STAINLESS STEEL SEAMLESS PIPE AND METHOD FOR MANUFACTURING SAME

Abstract

A stainless steel seamless pipe having high strength and excellent corrosion resistance. The stainless steel seamless pipe has a specified composition in which C, Si, Mn, Cr, Ni, Mo, Cu, and N satisfy a predetermined formula, a microstructure containing at least 25% martensitic phase, at most 65% ferrite phase, and at most 40% retained austenite phase by volume, and a yield strength of 758 MPa or more.

Claims

1. A stainless steel seamless pipe having a chemical composition comprising, by mass %: C: 0.06% or less; Si: 1.0% or less; P: 0.05% or less; S: 0.005% or less; Cr: more than 15.7% and 18.0% or less; Mo: 1.8% or more and 3.5% or less; Cu: 1.5% or more and 3.5% or less; Ni: 2.5% or more and 6.0% or less; Al: 0.10% or less; N: 0.10% or less; O: 0.010% or less; W: 0.5% or more and 2.0% or less; Co: 0.01% or more and 1.5% or less; and a balance being Fe and incidental impurities, wherein C, Si, Mn, Cr, Ni, Mo, Cu, and N satisfy the following formula (1):
13.0≤−5.9×(7.82+27C−0.91Si+0.21Mn−0.9Cr+Ni−1.1Mo+0.2Cu+11N)≤55.0  (1), where C, Si, Mn, Cr, Ni, Mo, Cu, and N represent a content of each element, by mass %, and a content is 0% for elements that are not contained, the stainless steel seamless pipe has a microstructure comprising at least 25% martensitic phase, at most 65% ferrite phase, and at most 40% retained austenite phase, by volume, and the stainless steel seamless pipe has a yield strength of 758 MPa or more.

2. The stainless steel seamless pipe according to claim 1, wherein the chemical composition further comprises, by mass %, at least one selected from the group consisting of Mn: 1.0% or less, and Nb: 0.30% or less.

3. The stainless steel seamless pipe according to claim 1, wherein the microstructure comprises at least 40% martensitic phase, at most 60% ferrite phase, and at most 30% retained austenite phase, by volume, and the stainless steel seamless pipe has a yield strength of 862 MPa or more.

4. The stainless steel seamless pipe according to claim 1, wherein the chemical composition further comprises at least one group selected from the following groups: Group A: at least one selected from the group consisting of, by mass %, V: 1.0% or less, B: 0.01% or less, and Ta: 0.3% or less, Group B: at least one selected from the group consisting of, by mass %, Ti: 0.3% or less, and Zr: 0.3% or less, and Group C: at least one selected from the group consisting of, by mass %, Ca: 0.01% or less, REM: 0.3% or less, Mg: 0.01% or less, Sn: 0.2% or less, and Sb: 1.0% or less.

5-6. (canceled)

7. A method for manufacturing the stainless steel seamless pipe of claim 1, the method comprising: forming a seamless steel pipe of predetermined dimensions from a steel pipe material; quenching by heating the seamless steel pipe to a temperature in a range of 850 to 1,150° C., and cooling the seamless steel pipe to a surface temperature of 50° C. or less at a cooling rate of air cooling or faster; and tempering by heating the quenched seamless steel pipe to a temperature in a range of 500 to 650° C.

8. The stainless steel seamless pipe according to claim 2, wherein the microstructure comprises at least 40% martensitic phase, at most 60% ferrite phase, and at most 30% retained austenite phase, by volume, and the stainless steel seamless pipe has a yield strength of 862 MPa or more.

9. The stainless steel seamless pipe according to claim 2, wherein the chemical composition further comprises at least one group selected from the following groups: Group A: at least one selected from the group consisting of, by mass %, V: 1.0% or less, B: 0.01% or less, and Ta: 0.3% or less, Group B: at least one selected from the group consisting of, by mass %, Ti: 0.3% or less, and Zr: 0.3% or less, and Group C: at least one selected from the group consisting of, by mass %, Ca: 0.01% or less, REM: 0.3% or less, Mg: 0.01% or less, Sn: 0.2% or less, and Sb: 1.0% or less.

10. The stainless steel seamless pipe according to claim 3, wherein the chemical composition further comprises at least one group selected from the following groups: Group A: at least one selected from the group consisting of, by mass %, V: 1.0% or less, B: 0.01% or less, and Ta: 0.3% or less, Group B: at least one selected from the group consisting of, by mass %, Ti: 0.3% or less, and Zr: 0.3% or less, and Group C: at least one selected from the group consisting of, by mass %, Ca: 0.01% or less, REM: 0.3% or less, Mg: 0.01% or less, Sn: 0.2% or less, and Sb: 1.0% or less.

11. The stainless steel seamless pipe according to claim 8, wherein the chemical composition further comprises at least one group selected from the following groups: Group A: at least one selected from the group consisting of, by mass %, V: 1.0% or less, B: 0.01% or less, and Ta: 0.3% or less, Group B: at least one selected from the group consisting of, by mass %, Ti: 0.3% or less, and Zr: 0.3% or less, and Group C: at least one selected from the group consisting of, by mass %, Ca: 0.01% or less, REM: 0.3% or less, Mg: 0.01% or less, Sn: 0.2% or less, and Sb: 1.0% or less.

12. A method for manufacturing the stainless steel seamless pipe of claim 2, the method comprising: forming a seamless steel pipe of predetermined dimensions from a steel pipe material; quenching by heating the seamless steel pipe to a temperature in a range of 850 to 1,150° C., and cooling the seamless steel pipe to a surface temperature of 50° C. or less at a cooling rate of air cooling or faster; and tempering by heating the quenched seamless steel pipe to a temperature in a range of 500 to 650° C.

13. A method for manufacturing the stainless steel seamless pipe of claim 3, the method comprising: forming a seamless steel pipe of predetermined dimensions from a steel pipe material; quenching by heating the seamless steel pipe to a temperature in a range of 850 to 1,150° C., and cooling the seamless steel pipe to a surface temperature of 50° C. or less at a cooling rate of air cooling or faster; and tempering by heating the quenched seamless steel pipe to a temperature in a range of 500 to 650° C.

14. A method for manufacturing the stainless steel seamless pipe of claim 4, the method comprising: forming a seamless steel pipe of predetermined dimensions from a steel pipe material; quenching by heating the seamless steel pipe to a temperature in a range of 850 to 1,150° C., and cooling the seamless steel pipe to a surface temperature of 50° C. or less at a cooling rate of air cooling or faster; and tempering by heating the quenched seamless steel pipe to a temperature in a range of 500 to 650° C.

15. A method for manufacturing the stainless steel seamless pipe of claim 8, the method comprising: forming a seamless steel pipe of predetermined dimensions from a steel pipe material; quenching by heating the seamless steel pipe to a temperature in a range of 850 to 1,150° C., and cooling the seamless steel pipe to a surface temperature of 50° C. or less at a cooling rate of air cooling or faster; and tempering by heating the quenched seamless steel pipe to a temperature in a range of 500 to 650° C.

16. A method for manufacturing the stainless steel seamless pipe of claim 9, the method comprising: forming a seamless steel pipe of predetermined dimensions from a steel pipe material; quenching by heating the seamless steel pipe to a temperature in a range of 850 to 1,150° C., and cooling the seamless steel pipe to a surface temperature of 50° C. or less at a cooling rate of air cooling or faster; and tempering by heating the quenched seamless steel pipe to a temperature in a range of 500 to 650° C.

17. A method for manufacturing the stainless steel seamless pipe of claim 10, the method comprising: forming a seamless steel pipe of predetermined dimensions from a steel pipe material; quenching by heating the seamless steel pipe to a temperature in a range of 850 to 1,150° C., and cooling the seamless steel pipe to a surface temperature of 50° C. or less at a cooling rate of air cooling or faster; and tempering by heating the quenched seamless steel pipe to a temperature in a range of 500 to 650° C.

18. A method for manufacturing the stainless steel seamless pipe of claim 11, the method comprising: forming a seamless steel pipe of predetermined dimensions from a steel pipe material; quenching by heating the seamless steel pipe to a temperature in a range of 850 to 1,150° C., and cooling the seamless steel pipe to a surface temperature of 50° C. or less at a cooling rate of air cooling or faster; and tempering by heating the quenched seamless steel pipe to a temperature in a range of 500 to 650° C.

Description

DETAILED DESCRIPTION

[0036] A stainless steel seamless pipe of the disclosed embodiments is a stainless steel seamless pipe having a composition that includes, in mass %, C: 0.06% or less, Si: 1.0% or less, P: 0.05% or less, S: 0.005% or less, Cr: more than 15.7% and 18.0% or less, Mo: 1.8% or more and 3.5% or less, Cu: 1.5% or more and 3.5% or less, Ni: 2.5% or more and 6.0% or less, Al: 0.10% or less, N: 0.10% or less, O: 0.010% or less, W: 0.5% or more and 2.0% or less, and Co: 0.01% or more and 1.5% or less, and in which C, Si, Mn, Cr, Ni, Mo, Cu, and N satisfy the following formula (1), and the balance is Fe and incidental impurities,

[0037] the stainless steel seamless pipe having a microstructure containing at least 25% martensitic phase, at most 65% ferrite phase, and at most 40% retained austenite phase by volume,

[0038] the stainless steel seamless pipe having a yield strength of 758 MPa or more,

13.0≤−5.9×(7.82+27C−0.91Si+0.21Mn−0.9Cr+Ni−1.1Mo+0.2Cu+11N)≤55.0  (1),

wherein C, Si, Mn, Cr, Ni, Mo, Cu, and N represent the content of each element in mass %, and the content is 0 (zero; mass %) for elements that are not contained.

[0039] The following describes the reasons for specifying the composition of a seamless steel pipe of the disclosed embodiments. In the following, “%” means percent by mass, unless otherwise specifically stated.

C: 0.06% or Less

[0040] C is an element that becomes incidentally included in the process of steelmaking. Corrosion resistance decreases when C is contained in an amount of more than 0.06%. For this reason, the C content is 0.06% or less. The C content is preferably 0.05% or less, more preferably 0.04% or less. Considering the decarburization cost, the C content is preferably 0.002% or more, more preferably 0.003% or more.

Si: 1.0% or Less

[0041] Si is an element that acts as a deoxidizing agent. However, hot workability and corrosion resistance decrease when Si is contained in an amount of more than 1.0%. For this reason, the Si content is 1.0% or less. The Si content is preferably 0.7% or less, more preferably 0.5% or less. It is not particularly required to set a lower limit, as long as the deoxidizing effect is obtained. However, in order to obtain a sufficient deoxidizing effect, the Si content is preferably 0.03% or more, more preferably 0.05% or more.

P: 0.05% or Less

[0042] P is an element that impairs the corrosion resistance, including carbon dioxide gas corrosion resistance, and sulfide stress cracking resistance. P is therefore contained preferably in as small an amount as possible in the disclosed embodiments. However, a P content of 0.05% or less is acceptable. For this reason, the P content is 0.05% or less. The P content is preferably 0.04% or less, more preferably 0.03% or less.

S: 0.005% or Less

[0043] S is an element that seriously impairs hot workability, and interferes with stable operations of hot working in the pipe manufacturing process. S exists as sulfide inclusions in steel, and impairs the corrosion resistance. S should therefore be contained preferably in as small an amount as possible. However, a S content of 0.005% or less is acceptable. For this reason, the S content is 0.005% or less. The S content is preferably 0.004% or less, more preferably 0.003% or less.

Cr: More than 15.7% and 18.0% or Less

[0044] Cr is an element that forms a protective coating on steel pipe surface, and contributes to improving corrosion resistance. The desired carbon dioxide gas corrosion resistance, the desired acid-environment corrosion resistance, and the desired sulfide stress cracking resistance cannot be provided when the Cr content is 15.7% or less. For this reason, Cr needs to be contained in an amount of more than 15.7%. With a Cr content of more than 18.0%, the ferrite fraction overly increases, and the desired strength cannot be provided. For this reason, the Cr content is more than 15.7% and 18.0% or less. The Cr content is preferably 16.0% or more, more preferably 16.3% or more. The Cr content is preferably 17.5% or less, more preferably 17.2% or less, further preferably 17.0% or less.

Mo: 1.8% or More and 3.5% or Less

[0045] By stabilizing the protective coating on steel pipe surface, Mo increases the resistance against pitting corrosion due to Cl.sup.− and low pH, and increases the carbon dioxide gas corrosion resistance and acid-environment corrosion resistance. Mo also increases the sulfide stress cracking resistance. Mo needs to be contained in an amount of 1.8% or more to obtain the desired corrosion resistance. The effects become saturated with a Mo content of more than 3.5%. For this reason, the Mo content is 1.8% or more and 3.5% or less. The Mo content is preferably 2.0% or more, more preferably 2.2% or more. The Mo content is preferably 3.3% or less, more preferably 3.0% or less, further preferably 2.8% or less, even more preferably less than 2.7%.

Cu: 1.5% or More and 3.5% or Less

[0046] Cu increases the retained austenite, and contributes to improving yield strength by forming a precipitate. This makes it possible to obtain high strength without decreasing low-temperature toughness. Cu also acts to strengthen the protective coating on steel pipe surface, and improve the carbon dioxide gas corrosion resistance and acid-environment corrosion resistance. Cu needs to be contained in an amount of 1.5% or more to obtain the desired strength and corrosion resistance, particularly carbon dioxide gas corrosion resistance. An excessively high Cu content results in decrease of hot workability of steel, and the Cu content is 3.5% or less. For this reason, the Cu content is 1.5% or more and 3.5% or less. The Cu content is preferably 1.8% or more, more preferably 2.0% or more. The Cu content is preferably 3.2% or less, more preferably 3.0% or less.

Ni: 2.5% or More and 6.0% or Less

[0047] Ni is an element that strengthens the protective coating on steel pipe surface, and contributes to improving corrosion resistance, particularly acid-environment corrosion resistance. By solid solution strengthening, Ni also increases the steel strength, and improves the toughness of steel. These effects become more pronounced when Ni is contained in an amount of 2.5% or more. A Ni content of more than 6.0% results in decrease of martensitic phase stability, and decreases the strength. For this reason, the Ni content is 2.5% or more and 6.0% or less. The Ni content is preferably more than 3.3%, more preferably 3.5% or more, further preferably 4.0% or more, even more preferably 4.2% or more. The Ni content is preferably 5.5% or less, more preferably 5.2% or less, even more preferably 5.0% or less.

Al: 0.10% or Less

[0048] Al is an element that acts as a deoxidizing agent. However, corrosion resistance decreases when Al is contained in an amount of more than 0.10%. For this reason, the Al content is 0.10% or less. The Al content is preferably 0.07% or less, more preferably 0.05% or less. It is not particularly required to set a lower limit, as long as the deoxidizing effect is obtained. However, in order to obtain a sufficient deoxidizing effect, the Al content is preferably 0.005% or more, more preferably 0.01% or more.

N: 0.10% or Less

[0049] N is an element that becomes incidentally included in the process of steelmaking. Nis also an element that increases the steel strength. However, when contained in an amount of more than 0.10%, N forms nitrides, and decreases the corrosion resistance. For this reason, the N content is 0.10% or less. The N content is preferably 0.08% or less, more preferably 0.07% or less. The N content does not have a specific lower limit. However, an excessively low N content leads to increased steel making cost. For this reason, the N content is preferably 0.002% or more, more preferably 0.003% or more.

O: 0.010% or Less

[0050] O (oxygen) exists as an oxide in steel, and causes adverse effects on various properties. For this reason, O is contained preferably in as small an amount as possible in the disclosed embodiments. An 0 content of more than 0.010% results in decrease of hot workability and corrosion resistance. For this reason, the 0 content is 0.010% or less.

W: 0.5% or More and 2.0% or Less

[0051] W is an element that contributes to improving steel strength, and that can increase carbon dioxide gas corrosion resistance and acid-environment corrosion resistance by stabilizing the protective coating on steel pipe surface. W also improves the sulfide stress cracking resistance. Particularly, W greatly improves corrosion resistance when contained with Mo. With a W content of 0.5% or more, the desired carbon dioxide gas corrosion resistance and the desired acid-environment corrosion resistance can be obtained. The effects become saturated with a W content of more than 2.0%. For this reason, W, when contained, is contained in an amount of 2.0% or less. The W content is preferably 0.8% or more, more preferably 1.0% or more. The W content is preferably 1.8% or less, more preferably 1.5% or less.

Co: 0.01% or More and 1.5% or Less

[0052] Co is an element that increases strength, in addition to improving corrosion resistance. In order to obtain the desired acid-environment corrosion resistance, Co is contained in an amount of 0.01% or more. The effects become saturated with a Co content of more than 1.5%. For this reason, the Co content is 0.01% or more and 1.5% or less in the disclosed embodiments. The Co content is preferably 0.05% or more, more preferably 0.10% or more. The Co content is preferably 1.0% or less, more preferably 0.5% or less.

[0053] In the disclosed embodiments, C, Si, Mn, Cr, Ni, Mo, Cu, and N are contained so as to satisfy the following formula (1), in addition to satisfying the foregoing composition.

13.0≤−5.9×(7.82+27C−0.91Si+0.21Mn−0.9Cr+Ni−1.1Mo+0.2Cu+11N)≤55.0  (1)

[0054] In the formula, C, Si, Mn, Cr, Ni, Mo, Cu, and N represent the content of each element in mass %, and the content is 0 (zero; mass %) for elements that are not contained.

[0055] In formula (1), the expression −5.9×(7.82+27C−0.91Si+0.21Mn−0.9Cr+Ni−1.1Mo+0.2Cu+11N) (hereinafter, referred to also as “middle polynomial of formula (1)”, or, simply, “middle value”) is determined as an index that indicates the likelihood of ferrite phase formation. With the alloy elements of formula (1) contained in adjusted amounts so as to satisfy formula (1), it is possible to stably produce a composite microstructure of martensitic phase and ferrite phase, or a composite microstructure of martensitic phase, ferrite phase, and retained austenite phase. When any of the alloy elements occurring in formula (1) is not contained, the value of the middle polynomial of formula (1) is calculated by regarding the content of such an element as zero percent.

[0056] When the value of the middle polynomial of formula (1) is less than 13.0, the ferrite phase decreases, and the manufacturing yield decreases.

[0057] On the other hand, when the value of the middle polynomial of formula (1) is more than 55.0, the ferrite phase becomes more than 65% by volume, and the desired strength cannot be provided.

[0058] For this reason, the formula (1) specified in the disclosed embodiments sets a left-hand value of 13.0 as the lower limit, and a right-hand value of 55.0 as the upper limit.

[0059] The lower-limit left-hand value of the formula (1) specified in the disclosed embodiments is preferably 15.0, more preferably 20.0. The right-hand value is preferably 50.0, more preferably 45.0, even more preferably 40.0.

[0060] In the disclosed embodiments, the balance in the composition above is Fe and incidental impurities.

[0061] In the disclosed embodiments, in addition to the foregoing basic components, the composition may further contain one or two or more optional elements (Mn, Nb, V, B, Ta, Ti, Zr, Ca, REM, Mg, Sn, and Sb), as follows.

[0062] Specifically, in the disclosed embodiments, the composition may additionally contain Mn: 1.0% or less, and Nb: 0.30% or less.

[0063] In the disclosed embodiments, the composition may additionally contain one or two or more selected from V: 1.0% or less, B: 0.01% or less, and Ta: 0.3% or less.

[0064] In the disclosed embodiments, the composition may additionally contain one or two selected from Ti: 0.3% or less, and Zr: 0.3% or less.

[0065] In the disclosed embodiments, the composition may additionally contain one or two or more selected from Ca: 0.01% or less, REM: 0.3% or less, Mg: 0.01% or less, Sn: 0.2% or less, and Sb: 1.0% or less.

Mn: 1.0% or Less

[0066] Mn, an optional element, is an element that acts as a deoxidizing agent and a desulfurizing agent, and improves hot workability and strength. Mn is contained in an amount of preferably 0.001% or more, more preferably 0.01% or more to obtain these effects. The effects become saturated with a Mn content of more than 1.0%. For this reason, Mn, when contained, is contained in an amount of 1.0% or less. The Mn content is preferably 0.8% or less, more preferably 0.6% or less.

Nb: 0.30% or Less

[0067] Nb, an optional element, is an element that increases strength, and improves corrosion resistance. The effects become saturated with a Nb content of more than 0.30%. For this reason, Nb, when contained, is contained in an amount of 0.30% or less. The Nb content is preferably 0.25% or less, more preferably 0.2% or less. The Nb content is preferably 0.01% or more, more preferably 0.05% or more, even more preferably more than 0.10%.

V: 1.0% or Less

[0068] V, an optional element, is an element that increases strength. The effect becomes saturated with a V content of more than 1.0%. For this reason, V, when contained, is contained in an amount of 1.0% or less. The V content is preferably 0.5% or less, more preferably 0.3% or less. The V content is preferably 0.01% or more, more preferably 0.03% or more.

B: 0.01% or Less

[0069] B, an optional element, is an element that increases strength. B also contributes to improving hot workability, and has the effect to reduce fracture and cracking during the pipe making process. On the other hand, a B content of more than 0.01% produces hardly any hot workability improving effect, and results in decrease of low-temperature toughness. For this reason, B, when contained, is contained in an amount of 0.01% or less. The B content is preferably 0.008% or less, more preferably 0.007% or less. The B content is preferably 0.0005% or more, more preferably 0.001% or more.

Ta: 0.3% or Less

[0070] Ta, an optional element, is an element that improves corrosion resistance, in addition to increasing strength. In order to obtain these effects, Ta is contained in an amount of preferably 0.001% or more. The effects become saturated with a Ta content of more than 0.3%. For this reason, Ta, when contained, is contained in a limited amount of 0.3% or less.

Ti: 0.3% or Less

[0071] Ti, an optional element, is an element that increases strength. In addition to this effect, Ti also has the effect to improve the sulfide stress cracking resistance. In order to obtain these effects, Ti is contained in an amount of preferably 0.0005% or more. A Ti content of more than 0.3% decreases toughness. For this reason, Ti, when contained, is contained in a limited amount of 0.3% or less.

Zr: 0.3% or Less

[0072] Zr, an optional element, is an element that increases strength. In addition to this effect, Zr also has the effect to improve the sulfide stress cracking resistance. In order to obtain these effects, Zr is contained in an amount of preferably 0.0005% or more. The effects become saturated with a Zr content of more than 0.3%. For this reason, Zr, when contained, is contained in a limited amount of 0.3% or less.

Ca: 0.01% or Less

[0073] Ca, an optional element, is an element that contributes to improving the sulfide stress corrosion cracking resistance by controlling the form of sulfide. In order to obtain this effect, Ca is contained in an amount of preferably 0.0005% or more. When Ca is contained in an amount of more than 0.01%, the effect becomes saturated, and Ca cannot produce the effect expected from the increased content. For this reason, Ca, when contained, is contained in a limited amount of 0.01% or less.

REM: 0.3% or Less

[0074] REM, an optional element, is an element that contributes to improving the sulfide stress corrosion cracking resistance by controlling the form of sulfide. In order to obtain this effect, REM is contained in an amount of preferably 0.0005% or more. When REM is contained in an amount of more than 0.3%, the effect becomes saturated, and REM cannot produce the effect expected from the increased content. For this reason, REM, when contained, is contained in a limited amount of 0.3% or less.

[0075] As used herein, “REM” means scandium (Sc; atomic number 21) and yttrium (Y; atomic number 39), as well as lanthanoids from lanthanum (La; atomic number 57) to lutetium (Lu; atomic number 71). As used herein, “REM concentration” means the total content of one or two or more elements selected from the foregoing REM elements.

Mg: 0.01% or Less

[0076] Mg, an optional element, is an element that improves corrosion resistance. In order to obtain this effect, Mg is contained in an amount of preferably 0.0005% or more. When Mg is contained in an amount of more than 0.01%, the effect becomes saturated, and Mg cannot produce the effect expected from the increased content. For this reason, Mg, when contained, is contained in a limited amount of 0.01% or less.

Sn: 0.2% or Less

[0077] Sn, an optional element, is an element that improves corrosion resistance. In order to obtain this effect, Sn is contained in an amount of preferably 0.001% or more. When Sn is contained in an amount of more than 0.2%, the effect becomes saturated, and Sn cannot produce the effect expected from the increased content. For this reason, Sn, when contained, is contained in a limited amount of 0.2% or less.

Sb: 1.0% or Less

[0078] Sb, an optional element, is an element that improves corrosion resistance. In order to obtain this effect, Sb is contained in an amount of preferably 0.001% or more. When Sb is contained in an amount of more than 1.0%, the effect becomes saturated, and Sb cannot produce the effect expected from the increased content. For this reason, Sb, when contained, is contained in a limited amount of 1.0% or less.

[0079] The following describes the reason for limiting the microstructure in the seamless steel pipe of the disclosed embodiments.

[0080] In addition to having the foregoing composition, the seamless steel pipe of the disclosed embodiments has a microstructure that contains at least 25% martensitic phase, at most 65% ferrite phase, and at most 40% retained austenite phase by volume.

[0081] In order to provide the desired strength, the seamless steel pipe of the disclosed embodiments contains at least 25% martensitic phase by volume. Preferably, the martensitic phase is at least 40% by volume. In embodiments, the ferrite is at most 65% by volume. With the ferrite phase, progression of sulfide stress corrosion cracking and sulfide stress cracking can be reduced, and excellent corrosion resistance can be obtained. If the ferrite phase precipitates in a large amount of more than 65% by volume, it might not be possible to provide the desired strength. The ferrite phase is preferably 5% or more by volume. The ferrite phase is preferably 60% or less, more preferably 55% or less, even more preferably 50% or less by volume.

[0082] The seamless steel pipe of the disclosed embodiments contains at most 40% austenitic phase (retained austenite phase) by volume, in addition to the martensitic phase and the ferrite phase. Ductility and toughness improve by the presence of the retained austenite phase. If the austenitic phase precipitates in a large amount of more than 40% by volume, it is not possible to provide the desired strength. For this reason, the retained austenite phase is 40% or less by volume. The retained austenite phase is preferably 5% or more by volume. The retained austenite phase is preferably 30% or less, more preferably 25% or less by volume.

[0083] For the measurement of the microstructure of the seamless steel pipe of the disclosed embodiments, a test specimen for microstructure observation is corroded with a Vilella's solution (a mixed reagent containing 2 g of picric acid, 10 ml of hydrochloric acid, and 100 ml of ethanol), and the structure is imaged with a scanning electron microscope (1,000 times magnification). The fraction of the ferrite phase microstructure (area ratio (%)) is then calculated with an image analyzer. The area ratio is defined as the volume ratio (%) of the ferrite phase.

[0084] Separately, an X-ray diffraction test specimen is ground and polished to have a measurement cross section (C cross section) orthogonal to the axial direction of pipe, and the fraction of the retained austenite (γ) phase microstructure is measured by an X-ray diffraction method. The fraction of the retained austenite phase microstructure is determined by measuring X-ray diffraction integral intensity for the (220) plane of the austenite phase (γ), and the (211) plane of the ferrite phase (α), and converting the calculated values using the following formula.

γ(volume ratio)=100/(1+(IαRγ/IγRα)),

wherein Iα is the integral intensity of α, Rα is the crystallographic theoretical value for α, Iγ is the integral intensity of γ, and Rγ is the crystallographic theoretical value for γ.

[0085] The fraction of the martensitic phase is the remainder other than the fractions of the ferrite phase and retained γ phase determined by the foregoing measurement method. As used herein, “martensitic phase” may contain at most 5% precipitate phase by volume, other than the martensitic phase, the ferrite phase, and the retained austenite phase.

[0086] The following describes a preferred method for manufacturing a stainless steel seamless pipe of the disclosed embodiments.

[0087] Preferably, a molten steel of the foregoing composition is made using a steelmaking process such as by using a converter, and formed into a steel pipe material, for example, a billet, using an ordinary method such as continuous casting, or ingot casting-billeting. The steel pipe material is then hot worked into a pipe using a known pipe manufacturing process, for example, the Mannesmann-plug mill process or the Mannesmann-mandrel mill process, to produce a seamless steel pipe of desired dimensions having the foregoing composition. The hot working may be followed by cooling. The cooling process is not particularly limited. After the hot working, the pipe is cooled to room temperature at a cooling rate about the same as air cooling, provided that the composition falls in the range of the disclosed embodiments.

[0088] In the disclosed embodiments, this is followed by a heat treatment that includes quenching and tempering.

[0089] In quenching, the steel pipe is reheated to a temperature of 850 to 1,150° C., and cooled at a cooling rate of air cooling or faster. The cooling stop temperature is 50° C. or less in terms of a surface temperature. When the heating temperature is less than 850° C., a reverse transformation from martensite to austenite does not occur, and the austenite does not transform into martensite during cooling, with the result that the desired strength cannot be provided. On the other hand, the crystal grains coarsen when the heating temperature exceeds 1,150° C. For this reason, the heating temperature of quenching is 850 to 1,150° C. The heating temperature of quenching is preferably 900° C. or more. The heating temperature of quenching is preferably 1,100° C. or less.

[0090] When the cooling stop temperature is more than 50° C., the austenite does not sufficiently transform into martensite, and the fraction of retained austenite becomes overly high. For this reason, the cooling stop temperature of the cooling in quenching is 50° C. or less in the disclosed embodiments.

[0091] Here, “cooling rate of air cooling or faster” means 0.01° C./s or more.

[0092] In quenching, the soaking retention time is preferably 5 to 30 minutes, in order to achieve a uniform temperature along a wall thickness direction, and prevent variation in the material.

[0093] In tempering, the quenched seamless steel pipe is heated to a heating temperature (tempering temperature) of 500 to 650° C. The heating may be followed by natural cooling. A tempering temperature of less than 500° C. is too low to produce the desired tempering effect as intended. When the tempering temperature is higher than 650° C., precipitation of intermetallic compounds occurs, and it is not possible to obtain desirable low-temperature toughness. For this reason, the tempering temperature is 500 to 650° C. The tempering temperature is preferably 520° C. or more. The tempering temperature is preferably 630° C. or less.

[0094] In tempering, the soaking retention time is preferably 5 to 90 minutes, in order to achieve a uniform temperature along a wall thickness direction, and prevent variation in the material.

[0095] After the heat treatment (quenching and tempering), the seamless steel pipe has a microstructure in which the martensitic phase, the ferrite phase, and the retained austenite phase are contained in a specific predetermined volume ratio. In this way, the stainless steel seamless pipe can have the desired strength and excellent corrosion resistance.

[0096] The stainless steel seamless pipe obtained in the disclosed embodiments in the manner described above is a high-strength steel pipe having a yield strength of 758 MPa or more, and has excellent corrosion resistance. Preferably, the yield strength is 862 MPa or more. Preferably, the yield strength is 1,034 MPa or less. The stainless steel seamless pipe of the disclosed embodiments can be used as a stainless steel seamless pipe for oil country tubular goods (a high-strength stainless steel seamless pipe for oil country tubular goods).

Examples

[0097] The disclosed embodiments are further described below through Examples.

[0098] Molten steels of the compositions shown in Table 1-1 and Table 1-2 (Steel Nos. A to BJ) were cast into steel pipe materials. The steel pipe material was heated, and hot worked into a seamless steel pipe measuring 83.8 mm in outer diameter and 12.7 mm in wall thickness, using a model seamless rolling mill. The seamless steel pipe was then cooled by air cooling. The heating of the steel pipe material before hot working was carried out at a heating temperature of 1,250° C.

[0099] Each seamless steel pipe was cut into a test specimen material, which was then subjected to quenching that included reheating to a temperature of 960° C., and cooling (water cooling) the test specimen to a cooling stop temperature of 30° C. with 20 minutes of retention in soaking. This was followed by tempering that included heating to a temperature of 575° C. or 620° C., and air cooling the test specimen with 20 minutes of retention in soaking. This produced steel pipe Nos. 1 to 65. In quenching, the water cooling was carried out at a cooling rate of 11° C./s. The air cooling (natural cooling) in tempering was carried out at a cooling rate of 0.04° C./s. The heating temperature of tempering is 575° C. for steel pipe Nos. 1 to 62, and 620° C. for steel pipe Nos. 63 to 65.

TABLE-US-00001 TABLE 1-1 Steel Composition (mass %) No. C Si Mn P S Cr Mo Cu Ni Nb Al N A 0.015 0.37 0.318 0.016 0.0013 16.69 2.48 2.51 4.52 0.107 0.026 0.029 B 0.008 0.32 0.360 0.017 0.0011 17.38 2.61 2.54 5.24 0.196 0.026 0.024 C 0.009 0.29 0.302 0.017 0.0012 16.95 2.46 2.57 5.05 0.204 0.025 0.019 D 0.057 0.31 0.342 0.017 0.0012 17.20 2.46 2.61 5.21 0.062 0.027 0.016 E 0.009 0.93 0.296 0.017 0.0011 16.77 2.48 2.58 5.16 0.069 0.027 0.026 F 0.014 0.28 0.940 0.015 0.0010 16.92 2.59 2.64 4.60 0.086 0.027 0.019 G 0.014 0.36 0.012 0.016 0.0010 16.73 2.57 2.53 4.97 0.101 0.026 0.032 H 0.011 0.31 0.329 0.043 0.0009 16.98 2.45 2.60 4.97 0.126 0.027 0.026 I 0.013 0.37 0.372 0.016 0.0042 17.13 2.55 2.61 4.34 0.136 0.025 0.032 J 0.010 0.34 0.275 0.017 0.0009 17.41 2.57 2.56 4.96 0.098 0.025 0.032 K 0.013 0.29 0.293 0.017 0.0011 15.76 2.60 2.60 4.79 0.171 0.025 0.023 L 0.015 0.33 0.324 0.017 0.0013 16.68 3.43 2.49 4.44 0.203 0.025 0.015 M 0.015 0.28 0.366 0.015 0.0010 16.96 1.84 2.64 5.09 0.122 0.026 0.025 N 0.009 0.37 0.286 0.015 0.0009 16.84 2.49 3.45 5.12 0.086 0.027 0.019 O 0.012 0.32 0.348 0.017 0.0012 16.67 2.56 1.55 4.86 0.201 0.028 0.029 P 0.014 0.35 0.300 0.015 0.0011 16.43 2.57 2.56 5.48 0.118 0.024 0.031 Q 0.013 0.32 0.359 0.016 0.0011 16.95 2.51 2.46 3.38 0.246 0.026 0.018 R 0.013 0.36 0.304 0.016 0.0012 16.91 2.63 2.47 4.73 0.280 0.027 0.025 S 0.009 0.34 0.346 0.015 0.0011 17.33 2.48 2.55 4.39 0.020 0.025 0.035 T 0.009 0.34 0.362 0.016 0.0009 17.20 2.62 2.48 4.82 0.228 0.092 0.024 U 0.010 0.28 0.286 0.015 0.0011 17.15 2.57 2.46 4.33 0.187 0.027 0.093 V 0.015 0.35 0.339 0.017 0.0010 16.98 2.51 2.51 4.61 0.055 0.026 0.019 W 0.008 0.31 0.375 0.014 0.0010 17.33 2.57 2.50 4.78 0.238 0.025 0.021 X 0.008 0.31 0.375 0.014 0.0010 17.33 2.57 2.50 4.78 0.238 0.025 0.021 Y 0.008 0.31 0.375 0.014 0.0010 17.33 2.57 2.50 4.78 0.238 0.025 0.021 Z 0.008 0.30 0.311 0.015 0.0013 16.82 2.56 2.56 4.77 0.076 0.024 0.023 AA 0.006 0.90 0.050 0.016 0.0011 16.33 3.48 1.54 3.37 0.050 0.023 0.009 AB 0.032 0.02 0.520 0.013 0.0009 16.09 2.29 2.58 4.98 0.110 0.024 0.039 AC 0.015 0.30 0.347 0.016 0.0013 16.70 2.50 2.64 4.81 0.075 0.025 0.025 Formula (1) (*3) Steel Composition (mass %) Middle No. O W Co Other value Result Remarks (*4) A 0.002 1.06 0.499 — 26.3 Satisfactory PS B 0.003 1.10 0.496 — 27.6 Satisfactory PS C 0.002 1.12 0.026 — 25.6 Satisfactory PS D 0.002 1.35 0.200 — 18.5 Satisfactory PS E 0.002 1.07 0.031 — 27.0 Satisfactory PS F 0.002 1.29 0.510 — 27.2 Satisfactory PS G 0.003 1.07 0.109 — 24.7 Satisfactory PS H 0.002 1.32 0.212 — 25.4 Satisfactory PS I 0.003 1.28 0.046 — 30.0 Satisfactory PS J 0.002 1.40 0.492 — 28.5 Satisfactory PS K 0.002 1.09 0.174 — 20.8 Satisfactory PS L 0.003 1.23 0.396 — 33.5 Satisfactory PS M 0.002 1.20 0.080 — 19.8 Satisfactory PS N 0.003 1.19 0.121 — 24.3 Satisfactory PS O 0.003 1.16 0.450 — 26.0 Satisfactory PS P 0.002 1.42 0.274 — 19.7 Satisfactory PS Q 0.002 1.17 0.502 — 35.3 Satisfactory PS R 0.002 1.29 0.263 — 27.7 Satisfactory PS S 0.002 1.30 0.522 — 30.7 Satisfactory PS T 0.002 1.35 0.507 — 29.3 Satisfactory PS U 0.002 1.28 0.027 — 26.7 Satisfactory PS V 0.009 1.26 0.339 — 28.0 Satisfactory PS W 0.002 1.92 0.416 — 30.0 Satisfactory PS X 0.002 0.88 0.416 — 30.0 Satisfactory PS Y 0.002 1.26 1.323 — 30.0 Satisfactory PS Z 0.002 1.22 0.020 — 27.0 Satisfactory PS AA 0.002 1.07 0.187 — 44.7 Satisfactory PS AB 0.003 1.23 0.396 — 13.6 Satisfactory PS AC 0.002 1.15 0.364 V: 0.05, B: 0.005 24.5 Satisfactory PS (*1) The balance is Fe and incidental impurities (*2) Underline means outside of the range of the disclosed embodiments (*3) Formula (1): 13.0 ≤ −5.9 × (7.82 + 27C − 0.91Si + 0.21 Mn − 0.9Cr + Ni − 1.1 Mo + 0.2Cu + 11N) ≤ 55.0 (*4) PS: Present Steel, CS: Comparative Steel

TABLE-US-00002 TABLE 1-2 Steel Composition (mass %) No. C Si Mn P S Cr Mo Cu Ni Nb Al N O AD 0.012 0.30 0.338 0.018 0.0012 16.64 2.62 2.58 4.45 0.135 0.028 0.027 0.003 AE 0.011 0.33 0.316 0.016 0.0013 17.16 2.48 2.49 4.94 0.204 0.027 0.034 0.002 AF 0.012 0.31 0.361 0.017 0.0009 16.45 2.47 2.50 4.89 0.091 0.025 0.035 0.003 AG 0.014 0.31 0.374 0.017 0.0010 17.33 2.57 2.62 5.09 0.151 0.027 0.024 0.003 AH 0.017 0.35 0.336 0.018 0.0013 17.11 2.63 2.58 4.99 0.058 0.025 0.026 0.003 Al 0.013 0.28 0.366 0.017 0.0011 16.51 2.55 2.51 4.28 0.104 0.026 0.019 0.003 AJ 0.014 0.29 0.304 0.017 0.0013 16.91 2.57 2.51 4.50 0.101 0.026 0.015 0.002 AK 0.014 0.33 0.346 0.017 0.0011 17.33 2.45 2.50 5.24 0.126 0.025 0.032 0.002 AL 0.011 0.28 0.362 0.017 0.0012 17.20 2.55 2.50 5.05 0.136 0.025 0.016 0.002 AM 0.013 0.37 0.286 0.015 0.0012 17.15 2.57 2.50 5.21 0.098 0.028 0.022 0.003 AN 0.068 0.30 0.284 0.015 0.0009 16.44 2.46 2.63 4.53 0.240 0.027 0.019 0.002 AO 0.013 1.08 0.316 0.016 0.0011 17.06 2.54 2.64 5.09 0.204 0.028 0.023 0.003 AP 0.012 0.37 0.004 0.016 0.0010 16.56 2.54 2.60 4.64 0.173 0.027 0.032 0.003 AQ 0.016 0.29 0.366 0.055 0.0010 16.91 2.64 2.49 4.70 0.159 0.027 0.015 0.002 AR 0.015 0.37 0.325 0.014 0.0055 17.30 2.48 2.47 5.21 0.058 0.025 0.032 0.002 AS 0.009 0.32 0.292 0.014 0.0009 17.56 2.54 2.50 4.89 0.181 0.026 0.016 0.002 AT 0.009 0.29 0.296 0.016 0.0012 15.38 2.52 2.53 5.19 0.074 0.026 0.022 0.003 AU 0.011 0.35 0.327 0.015 0.0009 17.18 1.72 2.49 4.74 0.092 0.025 0.020 0.003 AV 0.016 0.35 0.298 0.016 0.0009 17.02 2.57 1.42 4.62 0.129 0.025 0.027 0.002 AW 0.014 0.37 0.305 0.015 0.0010 16.42 2.59 2.50 5.59 0.131 0.028 0.016 0.002 AX 0.012 0.30 0.319 0.016 0.0010 16.41 2.49 2.58 2.90 0.073 0.026 0.025 0.002 AZ 0.012 0.35 0.351 0.015 0.0011 16.87 2.59 2.63 5.22 0.124 0.107 0.023 0.002 BA 0.017 0.31 0.356 0.016 0.0012 16.65 2.54 2.57 5.04 0.109 0.024 0.109 0.002 BB 0.010 0.37 0.333 0.016 0.0010 16.79 2.49 2.49 4.79 0.176 0.025 0.033 0.015 BC 0.010 0.37 0.333 0.016 0.0010 16.79 2.49 2.49 4.79 0.176 0.025 0.033 0.002 BD 0.011 0.28 0.346 0.017 0.0009 16.99 2.63 2.51 4.52 0.148 0.026 0.028 0.002 BE 0.007 0.94 0.020 0.016 0.0011 17.45 3.40 1.63 3.36 0.060 0.027 0.008 0.002 BF 0.016 0.35 0.337 0.014 0.0011 17.04 2.59 2.60 4.28 — 0.026 0.016 0.003 BG 0.009 0.32 0.292 0.014 0.0009 18.13 2.54 2.50 4.89 0.181 0.026 0.016 0.002 BH 0.014 0.37 0.305 0.015 0.0010 16.42 2.59 2.50 6.09 0.131 0.028 0.016 0.002 Bl 0.012 0.30 0.319 0.016 0.0010 16.41 2.49 2.58 2.41 0.073 0.026 0.025 0.002 BJ 0.006 0.95 0.022 0.016 0.0011 17.93 3.40 1.63 2.93 0.059 0.025 0.008 0.002 Formula (1) (*3) Steel Composition (mass %) Middle No. W Co Other value Result Remarks (*4) AD 1.34 0.054 V: 0.70 27.4 Satisfactory PS AE 1.09 0.118 Ta: 0.1 26.4 Satisfactory PS AF 1.22 0.246 Ti: 0.131, Zr: 0.161 22.4 Satisfactory PS AG 1.12 0.583 Ca: 0.006, Mq: 0.0050 26.8 Satisfactory PS AH 1.09 0.088 REM: 0.181 26.3 Satisfactory PS Al 1.05 0.239 Sb: 0.77 27.6 Satisfactory PS AJ 1.35 0.213 B: 0.007, Ti: 0.102, Zr: 0.201 28.8 Satisfactory PS AK 1.07 0.198 V: 0.06, REM: 0.183 25.0 Satisfactory PS AL 1.29 0.638 B: 0.004, Ti: 0.218, Sn: 0.143 27.2 Satisfactory PS AM 1.07 0.819 Zr: 0.198, Mq: 0.0019 26.1 Satisfactory PS AN 1.32 0.396 — 16.5 Satisfactory CS AO 1.19 0.517 — 29.7 Satisfactory CS AP 1.37 0.032 — 25.9 Satisfactory PS AQ 1.20 0.313 — 27.7 Satisfactory CS AR 1.44 0.097 — 25.3 Satisfactory CS AS 1.08 0.428 — 30.6 Satisfactory PS AT 1.36 0.291 — 16.7 Satisfactory CS AU 1.42 0.038 — 23.8 Satisfactory CS AV 1.40 0.063 — 29.3 Satisfactory CS AW 1.41 0.599 — 20.2 Satisfactory PS AX 1.22 0.108 — 34.6 Satisfactory PS AZ 1.07 0.487 — 24.3 Satisfactory CS BA 1.05 0.074 — 17.5 Satisfactory CS BB 1.05 0.333 — 25.8 Satisfactory CS BC 0.42 0.333 — 25.8 Satisfactory CS BD 1.39 0.004 — 29.0 Satisfactory CS BE 1.43 0.444 — 50.2 Satisfactory PS BF 1.11 0.486 — 30.7 Satisfactory PS BG 1.08 0.428 — 33.7 Satisfactory CS BH 1.41 0.599 — 17.3 Satisfactory CS Bl 1.22 0.108 — 37.5 Satisfactory CS BJ 1.15 0.402 — 55.5 Unsatisfactory CS (*1) The balance is Fe and incidental impurities (*2) Underline means outside of the range of the disclosed embodiments (*3) Formula (1): 13.0 ≤ −5.9 × (7.82 + 27C − 0.91Si + 0.21Mn − 0.9Cr + Ni − 1.1Mo + 0.2Cu + 11N) ≤ 55.0 (*4) PS: Example Steel, CS: Comparative Steel

[0100] A test specimen was taken from the heat-treated test material (seamless steel pipe), and subjected to microstructure observation, a tensile test, and a corrosion resistance test. The test methods are as follows.

(1) Microstructure Observation

[0101] A test specimen for microstructure observation was taken from the heat-treated test material in such an orientation that a cross section orthogonal to the pipe axis direction was exposed for observation. The test specimen for microstructure observation was corroded with a Vilella's solution (a mixed reagent containing 2 g of picric acid, 10 ml of hydrochloric acid, and 100 ml of ethanol), and the structure was imaged with a scanning electron microscope (1,000 times magnification). The fraction (area ratio (%)) of the ferrite phase microstructure was then calculated with an image analyzer. Here, the area ratio was calculated as the volume ratio (%) of the ferrite phase.

[0102] Separately, an X-ray diffraction test specimen was taken from the heat-treated test material. The test specimen was ground and polished to have a measurement cross section (C cross section) orthogonal to the axial direction of pipe, and the fraction of the retained austenite (γ) phase microstructure was measured by an X-ray diffraction method. The fraction of the retained austenite phase microstructure was determined by measuring X-ray diffraction integral intensity for the (220) plane of the austenite phase (γ), and the (211) plane of the ferrite phase (α), and converting the calculated values using the following formula.

γ(volume ratio)=100/(1+(IαRγ/IγRα)),

wherein Iα is the integral intensity of α, Rα is the crystallographic theoretical value for α, Iγ is the integral intensity of γ, and Rγ is the crystallographic theoretical value for γ. The fraction of the martensitic phase is the remainder other than the fractions of the ferrite phase and retained γ phase.

(2) Tensile Test

[0103] An API (American Petroleum Institute) arc-shaped tensile test specimen was taken from the heat-treated test material in such an orientation that the test specimen had a tensile direction along the pipe axis direction. The tensile test was conducted according to the API specifications to determine tensile properties (yield strength YS). The steel was determined as being high strength and acceptable when it had a yield strength YS of 758 MPa or more, and unacceptable when it had a yield strength YS of less than 758 MPa.

(3) Corrosion Resistance Test (Carbon Dioxide Gas Corrosion Resistance Test, and Acid-Environment Corrosion Resistance Test)

[0104] A corrosion test specimen measuring 3 mm in thickness, 30 mm in width, and 40 mm in length was prepared from the heat-treated test material by machining, and subjected to corrosion tests to evaluate carbon dioxide gas corrosion resistance and acid-environment corrosion resistance.

[0105] The corrosion test to evaluate carbon dioxide gas corrosion resistance was conducted by immersing the corrosion test specimen in a test solution (a 20 mass % NaCl aqueous solution; liquid temperature: 200° C.; an atmosphere of 30 atm CO.sub.2 gas) in an autoclave for 14 days (336 hours). The corrosion rate was determined from the calculated reduction in the weight of the tested specimen measured before and after the corrosion test. The steel was determined as being acceptable when it had a corrosion rate of 0.127 mm/y or less, and unacceptable when it had a corrosion rate of more than 0.127 mm/y.

[0106] The corrosion test to evaluate acid-environment corrosion resistance was conducted by immersing the test specimen for 40 minutes in a 15 mass % hydrochloric acid solution that had been heated to 80° C. The corrosion rate was determined from the calculated reduction in the weight of the tested specimen measured before and after the corrosion test. The steel was determined as being acceptable when it had a corrosion rate of 600 mm/y or less, and unacceptable when it had a corrosion rate of more than 600 mm/y.

(4) Sulfide Stress Cracking Resistance Test (SSC Resistance Test)

[0107] A round rod-shaped test specimen (diameter Ø: 6.4 mm) was prepared from the test specimen material by machining, in compliance with NACE TM0177, Method A, and was subjected to a sulfide stress cracking resistance test (SSC resistance test) Here, “NACE” stands for National Association of Corrosion Engineering.

[0108] The SSC resistance test was conducted by immersing the test specimen in a test solution (a 20 mass % NaCl aqueous solution; liquid temperature: 25° C.; an atmosphere of 0.1 atm H.sub.2S and 0.9 atm CO.sub.2) kept in an autoclave and having an adjusted pH of 3.5 with addition of acetic acid and sodium acetate, and applying a stress equal to 90% of the yield stress for 720 hours in the solution. The tested specimen was observed for the presence or absence of cracking. The steel was determined as being acceptable when it did not have a crack after the test. In Table 2, the open circle (∘) means no cracking, and the cross mark (x) means cracking is present.

The results are presented in Table 2.

TABLE-US-00003 TABLE 2 Microstructure Steel (volume %) Yield Acid-environment Steel pipe M F A strength Corrosion corrosion No. No. (*1) (*1) (*1) YS (MPa) rate (mm/y) rate (mm/y) SSC Remarks A 1 59 29 12 964 0.030 550.7 Acceptable Example B 2 53 32 15 931 0.020 500.2 Acceptable Example C 3 60 29 11 968 0.025 525.7 Acceptable Example D 4 52 19 29 927 0.078 589.1 Acceptable Example E 5 56 29 15 945 0.110 579.4 Acceptable Example F 6 54 31 15 976 0.027 533.7 Acceptable Example G 7 58 27 15 903 0.027 534.9 Acceptable Example H 8 56 29 15 949 0.095 567.2 Acceptable Example I 9 51 35 14 922 0.088 581.1 Acceptable Example J 10 48 33 19 887 0.021 506.8 Acceptable Example K 11 72 22  6 1031  0.081 576.3 Acceptable Example L 12 47 41 12 901 0.026 530.6 Acceptable Example M 13 66 21 13 1000  0.093 584.6 Acceptable Example N 14 58 26 16 991 0.020 507.9 Acceptable Example O 15 61 30  9 892 0.103 562.2 Acceptable Example P 16 63 21 16 887 0.027 532.8 Acceptable Example Q 17 53 45  2 888 0.045 575.9 Acceptable Example R 18 57 34  9 950 0.027 534.0 Acceptable Example S 19 48 35 17 879 0.026 527.6 Acceptable Example T 20 49 39 12 911 0.099 584.3 Acceptable Example U 21 53 31 16 951 0.091 575.9 Acceptable Example V 22 54 31 15 934 0.072 578.9 Acceptable Example W 23 48 39 13 903 0.023 516.3 Acceptable Example X 24 54 35 11 941 0.086 578.4 Acceptable Example Y 25 53 36 11 927 0.023 516.3 Acceptable Example Z 26 56 30 14 950 0.083 577.6 Acceptable Example AA 27 42 55  3 870 0.040 579.0 Acceptable Example AB 28 71 12 17 988 0.033 566.0 Acceptable Example AC 29 59 26 15 963 0.028 539.5 Acceptable Example AD 30 57 32 11 952 0.030 550.0 Acceptable Example AE 31 57 30 13 949 0.024 521.3 Acceptable Example AF 32 63 24 13 985 0.030 551.3 Acceptable Example AG 33 53 30 17 929 0.021 505.4 Acceptable Example AH 34 53 28 19 930 0.023 516.5 Acceptable Example AI 35 61 30  9 971 0.033 562.8 Acceptable Example AJ 36 59 29 12 964 0.030 550.7 Acceptable Example AK 37 51 32 17 931 0.020 500.2 Acceptable Example AL 38 55 31 14 968 0.021 521.9 Acceptable Example AM 39 55 30 15 968 0.019 540.9 Acceptable Example AN 40 63 18 19 982 0.143 618.3 Unacceptable Comparative Example AO 41 51 35 14 921 0.139 616.4 Unacceptable Comparative Example AP 42 58 31 11 858 0.030 549.5 Acceptable Example AQ 43 55 32 13 944 0.135 605.3 Unacceptable Comparative Example AR 44 51 28 21 918 0.139 611.9 Unacceptable Comparative Example AS 45 25 61 14 850 0.018 504.6 Acceptable Present Example AT 46 76 16  8 1051  0.144 617.3 Unacceptable Comparative Example AU 47 61 27 12 976 0.151 623.1 Unacceptable Comparative Example AV 48 53 35 12 858 0.140 618.7 Unacceptable Comparative Example AW 49 63 22 15 860 0.026 531.3 Acceptable Example AX 50 57 40  3 858 0.073 587.9 Acceptable Example AZ 52 55 30 15 940 0.130 631.2 Unacceptable Comparative Example BA 53 61 17 22 982 0.129 608.6 Unacceptable Comparative Example BB 54 60 29 11 969 0.132 613.1 Unacceptable Comparative Example BC 55 63 27 10 981 0.136 618.5 Unacceptable Comparative Example BD 56 53 34 13 930 0.027 638.1 Acceptable Comparative Example BE 57 29 61 10 805 0.032 558.5 Acceptable Example BF 58 52 30 18 896 0.051 579.4 Acceptable Example BG 59 23 51 26 705 0.015 502.7 Acceptable Comparative Example BH 60 32 22 46 721 0.029 536.9 Acceptable Comparative Example BI 61 42 40 18 706 0.036 561.9 Acceptable Comparative Example BJ 62  6 67 27 641 0.011 502.1 Acceptable Comparative Example A 63 38 29 33 831 0.029 548.9 Acceptable Example B 64 37 32 31 821 0.018 505.1 Acceptable Example C 65 39 29 32 840 0.026 526.7 Acceptable Example Underline means outside of the range of the disclosed embodiments (*1) M: Martensitic phase, F: Ferrite phase, A: Retained austenite phase

[0109] The stainless steel seamless pipes of the Examples all had high strength with a yield strength YS of 758 MPa or more. The stainless steel seamless pipes of the Examples also had excellent corrosion resistance (carbon dioxide gas corrosion resistance) in a CO.sub.2- and Cl.sup.−-containing high-temperature corrosive environment of 200° C., excellent acid-environment corrosion resistance, and excellent sulfide stress cracking resistance.