MARTENSITIC STAINLESS STEEL SEAMLESS PIPE FOR OIL COUNTRY TUBULAR GOODS, AND METHOD FOR MANUFACTURING SAME

Abstract

The disclosure is intended to provide a martensitic stainless steel seamless pipe for oil country tubular goods having high strength and excellent sulfide stress corrosion cracking resistance and a method for manufacturing thereof. The martensitic stainless steel seamless pipe for oil country tubular goods has a composition that contains, in mass %, C: 0.0100% or more, Si: 0.5% or less, Mn: 0.25 to 0.50%, P: 0.030% or less, S: 0.005% or less, Ni: 4.6 to 8.0%, Cr: 10.0 to 14.0%, Mo: 1.0 to 2.7%, Al: 0.1% or less, V: 0.005 to 0.2%, N: 0.1% or less, Ti: 0.06 to 0.25%, Cu: 0.01 to 1.0%, and Co: 0.01 to 1.0%, in which C, Mn, Cr, Cu, Ni, Mo, W, Nb, N, and Ti satisfy predetermined relations, and the balance is Fe and incidental impurities. The martensitic stainless steel seamless pipe has a yield stress of 758 MPa or more.

Claims

1. A martensitic stainless steel seamless pipe for oil country tubular goods, the martensitic stainless steel seamless pipe having a yield stress of 758 MPa or more and a composition comprising, in mass %, C: 0.0100% or more, Si: 0.5% or less, Mn: 0.25 to 0.50%, P: 0.030% or less, S: 0.005% or less, Ni: 4.6 to 8.0%, Cr: 10.0 to 14.0%, Mo: 1.0 to 2.7%, Al: 0.1% or less, V: 0.005 to 0.2%, N: 0.1% or less, Ti: 0.06 to 0.25%, Cu: 0.01 to 1.0%, Co: 0.01 to 1.0%, and a balance including Fe and incidental impurities, the composition satisfying; (i) all of the relations in formula (4) below, wherein value (1), value (2), and value (3) are obtained from formulae (1), (2), and (3), respectively, and (ii) formulae (5) or (6) below,
−109.37C+7.307Mn+6.399Cr+6.329Cu+11.343Ni−13.529Mo+1.276W+2.925Nb+196.775N−2.621Ti−120.307  Formula (1)
−0.0278Mn+0.0892Cr+0.00567Ni+0.153Mo−0.0219W−1.984N+0.208Ti−1.83  Formula (2)
−1.324C+0.0533Mn+0.0268Cr+0.0893Cu+0.00526Ni+0.0222Mo−0.0132W−0.473N−0.5Ti−0.514  Formula (3)
−35.0≤value of (1)≤45.0, −0.600≤value of (2)≤−0.250, and −0.400≤value of (3)≤0.010  Formula (4)
Ti<6.0C  Formula (5)
10.1C<Ti,  Formula (6) wherein C, Mn, Cr, Cu, Ni, Mo, W, Nb, N, and Ti represent a content of each element in mass %, and the content is zero percent for element that is not present.

2. The martensitic stainless steel seamless pipe for oil country tubular goods according to claim 1, wherein the composition further comprises, in mass %, at least one element selected from Group (A) and/or Group (B): Group (A): Nb: 0.1% or less, and W: 1.0% or less, and Group (B): Ca: 0.010% or less, REM: 0.010% or less, Mg: 0.010% or less, and B: 0.010% or less.

3. (canceled)

4. A method for manufacturing a martensitic stainless steel seamless pipe for oil country tubular goods, the method comprising: forming a steel pipe from a steel pipe material having a composition comprising, in mass %, C: 0.0100% or more, Si: 0.5% or less, Mn: 0.25 to 0.50%, P: 0.030% or less, S: 0.005% or less, Ni: 4.6 to 8.0%, Cr: 10.0 to 14.0%, Mo: 1.0 to 2.7%, Al: 0.1% or less, V: 0.005 to 0.2%, N: 0.1% or less, Ti: 0.06 to 0.25%, Cu: 0.01 to 1.0%, Co: 0.01 to 1.0%, and a balance including Fe and incidental impurities, the composition satisfying: (i) all of the relations in formula (4) below, wherein value (1), value (2), and value (3) are obtained from formulae (1), (2), and (3), respectively, and (ii) formulae (5) or (6) below,
−109.37C+7.307Mn+6.399Cr+6.329Cu+11.343Ni−13.529Mo+1.276W+2.925Nb+196.775N−2.621Ti−120.307  Formula (1)
−0.0278Mn+0.0892Cr+0.00567Ni+0.153Mo−0.0219W−1.984N+0.208Ti−1.83  Formula (2)
−1.324C+0.0533Mn+0.0268Cr+0.0893Cu+0.00526Ni+0.0222Mo−0.0132W−0.473N−0.5Ti−0.514  Formula (3)
−35.0≤value of (1)≤45.0, −0.600≤value of (2)≤−0.250, and −0.400≤value of (3)≤0.010  Formula (4)
Ti<6.0C  Formula (5)
10.1C<Ti,  Formula (6) wherein C, Mn, Cr, Cu, Ni, Mo, W, Nb, N, and Ti represent a content of each element in mass %, and the content is zero percent for element that is not present; quenching the steel pipe by heating the steel pipe to a temperature equal to or greater than an Ac.sub.3 transformation point, and cooling the steel pipe to a cooling stop temperature of 100° C. or less; and tempering the steel pipe at a temperature equal to or less than an Ac.sub.1 transformation point.

5. A method for manufacturing a martensitic stainless steel seamless pipe for oil country tubular goods, the method comprising: forming a steel pipe from a steel pipe material having a composition comprising, in mass %, C: 0.0100% or more, Si: 0.5% or less, Mn: 0.25 to 0.50%, P: 0.030% or less, S: 0.005% or less, Ni: 4.6 to 8.0%, Cr: 10.0 to 14.0%, Mo: 1.0 to 2.7%, Al: 0.1% or less, V: 0.005 to 0.2%, N: 0.1% or less, Ti: 0.06 to 0.25%, Cu: 0.01 to 1.0%, Co: 0.01 to 1.0%, at least one element selected from Group (A) and/or Group (B): Group (A): Nb: 0.1% or less, and W: 1.0% or less, and Group (B): Ca: 0.010% or less, REM: 0.010% or less, Mg: 0.010% or less, and B: 0.010% or less, and a balance including Fe and incidental impurities, the composition satisfying: (i) all of the relations in formula (4) below, wherein value (1), value (2), and value (3) are obtained from formulae (1), (2), and (3), respectively, and (ii) formulae (5) or (6) below,
−109.37C+7.307Mn+6.399Cr+6.329Cu+11.343Ni−13.529Mo+1.276W+2.925Nb+196.775N−2.621Ti−120.307  Formula (1)
−0.0278Mn+0.0892Cr+0.00567Ni+0.153Mo−0.0219W−1.984N+0.208Ti−1.83  Formula (2)
−1.324C+0.0533Mn+0.0268Cr+0.0893Cu+0.00526Ni+0.0222Mo−0.0132W−0.473N−0.5Ti−0.514  Formula (3)
−35.0≤value of (1)≤45.0, −0.600≤value of (2)≤−0.250, and −0.400≤value of (3)≤0.010  Formula (4)
Ti<6.0C  Formula (5)
10.1C<Ti,  Formula (6) wherein C, Mn, Cr, Cu, Ni, Mo, W, Nb, N, and Ti represent a content of each element in mass %, and the content is zero percent for element that is not present; quenching the steel pipe by heating the steel pipe to a temperature equal to or greater than an Ac.sub.3 transformation point, and cooling the steel pipe to a cooling stop temperature of 100° C. or less; and tempering the steel pipe at a temperature equal to or less than an Ac.sub.1 transformation point.

Description

DETAILED DESCRIPTION

[0026] A martensitic stainless steel seamless pipe for oil country tubular goods of the present application contains, in mass %, C: 0.0100% or more, Si: 0.5% or less, Mn: 0.25 to 0.50%, P: 0.030% or less, S: 0.005% or less, Ni: 4.6 to 8.0%, Cr: 10.0 to 14.0%, Mo: 1.0 to 2.7%, Al: 0.1% or less, V: 0.005 to 0.2%, N: 0.1% or less, Ti: 0.06 to 0.25%, Cu: 0.01 to 1.0%, Co: 0.01 to 1.0%, and the balance Fe and incidental impurities,

[0027] the composition satisfying all of the relations in the formula (4) below with values of the following formulae (1), (2), and (3), and also satisfying the formula (5) or (6) below, the martensitic stainless steel seamless pipe having a yield stress of 758 MPa or more.

−109.37C+7.307Mn+6.399Cr+6.329Cu+11.343Ni−13.529Mo+1.276W+2.925Nb+196.775N−2.621Ti−120.307  Formula (1)

−0.0278Mn+0.0892Cr+0.00567Ni+0.153Mo−0.0219W−1.984N+0.208Ti−1.83  Formula (2)

−1.324C+0.0533Mn+0.0268Cr+0.0893Cu+0.00526Ni+0.0222Mo−0.0132W−0.473N−0.5Ti−0.514  Formula (3)

−35.0≤value of (1)≤545.0, −0.600≤value of (2)≤−0.250, and −0.400≤value of (3)≤0.010  Formula (4)

Ti<6.0C  Formula (5)

10.1C<Ti  Formula (6)

[0028] In the formulae, C, Mn, Cr, Cu, Ni, Mo, W, Nb, N, and Ti represent the content of each element in mass %, and the content is 0 (zero) percent for elements that are not contained.

[0029] The following describes the reasons for specifying the composition of a steel pipe of the disclosed embodiments. In the following, “%” means percent by mass, unless otherwise specifically stated.

C: 0.0100% or More

[0030] C is an important element involved in the strength of the martensitic stainless steel, and is effective at improving strength. C is also an element that contributes to improving corrosion resistance, and improves the sulfide stress corrosion cracking resistance. For these reasons, the C content is limited to 0.0100% or more in the disclosed embodiments. However, when C is contained in excess amounts, the hardness increases, and the steel becomes more susceptible to sulfide stress corrosion cracking. For this reason, carbon is contained in an amount of preferably 0.0400% or less. That is, the preferred C content is 0.0100 to 0.0400%. The C content is more preferably 0.0100 to 0.0300%, further preferably 0.0100 to 0.0200%.

Si: 0.5% or Less

[0031] Si acts as a deoxidizing agent, and is contained in an amount of preferably 0.05% or more. A Si content of more than 0.5% impairs carbon dioxide corrosion resistance and hot workability. For this reason, the Si content is limited to 0.5% or less. From the viewpoint of stably providing strength, the Si content is preferably 0.10% or more. The Si content is preferably 0.30% or less. More preferably, the Si content is 0.25% or less.

Mn: 0.25 to 0.50%

[0032] Mn is an element that improves strength. By contributing to repassivation, Mn improves the sulfide stress corrosion cracking resistance. Because Mn is an austenite forming element, Mn reduces formation of delta ferrite, which causes cracking or defect during pipe manufacture. A Mn content of 0.25% or more is needed to obtain these effects. When added in excess amounts, Mn precipitates into MnS, and impairs the sulfide stress corrosion cracking resistance. For this reason, the Mn content is limited to 0.25 to 0.50%. Preferably, the Mn content is 0.40% or less.

P: 0.030% or Less

[0033] P is an element that impairs carbon dioxide corrosion resistance, pitting corrosion resistance, and sulfide stress corrosion cracking resistance, and should desirably be contained in as small an amount as possible in the disclosed embodiments. However, an excessively small P content increases the manufacturing cost. For this reason, the P content is limited to 0.030% or less, which is a content range that does not cause a severe impairment of characteristics, and that is economically practical in industrial applications. Preferably, the P content is 0.015% or less.

S: 0.005% or Less

[0034] S is an element that seriously impairs hot workability, and should desirably be contained in as small an amount as possible. A reduced S content of 0.005% or less enables pipe production using an ordinary process, and the S content is limited to 0.005% or less in the disclosed embodiments. Preferably, the S content is 0.002% or less.

Ni: 4.6 to 8.0%

[0035] Ni strengthens the protective coating, and improves the corrosion resistance. That is, Ni contributes to improving the sulfide stress corrosion cracking resistance. Ni also increases steel strength by forming a solid solution. Ni needs to be contained in an amount of 4.6% or more to obtain these effects. With a Ni content of more than 8.0%, the martensitic phase becomes less stable, and the strength decreases. For this reason, the Ni content is limited to 4.6 to 8.0%. The Ni content is preferably 4.6 to 7.6%, more preferably 4.6 to 6.8%.

Cr: 10.0 to 14.0%

[0036] Cr is an element that forms a protective coating, and improves the corrosion resistance. The required corrosion resistance for oil country tubular goods can be provided when Cr is contained in an amount of 10.0% or more. A Cr content of more than 14.0% facilitates ferrite formation, and a stable martensitic phase cannot be provided. For this reason, the Cr content is limited to 10.0 to 14.0%. The Cr content is preferably 11.0% or more, more preferably 11.2% or more. The Cr content is preferably 13.5% or less.

Mo: 1.0 to 2.7%

[0037] Mo is an element that improves the resistance against pitting corrosion by Cl.sup.−. Mo needs to be contained in an amount of 1.0% or more to obtain the corrosion resistance necessary for a severe corrosive environment. Mo is also an expensive element, and a Mo content of more than 2.7% increases the manufacturing cost. A Mo content of more than 2.7% also produces areas of higher Mo concentrations in the passive film, which promote breaking of the passive film, and impair the sulfide stress corrosion cracking resistance. For this reason, the Mo content is limited to 1.0 to 2.7%. The Mo content is preferably 1.2% or more, more preferably 1.5% or more. The Mo content is preferably 2.6% or less, more preferably 2.5% or less.

Al: 0.1% or Less

[0038] Al acts as a deoxidizing agent, and an Al content of 0.01% or more is preferred for obtaining this effect. However, Al has an adverse effect on toughness when contained in an amount of more than 0.1%. For this reason, the Al content is limited to 0.1% or less in the disclosed embodiments. The Al content is preferably 0.01% or more, and is preferably 0.03% or less.

V: 0.005 to 0.2%

[0039] V needs to be contained in an amount of 0.005% or more to improve steel strength through precipitation hardening, and to improve sulfide stress corrosion cracking resistance. Because a V content of more than 0.2% impairs toughness, the V content is limited to 0.005 to 0.2% in the disclosed embodiments. The V content is preferably 0.008% or more, and is preferably 0.18% or less.

N: 0.1% or Less

[0040] N is an element that acts to increase strength by forming a solid solution in the steel, in addition to improving pitting corrosion resistance. However, N forms various nitride inclusions, and impairs pitting corrosion resistance when contained in an amount of more than 0.1%. For this reason, the N content is limited to 0.1% or less in the disclosed embodiments. Preferably, the N content is 0.010% or less.

Ti: 0.06 to 0.25%

[0041] When contained in an amount of 0.06% or more, Ti reduces the solid-solution carbon by forming carbides, and improves the sulfide stress corrosion cracking resistance by reducing hardness. However, when contained in an amount of more than 0.25%, Ti generates TiN in the form of an inclusion, which potentially becomes an initiation point of pitting corrosion, and impairs the sulfide stress corrosion cracking resistance. For this reason, the Ti content is limited to 0.06 to 0.25%. The Ti content is preferably 0.08% or more. The Ti content is preferably 0.15% or less.

Cu: 0.01 to 1.0%

[0042] Cu is contained in an amount of 0.01% or more to strengthen the protective coating, and improve the sulfide stress corrosion cracking resistance. However, when contained in an amount of more than 1.0%, Cu precipitates into CuS, and impairs hot workability. Because Cu is an austenite forming element, Cu, when contained in an amount of more than 1.0%, increases the amount of retained austenite, and impairs the sulfide stress corrosion cracking resistance as a result of increased hardness. For this reason, the Cu content is limited to 0.01 to 1.0%. The Cu content is preferably 0.01 to 0.8%, more preferably 0.01 to 0.5%.

Co: 0.01 to 1.0%

[0043] Co is an element that improves the pitting corrosion resistance, in addition to reducing hardness by raising the Ms point and promoting a transformation. Co needs to be contained in an amount of 0.01% or more to obtain these effects. However, an excessively high Co content may impair toughness, and increases the material cost. When contained in an amount of more than 1.0%, Co increases the amount of retained austenite, and impairs the sulfide stress corrosion cracking resistance as a result of increased hardness. For this reason, the Co content is limited to 0.01 to 1.0% in the disclosed embodiments. The Co content is preferably 0.03% or more, and is preferably 0.6% or less.

[0044] In the disclosed embodiments, C, Mn, Cr, Cu, Ni, Mo, N, and Ti are contained in the foregoing amounts, and these elements, with optionally contained W and Nb, are contained in such amounts that the values of the following formulae (1), (2), and (3) satisfy the formula (4) below.

−109.37C+7.307Mn+6.399Cr+6.329Cu+11.343Ni−13.529Mo+1.276W+2.925Nb+196.775N−2.621Ti−120.307  Formula (1)

−0.0278Mn+0.0892Cr+0.00567Ni+0.153Mo−0.0219W−1.984N+0.208Ti−1.83  Formula (2)

−1.324C+0.0533Mn+0.0268Cr+0.0893Cu+0.00526Ni+0.0222Mo−0.0132W−0.473N−0.5Ti−0.514  Formula (3)

−35.0≤value of (1)≤545.0, −0.600≤value of (2)≤−0.250, and −0.400≤value of (3)≤0.010  Formula (4)

[0045] In the formulae, C, Mn, Cr, Cu, Ni, Mo, W, Nb, N, and Ti represent the content of each element in mass % (the content is 0 (zero) percent for elements that are not contained).

[0046] Formula (1) correlates with an amount of retained austenite (retained γ). By reducing the value of (1), the retained austenite decreases, and the sulfide stress corrosion cracking resistance improves as a result of decreased hardness.

[0047] Formula (2) correlates with repassivation potential. A passive film regenerates more easily, and repassivation improves when C, Mn, Cr, Cu, Ni, Mo, N, and Ti (and, optionally, W and Nb) are contained in such amounts that the value of formula (1) satisfies the range of formula (4), and when Mn, Cr, Ni, Mo, N, and Ti (and, optionally, W) are contained in such amounts that the value of formula (2) satisfies the range of formula (4).

[0048] Formula (3) correlates with pitting corrosion potential. It is possible to reduce generation of pitting corrosion, which becomes an initiation point of sulfide stress corrosion cracking, and to greatly improve sulfide stress corrosion cracking resistance when C, Mn, Cr, Cu, Ni, Mo, N, and Ti (and, optionally, W and Nb) are contained in such amounts that the value of formula (1) satisfies the range of formula (4), and when C, Mn, Cr, Cu, Ni, Mo, N, and Ti (and, optionally, W) are contained in such amounts that the value of formula (3) satisfies the range of formula (4).

[0049] It should be noted here that, with the value of (1) satisfying the range of formula (4), the hardness increases when the value of (1) is 10 or more. However, with the value of (2) and the value of (3) satisfying the ranges of formula (4), it is possible to achieve notable regeneration of passive film, and great reduction of pitting corrosion, with the result that the sulfide stress corrosion cracking resistance improves.

[0050] Preferably, the value of (1) is −30.0 or more. The value of (1) is preferably 45.0 or less, more preferably 40.0 or less.

[0051] The value of (2) is preferably −0.550 or more, more preferably −0.530 or more. Preferably, the value of (2) is −0.255 or less.

[0052] The value of (3) is preferably −0.350 or more, more preferably −0.320 or more. Preferably, the value of (3) is 0.008 or less.

[0053] C and Ti are contained so as to satisfy the following formula (5) or (6).

Ti<6.0C  Formula (5)

10.1C<Ti  Formula (6)

[0054] In the formulae, C and Ti represent the content of each element in mass % (the content is 0 (zero) percent for elements that are not contained).

[0055] C and Ti are elements involved in hardness. It is possible to decrease hardness by containing Ti. However, when contained, Ti forms Ti-base inclusions, and impairs the sulfide stress corrosion cracking resistance. The hardness decreases with reduced C contents. However, it becomes difficult to obtain the desired strength. By containing C and Ti so as to satisfy the formula (5) or (6), the impairment of sulfide stress corrosion cracking resistance due to inclusions, and the detrimental effect of inclusions on strength can be minimized, and the sulfide stress corrosion cracking resistance improves as a result of decreased hardness. In formula (5), Ti is preferably more than 4.4C. In formula (6), Ti is preferably less than 20.0C.

[0056] The balance is Fe and incidental impurities in the composition.

[0057] In addition to these basic components, the composition may further contain at least one optional element selected from Nb: 0.1% or less, and W: 1.0% or less, as needed. Nb forms carbides, and can reduce hardness by reducing solid-solution carbon. However, Nb may impair toughness when contained in excessively large amounts. W is an element that improves the pitting corrosion resistance. However, W may impair toughness, and increases the material cost when contained in excessively large amounts. For this reason, Nb, when contained, is contained in a limited amount of 0.1% or less, and W, when contained, is contained in a limited amount of 1.0% or less.

[0058] One or more selected from Ca: 0.010% or less, REM: 0.010% or less, Mg: 0.010% or less, and B: 0.010% or less may be contained as optional elements, as needed. Ca, REM, Mg, and B are elements that improve the corrosion resistance by controlling the shape of inclusions. The desired contents for providing this effect are Ca: 0.0005% or more, REM: 0.0005% or more, Mg: 0.0005% or more, and B: 0.0005% or more. Ca, REM, Mg, and B impair toughness and carbon dioxide corrosion resistance when contained in amounts of more than Ca: 0.010%, REM: 0.010%, Mg: 0.010%, and B: 0.010%. For this reason, the contents of Ca, REM, Mg, and B, when contained, are limited to Ca: 0.010% or less, REM: 0.010% or less, Mg: 0.010% or less, and B: 0.010% or less.

[0059] In the disclosed embodiments, aside from the dominant-phase martensite, the microstructure may include delta ferrite and retained austenite, though the microstructure is not particularly limited. Preferably, delta ferrite should be reduced as much as possible because delta ferrite causes cracking or defect during pipe manufacture. Retained austenite leads to increased hardness, and is contained in an amount of preferably 0.0 to 10.5% by volume.

[0060] The following describes a preferred method for manufacturing a stainless steel seamless pipe for oil country tubular goods of the present application.

[0061] In the present application, a steel pipe material of the foregoing composition is used. However, the method of production of a stainless steel seamless pipe used as a steel pipe material is not particularly limited, and any known seamless pipe manufacturing method may be used.

[0062] Preferably, a molten steel of the foregoing composition is made into steel using a smelting process such as by using a converter, and formed into a steel pipe material, for example, a billet, using a method such as continuous casting, or ingot casting-blooming. The steel pipe material is then heated, and 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 the foregoing composition.

[0063] The process after the production of the steel pipe from the steel pipe material is not particularly limited. Preferably, the steel pipe is subjected to quenching in which the steel pipe is heated to a temperature equal to or greater than the Ac.sub.3 transformation point, and cooled to a cooling stop temperature of 100° C. or less, followed by tempering at a temperature equal to or less than the Ac.sub.1 transformation point.

Quenching

[0064] In the disclosed embodiments, the steel pipe is subjected to quenching in which the steel pipe is reheated to a temperature equal to or greater than the Ac.sub.3 transformation point, held for preferably at least 5 min, and cooled to a cooling stop temperature of 100° C. or less. This makes it possible to produce a refined, tough martensitic phase. When the heating temperature of quenching is less than the Ac.sub.3 transformation point, the microstructure cannot be heated into the austenite single-phase region, and a sufficient martensitic microstructure does not occur in the subsequent cooling, with the result that the desired high strength cannot be obtained. For this reason, the quenching heating temperature is limited to a temperature equal to or greater than the Ac.sub.3 transformation point. The cooling method is not particularly limited. Typically, the steel pipe is air cooled (at a cooling rate of 0.05° C./s or more and 20° C./s or less) or water cooled (at a cooling rate of 5° C./s or more and 100° C./s or less). The cooling rate conditions are not limited either.

Tempering

[0065] The quenched steel pipe is tempered. The tempering is a process in which the steel pipe is heated to a temperature equal to or less than the Ac.sub.1 transformation point, held for preferably at least 10 min, and air cooled. When the tempering temperature is higher than the Ac.sub.1 transformation point, the martensitic phase precipitates after the tempering, and it is not possible to provide the desired high toughness and excellent corrosion resistance. For this reason, the tempering temperature is limited to a temperature equal to or less than the Ac.sub.1 transformation point. The Ac.sub.3 transformation point (° C.) and Ac.sub.1 transformation point (° C.) can be measured by a Formaster test by giving a heating and cooling temperature history to a test piece, and finding the transformation point from a microdisplacement due to expansion and contraction.

EXAMPLES

[0066] The disclosed embodiments are further described below through Examples.

[0067] Molten steels containing the components shown in Table 1 were made into steel with a converter, and cast into billets (steel pipe material) by continuous casting. The billet was hot worked into a pipe with a model seamless rolling mill, and cooled by air cooling or water cooling to produce a seamless steel pipe measuring 83.8 mm in outer diameter and 12.7 mm in wall thickness.

[0068] The formulae (1), (2), and (3) presented in Table 1 are as follows. The table shows whether the values of these formulae satisfy the formula (4) below.

[0069] The formulae (5) and (6) presented in Table 1 are as follows. The table shows whether the steels satisfy which of formulae (5) and (6), and a steel satisfying neither of these formulae is indicated by “out of range”.

−109.37C+7.307Mn+6.399Cr+6.329Cu+11.343Ni−13.529Mo+1.276W+2.925Nb+196.775N−2.621Ti−120.307  Formula (1)

−0.0278Mn+0.0892Cr+0.00567Ni+0.153Mo−0.0219W−1.984N+0.208Ti−1.83  Formula (2)

−1.324C+0.0533Mn+0.0268Cr+0.0893Cu+0.00526Ni+0.0222Mo−0.0132W−0.473N−0.5Ti−0.514  Formula (3)

−35.0≤value of (1)≤45.0, −0.600≤value of (2)≤−0.250, and −0.400≤value of (3)≤0.010  Formula (4)

Ti<6.0C  Formula (5)

10.1C<Ti  Formula (6)

[0070] In the formulae, C, Mn, Cr, Cu, Ni, Mo, W, Nb, N, and Ti represent the content of each element in mass % (the content is 0 (zero) percent for elements that are not contained).

[0071] Each seamless steel pipe was cut to obtain a test material, which was then subjected to quenching and tempering under the conditions shown in Table 2. In quenching, the steel pipes were cooled by air cooling (cooling rate: 0.5° C./s) or water cooling (cooling rate: 25° C./s).

[0072] An arc-shaped tensile test specimen specified by API standard was taken from the quenched and tempered test material, and the tensile properties (yield stress YS, tensile stress TS) were determined in a tensile test conducted according to the API specification. A test piece (4-mm diameter×10 mm) was taken from the quenched test material, and the Ac.sub.3 and Ac.sub.1 points (° C.) in Table 2 were measured in a Formaster test. Specifically, the test piece was heated to 500° C. at 5° C./s, and further heated to 920° C. at 0.25° C./s. The test piece was then held for 10 minutes, and cooled to room temperature at 2° C./s. The Ac.sub.3 and Ac.sub.1 points (° C.) were determined by detecting the expansion and contraction occurring in the test piece with this temperature history.

[0073] The SSC test was conducted according to NACE TM0177, Method A. The test environment was created by adjusting the pH of a test solution (a 0.165 mass % NaCl aqueous solution; liquid temperature: 25° C.; H.sub.2S: 1 bar; CO.sub.2 bal.) to 3.5 with the addition of sodium acetate and hydrochloric acid. In the test, a stress 90% of the yield stress was applied for 720 hours in the solution. Samples were determined as being acceptable when there was no crack in the test piece after the test, and unacceptable when the test piece had a crack after the test.

[0074] The results are presented in Table 2.

TABLE-US-00001 TABLE 1 Steel Composition (mass %) No. C Si Mn P S Ni Cr Mo Al V N Ti Cu Co Nb, W A 0.0108 0.18 0.31 0.014 0.001 5.84 11.9 1.88 0.027 0.046 0.0041 0.062 0.16 0.27 — B 0.0106 0.20 0.28 0.015 0.001 5.90 12.0 1.91 0.040 0.044 0.0052 0.060 0.21 0.23 — C 0.0112 0.19 0.35 0.017 0.001 6.03 12.4 2.20 0.042 0.014 0.0065 0.114 0.07 0.09 — D 0.0116 0.20 0.29 0.015 0.001 5.85 11.9 2.02 0.032 0.038 0.0070 0.069 0.15 0.16 — E 0.0134 0.18 0.47 0.016 0.001 7.56 13.8 1.21 0.038 0.020 0.0064 0.152 0.50 0.41 W: 0.11 F 0.0155 0.17 0.27 0.014 0.001 4.81 11.2 2.59 0.039 0.024 0.0087 0.209 0.09 0.08 — G 0.0133 0.20 0.26 0.015 0.001 7.23 13.0 2.33 0.032 0.051 0.0048 0.171 0.31 0.32 — H 0.0124 0.19 0.48 0.015 0.001 6.64 11.8 1.31 0.042 0.044 0.0041 0.070 0.42 0.24 Nb: 0.04 I 0.0108 0.18 0.49 0.013 0.001 5.99 13.9 2.03 0.039 0.044 0.0055 0.062 0.98 0.14 Nb: 0.02 J 0.0133 0.19 0.26 0.014 0.001 5.46 11.0 1.74 0.044 0.037 0.0079 0.245 0.02 0.06 — K 0.0094 0.21 0.43 0.015 0.001 5.21 11.6 1.94 0.029 0.015 0.0103 0.117 0.34 0.33 — L 0.0134 0.17 0.23 0.013 0.001 6.72 12.0 1.64 0.045 0.025 0.0143 0.210 0.47 0.32 — M 0.0146 0.18 0.39 0.014 0.001 4.52 13.8 1.36 0.037 0.045 0.0063 0.076 0.24 0.41 — N 0.0105 0.19 0.44 0.015 0.001 6.37 12.3 2.81 0.038 0.017 0.0074 0.148 0.17 0.16 Nb: 0.02 O 0.0151 0.20 0.29 0.014 0.001 5.29 12.8 1.65 0.041 0.033 0.0043 0.054 0.51 0.38 — P 0.0119 0.18 0.40 0.016 0.001 5.63 11.7 1.93 0.042 0.028 0.0053 0.128 1.09 0.17 Nb: 0.04 Q 0.0108 0.17 0.47 0.014 0.001 6.28 11.5 2.68 0.039 0.015 0.0134 0.062 0.67 1.08 — R 0.0124 0.19 0.42 0.015 0.001 7.72 13.5 1.21 0.040 0.009 0.0156 0.067 0.83 0.31 Nb: 0.02, W: 0.56 S 0.0487 0.20 0.27 0.014 0.001 4.71 10.8 2.63 0.036 0.013 0.0041 0.211 0.05 0.24 — T 0.0477 0.20 0.25 0.015 0.001 7.74 13.8 2.62 0.033 0.048 0.0048 0.208 0.84 0.41 — U 0.0117 0.19 0.45 0.015 0.001 4.79 11.1 1.44 0.040 0.029 0.0142 0.065 0.03 0.15 Nb: 0.04, W: 0.55 V 0.0102 0.18 0.49 0.016 0.001 7.98 13.9 1.98 0.029 0.015 0.0039 0.060 1.00 0.42 — W 0.0744 0.19 0.25 0.013 0.001 4.62 10.2 2.03 0.041 0.042 0.0200 0.250 0.01 0.04 W: 0.91 X 0.0123 0.20 0.33 0.014 0.001 5.13 11.2 2.55 0.035 0.013 0.0069 0.097 0.57 0.26 — Y 0.0234 0.18 0.51 0.014 0.001 5.14 11.7 2.45 0.046 0.037 0.0165 0.128 0.44 0.19 — Z 0.0114 0.20 0.35 0.015 0.001 8.06 12.4 1.56 0.040 0.021 0.0079 0.065 0.26 0.08 — AA 0.0331 0.17 0.34 0.016 0.001 6.54 13.1 2.31 0.037 0.004 0.0210 0.140 0.50 0.50 — AB 0.0157 0.19 0.42 0.015 0.001 6.33 12.2 2.62 0.044 0.064 0.0097 0.190 — 0.53 — Value of Value of Value of Steel Composition (mass %) formula formula formula Formula (5) No. Ca, REM, Mg, B (1) (*1) (2) (*2) (3) (*3) Ti/C or (6) (*4) Remarks A — −0.6 −0.452 −0.139 5.7 (5) Compliant Example B —  0.6 −0.440 −0.132 5.7 (5) Compliant Example C —  0.4 −0.352 −0.151 10.2 (6) Compliant Example D B: 0.002 −2.1 −0.434 −0.144 5.9 (5) Compliant Example E — 43.5 −0.368 −0.106 11.3 (6) Compliant Example F Ca: 0.003 −27.1  −0.389 −0.238 13.5 (6) Compliant Example G Ca: 0.002, REM: 0.002 16.3 −0.254 −0.140 12.9 (6) Compliant Example H — 18.3 −0.546 −0.124 5.6 (5) Compliant Example I Ca: 0.002 18.7 −0.257  0.001 5.7 (5) Compliant Example J Mg: 0.003 −10.0  −0.524 −0.280 18.4 (6) Compliant Example K — −7.2 −0.477 −0.155 12.4 (6) Comparative Example L — 16.0 −0.462 −0.196 15.7 (6) Comparative Example M —  4.7 −0.373 −0.108 5.2 (5) Comparative Example N — −3.1 −0.263 −0.141 14.1 (6) Comparative Example O —  3.7 −0.411 −0.095 3.6 (5) Comparative Example P —  1.7 −0.454 −0.092 10.8 (6) Comparative Example Q — −2.8 −0.385 −0.080 5.7 (5) Comparative Example R — 47.9 −0.438 −0.053 5.4 (5) Comparative Example S — −36.1  −0.409 −0.294 4.3 (5) Comparative Example T — 22.7 −0.128 −0.126 4.4 (5) Comparative Example U — −8.8 −0.632 −0.195 5.6 (5) Comparative Example V — 41.8 −0.251  0.015 5.9 (5) Comparative Example W — −31.9  −0.598 −0.402 3.4 (5) Comparative Example X Ca: 0.002 −19.2  −0.414 −0.130 7.9 Out of Comparative Example range Y — −13.4  −0.403 −0.155 5.5 (5) Comparative Example Z — 33.7 −0.451 −0.114 5.7 (5) Comparative Example AA — 12.2 −0.293 −0.138 4.2 (5) Comparative Example AB — −3.1 −0.296 −0.194 12.1 (6) Comparative Example * Underline means outside the range of the application • The balance is Fe and incidental impurities (*1) Formula (1): −109.37C + 7.307Mn + 6.399Cr + 6.329Cu + 11.343Ni − 13.529Mo + 1.276W + 2.925Nb + 196.775N − 2.621Ti − 120.307 (*2) Formula (2): −0.0278Mn + 0.0892Cr + 0.00567Ni + 0.153Mo − 0.0219W − 1.984N + 0.208Ti − 1.83 (*3) Formula (3): −1.324C + 0.0533Mn + 0.0268Cr + 0.0893Cu + 0.00526Ni + 0.0222Mo − 0.0132W − 0.473N − 0.5Ti − 0.514 (*4) Formula (5): Ti < 6.0C, Formula (6): 10.1C < Ti

TABLE-US-00002 TABLE 2 SSC Quenching Tensile properties resistance Cooling Tempering Yield Tensile test Steel Ac.sub.3 Heating Holding stop Ac.sub.1 Heating Holding stress stress Presence pipe Steel point temp. time Cooling temp. point temp. time YS TS or absence No. No. (° C.) (° C.) (min) method (° C.) (° C.) (° C.) (min) (MPa) (MPa) of cracking Remarks 1 A 745 920 20 Water cooling 25 645 595 60 818 852 Absent Present Example 2 B 750 920 20 Air cooling 25 650 605 60 787 846 Absent Present Example 3 C 755 920 20 Water cooling 25 645 550 30 823 857 Absent Present Example 4 D 745 920 20 Air cooling 25 645 510 30 859 881 Absent Present Example 5 E 740 810 20 Water cooling 25 655 600 45 769 819 Absent Present Example 6 F 730 810 20 Air cooling 25 640 560 45 826 869 Absent Present Example 7 G 775 920 20 Water cooling 25 660 580 60 798 844 Absent Present Example 8 H 750 920 20 Water cooling 25 640 500 60 865 901 Absent Present Example 9 I 745 900 20 Water cooling 25 655 600 30 778 815 Absent Present Example 10 J 730 920 20 Air cooling 25 640 585 60 800 839 Absent Present Example 11 A 745 705 20 Water cooling 25 645 595 60 715 804 Absent Comparative Example 12 B 750 920 20 Air cooling 25 650 680 60 688 780 Absent Comparative Example 13 K 740 920 20 Air cooling 25 635 565 60 804 864 Present Comparative Example 14 L 735 810 20 Water cooling 25 650 580 45 796 847 Present Comparative Example 15 M 750 810 20 Air cooling 25 650 595 45 777 835 Present Comparative Example 16 N 745 900 20 Water cooling 25 660 575 30 823 894 Present Comparative Example 17 O 745 810 20 Air cooling 25 645 600 60 762 856 Present Comparative Example 18 P 755 810 20 Water cooling 25 650 525 30 851 896 Present Comparative Example 19 Q 760 920 20 Water cooling 25 660 585 30 819 871 Present Comparative Example 20 R 760 920 20 Air cooling 25 655 545 60 833 896 Present Comparative Example 21 S 740 810 20 Air cooling 25 640 570 60 824 886 Present Comparative Example 22 T 765 920 20 Water cooling 25 660 535 45 842 895 Present Comparative Example 23 U 750 920 20 Water cooling 25 645 585 60 786 883 Present Comparative Example 24 V 750 920 20 Air cooling 25 650 595 60 768 846 Present Comparative Example 25 W 745 920 20 Air cooling 25 645 555 60 841 897 Present Comparative Example 26 X 735 900 20 Water cooling 25 640 585 60 793 872 Present Comparative Example 27 Y 760 920 20 Water cooling 25 650 590 60 806 846 Present Comparative Example 28 Z 725 810 20 Air cooling 25 635 600 60 747 809 Absent Comparative Example 29 AA 750 900 20 Air cooling 25 640 590 30 782 829 Present Comparative Example 30 AB 755 920 20 Water cooling 25 645 580 45 812 855 Present Comparative Example * Underline means outside the range of the application

[0075] The steel pipes of the present examples all had high strength with a yield stress of 758 MPa or more, demonstrating that the steel pipes were martensitic stainless steel seamless pipes having excellent SSC resistance that do not crack even when placed under a stress in a H.sub.2S-containing environment. On the other hand, in Comparative Examples outside the range of the application, the steel pipes did not have the desired high strength or desirable SSC resistance.