Patent Application
20110132501
This disclosure relates to a martensitic stainless steel seamless tube for oil country tubular goods, 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.
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.
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.
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.
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. 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.
It could therefore be helpful 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. “Excellent low-temperature toughness” implies a case where a fracture transition temperature vTrs in a Charpy impact test is −40° C. or below.
We thus provide:
C−31/4Nb+7/6N−9/4Al≦−0.30 (1).
C−31/4Nb+7/6N−9/4Al≦−0.30 (1).
Hot straitening of our stainless steel tube 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.
We 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, we 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. We 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. Hence, the increase of yield strength YS after straitening is decreased to 15 MPa or less.
Reasons for limiting the composition of seamless steel tube for OCTG are explained. Hereinafter, mass % is simply described as % unless otherwise specified.
The seamless steel tube for OCTG 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
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, 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 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. Hence, 10% is set to a lower limit of Cr content. 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
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
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 is an important element. Nb is an element which forms a carbide, and increases strength of steel through precipitation strengthening by Nb carbide. Further, 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%.
Although the above-mentioned components are basic components of the stainless steel, 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
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 is an element which increases strength of the stainless steel. 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
P is an element which deteriorates corrosion resistance such as CO2 corrosion resistance. Hence, it is desirable to reduce the content of P as much as possible. 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
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 5 to 0.010% or less. Hence, the content of S is limited to 0.010% or less. The content of S is preferably limited to 0.003% or less.
Al: 0.10% or less
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.
It is preferable that the content of Nb falls within the above-mentioned content range and, further, a following formula (I) is satisfied in view of the relationship between the content Nb and the contents of C, Al and N:
C−31/4Nb+7/6N−9/4Al≦−0.30 (1).
(C, Nb, N, Al: contents of respective elements (mass %)).
In the formula (I), 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 %.
When the Nb content does not satisfy the above-mentioned formula (I), 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).
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:
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.
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%.
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%.
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.
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.
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%.
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 0 is allowable.
Next, the structure of seamless steel tube for OCTG is explained.
The seamless steel tube for OCTG 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 does not contain M3c type Cr type carbide.
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.
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.
Next, a manufacturing method of a seamless steel tube for OCTG 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.
Although a manufacturing method of a starting material which has the above-mentioned composition is not particularly limited, 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.
The starting material (seamless steel tube) is firstly subject to quenching.
Quenching implies treatment in which the seamless steel tube is reheated to a quenching temperature of A3 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 Ac3 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. 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 Ac3 transformation temperature or above. The heating temperature is preferably set to 1000° C. or below.
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 possesses the high quenching property. 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.
The seamless steel tube to which quenching is applied is subsequently subject to tempering.
Tempering is an important treatment for ensuring excellent low-temperature toughness. In the tempering, the seamless steel tube is heated to a tempering temperature which is 550° C. or more, and preferably Ac1 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. 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 Ac1 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.
Further, 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. The desired irregularities (ΔYS) of yield strength YS is 15 MPa or less.
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.
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).
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.
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.
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:
ΔYS=(YS of straightened steel tube)−(YS of non-straightened steel tube).
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.
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.
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 (168 h). 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.
An obtained result of the test is shown in Table 4 and Table 5.
All of our examples (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 our range, 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.
A
0.026
−0.29
B
—
−0.07
H
0.025
—
0.066
I
—
−0.01
J
0.025
—
0.060
—
−0.04
O
0.030
—
−0.05
P
0.040
0.02
−0.07
Q
−0.28
R
—
−0.03
S
14.5
3.5
—
−0.05
A
A
B
400
H
H
I
O
350
300
P
400
350
Q
R
S
A
A
−35
B
—
H
−20
H
642
I
—
—
O
P
0.013
Q
−30
R
−30
S
−25
| Number | Date | Country | Kind |
|---|---|---|---|
| 2008-226578 | Sep 2008 | JP | national |
This is a §371 of International Application No. PCT/JP2008/073918, with an international filing date of Dec. 24, 2008 (WO 2010/026672 A1, published Mar. 11, 2010), which is based on Japanese Patent Application No. 2008-226578, filed Sep. 4, 2008, the subject matter of which is incorporated by reference.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2008/073918 | 12/24/2008 | WO | 00 | 2/17/2011 |
