SEAMLESS PIPE OF MARTENSITIC STAINLESS STEEL FOR OIL WELL PIPE AND PROCESS FOR PRODUCING THE SAME

Abstract

 A seamless steel tube for OCTG which possesses both of high strength of yield strength YS of 95 ksi grade (665 to 758 MPa) and excellent low-temperature toughness, and a manufacturing method of the seamless steel tube are provided. To be more specific, to a stainless steel seamless tube having the composition which contains, by mass%, 0.020% or less C, 10 to 14% Cr, 3% or less Ni, 0.05% or less N, 0.03 to 0.2% Nb, and optionally further, 1.0% or less Si, 0.1 to 2.0% Mn, 0.020% or less P, 0.010% or less S, 0.10% or less Al, and Fe and unavoidable impurities as a balance, quenching in which the seamless tube is heated at a quenching temperature of A_c3 transformation temperature or above and, thereafter, the seamless tube is cooled to a temperature range of 100°C or less at a cooling rate of air cooling or more, and tempering which follows the quenching and in which the seamless tube is heated at a tempering temperature of 550°C or above and is cooled are applied. Due to such treatment, a martensitic stainless steel seamless tube for country tubular goods which has the tempered martensitic structure where a precipitated Nb quantity is 0.020% or more, and possesses both of high strength at yield strength of 95 ksi grade and the excellent low-temperature toughness of fracture transition temperature vTrs of -40°C or below, and allows the hot straightening. In addition to the above-mentioned composition, the seamless steel tube may further contain one or two kinds of elements selected from Cu, Mo and/or one or two kinds selected from V, Ti, B in addition to the above-mentioned composition.

Description

Technical Field

[0001] The present invention relates to a martensitic stainless steel seamless tube for oil country tubular goods, and more particularly to a seamless steel tube for OCTG which possesses both of high strength of 95 ksi (655 MPa) in terms of yield strength (YS) and excellent low-temperature toughness, and a manufacturing method thereof.

Background of the Invention

[0002] Recently, in view of sharp rise of oil price and exhaustion of petroleum expected in near future, the development of oil wells which have not been considered as targets of development because of extreme depths thereof, the development of oil wells or gas fields in a severe corrosion environment which contains carbon dioxide gas, chlorine ions or the like, the development of oil wells in a severe drilling environment such as a cold district or a seabed and the like have been promoted. A steel tube for an oil well used in such a severe environment is required to be made of a material which possesses high strength, excellent corrosion resistance and excellent toughness.

[0003] Conventionally, in oil wells and gas fields which contain carbon dioxide CO₂, chlorine ions Cl^- or the like, a 13%-Cr martensitic stainless steel tube has been popularly used as an oil country tubular good used in digging.

[0004] For example, JP-A-2002-363708 proposes martensitic stainless steel which contains 0.01 to 0.1% of C, 9 to 15% of Cr and 0.1% or less of Ni. Although the stainless steel exhibits relatively high C content so that the stainless steel possesses high strength, the stainless steel possesses high toughness. Accordingly, the stainless steel is preferably used for manufacturing oil country tubular goods. According to a technique disclosed in JP-A-2002-363708, it is estimated that by reducing a quantity of carbide present in a prior austenite grain boundary to 0.5 volume% or less, by setting a maximum minor axis of carbide to 10 to 200 nm and by setting a ratio between average Cr concentration and average Fe concentration in carbide to 0.4 or less, the precipitation of M₂₃C₆-type carbide is suppressed and the precipitation of M₃C-type carbide is accelerated thus largely improving toughness. To adjust the structure and the composition of such carbide within desired ranges, according to the technique disclosed in JP-A-2002-363708, the stainless steel is subject to tempering at a temperature of 450°C or below in such a manner that the stainless steel is subject to air cooling (standing to cool) after hot working, is held in air cooling (standing to cool) after solution treatment, or is subject to air cooling (standing to cool) after solution treatment.

[0005] However, when the stainless steel is subject to low-temperature tempering at 450°C or below using the technique disclosed in JP-A-2002-363708, straightening which follows the tempering treatment is performed at a low temperature and hence, processing strain is induced in the stainless steel in straightening thus giving rise to a drawback that irregularities in steel tube properties, particularly irregularities in yield strength YS are increased.

[0006] The present invention has been made to overcome such drawbacks of the prior art, and it is an object of the present invention to provide a seamless steel tube for OCTG which possesses both of high strength of yield strength YS of 95 ksi grade (655 to 758 MPa) or more and excellent low-temperature toughness, and a method of manufacturing the seamless steel; tube in a stable manner. Here, "excellent low-temperature toughness" implies a case where a fracture transition temperature vTrs in a Charpy impact test is -40°C or below.

Disclosure of the Invention

[0007] That is, the gist of the present invention lies in the followings.

(1) A martensitic stainless steel seamless tube for oil country tubular goods possessing both of high strength of yield strength of 95 ksi or more and excellent low-temperature toughness in which a fracture transition temperature vTrs in a Charpy impact test is -40°C or below, the seamless tube having the composition which contains, by mass%, 0.020% or less C, 10 to 14% Cr, 3% or less Ni, 0.03 to 0.2% Nb, 0.05% or less N, and Fe and unavoidable impurities as a balance, and having the structure where a precipitated Nb quantity is 0.020% or more in terms of Nb.

(2) A martensitic stainless steel seamless tube for oil country tubular goods possessing both of high strength of yield strength of 95 ksi or more and excellent low-temperature toughness in which a fracture transition temperature vTrs in a Charpy impact test is -40°C or below, the seamless tube having the composition which contains, by mass%, 0.020% or less C, 1.0% or less Si, 0.1 to 2.0% Mn, 0.020% or less P, 0.010% or less S, 0.10% or less Al, 10 to 14% Cr, 3% or less Ni, 0.03 to 0.2% Nb, 0.05% or less N, and Fe and unavoidable impurities as a balance, and having the structure where a precipitated Nb quantity is 0.020% or more in terms of Nb.

(3) In the martensitic stainless steel seamless tube for oil country tubular goods having the constitution (1) or (2), the seamless steel tube has the composition which contains said Nb such that a following formula (1) is satisfied in terms of C, Al and N.

        C-31/4Nb+7/6N-9/4Al≤-0.30     (1)

(Here, C, Nb, N, Al: contents of respective elements (mass%)). In the stainless steel having the constitution (1), it is treated that Al content is 0 mass%.

(4) In the martensitic stainless steel seamless tube for oil country tubular goods having any one of the constitutions (1) to (3), the composition of the seamless tube contains, by mass%, in addition to the above-mentioned composition, one or two kinds of elements selected from 2.0% or less Cu, and 2.0% or less Mo.

(5) In the martensitic stainless steel seamless tube for oil country tubular goods having any one of the constitutions (1) to (4), the composition of the seamless tube contains, by mass%, in addition to the above-mentioned composition, one kind, two or more kinds of elements selected from 0.20% or less V, 0.10% or less Ti and 0.005% or less B.

(6) A manufacturing method of a martensitic stainless steel seamless tube for oil country tubular goods possessing both of high strength and excellent low-temperature toughness, the method comprising: quenching in which a seamless tube having the composition which contains, by mass%, 0.020% or less C, 10 to 14% Cr, 3% or less Ni, 0.03 to 0.2% Nb, 0.05% or less N, and Fe and unavoidable impurities as a balance is heated at a quenching temperature of A_c3 transformation temperature or above and, thereafter, the seamless tube is cooled to a temperature range of 100°C or less from the quenching temperature at a cooling rate of air cooling or more; and tempering which follows the quenching and in which the seamless tube is heated at a tempering temperature of 550°C or more and A_c1 transformation temperature or below and is cooled.

(7) A manufacturing method of a martensitic stainless steel seamless tube for oil country tubular goods possessing both of high strength and excellent low-temperature toughness, the method comprising: quenching in which a seamless tube having the composition which contains, by mass%, 0.020% or less C, 1.0% or less Si, 0.1 to 2.0% Mn, 0.020% or less P, 0.010% or less S, 0.10% or less Al, 10 to 14% Cr, 3% or less Ni, 0.03 to 0.2% Nb., 0.05% or less N, and Fe and unavoidable impurities as a balance is heated at a quenching temperature of A_c3 transformation temperature or above and, thereafter, the seamless tube is cooled to a temperature range of 100°C or less from the quenching temperature at a cooling rate of air cooling or more; and tempering which follows the quenching in which the seamless tube is heated at a tempering temperature of 550°C or more and A_c1 transformation temperature or below and is cooled.

(8) In the manufacturing method of a martensitic stainless steel seamless tube for oil country tubular goods having the constitution (6) or (7), the seamless steel tube has the composition which contains said Nb such that a following formula (1) is satisfied in terms of C, Al and N.

        C-31/4Nb+7/6N-9/4Al≤-0.30     (1)

(Here, C, Nb, N, Al: contents of respective elements (mass%)). In the stainless steel having the constitution (6), it is treated that Al content is 0 mass%.

(9) In the manufacturing method of a martensitic stainless steel seamless tube for oil country tubular goods having any one of the constitutions (6) to (8), the composition of the seamless tube contains, by mass%, in addition to said composition, one or two kinds of elements selected from 2.0% or less Cu, and 2.0% or less Mo.

(10) In the manufacturing method of a martensitic stainless steel seamless tube for oil country tubular goods having any one of the constitutions (6) to (9), the composition of the seamless tube contains, by mass%, in addition to said composition, one kind, two or more kinds of elements selected from 0.20% or less V, 0.10% or less Ti and 0.005% or less B.

(11) In the manufacturing method of a martensitic stainless steel seamless tube for oil country tubular goods having any one of the constitutions (6) to (10), straightening is performed in a temperature range of 450°C or more during cooling after said tempering.

(12) A martensitic stainless steel seamless tube for oil country tubular goods obtained by the manufacturing method of a martensitic stainless steel seamless tube for oil country tubular goods described in (11), wherein an increment ΔYS of yield strength by the straightening is 15 MPa or less.

Advantage of the Invention

[0008] According to the present invention, hot straitening can be realized. Accordingly, even when straitening is performed, the increase of yield strength is small so that it is possible to manufacture easily and stably a seamless steel tube for OCTG which possesses both of high strength of yield strength YS of 95 ksi grade (655 to 758 MPa) or more and excellent low-temperature toughness where a fracture transition temperature vTrs is -40°C or below thus acquiring remarkable industrial advantageous effects.

Best Mode for Carrying Out the Invention

[0009] Inventors of the present invention, to realize the above-mentioned object, have extensively studied the influence of content composition and heat treatment conditions exerted on a change of toughness brought about by increasing strength of a 13Cr martensitic stainless steel tube. As a result of the study, the inventors have found that it is possible to prevent the deterioration of toughness attributed to grain boundary precipitation of M₂₃C₆ type Cr-based carbide by adopting a content system in which a C content is limited to 0.02% or less, a Cr content is set to a value which falls within approximately 10% to 14% Cr where corrosion resistance is not deteriorated, Ni content is set to a relatively low value of 3% or less, and a relatively large quantity of, that is, 0.03% or more of Nb is contained in the content system. It is also found that, due to such content composition, even when tempering is applied to the steel tube at a high temperature of 550°C or more after quenching, it is possible to manufacture the steel tube which ensures high strength of yield strength YS of 95 ksi grade (655 to 758 MPa) or more and possesses high toughness where vTrs is -40°C or below. Further, a straitening temperature can be set to a high temperature of 450°C or more and hence, the increase of yield strength YS after straitening is decreased to 15 MPa or less.

[0010] The present invention has been completed by further continuing studies based on the above-mentioned findings.

[0011] Firstly, reasons for limiting the composition of seamless steel tube for OCTG according to the present invention are explained. Hereinafter, mass% is simply described as % unless otherwise specified.

[0012] The seamless steel tube for OCTG according to the present invention is a martensitic stainless steel seamless tube which adopts the composition which contains 0.020% or less C, 10 to 14% Cr, 3% or less Ni, 0.03 to 0.2% Nb, 0.05% or less N, and Fe and unavoidable impurities as a balance as the basic composition. The martensitic stainless steel seamless tube may adopt the composition which contains 0.020% or less C, 1.0% or less Si, 0.1 to 2.0% Mn, 0.020% or less P, 0.010% or less S, 0.10% or less Al, 10 to 14% Cr, 3% or less Ni, 0.03 to 0.2% Nb, 0.05% or less N, and Fe and unavoidable impurities as a balance as the basic composition.

C: 0.020% or less

[0013] C is an important element relating to strength of martensitic stainless steel. Although it is desirable that the stainless steel contains 0.003% or more of C to ensure desired high strength, when a content of C exceeds 0.020%, toughness and also corrosion resistance are liable to be lowered. Accordingly, in the present invention, the content of C is limited to 0.020% or less. From a view point of stable assuring of strength and toughness, the content of C is preferably limited to a value which falls within a range from 0.003% to 0.015%.

Cr: 10 to 14%

[0014] Cr is an element which enhances corrosion resistance due to the formation of a protective film, and is an element which effectively contributes to the enhancement of CO₂ corrosion resistance and CO₂ stress corrosion cracking resistance. When the stainless steel contains 10% or more of Cr, the stainless steel can ensure required corrosion resistance as oil country tubular goods and hence, 10% is set to a lower limit of Cr content in the present invention. On the other hand, when the stainless steel contains a large quantity of Cr exceeding 14%, ferrite is easily formed so that the addition of a large quantity of expensive austenite forming element becomes necessary for ensuring stability of a martensite phase or for preventing the lowering of hot workability whereby the content of Cr exceeding 14% is economically disadvantageous. Accordingly, the content of Cr is limited to a value which falls within a range from 10 to 14%. From a view point of ensuring the more stable structure and the more stable hot workability, the content of Cr is preferably limited to a value which falls within a range from 10.5 to 11.5%.

Ni: 3% or less

[0015] Ni is an element which has a function of strengthening a protective film, and enhances corrosion resistance such as CO₂ corrosion resistance. Although it is desirable that the stainless steel tube contains 0.1% or more of Ni to acquire such an advantageous effect, when the content of Ni exceeds 3%, this leads to only a sharp rise of a manufacturing cost. Accordingly, the content of Ni is limited to a value which falls within a range not more than 3%. The content of Ni is preferably limited to a value which falls within a range from 1.5 to 2.5%.

N: 0.05% or less

[0016] N is an element which remarkably enhances pitting corrosion resistance, and such an advantageous effect becomes outstanding when the content of N becomes 0.003% or more. On the other hand, when the content of N exceeds 0.05%, various nitrides are formed thus lowering toughness. Accordingly, the content of N is limited to 0.05% or less. The content of N is preferably limited to a value which falls within a range from 0.01 to 0.02%.

Nb: 0.03 to 0.2%

[0017] Nb is an important element in the present invention. Nb is an element which forms a carbide, and increases strength of steel through precipitation strengthening by Nb carbide. Further, in the present invention, Nb plays an important role for preventing the grain boundary precipitation of M₂₃C₆ type Cr carbide thus enhancing toughness. To acquire such an advantageous effect, it is necessary to set the content of Nb to 0.03% or more, and more preferably to a value which exceeds 0.03%. Further, from a view point of acquiring higher strengthening and higher toughness, it is preferable to set the content of Nb to 0.06% or more. On the other hand, when the content of Nb exceeds 0.2%, toughness is lowered. Accordingly, the content of Nb is limited to a value which falls within a range from 0.03 to 0.2%. The content of Nb is preferably limited to a value which falls within a range from 0.03% to 0.15%. The content of Nb is more preferably limited to a value which falls within a range from 0.06 to 0.15%.

[0018] Although the above-mentioned components are basic components of the stainless steel of the present invention, in the present invention, it is preferable to adopt the composition which contains 1.0% or less Si, 0.1 to 2.0% Mn, 0.020% or less P, 0.010% or less S, and 0.10% or less Al in addition to these basic components as the basic composition.

Si: 1.0% or less

[0019] Si is an element which functions as deoxidizing agent in a usual steel making process. Although it is desirable to set the content of Si to 0.1% or more, when the content of Si exceeds 1.0%, toughness is lowered and cold workability property is also lowered. Accordingly, the content of Si is limited to 1.0% or less. The content of Si is preferably limited to a value which falls within a range from 0.1 to 0.3%.

Mn: 0.1 to 2.0%

[0020] Mn is an element which increases strength of the stainless steel. In the present invention, although it is desirable that the content of Mn is 0.1% or more to allow the stainless steel to ensure strength necessary for a steel tube for oil country tubular goods, when the content of Mn exceeds 2.0%, toughness is adversely influenced by Mn. Accordingly, the content of Mn is limited to a value which falls within a range from 0.1 to 2.0%. The content of Mn is preferably limited to a value which falls within a range from 0.5 to 1.5%.

P: 0.020% or less

[0021] P is an element which deteriorates corrosion resistance such as CO₂ corrosion resistance and hence, it is desirable to reduce the content of P as much as possible in the present invention. However, the extreme reduction of the content of P pushes up a manufacturing cost. As a range of the content of P which can realize the relatively inexpensive industrial manufacture of the stainless steel, and prevents the deterioration of corrosion resistance such as CO₂ corrosion resistance, the content P is limited to 0.020% or less. The content of P is preferably limited to 0.015% or less.

S: 0.010% or less

[0022] S is an element which remarkably deteriorates hot workability in a tube manufacturing step. Although it is desirable to decrease the content of S as much as possible, the tube can be manufactured in a usual step by decreasing the content of S to 0.010% or less and hence, the content of S is limited to 0.010% or less. Here, the content of S is preferably limited to 0.003% or less.

Al: 0.10% or less

[0023] Al is an element which possesses a strong deoxidizing action. To acquire such an advantageous effect, it is desirable that the stainless steel contains 0.001% or more of Al. However, when the content of Al exceeds 0.10%, Al adversely influences toughness. Accordingly, the content of Al is limited to 0.10% or less. The content of Al is preferably limited to 0.05% or less.

[0024] In the present invention, it is preferable that the content of Nb falls within the above-mentioned content range and, further, a following formula (1) is satisfied in view of the relationship between the content Nb and the contents of C, Al and N.

(Here, C, Nb, N, Al: contents of respective elements (mass%)).

[0025] In the formula (1), when an addition content of Al is at a level of unavoidable impurities, the calculation is made assuming the content of Al as 0 mass%.

[0026] When the Nb content does not satisfy the above-mentioned formula (1), the stainless steel cannot possess both of desired high strength (yield strength: 95 ksi or more) and high toughness (fracture transition temperature vTrs in a Charpy impact test is -40°C or below).

[0027] In the present invention, in addition to the above-mentioned basic components, the stainless steel may include one or two kinds of components in a group A and a group B described hereinafter.

[0028] Group A: one kind, or two kinds selected from 2.0% or less Cu, 2.0% or less Mo.

[0029] Group B: one kind or two or more kinds selected from 0.20% or less V, 0.10% or less Ti, 0.005% or less B.

[0030] Group A: one kind or two kinds selected from 2.0% or less Cu, 2.0% or less Mo.

[0031] Both Cu and Mo are elements which have a function of enhancing corrosion resistance and the stainless steel may selectively contain these elements when necessary.

[0032] Cu is an element which has a function of strengthening a protective film thus enhancing pitting resistance, and it is desirable to set the content of Cu to 0.2% or more to acquire such an advantageous effect. On the other hand, when the content of Cu exceeds 2.0%, Cu or a part of Cu compound precipitates thus lowering toughness. Accordingly, when the stainless steel contains Cu, the content of Cu is preferably limited to 2.0% or less. The content of Cu is more preferably limited to a value which falls within a range from 0.2 to 1.0%.

[0033] Further, Mo is an element which has a function of increasing resistance against pitting by Cl^-, and it is desirable to set the content of Mo to 0.2% or more to acquire such an advantageous effect. On the other hand, when the content of Mo exceeds 2.0%, the strength of the stainless steel is lowered and, at the same time, a manufacturing cost sharply rises. Accordingly, the content of Mo is preferably limited to 2.0% or less. The content of Mo is more preferably limited to a value which falls within a range from 0.2 to 1.0%.

[0034] Group B: one kind or two or more kinds selected from 0.20% or less V, 0.10% or less Ti, 0.005% or less B.

[0035] All of V, Ti and B are elements which increase strength of the stainless steel, and the stainless steel may selectively contain one kind or two or more kinds of these elements when necessary.

[0036] To acquire such an advantageous effect, it is desirable that the stainless steel contains 0.02 or more V, 0.02% or more Ti, 0.0015% or more B. On the other hand, when the content of V exceeds, 0.20%, the content of Ti exceeds 0.10%, or the content of B exceeds 0.005%, toughness is lowered. Accordingly, when the stainless steel contains these elements, it is desirable to set the content of V to 0.20 or less, the content of Ti to 0.10% or less, and the content of B to 0.005% or less. It is more preferable to set the content of V to 0.02 to 0.10%, the content of Ti to 0.02 to 0.05%, and the content of B to 0.0015 to 0.0040%.

[0037] A balance of the stainless steel except for the above-mentioned components is formed of Fe and unavoidable impurities. As unavoidable impurities, 0.010% or less of O is allowable.

[0038] Next, the structure of seamless steel tube for OCTG of the present invention is explained.

[0039] The seamless steel tube for OCTG of the present invention has the structure which is mainly in a tempered martensite phase and in which precipitated Nb is dispersed. As the structure other than the tempered martensite phase, the structure may contain 5 volume% or less of a delta ferrite and 5 volume% or less of austenite respectively. Further, from a viewpoint of workability, the content of delta ferrite may preferably be set to 2 volume% or less. Also from a viewpoint of stability of strength, the content of austenite may preferably be set to 2 volume% or less. Due to such structure, it is possible to provide a steel tube which has desired high strength (yield strength: 95 ksi or more) and high toughness (fracture transition temperature vTrs in a Charpy impact test: -40°C or below) and, at the same time, also possesses sufficient corrosion resistance as a tube for oil country tubular goods. A precipitated Nb quantity is set to 0.020 mass% or more in terms of Nb. When the precipitated Nb quantity is less than 0.020 mass%, the grain boundary precipitation of M₂₃C₆ type Cr carbide which adversely influences toughness cannot be suppressed so that toughness is lowered. The precipitated Nb quantity is preferably set to 0.025 mass % or more in terms of Nb. The seamless steel tube for OCTG of the present invention does not contain M₃c type Cr type carbide.

[0040] Here, the precipitated Nb quantity is determined such that an electrolytic residue obtained by electrolytic extraction using an electrolytic extraction method is subject to a chemical analysis thus obtaining a quantity of Nb contained in the electrolytic residue, and the obtained quantity of Nb is used as the precipitated Nb quantity contained in a sample.

[0041] Here, the precipitated Nb is mainly formed on Nb carbide or Nb carbonitride. The precipitated Nb is a precipitated material having a spherical shape with an average particle size of 3 nm to 15 nm.

[0042] Next, a manufacturing method of a seamless steel tube for OCTG of the present invention is explained. Using the stainless steel seamless tube having the above-mentioned composition as a starting material, quenching and tempering are applied to the seamless steel tube. Further, straightening may also be applied to the seamless steel tube for straightening a deformed steel tube shape when necessary.

[0043] Although a manufacturing method of a starting material which has the above-mentioned composition is not particularly limited in the present invention, it is preferable that molten steel having the above-mentioned composition is produced by a usually known melting method such as a steel converter, an electric furnace, a vacuum melting furnace, a molten state is formed into a steel tube raw material such as billets by a usual method such as continuous casting, ingot casting or blooming method. Then, these steel tube material is heated and is formed into a seamless steel tube having a desired size by hot working using manufacturing steps of a usual Mannesmann-plug mill method or a usual Mannesmann-mandrel mill method, and the seamless steel tube is used as a starting raw material. A seamless steel tube may be manufactured by hot extruding using a press method. Further, after forming the seamless steel tube, it is desirable to cool the seamless steel tube to a room temperature at a cooling rate of air cooling or more.

[0044] The starting material (seamless steel tube) is firstly subject to quenching.

[0045] Quenching in the present invention implies treatment in which the seamless steel tube is reheated to a quenching temperature of A_c3 transformation temperature or above and, thereafter, the seamless steel tube is cooled to a temperature zone of 100°C or below from the quenching temperature at a cooling rate of air cooling or more. Due to such quenching, it is possible to form the structure of the starting material into the fine martensitic structure. When the quenching heating temperature is below the A_c3 transformation temperature, the temperature of the seamless steel tube cannot be heated at a austenite single phase region so that the sufficient martensitic structure cannot be formed by cooling after heating and hence, the seamless steel tube cannot ensure desired strength (yield strength: 95 ksi or more). Accordingly, the heating temperature of quenching is limited to an A_c3 transformation temperature or above. The heating temperature is preferably set to 1000°C or below.

[0046] Further, cooling from the quenching heating temperature is performed until a temperature zone of 100°C or below at a cooling rate of air cooling or more. The starting material used in the present invention possesses the high quenching property and hence, when the starting material is cooled down to the temperature zone of 100°C or below at a cooling rate of approximately air cooling, the seamless steel tube can acquire the sufficient quenching structure (martensitic structure). Further, it is preferable to set a holding time of the starting material at the quenching temperature to 10 minutes or more from a viewpoint of homogeneous heating.

[0047] The seamless steel tube to which quenching is applied is subsequently subject to tempering.

[0048] In the present invention, tempering is important treatment for ensuring excellent low-temperature toughness. In the tempering performed in the present invention, the seamless steel tube is heated to a tempering temperature which is 550°C or more, and preferably A_c1 transformation temperature or below, the heating is preferably held for 30 minutes or more and, thereafter, the seamless steel tube is cooled down preferably to a room temperature preferably at a cooling rate of air cooling or more. Due to such tempering, it is possible to produce the seamless steel tube which possesses both high strength of YS of 95 ksi or more and the excellent low-temperature toughness of vTrs of -40°C or below. When the temperature at straightening is set to a tempering temperature or above, the structure is changed and hence, there is no way but to set the temperature at straightening to the tempering temperature or below when the tempering temperature is less than 550°C. Accordingly, as described later, irregularities of yield strength YS are liable to occur. On the other hand, when the tempering temperature exceeds the A_c1 transformation temperature, an austenite phase is formed and the austenite phase is transformed into quenched martensite at the time of cooling. Since the quenched martensite has many mobile dislocations, when the quenched martensite precipitates, yield strength YS is lowered. Further, from a viewpoint of acquiring the sufficient martensite, it is preferable to perform cooling from the tempering temperature at a cooling rate of air cooling or more.

[0049] Further, in the present invention, when necessary, straightening may be applied to the seamless steel tube for straightening the deformed steel tube shape following the tempering. It is preferable to perform straightening in a temperature zone of 450°C or above. When the temperature at the straightening is less than 450°C, a working strain is locally generated in the steel tube at the time of performing straightening so that irregularities are liable to occur in mechanical properties, and particularly yield strength YS. Accordingly, when straightening is performed, the straightening is performed within a temperature zone of 450°C or above. Here, the desired irregularities (ΔYS) of yield strength YS in the present invention is 15 MPa or less.

[0050] The seamless steel tube manufactured by the above-mentioned manufacturing method is formed into the martensitic stainless steel seamless tube which has the above-mentioned composition and structure, and possesses high strength of yield strength of 95 ksi or more (655 MPa or more) and the excellent low-temperature toughness of fracture transition temperature vTrs of -40°C or below in a Charpy impact test, and further possesses the sufficient corrosion resistance as the oil country tubular goods.

Embodiment

[0051] The molten steel having the composition shown in Table 1 is degassed and, thereafter, billets (size: 207 mmφ) are formed by casting by a continuous casting method and are used as steel tube materials. These steel tube materials are heated, and are formed into tubes by hot working through Mannesmann-method manufacturing steps and, thereafter, the tubes are air-cooled so as to form seamless steel tubes (outer diameter of 177.8 mmφ × wall thickness of 12.65 mm).

[0052] Specimens (steel tubes) are sampled from the obtained seamless steel tubes, and quenching and tempering are applied and optional straightening is further applied to the specimens (steel tubes) under conditions shown in Table 2 and Table 3.

[0053] Electrolytic extraction specimens are sampled from the specimens (steel tubes) to which quenching and tempering are applied and optional straightening is further applied. Using the sampled electrolytic extraction specimen, a quantity of Nb contained in an obtained electrolytic residue is obtained using an electrolytic extraction method and is determined as a quantity of precipitated Nb contained in the specimen.

[0054] Further, from the specimen (steel tubes) to which quenching and tempering are applied and optional straightening is further applied, a strip specimen specified by API standard 5CT is sampled, and a tensile test is carried out on the strip specimen thus obtaining tensile characteristics (yield strength YS, tensile strength TS) of the strip specimen. With respect to the specimen (steel tube) to which the straightening is applied, an increment ΔYS of YS by straightening is obtained. The tensile test is carried out on the non-straightened steel tubes manufactured under the same condition except for straightening thus obtaining tensile characteristics (yield strength YS, tensile strength TS) of the strip specimens. An increment ΔYS of YS by straightening is calculated by a following formula.

[0055] Further, V-notch specimens (thickness: 10 mm) are sampled from specimens to which quenching and tempering are applied and optional straightening is further applied in accordance with the stipulation of JIS Z 2242. The V-notch specimens are subject to a Charpy impact test where a fracture transition temperature vTrs is obtained and toughness is evaluated.

[0056] Further, a corrosion specimen having a thickness of 3 mm, a width of 30 mm and a length of 40 mm is prepared by machining from the specimens, and a corrosion test is carried on the corrosion specimen.

[0057] The corrosion test is carried out in such a manner that the corrosion specimen is immersed into a test solution: 20% NaCl aqueous solution (temperature of the test solution: 80°C, under CO₂ gas atmosphere of 30 atmospheric pressure) held in an autoclave, and an test duration is 1 week (168h). With respect to the specimen after the corrosion test, the weight of the specimen is measured and a corrosion rate is calculated based on the reduction of weight before and after the corrosion test.

[0058] An obtained result of the test is shown in Table 4 and Table 5.

[0059] All examples of the present invention (steel tubes No. 4 to 13, 18 to 23, 29 and 30) are martensitic stainless steel seamless tubes which exhibit the sufficient corrosion resistance, possess both of high strength of YS of 95 ksi or more (655 MPa or more) and excellent low-temperature toughness of vTrs of -40°C or less, and allows hot straightening at a temperature of 450°C or more so that an increment of yield strength is small and the difference ΔYS of the average YS is small (15 MPa or less) even when straightening is applied. On the other hand, with respect to comparison examples (steel tubes No. 1 to 3, 14 to 17, 24 to 28) which are outside the range of the present invention, strength is insufficient (YS: less than 95 ksi) or the low-temperature toughness is low (vTrs exceeding -40°C) so that desired high strength and desired high toughness cannot be ensured. Further, an increment of yield strength (ΔYS exceeding 15 MPa) after straightening is increased.

Table 1

Steel No.	Chemical component (mass%)														Remarks
	C	Si	Mn	P	S	Al	Cr	Ni	N	Nb	Formula (1)*		Cu, Mo	V, Ti, B
											Left side value	Appropriate
A	0.026	0.16	1.20	0.016	0.0011	0.021	11.0	1.5	0.025	0.04	-0.29	×	Cu:0.5	V:0.02	Comparison example
B	0.010	0.12	1.14	0.019	0.0010	0.039	10.9	2.3	0.005	=	0.07	×	Cu:0.51	-	Comparison example
C	0.010	0.15	1.21	0.016	0.0009	0.022	10.9	2.0	0.008	0.04	-0.33	o	Cu:0.5	V:0.03	Example of present invention
D	0.009	0.15	1.23	0.003	0.0010	0.025	11.1	2.0	0.009	0.04	-0.35	o	Cu:0.5	V:0.03	Example of present invention
E	0.009	0.15	1.23	0.017	0.0007	0.024	11.1	2.0	0.010	0.04	-0.34	o	Cu:0.5	V:0.07	Example of present invention
F	0.011	0.15	1.21	0.017	0.0007	0.030	10.9	2.0	0.010	0.07	-0.59	o	Cu:0.5	V:0.03	Example of present invention
G	0.009	0.16	1.20	0.017	0.0008	0.030	11.1	2.0	0.010	0.04	-0.36	o	Cu:0.5	V:0.03, Ti:0.035	Example of present invention
H	0.025	0.19	2.50	0.017	0.0020	0.024	13.2	=	0.066	0.08	-0.60	o	-	V:0.03	Comparison example
I	0.011	0.16	1.23	0.017	0.0020	0.023	11.1		2.0 0.025	=	-0.01	×	Cu:0.5	V:0.03, B:0.0028	Comparison example
J	0.025	0.20	2.02	0.017	0.0011	0.025	11.0	=	0.060	=	-0.04	×	Cu:2.5	V:0.03	Comparison example
K	0.010	0.15	1.20	0.015	0.0010	0.025	11.0	2.5	0.010	0.10	-0.81	o	Cu:0.5	V:0.03	Example of present invention
L	0.010	0.15	1.70	0.015	0.0010	0.025	11.0	1.6	0.010	0.14	-1.12	o	Cu:0.5	V:0.03	Example of present invention
M	0.010	0.15	1.20	0.015	0.0010	0.025	11.0	2.0	0.010	0.07	-0.58	o	Cu:0.5, Mo:0.2	V:0.03	Example of present invention
N	0.010	0.15	1.20	0.015	0.0010	0.025	11.0	2.5	0.010	0.07	-0.58	o	Cu:0.5	V:0.13	Example of present invention
O	0.030	0.25	0.52	0.013	0.0009	0.004	10.8	1.2	0.027	=	-0.05	×	Cu:1.0, Mo:0.2	V:0.04	Comparison example
P	0.040	0.22	0.66	0.016	0.0011	0.001	11.6	1.7	0.051	0.02	-0.07	×	-	V:0.04, Ti:0.10, B:0.001	Comparison example
Q	0.017	0.17	1.24	0.015	0.0411	0.023	11.4	2.0	0.020	0.04	-0.28	×	Cu:0.5	V:0.04	Comparison example
R	0.008	0.21	0.84	0.015	0.0010	0.020	9.4	2.3	0.010	=	-0.03	×	-	-	Comparison example
S	0.008	0.18	1.21	0.015	0.0010	0.030	14.5	3.5	0.010	=	-0.05	×	Cu:0.5	-	Comparison example
T	0.015	-**	-**	-**	-**	0.023	11.3	2.1	0.020	0.04	-0.32	o	-	-	Example of present invention
U	0.010	-**	-**	-**	-**	-**	11.0	1.5	0.020	0.05	-0.32	o	-	-	Example of present invention

*) C-31/4Nb+7/6N-9/4Al ≤ -0.30 **) not added

Table 2

Steel tube No.	Steel No.	Quenching			Tempering			Straightening	Remarks
		Heating temperature (°C)	Cooling	Cooling stop temperature (°C)	Tempering temperature (°C)	Cooling	Cooling stop temperature (°C)	Straightening temperature (°C)
1	A	850	air cooling	25	550 example	air cooling	25	-	Comparison
2	A	850	air cooling	25	650 example	air cooling	25	-	Comparison
3	B	810	air cooling	25	500 example	air cooling	25	400	Comparison
4	C	850	air cooling	25	650	air cooling	25	-	Example of present invention
5	C	850	air cooling	25	675	air cooling	25	-	Example of present invention
6	C	850	air cooling	25	650	air cooling	25	550	Example of present invention
7	D	850	air cooling	25	650	air cooling	25	-	Example of present invention
8	E	850	air cooling	25	650	air cooling	25	-	Example of present invention
9	F	850	air cooling	25	600	air cooling	25	-	Example of present invention
10	F	850	air cooling	25	700	air cooling	25	-	Example of present invention
11	F	850	air cooling	25	625	air cooling	25	-	Example of present invention
12	F	850	air cooling	25	600	air cooling	25	500	Example of present invention
13	G	850	air cooling	25	650	air cooling	25	-	Example of present invention
14	H	850	air cooling	25	600 example	air cooling	25	-	Comparison
15	H	850	air cooling	25	700 example	air cooling	25	-	Comparison
16	I	850	air cooling	25	600 example	air cooling	25	-	Comparison

Table 3

Steel tube No.	Steel No.	Quenching			Tempering			Straightening	Remarks
		Heating temperature (°C)	Cooling	Cooling stop temperature (°C)	Tempering temperature (°C)	Cooling	Cooling stop temperature (°C)	Straightening temperature (°C)
17	J	850	air cooling	25	600	air cooling	25	-	Comparison example
18	K	850	air cooling	25	600	air cooling	25	-	Example of present invention
19	K	850	air cooling	25	600	air cooling	25	525	Example of present invention
20	L	850	air cooling	25	600	air cooling	25	-	Example of present invention
21	L	850	air cooling	25	600	air cooling	25	510	Example of present invention
22	M	850	air cooling	25	600	air cooling	25	-	Example of present invention
23	N	850	air cooling	25	600	air cooling	25	-	Example of present invention
24	O	920	air cooling	25	350	air cooling	25	300	Comparison example
25	P	920	air cooling	25	400	air cooling	25	350	Comparison example
26	Q	850	air cooling	25	600	air cooling	25	-	Comparison example
27	R	850	air cooling	25	550	air cooling	25	-	Comparison example
28	S	850	air cooling	25	550	air cooling	25	-	Comparison example
29	T	850	air cooling	25	675	air cooling	25	-	Example of present invention
30	U	850	air cooling	25	675	air cooling	25	-	Example of present invention

Table 4

Steel tube No.	Steel No.	Precipitate(s)	Tensile characteristic			Toughness	Corrosion resistance	Remarks
		Precipitated Nb amount (in terms of Nb) (mass%)	YS (MPa)	TS (MPa)	Increment of YS by straightening WAYS (MPa)	vTrs (°C)	Corrosion rate (mm/y)
1	A	0.024	825	1010	-	10	0.05	Comparison example
2	A	0.025	716	840	-	-35	0.05	Comparison example
3	B	=	870	903	33	-75	0.03	Comparison example
4	C	0.028	738	823	-	-40	0.035	Example of present invention
5	C	0.029	661	815	-	-70	0.035	Example of present invention
6	C	0.028	740	823	2	-40	0.035	Example of present invention
7	D	0.026	738	823	-	-40	0.035	Example of present invention
8	E	0.024	759	839	-	-50	0.035	Example of present invention
9	F	0.052	838	873	-	-50	0.035	Example of present invention
10	F	0.054	712	869	-	-100	0.035	Example of present invention
11	F	0.053	795	853	-	-50	0.035	Example of present invention
12	F	0.052	848	878	10	-50	0.035	Example of present invention
13	G	0.027	679	723	-	-50	0.035	Example of present invention
14	H	0.056	753	892	-	-20	0.06	Comparison example
15	H	0.058	642	771	-	-60	0.06	Comparison example
16	I	=	698	791	-	10	0.035	Comparison example

Table 5

Steel tube No.	Steel No.	Precipitate(s)	Tensile characteristic			Toughness	Corrosion resistance	Remarks
		Precipitated Nb amount (in terms of Nb) (mass%)	YS (MPa)	TS (MPa)	Increment of YS by straightening ΔYS (MPa)	vTrs (°C)	Corrosion rate (mm/y)
17	J	=	812	930	-	10	0.10	Comparison example
18	K	0.071	849	875	-	-50	0.03	Example of present invention
19	K	0.071	852	875	3	-50	0.03	Example of present invention
20	L	0.102	863	894	-	-55	0.05	Example of present invention
21	L	0.102	868	897	5	-55	0.05	Example of present invention
22	M	0.049	838	873	-	-60	0.04	Example of present invention
23	N	0.048	896	923	-	-55	0.03	Example of present invention
24	O	-	855	1053	47	-50	0.06	Comparison example
25	P	0.013	833	1094	42	-55	0.04	Comparison example
26	Q	0.022	789	877	-	-30	0.04	Comparison example
27	R	-	680	761	-	-30	0.04	Comparison example
28	S	-	699	803	-	-25	0.01	Comparison example
29	T	0.029	670	821	-	-60	0.04	Example of present invention
30	U	0.030	660	807	-	-60	0.03	Example of present invention

Claims

1. A martensitic stainless steel seamless tube for oil country tubular goods possessing both of high strength of yield strength of 95 ksi or more and excellent low-temperature toughness in which a fracture transition temperature vTrs in a Charpy impact test is -40°C or below, wherein the seamless tube has the composition which contains, by mass%, 0.020% or less C, 10 to 14% Cr, 3% or less Ni, 0.03 to 0.2% Nb, 0.05% or less N, and Fe and unavoidable impurities as a balance, and has the structure where a precipitated Nb quantity is 0.020% or more in terms of Nb.

2. A martensitic stainless steel seamless tube for oil country tubular goods possessing both of high strength of yield strength YS of 95 ksi or more and excellent low-temperature toughness in which a fracture transition temperature vTrs in a Charpy impact test is -40°C or below, wherein the seamless tube has the composition which contains, by mass%, 0.020% or less C, 1.0% or less Si, 0.1 to 2.0% Mn, 0. 020% or less P, 0.10% or less S, 0.10% or less Al, 10 to 14% Cr, 3% or less Ni, 0.03 to 0.2% Nb, 0.05% or less N, and Fe and unavoidable impurities as a balance, and has the structure where a precipitated Nb quantity is 0.020% or more in terms of Nb.

3. The martensitic stainless steel seamless tube for oil country tubular goods according to claim 1 or 2, wherein the seamless steel tube has the composition which contains said Nb such that a following formula (1) is satisfied in terms of C, Al and N.


(Here, C, Nb, N, Al: contents of respective elements (mass%)

4. The martensitic stainless steel seamless tube for oil country tubular goods according to any one of claims 1 to 3, wherein the composition of the seamless tube contains, by mass%, in addition to said composition, one or two kinds of elements selected from 2.0% or less Cu, and 2.0% or less Mo.

5. The martensitic stainless steel seamless tube for oil country tubular goods according to any one of claims 1 to 4, wherein the composition of the seamless tube contains, by mass%, in addition to said composition, one kind, two or more kinds of elements selected from 0.20% or less V, 0.10% or less Ti and 0.005% or less B.

6. A manufacturing method of a martensitic stainless steel seamless tube for oil country tubular goods, the method comprising: quenching in which a stainless steel seamless tube having the composition which contains, by mass%, 0.020% or less C, 10 to 14% Cr, 3% or less Ni, 0.03 to 0.2% Nb, 0.05% or less N, and Fe and unavoidable impurities as a balance is heated at a quenching temperature of A_c3 transformation temperature or above and, thereafter, the seamless tube is cooled to a temperature range of 100°C or less from the quenching temperature at a cooling rate of air cooling or more; and tempering which follows the quenching and in which the seamless tube is heated at a tempering temperature of 550°C or more and A_c1 transformation temperature or below and is cooled.

7. A manufacturing method of a martensitic stainless steel seamless tube for oil country tubular goods, the method comprising: quenching in which a stainless steel seamless tube having the composition which contains, by mass%, 0.020% or less C, 1.0% or less Si, 0.1 to 2.0% Mn, 0.020% or less P, 0.010% or less S, 0.10% or less Al, 10 to 14% Cr, 3% or less Ni, 0.03 to 0.2% Nb, 0.05% or less N, and Fe and unavoidable impurities as a balance is heated at a quenching temperature of A_c3 transformation temperature or above and, thereafter, the seamless tube is cooled to a temperature range of 100°C or less from the quenching temperature at a cooling rate of air cooling or more; and tempering which follows the quenching and in which the seamless tube is heated at a tempering temperature of 550°C or more and A_c1 transformation temperature or below and is cooled.

8. The manufacturing method of a martensitic stainless steel seamless tube for oil country tubular goods according to claim 6 or 7, wherein the seamless steel tube has the composition which contains said Nb such that a following formula (1) is satisfied in terms of C, Al and N.


(Here, C, Nb, N, Al: contents of respective elements (mass%))

9. The manufacturing method of a martensitic stainless steel seamless tube for oil country tubular goods according to any one of claims 6 to 8, wherein the composition of the seamless tube contains, by mass%, in addition to said composition, one or two kinds of elements selected from 2.0% or less Cu, and 2.0% or less Mo.

10. The manufacturing method of a martensitic stainless steel seamless tube for oil country tubular goods according to any one of claims 6 to 9, wherein the composition of the seamless tube contains, by mass%, in addition to said composition, one kind, two or more kinds of elements selected from 0.10% or less V, 0.10% or less Ti and 0.005% or less B.

11. The manufacturing method of a martensitic stainless steel seamless tube for oil country tubular goods according to any one of claims 6 to 10, wherein straightening is performed in a temperature range of 450°C or more during cooling after said tempering.

12. A martensitic stainless steel seamless tube for oil country tubular goods obtained by the manufacturing method of a martensitic stainless steel seamless tube for oil country tubular goods according to claim 11, wherein an increment ΔYS of yield strength by the straightening is 15 MPa or less.