in the quenching treatment, the soaking time is preferably set to 5 to 30 minutes in order to uniform the temperature in the wall thickness direction and prevent variations in the quality of the material.

The tempering treatment is a treatment in which the seamless steel pipe that has undergone the quenching treatment is heated to a tempering temperature of 500° C. to 650° C. In addition, after this heating, it is possible to cool the seamless steel pipe in the air.

When the tempering temperature is lower than 500° C., the tempering temperature is too low, which makes it impossible to expect a desired tempering effect. On the other hand, when the tempering temperature is a high temperature exceeding 650° C., an intermetallic compound is precipitated, which makes it impossible to obtain excellent low-temperature toughness. Therefore, the tempering temperature is set to a temperature within a range of 500° C. to 650° C. The tempering temperature is preferably 520° C. or higher. The tempering temperature is preferably 630° C. or lower.

In addition, in the tempering treatment, the holding time (soaking holding time) is preferably set to 5 to 90 minutes in order to uniform the temperature in the wall thickness direction and prevent variations in the quality of the material.

When the above-described heat treatment (the quenching treatment and the tempering treatment) is carried out, the microstructure of the seamless steel pipe becomes a microstructure including a martensitic phase, a ferritic phase, and a retained austenitic phase that are specified by predetermined volume fractions. This makes it possible to obtain a stainless steel seamless pipe having a desired strength and excellent corrosion resistance.

Hitherto, stainless steel seamless pipes that are obtained in accordance with aspects of the present invention are high-strength steel pipes having a yield strength of 758 MPa or higher and have excellent corrosion resistance and an elevated temperature strength. The yield strength is preferably 862 MPa or higher. The yield strength is preferably 1034 MPa or lower. The stainless steel seamless pipe according to aspects of the present invention can be made into a stainless steel seamless pipe for an oil well (high strength stainless steel seamless pipe for an oil well).

EXAMPLES

Hereinafter, aspects of the present invention will be further described based on examples. The present invention is not limited to the following examples.

Steel pipe materials were cast using molten steels having a component composition shown in Table 1-1 and Table 1-2. After that, the steel pipe materials were heated and made into pipes by hot working using a model seamless rolling mill, thereby producing seamless steel pipes having an outer diameter of 83.8 mm and a wall thickness of 12.7 mm. The seamless steel pipes were air-cooled. At this time, the heating temperature of the steel pipe materials before the hot working was set to 1250° C.

Test piece materials were cut out from the obtained seamless steel pipes, and a quenching treatment was carried out by reheating the test piece materials to a heating temperature of 960° C., holding the test piece materials for a soaking holding time of 20 minutes, and cooling (water-cooling) the test piece materials to a cooling stop temperature of 30° C. In addition, furthermore, a tempering treatment was carried out by holding the test piece materials at a heating temperature (tempering temperature) of 575° C. for a soaking holding time of 20 minutes, at a heating temperature (tempering temperature) of 525° C. for a soaking holding time of 20 minutes, or at a heating temperature (tempering temperature) of 620° C. for a soaking holding time of 40 minutes and then air-cooling the test piece materials. The cooling rate during the water cooling in the quenching treatment was 11° C./s, and the cooling rate during the air cooling (cooling in the air) in the tempering treatment was 0.04° C./s. Blank cells in Table 1-1 and Table 1-2 indicate that the corresponding element is intentionally not added, which includes not only a case where the element is not contained (0%) but also a case where the element is unavoidably contained.

Steel No.	Component composition (mass%)	Formula (1) (*3)	Remarks
C	Si	Mn	P	S	Cr	Mo	Cu	Ni	Al	N	O	V	Others	Central value	Adequacy
A	0.017	0.36	0.261	0.012	0.0009	17.03	2.77	2.15	4.87	0.026	0.020	0.001	0.283		28.6	O	Present Example
B	0.010	0.26	0.331	0.019	0.0010	16.98	2.47	2.77	4.15	0.022	0.019	0.002	0.332		30.5	O	Present Example
C	0.013	0.28	0.384	0.018	0.0013	17.26	2.75	3.16	4.99	0.027	0.017	0.002	0.369		28.0	O	Present Example
D	0.059	0.30	0.271	0.014	0.0012	17.40	2.41	2.43	5.28	0.027	0.020	0.003	0.293		18.5	O	Present Example
E	0.018	0.92	0.221	0.016	0.0015	16.49	2.54	2.82	5.14	0.026	0.010	0.003	0.316		25.4	O	Present Example
F	0.011	0.33	0.837	0.017	0.0012	16.96	2.39	1.86	4.14	0.027	0.012	0.002	0.220		31.0	O	Present Example
G	0.011	0.25	0.023	0.017	0.0010	16.62	2.61	3.13	4.23	0.025	0.015	0.001	0.313		29.0	O	Present Example
H	0.008	0.32	0.148	0.049	0.0015	16.80	2.35	3.05	4.50	0.023	0.018	0.001	0.193		27.3	O	Present Example
I	0.010	0.31	0.190	0.019	0.0041	16.74	2.48	1.85	4.95	0.025	0.016	0.004	0.325		26.3	O	Present Example
J	0.014	0.26	0.283	0.015	0.0015	17.95	2.53	2.80	4.75	0.025	0.011	0.004	0.296		32.4	O	Present Example
K	0.013	0.32	0.310	0.016	0.0014	15.76	2.56	1.86	5.39	0.029	0.022	0.002	0.272		18.0	O	Present Example
L	0.008	0.29	0.203	0.014	0.0011	17.04	3.73	3.18	4.79	0.026	0.015	0.004	0.310		35.6	O	Present Example
M	0.009	0.32	0.183	0.014	0.0010	17.32	1.66	2.68	4.53	0.027	0.022	0.004	0.189		25.3	O	Present Example
N	0.011	0.30	0.234	0.013	0.0015	16.70	2.90	3.81	4.20	0.024	0.018	0.001	0.200		30.5	O	Present Example
O	0.018	0.29	0.259	0.015	0.0010	16.80	2.32	3.15	5.09	0.025	0.012	0.004	0.300		22.0	O	Present Example
P	0.017	0.23	0.374	0.015	0.0012	16.74	2.33	1.18	4.27	0.026	0.012	0.003	0.330		28.6	O	Present Example
Q	0.012	0.36	0.171	0.016	0.0010	16.93	2.53	2.75	5.86	0.023	0.012	0.004	0.300		21.4	O	Present Example
T	0.014	0.30	0.261	0.015	0.0009	16.91	3.27	2.26	3.14	0.026	0.018	0.002	0.320		41.6	O	Present Example
U	0.015	0.29	0.374	0.016	0.0010	16.54	3.00	2.78	4.53	0.089	0.013	0.004	0.282		29.0	O	Present Example
V	0.018	0.29	0.259	0.015	0.0010	16.80	2.32	2.80	5.09	0.025	0.085	0.003	0.279		17.6	O	Present Example
W	0.011	0.31	0.206	0.014	0.0008	16.96	2.48	3.16	5.38	0.024	0.015	0.009	0.321		23.2	O	Present Example
X	0.011	0.27	0.333	0.015	0.0008	16.76	2.23	2.07	5.51	0.024	0.014	0.002	0.913		20.8	O	Present Example
Y	0.008	0.36	0.150	0.016	0.0006	16.52	3.24	3.18	4.82	0.025	0.011	0.003	0.389		30.2	O	Present Example
Z	0.010	0.32	0.258	0.015	0.0011	16.77	2.36	1.92	5.46	0.026	0.017	0.002	0.127		22.4	O	Present Example
AA	0.012	0.35	0.288	0.017	0.0009	17.02	2.80	1.87	5.47	0.026	0.020	0.002	0.185		26.2	O	Present Example
AB	0.002	0.90	0.267	0.015	0.0010	17.78	3.78	1.82	4.29	0.018	0.006	0.001	0.335		49.1	O	Present Example
AC	0.034	0.06	0.426	0.015	0.0008	16.01	1.79	2.65	4.72	0.027	0.007	0.003	0.379		13.4	O	Present Example
AD	0.005	0.30	0.380	0.016	0.0010	16.63	2.54	2.88	5.22	0.026	0.017	0.001	0.330	W:1.41	23.7	O	Present Example
AE	0.008	0.29	0.253	0.016	0.0013	16.55	2.55	1.81	4.46	0.022	0.017	0.002	0.282	Nb:0.07	28.7	O	Present Example
(*1) The remainder other than the above-described components are Fe and unavoidable impurities.
(*2) Underlined values are outside the scope of the invention.
(*3) Formula (1): 13.0 ≤ -5.9 x (7.82 + 27C - 0.91 Si + 0.21 Mn - 0.9Cr + Ni - 1.1 Mo + 0.2Cu + 11N) ≤ 50.0

Steel No.	Component composition (mass%)	Formula (1) (*3)	Remarks
C	Si	Mn	P	S	Cr	Mo	Cu	Ni	Al	N	O	V	Others	Central value	Adequacy
AF	0.009	0.24	0.240	0.013	0.0014	16.70	2.53	3.00	4.11	0.027	0.020	0.003	0.282	B:0.003,Ta:0.25,Co:1.30,Ti:0.22,Zr:0.24	29.4	O	Present Example
AG	0.014	0.23	0.374	0.014	0.0011	17.32	3.23	3.07	4.22	0.028	0.015	0.003	0.250	Ca:0.009,REM:0.22,Mg:0.005,Sn:0.95,Sb:0.93	35.8	O	Present Example
AH	0.013	0.30	0.160	0.017	0.0007	17.01	2.58	3.19	4.56	0.027	0.015	0.003	0.262	W:1.03,Nb:0.05	28.6	O	Present Example
Al	0.010	0.30	0.381	0.015	0.0014	16.93	2.36	2.39	4.15	0.027	0.014	0.002	0.328	W:0.23,B:0.004	30.4	O	Present Example
AJ	0.012	0.30	0.282	0.015	0.0012	16.44	2.86	2.79	4.83	0.027	0.016	0.001	0.379	W:0.25,Sn:0.22	26.2	O	Present Example
AK	0.008	0.36	0.150	0.016	0.0006	16.52	3.24	3.18	4.82	0.025	0.011	0.003	0.338	Nb:0.03,Ta:0.06	30.2	O	Present Example
AL	0.014	0.27	0.265	0.016	0.0008	16.96	3.11	2.76	4.54	0.024	0.011	0.002	0.214	Nb:0.07,Sb:0.27	32.2	O	Present Example
AM	0.011	0.28	0.247	0.016	0.0012	16.64	2.47	3.12	5.31	0.028	0.015	0.002	0.259	Ti:0.25,Ca:0.004	21.7	O	Present Example
AN	0.012	0.25	0.408	0.014	0.0012	16.79	2.45	2.45	4.37	0.025	0.016	0.003	0.231	W:1.35,Nb:0.04,Zr:0.22,REM:0.21	28.1	O	Present Example
AO	0.066	0.33	0.248	0.015	0.0015	17.19	3.00	2.67	4.98	0.027	0.012	0.002	0.255		22.3	O	Comparative Example
AP	0.008	1.28	0.302	0.015	0.0006	16.51	3.15	1.98	4.34	0.024	0.016	0.003	0.366		38.2	O	Comparative Example
AQ	0.005	0.30	0.928	0.016	0.0010	16.63	2.54	2.88	5.22	0.026	0.017	0.001	0.322		23.0	O	Comparative Example
AR	0.014	0.29	0.289	0.056	0.0012	16.53	2.77	1.83	5.17	0.027	0.020	0.002	0.240		24.6	O	Comparative Example
AS	0.014	0.29	0.330	0.016	0.0058	16.60	2.65	2.58	4.63	0.028	0.021	0.003	0.240		26.4	O	Comparative Example
AT	0.017	0.28	0.402	0.016	0.0014	18.13	3.24	3.12	4.64	0.027	0.014	0.003	0.399		37.5	O	Comparative Example
AU	0.012	0.28	0.294	0.017	0.0009	15.52	2.66	2.08	4.77	0.027	0.018	0.003	0.224		21.0	O	Comparative Example
AV	0.014	0.30	0.280	0.017	0.0007	17.36	4.03	2.55	4.30	0.023	0.017	0.003	0.249		41.7	O	Comparative Example
AW	0.009	0.25	0.364	0.014	0.0010	16.50	1.47	1.93	5.23	0.024	0.014	0.003	0.181		16.4	O	Comparative Example
AX	0.014	0.32	0.330	0.016	0.0010	16.85	2.99	1.02	5.40	0.027	0.021	0.002	0.350		27.4	O	Comparative Example
AY	0.014	0.30	0.297	0.015	0.0012	17.36	2.70	3.16	6.17	0.024	0.014	0.003	0.209		21.5	O	Comparative Example
AZ	0.011	0.26	0.388	0.013	0.0008	16.64	2.51	3.08	5.14	0.131	0.013	0.003	0.216		22.9	O	Comparative Example
BA	0.012	0.26	0.262	0.017	0.0011	17.04	2.76	1.84	4.41	0.028	0.115	0.001	0.398		25.8	O	Comparative Example
BB	0.014	0.36	0.274	0.016	0.0013	16.60	2.44	2.43	4.58	0.025	0.014	0.013	0.266		26.4	O	Comparative Example
BC	0.014	0.27	0.265	0.016	0.0008	16.96	3.11	2.76	4.54	0.024	0.011	0.002	0.098		32.2	O	Comparative Example
BD	0.004	0.92	0.299	0.017	0.0014	17.93	3.78	1.80	3.89	0.019	0.014	0.002	0.197		51.6	X	Comparative Example
BE	0.017	0.28	0.350	0.016	0.0010	16.50	2.55	1.65	4.12	0.025	0.013	0.003	0.383	Ca:0.005	29.3	O	Present Example
BF	0.014	0.30	0.253	0.016	0.0007	16.85	2.53	1.88	4.56	0.026	0.014	0.002	0.251	Co:0.05	28.8	O	Present Example
BG	0.012	0.25	0.240	0.015	0.0009	17.36	2.58	3.00	4.35	0.025	0.022	0.002	0.183	Sb:0.02	31.3	O	Present Example
BH	0.011	0.32	0.374	0.013	0.0014	16.64	2.36	3.07	4.83	0.025	0.019	0.002	0.153	W:1.01	23.7	O	Present Example
(*1) The remainder other than the above-described components are Fe and unavoidable impurities.
(*2) Underlined values are outside the scope of the invention.
(*3) Formula (1): 13.0 ≤ 5.9 × (7.82 + 27C - 0.91Si + 0.21 Mn - 0.9Cr + Ni - 1.1 Mo + 0.2Cu + 11N) ≤ 50.0

Test pieces were collected from the obtained heat treatment-finished test piece materials (seamless steel pipes), and microstructure observation, tensile tests, elevated temperature tensile tests, and corrosion resistance tests were carried out. The test methods were as described below.

Microstructure Observation

From each of the obtained heat treatment-finished test material, a test piece for microstructure observation was collected such that a cross section orthogonal to the pipe axial direction became an observation surface. The obtained test piece for microstructure observation was corroded with a Vilella’s reagent (a reagent obtained by mixing picric acid, hydrochloric acid, and ethanol in fractions of 2 g, 10 ml, and 100 ml, respectively), an image of the microstructure was captured with a scanning electron microscope (magnification: 1000 times), and the microstructural fraction (area ratio (%)) of the ferritic phase was calculated using an image analyzer. This area ratio was defined as the volume fraction (%) of the ferritic phase.

In addition, a test piece for X-ray diffraction was collected from the obtained heat treatment-finished test material, ground and polished such that a cross section orthogonal to the pipe axial direction (C cross section) became a measurement surface, and the microstructural fraction of a retained austenite (γ) phase was measured using the X-ray diffraction method. The microstructural fraction of the retained austenitic phase was obtained by measuring the diffraction X-ray integrated intensities of the (220) plane of γ and the (211) plane of α (ferrite) and converting the diffraction X-ray integrated intensities using the following equation.

$\text{γ}\left(\text{volume fraction}\right)=100/\left(1+\left(\text{I}\text{α}\text{R}\text{γ}/\text{I}\text{γ}\text{R}\text{α}\right)\right)$

Here, Iα indicates the integrated intensity of α, Rα indicates the crystallographic theoretical calculation value of α, Iγ indicates the integrated intensity of γ, and Rγ indicates the crystallographic theoretical calculation value of γ.

The fraction of a martensitic phase is the remainder other than the ferritic phase and the retained γ phase.

Tensile Test

From the obtained heat treatment-finished test materials, rod-shaped test pieces were taken such that the pipe axial direction became a tensile direction, tensile tests were carried out based on the specification of JIS Z 2241 (2011), and the yield stress (YS) was obtained as the 0.2% proof stress. Here, test pieces having a yield strength (YS) of 758 MPa or higher were regarded as having a high strength and determined as pass, and test pieces having a yield strength of lower than 758 MPa were determined as fail.

Elevated Temperature Tensile Test

From the obtained heat treatment-finished test materials, rod-shaped test pieces were taken such that the pipe axial direction became a tensile direction, tensile tests were carried out at 200° C. based on the specification of JIS G 0567 (2012), and the yield stress (YS) was obtained as the 0.2% proof stress. Here, the same heat treatment was carried out on the same steel, in a case where the ratio of the 0.2% proof stress at 200° C. to the yield stress (0.2% proof stress) obtained from the tensile test (2) was 0.85 or more, test pieces were determined as pass, and, in a case where the ratio was less than 0.85, test pieces were determined as fail.

Corrosion Resistance Test (Carbon Dioxide Corrosion Resistance Test and Sulfide Stress Cracking Resistance Test)

From the obtained heat treatment-finished test materials, corrosion test pieces having a thickness of 3 mm, a width of 30 mm, and a length of 40 mm were produced by machining. Corrosion tests were carried out using the corrosion test pieces, and the carbon dioxide corrosion resistance was evaluated.

The corrosion test for evaluating the carbon dioxide corrosion resistance was carried out by immersing the corrosion test piece in a test solution held in an autoclave: 20 mass% NaCl aqueous solution (liquid temperature: 200° C., CO₂ gas atmosphere of 30 atm) for an immersion period set to 14 days (336 hours). The weight of the test piece after the test was measured, and the corrosion rate calculated from the weight reduced before and after the corrosion test was obtained. Test pieces having a corrosion rate of 0.127 mm/y or slower were determined as pass, and test pieces exceeding 0.127 mm/y were determined as fail.

Furthermore, round rod-shaped test pieces (diameter: 3.81 mm) were prepared from the obtained test piece materials by machining, and sulfide stress cracking resistance tests (SSC resistance tests) were carried out.

In the SSC resistance test, the test pieces were immersed in an aqueous solution having a pH adjusted to 3.0 by adding acetic acid and sodium acetate to a test solution held in an autoclave: 0.165 mass% NaCl aqueous solution (liquid temperature: 25° C., CO₂ gas of 0.99 atm, H₂S atmosphere of 0.01 atm) and exposed for 720 hours in a state in which 90% of the yield stress was applied to the test pieces, and whether or not the test pieces after the test broke or cracked was observed. Test pieces that neither broke nor cracked were determined as pass (indicated by a symbol “0” in Table 2-1 and Table 2-2), and test pieces that broke or cracked were determined as fail (indicated by a symbol “X” in Table 2-1 and Table 2-2).

The obtained results are shown in Table 2-1 and Table 2-2.

Steel No.	Steel pipe No.	Tempering temperature (°C)	Soaking holding time (minutes)	Microstructure (volume%)	Yield strength YS (MPa)	Elevated temperature strength (*2)	Corrosion rate (mm/y)	SSC resistance	Remarks
M (*1)	F (*1)	A (*1)
A	1	575	20	53	34	13	929	0.87	0.028	O	Present Example
A	2	620	40	35	36	29	802	0.86	0.030	O	Present Example
B	3	575	20	58	31	11	944	0.88	0.026	O	Present Example
B	4	620	40	37	33	30	827	0.87	0.022	O	Present Example
C	5	575	20	50	32	18	915	0.86	0.025	O	Present Example
C	6	620	40	34	35	31	793	0.90	0.029	O	Present Example
D	7	575	20	44	29	27	880	0.89	0.106	O	Present Example
E	8	575	20	57	32	11	935	0.89	0.058	O	Present Example
F	9	575	20	54	24	22	961	0.90	0.027	O	Present Example
G	10	575	20	60	33	7	888	0.90	0.025	O	Present Example
H	11	575	20	62	30	8	948	0.86	0.079	O	Present Example
I	12	575	20	60	32	8	956	0.89	0.025	O	Present Example
J	13	575	20	47	32	21	893	0.88	0.020	O	Present Example
K	14	575	20	70	24	6	996	0.88	0.098	O	Present Example
L	15	575	20	42	34	24	870	0.90	0.021	O	Present Example
M	16	575	20	61	28	11	959	0.87	0.079	O	Present Example
N	17	575	20	56	32	12	936	0.88	0.016	O	Present Example
O	18	575	20	60	29	11	921	0.88	0.019	O	Present Example
P	19	575	20	62	34	4	937	0.90	0.037	O	Present Example
Q	20	575	20	50	31	19	887	0.88	0.023	O	Present Example
T	21	575	20	53	34	13	927	0.87	0.031	O	Present Example
U	22	575	20	56	30	14	936	0.86	0.062	O	Present Example
V	23	575	20	53	28	19	923	0.90	0.069	O	Present Example
W	24	575	20	53	31	16	924	0.89	0.070	O	Present Example
X	25	575	20	60	27	13	944	0.88	0.028	O	Present Example
Y	26	575	20	61	28	11	947	0.90	0.036	O	Present Example
Z	27	575	20	40	47	13	875	0.85	0.027	O	Present Example
AA	28	575	20	69	24	7	991	0.89	0.025	O	Present Example
AB	29	575	20	40	56	4	947	0.88	0.027	O	Present Example
AC	30	575	20	69	25	6	963	0.88	0.031	O	Present Example
AD	31	575	20	56	33	11	945	0.90	0.031	O	Present Example
AE	32	575	20	52	31	17	919	0.90	0.026	O	Present Example
Underlined values are outside the scope of the invention.
(*1) M: Martensitic phase, F: Ferritic phase, A: Retained austenitic phase
(*2) The elevated temperature strength refers to the ratio of the yield stress (0.2% proof stress) at 200° C. to the yield stress (0.2% proof stress) at room temperature.

Steel No.	Steel pipe No.	Tempering temperature (°C)	Soaking holding time (minutes)	Microstructu re (volume%)	Yield strength YS (MPa)	Elevated temperature strength (*2)	Corrosion rate (mm/y)	SSC resistance	Remarks
M (*1)	F (*1)	A (*1)
AF	33	575	20	46	36	18	901	0.89	0.024	O	Present Example
AG	34	575	20	56	30	14	928	0.90	0.027	O	Present Example
AH	35	575	20	62	31	7	954	0.88	0.024	O	Present Example
Al	36	575	20	55	33	12	936	0.87	0.027	O	Present Example
AJ	37	575	20	52	33	15	923	0.90	0.025	O	Present Example
AK	38	575	20	55	31	14	918	0.90	0.023	O	Present Example
AL	39	575	20	55	31	14	936	0.89	0.027	O	Present Example
AM	40	575	20	51	32	17	922	0.88	0.024	O	Present Example
AN	41	575	20	58	33	9	945	0.90	0.026	O	Present Example
P	42	525	20	66	34	0	942	0.90	0.035	O	Present Example
AO	43	575	20	42	28	30	869	0.88	0.136	X	Comparative Example
AP	44	575	20	56	34	10	932	0.90	0.138	X	Comparative Example
AQ	45	575	20	62	28	10	946	0.90	0.085	X	Comparative Example
AR	46	575	20	60	31	9	940	0.89	0.151	X	Comparative Example
AS	47	575	20	37	37	26	956	0.90	0.076	X	Comparative Example
AT	48	575	20	29	55	16	731	0.88	0.015	O	Comparative Example
AU	49	575	20	42	36	22	867	0.88	0.156	X	Comparative Example
AV	50	575	20	29	46	25	740	0.90	0.018	O	Comparative Example
AW	51	575	20	55	30	15	891	0.90	0.154	X	Comparative Example
AX	52	575	20	46	31	23	899	0.89	0.135	X	Comparative Example
AY	53	575	20	29	28	43	719	0.90	0.026	O	Comparative Example
AZ	54	575	20	53	29	18	909	0.88	0.159	X	Comparative Example
BA	55	575	20	60	29	11	947	0.87	0.152	X	Comparative Example
BB	56	575	20	58	31	11	949	0.90	0.137	X	Comparative Example
BC	57	575	20	53	32	15	946	0.73	0.045	O	Comparative Example
BD	58	575	20	20	63	17	658	0.85	0.037	O	Comparative Example
BE	59	575	20	56	29	15	897	0.88	0.035	O	Present Example
BF	60	575	20	52	30	18	888	0.87	0.039	O	Present Example
BG	61	575	20	61	27	12	925	0.86	0.027	O	Present Example
BH	62	575	20	64	22	14	941	0.86	0.040	O	Present Example
Underlined values are outside the scope of the invention.
(*1) M: Martensitic phase, F: Ferritic phase, A: Retained austenitic phase
(*2) The elevated temperature strength refers to the ratio of the yield stress (0.2% proof stress) at 200° C. to the yield stress (0.2% proof stress) at room temperature.

As shown in Table 2-1 and Table 2-2, the present invention examples were all stainless steel seamless pipes having a high strength of 758 MPa or higher in terms of the yield strength of YS, excellent corrosion resistance (carbon dioxide corrosion resistance) under highly corrosive high-temperature (200° C.) environments containing CO₂ and Cl^-, excellent sulfide stress cracking resistance, and an excellent elevated temperature strength.

Claims

1. A stainless steel seamless pipe having a component composition comprising, in terms of mass%:

C: 0.06% or less;

Si: 1.0% or less;

Mn: 0.01% or more and 0.90% or less;

P: 0.05% or less;

S: 0.005% or less;

Cr: 15.70% or more and 18.00% or less;

Mo: 1.60% or more and 3.80% or less;

Cu: 1.10% or more and 4.00% or less;

Ni: 3.0% or more and 6.0% or less;

Al: 0.10% or less;

N: 0.10% or less;

O: 0.010% or less;

V: 0.120% or more and 1.000% or less,

C, Si, Mn, Cr, Ni, Mo, Cu, and N satisfying Formula (1) below; and

a remainder consisting of Fe and unavoidable impurities,

wherein the stainless steel seamless pipe has a microstructure including, in terms of volume fraction, 30% or more of a martensitic phase, 60% or less of a ferritic phase, and 40% or less of a retained austenitic phase, and

a yield strength is 758 MPa or higher, 13.0 ≤ − 5.9 × 7.82 + 27 C − 0.91Si + 0.21Mn − 0.9Cr + Ni − 1.1Mo + 0.2 Cu + 11Ν ≤ 50.0 ­­­(1)

here, each of C, Si, Mn, Cr, Ni, Mo, Cu, and N is the content (mass%) of each element and is regarded as zero in a case of being not contained.

2. The stainless steel seamless pipe according to claim 1,

wherein the volume fraction of the martensitic phase is 40% or more, the volume fraction of the retained austenitic phase is 30% or less, and the yield strength is 862 MPa or higher.

3. The stainless steel seamless pipe according to claim 1, further comprises, in addition to the component composition, in terms of mass%, one group or two or more groups selected from Group A to Group D below:

Group A: W: 3.0% or less,

Group B: Nb: less than 0.10%,

Group C: one or two or more selected from B: 0.010% or less, Ta: 0.3% or less, Co: 1.5% or less, Ti: 0.3% or less, and Zr: 0.3% or less, and

Group D: one or two or more selected from Ca: 0.01% or less, REM: 0.3% or less, Mg: 0.01% or less, Sn: 1.0% or less, and Sb: 1.0% or less.

4. The stainless steel seamless pipe according to claim 2, further comprises, in addition to the component composition, in terms of mass%, one group or two or more groups selected from Group A to Group D below:

Group A: W: 3.0% or less,

Group B: Nb: less than 0.10%,

Group C: one or two or more selected from B: 0.010% or less, Ta: 0.3% or less, Co: 1.5% or less, Ti: 0.3% or less, and Zr: 0.3% or less, and

Group D: one or two or more selected from Ca: 0.01% or less, REM: 0.3% or less, Mg: 0.01% or less, Sn: 1.0% or less, and Sb: 1.0% or less.

5. A method for manufacturing the stainless steel seamless pipe according to claim 1, the method comprising:

heating a steel pipe material having the component composition at a temperature within a heating temperature range of 1100° C. to 1350° C. to make the steel pipe material into a seamless steel pipe by hot-working;

subsequently, carrying out a quenching treatment by reheating the seamless steel pipe to a temperature within a heating temperature range of 850° C. to 1150° C. and cooling the seamless steel pipe to a cooling stop temperature of 50° C. or lower at a cooling rate for air cooling or faster; and

thereafter, carrying out a tempering treatment by heating the seamless steel pipe to a temperature within a tempering temperature range of 500° C. to 650° C.

6. A method for manufacturing the stainless steel seamless pipe according to claim 2, the method comprising:

heating a steel pipe material having the component composition at a temperature within a heating temperature range of 1100° C. to 1350° C. to make the steel pipe material into a seamless steel pipe by hot-working;

subsequently, carrying out a quenching treatment by reheating the seamless steel pipe to a temperature within a heating temperature range of 850° C. to 1150° C. and cooling the seamless steel pipe to a cooling stop temperature of 50° C. or lower at a cooling rate for air cooling or faster; and

thereafter, carrying out a tempering treatment by heating the seamless steel pipe to a temperature within a tempering temperature range of 500° C. to 650° C.

7. A method for manufacturing the stainless steel seamless pipe according to claim 3, the method comprising:

heating a steel pipe material having the component composition at a temperature within a heating temperature range of 1100° C. to 1350° C. to make the steel pipe material into a seamless steel pipe by hot-working;

subsequently, carrying out a quenching treatment by reheating the seamless steel pipe to a temperature within a heating temperature range of 850° C. to 1150° C. and cooling the seamless steel pipe to a cooling stop temperature of 50° C. or lower at a cooling rate for air cooling or faster; and

thereafter, carrying out a tempering treatment by heating the seamless steel pipe to a temperature within a tempering temperature range of 500° C. to 650° C.

8. A method for manufacturing the stainless steel seamless pipe according to claim 4, the method comprising:

heating a steel pipe material having the component composition at a temperature within a heating temperature range of 1100° C. to 1350° C. to make the steel pipe material into a seamless steel pipe by hot-working;

subsequently, carrying out a quenching treatment by reheating the seamless steel pipe to a temperature within a heating temperature range of 850° C. to 1150° C. and cooling the seamless steel pipe to a cooling stop temperature of 50° C. or lower at a cooling rate for air cooling or faster; and

thereafter, carrying out a tempering treatment by heating the seamless steel pipe to a temperature within a tempering temperature range of 500° C. to 650° C.