JP6801376B2 - Seamless steel pipe for high-strength low-alloy oil wells and its manufacturing method - Google Patents

Description

The present invention relates to a seamless steel pipe for a high-strength low-alloy oil well and a method for producing the same, and more particularly to a seamless steel pipe for a high-strength low-alloy oil well having excellent hydrogen embrittlement resistance and a method for producing the same.

Steel pipes for oil wells are used as casings or tubing in oil wells and gas wells. Hereinafter, the oil well and the gas well are collectively referred to as an oil well. As the steel pipe for oil wells, a steel pipe having a yield strength of 95 to 125 ksi (655 to 862 MPa) class has been conventionally used.

Due to the need to secure energy such as oil and natural gas, oil field development is becoming deeper. Steel pipes for oil wells used for oil field development are connected underground and used, but they are being developed at depths of several thousand meters underground, and the external pressure applied to the steel pipes for oil wells increases as the depth increases. Will also increase. Therefore, it is necessary to increase the yield strength of steel pipes for oil wells, and there is a growing need for steel pipes for oil wells having a high strength of 160 ksi (1103 MPa) class, which exceeds the 95 to 125 ksi class conventionally used.

On the other hand, when the strength is increased to 160 ksi or more, the amount of hydrogen invaded into the steel increases, so that there is a concern that hydrogen embrittlement represented by delayed fracture may occur. Even in an environment that does not generally contain hydrogen sulfide, the effects of hydrogen are not small, and measures against hydrogen embrittlement are essential. That is, it is an issue to achieve both the target yield strength and the hydrogen embrittlement resistance.

Japanese Patent Application Laid-Open No. 61-64815 states that steel having a predetermined chemical composition is hardened after being adjusted so that the austenite particle size is 8.5 or more, and tempered under the condition that the tempering parameter _PLM is 16800 or more. A method for producing high-strength steel is described. According to the same publication, the steel produced by this production method has a yield strength of more than 150 ksi and is also excellent in delayed fracture property.

Japanese Patent Application Laid-Open No. 62-40345 describes a high-strength oil well steel pipe in which X in the formula (Mo × 1/16 + V × 3/17) / C = X satisfies 0.5 ≦ X ≦ 1.5. Has been done. According to the same publication, this steel pipe for oil wells has a yield strength of more than 150 ksi and is also excellent in delayed fracture property.

JP-A-61-64815 Japanese Unexamined Patent Publication No. 62-40345 Japanese Unexamined Patent Publication No. 2001-2711134

Even with the techniques described in JP-A-61-64815 and JP-A-62-40345, the yield strength and hydrogen embrittlement resistance can be achieved at the same time while stably ensuring the yield strength. Things can be difficult.

An object of the present invention is to obtain a seamless steel pipe for a high-strength low-alloy oil well having excellent hydrogen embrittlement resistance and a method for producing the same.

The seamless steel pipe for high-strength low-alloy oil wells according to one embodiment of the present invention has a chemical composition of 0.25 to 0.50% by mass, Si: 0.05 to 1.00%, Mn: 0.1 to 1.0%, P: 0.030% or less, S: 0.0050% or less, Al: 0.005 to 0.1%, Cr: 0.10 to 1.20%, N: 0 0.01% or less, Mo: 1.1 to 2.5%, Ti: 0.002 to 0.020%, Nb: 0.050% or less, V: 0.01 to 0.30%, B: 0. 0050% or less, O: 0.0050% or less, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, REM: 0 to 0.0050%, Cu: 0 to 1.0%, Ni: 0 to 1.0%, Co: 0 to 1.0%, W: 0 to 1.0%, balance: Fe and impurities, having a yield strength of 1103-1206 MPa, and the chemical composition having the following formula ( It satisfies 1) and the formula (2), and has an austenite grain having an austenite crystal grain size number of 8.5 or more according to ASTM E112-10.
Vc90 ≤ 1.0 (1)
Mo / Cr ≧ 2.0 (2)
However, logVc90 = 2.94-0.75 (2.7C + 0.4Si + Mn + 0.45Ni + 0.45Cu + 0.8Cr + 2Mo)
The content of the corresponding element is substituted in mass% for the element symbol in the above formula. log is the common logarithm.

The method for producing a seamless steel pipe for a high-strength low-alloy oil well according to an embodiment of the present invention has a chemical composition of 0.25 to 0.50% by mass, Si: 0.05 to 1.00%. , Mn: 0.1 to 1.0%, P: 0.030% or less, S: 0.0050% or less, Al: 0.005 to 0.1%, Cr: 0.10 to 1.20%, N: 0.01% or less, Mo: 1.1 to 2.5%, Ti: 0.002 to 0.020%, Nb: 0.050% or less, V: 0.01 to 0.30%, B : 0.0050% or less, O: 0.0050% or less, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, REM: 0 to 0.0050%, Cu: 0 to 1.0% , Ni: 0-1.0%, Co: 0-1.0%, W: 0-1.0%, balance: Fe and the process of preparing the steel material which is an impurity, and the steel material is hot-worked. The step of making the steel pipe into a steel pipe, the step of cooling the steel pipe to room temperature at a speed equal to or higher than air cooling, the step of tempering the cooled steel pipe, and the step of tempering the tempered steel pipe to Ac ₃ transformation. It comprises a step of reheating to a temperature of 1000 ° C. or lower and tempering, and a step of tempering the hardened steel pipe again, and the chemical composition satisfies the following formulas (1) and (2). In the step of tempering again, the tempering parameter TP defined by the following formula (3) is 18200 to 18400.
Vc90 ≤ 1.0 (1)
Mo / Cr ≧ 2.0 (2)
TP = (T + 273) (log (t) +20) (3)
However, logVc90 = 2.94-0.75 (2.7C + 0.4Si + Mn + 0.45Ni + 0.45Cu + 0.8Cr + 2Mo)
The content of the corresponding element is substituted in mass% for the element symbol in the above formula. log is the common logarithm. In the formula (3), T is the tempering temperature and t is the tempering holding time. The unit of T is ° C. and the unit of t is time.

According to the present invention, a seamless steel pipe for high-strength low-alloy oil wells having excellent hydrogen embrittlement resistance and a method for producing the same can be obtained.

FIG. 1 is a flow chart showing an example of a method for manufacturing a seamless steel pipe for a high-strength low-alloy oil well. FIG. 2 is a scatter diagram showing the relationship between the tempering parameter TP and the yield strength YS. FIG. 3 is a scatter diagram showing the relationship between the yield strength YS and the crack length ratio CLR.

The present inventors have investigated the hydrogen embrittlement resistance of a seamless steel pipe for low alloy wells having a high strength of 160 ksi (1103 MPa) or more.

As a method for stably obtaining a high yield strength, it is conceivable to improve the hardenability of steel and to obtain a structure having a high volume fraction of the hardened structure.

Further, as a method for improving the hydrogen embrittlement resistance, it is conceivable to raise the tempering temperature or lengthen the tempering time. By raising the tempering temperature or lengthening the tempering time, the carbides can be spheroidized and the density of dislocations, which are hydrogen trap sites, can be reduced. On the other hand, dislocations also contribute to increasing the strength of steel, so if the tempering temperature is simply increased or the tempering time is lengthened, the required yield strength cannot be maintained.

Mo is known as an element that improves hardenability and temper softening resistance of steel. However, it is considered that if Mo is contained in an amount exceeding 1.0% by mass, needle-shaped MC-type alloy carbides are precipitated and the hydrogen embrittlement resistance is lowered (for example, JP-A-2001-2711134). See.).

The present inventors have focused on the fact that the morphology of carbides can be controlled by further increasing the Mo content and adjusting the ratio to the Cr content. Specifically, the Mo content more than 1.1 mass%, and by a Mo / Cr less than 2.0, in place of the MC-type alloy carbides, of _{M 2} C type, such as Mo ₂ C Alloy carbides can be deposited. M 2 _C-type alloys carbides traps the hydrogen, to improve the hydrogen embrittlement resistance. Therefore, it is possible to improve the yield strength by improving the temper softening resistance and the hydrogen embrittlement resistance.

Based on the above findings, the present invention has been completed. Hereinafter, a seamless steel pipe for a high-strength low-alloy oil well according to an embodiment of the present invention will be described in detail.

[Chemical composition]
The seamless steel pipe for high-strength low-alloy oil wells according to this embodiment has the chemical composition described below. In the following description, "%" of elemental content means mass%.

C: 0.20 to 0.50%
Carbon (C) enhances the hardenability of steel and enhances the strength of steel. If the C content is less than 0.20%, this effect cannot be sufficiently obtained. On the other hand, when the C content exceeds 0.50%, the burning sensitivity of the steel becomes high. Therefore, the C content is 0.25 to 0.50%. The lower limit of the C content is preferably 0.22%. The upper limit of the C content is preferably 0.40%, more preferably 0.30%.

Si: 0.05 to 1.00%
Silicon (Si) deoxidizes steel. If the Si content is less than 0.05%, this effect cannot be sufficiently obtained. On the other hand, if the Si content exceeds 1.00%, the hydrogen embrittlement resistance of the steel deteriorates. Therefore, the Si content is 0.05 to 1.00%. The lower limit of the Si content is preferably 0.10%, more preferably 0.20%. The upper limit of the Si content is preferably 0.80%, more preferably 0.50%.

Mn: 0.1 to 1.0%
Manganese (Mn) enhances the hardenability of steel and contributes to the improvement of strength. If the Mn content is less than 0.1%, this effect cannot be sufficiently obtained. On the other hand, if the Mn content exceeds 1.0%, the hydrogen embrittlement resistance of the steel deteriorates. Therefore, the Mn content is 0.1 to 1.0%. The lower limit of the Mn content is preferably 0.2%, more preferably 0.3%. The upper limit of the Mn content is preferably 0.8%, more preferably 0.6%.

P: 0.030% or less Phosphorus (P) is an impurity. P segregates at the grain boundaries and reduces the hydrogen embrittlement resistance of the steel. Therefore, the P content is 0.030% or less. It is preferable that the P content is as low as possible. The P content is preferably 0.020% or less.

S: 0.0050% or less Sulfur (S) is an impurity. S lowers the hydrogen embrittlement resistance of steel. Therefore, the S content is 0.0050% or less. It is preferable that the S content is as low as possible. The S content is preferably 0.0020% or less.

Al: 0.005-0.1%
Aluminum (Al) deoxidizes steel. If the Al content is less than 0.005%, this effect cannot be sufficiently obtained. On the other hand, when the Al content exceeds 0.1%, the inclusions become coarse and the hydrogen embrittlement resistance of the steel deteriorates. Therefore, the Al content is 0.005 to 0.1%. The lower limit of the Al content is preferably 0.01%. The upper limit of the Al content is preferably 0.08%, more preferably 0.06%. The Al content in the present specification means the content of acid-soluble Al (so-called Sol.Al).

Cr: 0.10 to 1.20%
Chromium (Cr) enhances the hardenability of steel and contributes to the improvement of strength. Cr also has an action of spheroidizing carbides when contained together with an appropriate amount of Mo, and contributes to improvement of hydrogen embrittlement resistance. If the Cr content is less than 0.10%, this effect cannot be sufficiently obtained. On the other hand, when the Cr content exceeds 1.20%, _{M 2} C-type alloys precipitation of carbides is prevented. Therefore, the Cr content is 0.10 to 1.20%. The lower limit of the Cr content is preferably 0.20%, more preferably 0.40%. The upper limit of the Cr content is preferably 1.0%, more preferably 0.8%.

N: 0.01% or less Nitrogen (N) is an impurity. N forms nitride-based inclusions and reduces the hydrogen embrittlement resistance of steel. Therefore, the N content is 0.01% or less. It is preferable that the N content is as low as possible. The upper limit of the N content is preferably 0.006%, more preferably 0.006%. From the viewpoint of cost, the lower limit of the N content is preferably 0.001%.

Mo: 1.1-2.5%
Molybdenum (Mo) enhances the hardenability of steel and contributes to the improvement of strength. Mo also increases the temper softening resistance of steel. Mo also precipitates as M 2 _C-type alloys carbides, improving the hydrogen embrittlement resistance of the steel. If the Mo content is less than 1.1%, these effects cannot be sufficiently obtained. On the other hand, when the Mo content exceeds 2.5%, the precipitate becomes excessive and the hydrogen embrittlement resistance deteriorates. Therefore, the Mo content is 1.1 to 2.5%. The lower limit of the Mo content is preferably 1.15%, more preferably 1.2%. The upper limit of the Mo content is preferably 2.0%, more preferably 1.8%.

Ti: 0.002-0.020%
Titanium (Ti) forms a nitride or carbonitride and contributes to the refinement of crystal grains by the pinning effect. If the Ti content is less than 0.002%, this effect cannot be sufficiently obtained. On the other hand, when the Ti content exceeds 0.020%, the nitride-based inclusions become coarse and the hydrogen embrittlement resistance of the steel deteriorates. Therefore, the Ti content is 0.002 to 0.020%. The lower limit of the Ti content is preferably 0.003%. The upper limit of the Ti content is preferably 0.009%.

Nb: 0.050% or less Niobium (Nb) forms carbides and contributes to the refinement of crystal grains by the pinning effect. This effect can be obtained if even a small amount of Nb is contained. On the other hand, when the Nb content exceeds 0.020%, the nitride-based inclusions become coarse and the hydrogen embrittlement resistance of the steel deteriorates. Therefore, the Nb content is 0.2% or less. The lower limit of the Nb content is preferably 0.01%, more preferably 0.02%. The upper limit of the Nb content is preferably 0.040%, more preferably 0.030%.

V: 0.01 to 0.30%
Vanadium (V) forms fine alloy carbides and increases the temper softening resistance of steel. If the V content is less than 0.01%, this effect cannot be sufficiently obtained. On the other hand, when the V content exceeds 0.30%, the precipitate becomes excessive and the hydrogen embrittlement resistance deteriorates. Therefore, the V content is 0.01 to 0.30%. The lower limit of the V content is preferably 0.04%, more preferably 0.06%. The upper limit of the V content is preferably 0.25%, more preferably 0.20%.

B: 0.0050% or less Boron (B) enhances hardenability of steel and contributes to improvement of strength. This effect can be obtained if even a small amount of B is contained. On the other hand, when the B content exceeds 0.0050%, nitride-based inclusions are formed and the hydrogen embrittlement resistance of the steel deteriorates. Therefore, the B content is 0.0050% or less. The lower limit of the B content is preferably 0.0001%, more preferably 0.0005%. The upper limit of the B content is preferably 0.0040%.

O: 0.0050% or less Oxygen (O) is an impurity. O forms an oxide and lowers the hydrogen embrittlement resistance and toughness of steel. Therefore, the O content is 0.0050% or less. The O content is preferably as low as possible. The O content is preferably 0.0030% or less.

The rest of the chemical composition of the seamless steel pipe for high-strength low-alloy oil wells according to this embodiment is Fe and impurities. The impurities referred to here refer to elements mixed from ores and scraps used as raw materials for steel, or elements mixed from the environment of the manufacturing process.

The chemical composition of the seamless steel pipe for high-strength low-alloy oil wells according to the present embodiment may further contain an element described below in place of a part of Fe. The elements described below are all selective elements. That is, the chemical composition of the seamless steel pipe for high-strength low-alloy oil wells according to this embodiment does not have to contain a part or all of the following elements.

Ca: 0 to 0.0050%
Mg: 0 to 0.0050%
REM: 0-0.0050%
Calcium (Ca), magnesium (Mg), and rare earth elements (REM) atomize sulfides and contribute to the improvement of hydrogen embrittlement resistance. This effect can be obtained if any of these elements are contained. On the other hand, when the content of each of these elements exceeds 0.0050%, oxide-based inclusions are formed and the hydrogen embrittlement resistance of the steel is lowered. Therefore, the respective contents of Ca, Mg, and REM are 0 to 0.0050%. The lower limit of the content of each of these elements is preferably 0.001%. The upper limit of the content of each of these elements is preferably 0.0025%.

Note that REM is a general term for 17 elements in which Y and Sc are combined with 15 elements of lanthanoid, and one or more of these elements can be contained. REM content means the total content of these elements.

Cu: 0-1.0%
Ni: 0-1.0%
Copper (Cu) and nickel (Ni) enhance the hardenability of steel and contribute to the improvement of strength. This effect can be obtained if any of these elements are contained. On the other hand, if the content of each of these elements exceeds 1.0%, the hydrogen embrittlement resistance property deteriorates. Therefore, the respective contents of Cu and Ni are 0 to 1.0%. The lower limit of the content of each of these elements is preferably 0.01%. The upper limit of the content of each of these elements is preferably 0.5%, more preferably 0.1%.

Co: 0-1.0%
W: 0 to 1.0%
Cobalt (Co) and W (tungsten) reduce the corrosion rate and enhance the hydrogen embrittlement resistance of steel. This effect can be obtained if any of these elements are contained. On the other hand, if the content of each of these elements exceeds 1.0%, the effect is saturated and the cost increases. Therefore, the respective contents of Co and W are 0 to 1.0%. The lower limit of the content of each of these elements is preferably 0.01%. The upper limit of the content of each of these elements is preferably 0.8%.

The chemical composition of the seamless steel pipe for high-strength low-alloy oil wells according to the present embodiment further satisfies the following formulas (1) and (2).
Vc90 ≤ 1.0 (1)
Mo / Cr ≧ 2.0 (2)
However, logVc90 = 2.94-0.75 (2.7C + 0.4Si + Mn + 0.45Ni + 0.45Cu + 0.8Cr + 2Mo)
The content of the corresponding element is substituted in mass% for the element symbol in the above formula. log is the common logarithm.

[About equation (1)]
Vc90 is an index of hardenability, and specifically, is a critical cooling rate (° C./sec) for obtaining a martensite structure having a volume fraction of 90% or more. In the present embodiment, the chemical composition is such that Vc90 is 1.0 or less. If the chemical composition has a Vc90 of more than 1.0, the hardenability is not sufficient and it becomes difficult to obtain the required yield strength. Vc90 is preferably 0.8 or less.

[About equation (2)]
In the present embodiment, the Cr content and the Mo content are set within the above ranges, and the ratio Mo / Cr of the Mo content to the Cr content expressed in mass% is set to 2.0 or more. Mo forms Mo ₂ C as described above and contributes to the improvement of hydrogen embrittlement resistance. However, increasing Cr content hinders the formation of Mo ₂ C. If Mo / Cr is less than 2.0, the formation of Mo ₂ C becomes insufficient due to the influence of Cr. The value of Mo / Cr is preferably 2.3 or more.

[Structure, yield strength, hydrogen embrittlement resistance]
The structure of the seamless steel pipe for high-strength low-alloy oil wells according to the present embodiment preferably has tempered martensite having a volume fraction of 90% or more, and further contains carbides precipitated during tempering. The structure of the seamless steel pipe for high-strength low-alloy oil wells may contain precipitates, inclusions, and retained austenite other than carbides such as nitrides and carbonitrides. However, since retained austenite causes variation in intensity, the volume fraction is preferably 5% or less.

The volume fraction of retained austenite is measured as follows. Take a test piece including the central part in the thickness direction of the steel pipe. Polish the surface of the collected test piece. X-ray diffraction is performed on the polished test piece using CoKα rays as incident X-rays. From the integrated intensities of the (220), (200), and (111) planes of the α phase and the (220), (200), and (111) planes of austenite, the volume fraction of retained austenite. Quantify.

The structure of the seamless steel pipe for high-strength low-alloy oil wells according to the present embodiment further has an old austenite grain having a crystal grain size number of 8.5 or more according to ASTM E112-10. When the old austenite grains are coarse grains having a crystal grain size number of less than 8.5, the required hydrogen embrittlement resistance cannot be obtained. The old austenite grains preferably have a crystal grain size number of 9.0 or more.

For the grain size number of the former austenite grains, a test piece is cut out from each steel pipe and embedded in a resin so that the cross section perpendicular to the length direction (tube manufacturing direction) of the steel pipe becomes the test surface, and corroded with a saturated aqueous solution of picric acid. The former austenite grain boundaries are revealed by the Bechet-Beaujard method, and the measurement is performed according to ASTM E112-10.

The crystal grain size number of the old austenite grains may be measured using a steel material after quenching and before tempering (so-called as-quenched steel material), or may be measured using a tempered steel material. Regardless of which steel material is used, the crystal grain size numbers of the former austenite grains are almost the same.

For the tempered steel material, the ASTM particle size number of the former austenite crystal grains can be obtained from the crystal orientation relationship by using a method such as electron backscatter diffraction (EBSD). In this case, the metallographic structure of the seamless steel pipe after tempering is measured by EBSD as follows. A sample is taken from the center position of the wall thickness of the cross section of the seamless steel pipe after tempering (the cross section perpendicular to the axial direction of the seamless steel pipe). Using the collected sample, crystal orientation analysis was performed by EBSD in an observation range of 500 × 500 μm ² , and the boundary between grains whose Generation Angle was in the range of 15 to 51 ° was defined as the old austenite grain boundary and drawn. , The crystal grain size number is obtained according to ASTM E112-10 based on the drawn drawing.

The seamless steel pipe for high-strength low-alloy oil wells according to the present embodiment has a yield strength of 160 ksi (1103 MPa) to 175 ksi (1206 MPa). The lower limit of the yield strength of the seamless steel pipe for high-strength low-alloy oil wells according to the present embodiment is preferably 165 ksi (1138 MPa).

The seamless steel pipe for high-strength low-alloy oil wells according to this embodiment has excellent hydrogen embrittlement resistance. High strength low alloy seamless steel oil country tubular goods according to the present embodiment, preferably, under 4 ° C. environment for 96 hours in a test solution saturated with _{H 2} S gas partial pressure 0.1bar to solution B liquid immersion The CLR after this is 5% or less.

[Production method]
Hereinafter, an example of a method for manufacturing a seamless steel pipe for a high-strength low-alloy oil well according to the present embodiment will be described. This manufacturing method is merely an example, and the manufacturing method of a seamless steel pipe for a high-strength low-alloy oil well according to the present embodiment is not limited to this.

FIG. 1 is a flow chart showing an example of a method for manufacturing a seamless steel pipe for a high-strength low-alloy oil well. This manufacturing method includes a step of preparing a steel material (step S1), a step of hot-working the steel material to make a steel pipe (step S2), a step of replenishing the steel pipe (step S3), and a steel material. It includes a step of cooling the pipe (step S4), a step of tempering the cooled steel raw pipe (step S5), and a step of performing reheating quenching and tempering one or more times (steps S6 to S9). Hereinafter, each step will be described in detail.

[Steel material preparation process (step S1)]
Steel with the above chemical composition is melted and refined by a well-known method. Subsequently, the molten steel is made into a continuous cast material by a continuous casting method. The continuous cast material is, for example, a slab, bloom, or billet. The molten steel may be made into an ingot by the ingot method. Slabs, blooms, or ingots are made into billets by hot working.

[Hot working process (step S2)]
The billet obtained by continuous casting or hot working is hot worked to produce a steel tube. Hot working is, for example, the Mannesmann method. The steel tube may be manufactured by other hot working.

[Replenishment step (step S3)]
After the hot working step (step S2) and before the cooling step (step S4), the steel pipe is heated to a temperature equal to or higher than the Ac ₃ transformation point (step S3). The heat replenishing step (step S3) is an arbitrary step. That is, the cooling step (step S4) may be carried out without carrying out the heat supplementing step (step S3). If the heat supplementing step (step S3) is carried out, the temperature of the steel pipe can be made uniform, and a more uniform structure can be obtained.

[Cooling step (step S4)]
The hot-worked steel pipe or the reheated steel pipe is cooled to room temperature at a cooling rate equal to or higher than air cooling (step S4). Water cooling is preferable as the cooling method. That is, it is preferable to carry out direct quenching in which the steel pipe formed by hot working is water-cooled, or in-line quenching in which water is cooled after supplementing heat. By quenching the steel pipe, the structure becomes a hardened structure. The hardened structure obtained at this time undergoes reverse transformation to austenite when the temperature rises during reheating and quenching, so that the crystal grains can be made finer.

[Tempering step (step S5)]
The cooled steel pipe is tempered (step S5). The tempering temperature is preferably less than or equal to the Ac ₁ transformation point.

[Reheat quenching step and tempering step (steps S6 to S9)]
The steel pipe after the tempering step (step S5) is subjected to reheating quenching and tempering at least once (steps S6 to S9). FIG. 1 shows a case where reheating quenching and tempering are performed twice, but the number of times of reheating quenching and tempering may be one, or may be three or more. As the number of times of reheating quenching and tempering is increased, the crystal grains can be made finer.

The quenching temperature in the final quenching step (step S8) is at least the Ac ₃ transformation point and at least 1000 ° C. If the quenching temperature is less than the Ac ₃ transformation point, a structure having a high volume fraction of martensite structure cannot be obtained. Further, when the quenching temperature is higher than 1000 ° C., the austenite grains become coarse and a fine structure cannot be obtained.

The conditions of the quenching step (step S6) other than the final quenching step (step S8) are not particularly limited, but as in the final quenching step (step S8), the quenching temperature may be the Ac ₃ transformation point or more and 1000 ° C. or less. preferable.

In the final tempering step (step S9), the tempering parameter TP defined by the following formula (3) is set to 18200 to 18400.
TP = (T + 273) (log (t) +20) (3)
In formula (3), T is the tempering temperature, t is the tempering holding time, and log is the common logarithm. The unit of T is ° C. and the unit of t is time.

If the tempering parameter TP is less than 18200, it becomes difficult to obtain the required hydrogen embrittlement resistance. On the other hand, if the tempering parameter TP exceeds 18400, it becomes difficult to obtain the required yield strength.

The upper limit of the tempering temperature T in the final tempering step (step S9) is preferably the Ac ₁ transformation point, and more preferably 700 ° C. The lower limit of the tempering temperature T is preferably 600 ° C., more preferably 650 ° C.

The conditions of the tempering step (step S7) other than the final tempering step (step S9) are not particularly limited, but as in the final tempering step (step S9), the tempering temperature is preferably Ac ₁ transformation point or less.

The above is an example of a method for manufacturing a seamless steel pipe for a high-strength low-alloy oil well. According to this production method, it has a yield strength of 1103-1206 MPa, has old austenite grains having a crystal grain size number of 8.5 or more according to ASTM E112-10, and has excellent hydrogen embrittlement resistance and high strength. A seamless steel pipe for low alloy oil wells can be obtained.

Hereinafter, the present invention will be described in more detail with reference to Examples. The present invention is not limited to these examples.

After heating the slabs of steels A to G having the chemical compositions shown in Table 1 at 1250 ° C. for 30 minutes, hot pipe making was carried out, and a seamless steel pipe having an outer diameter of 244.48 mm and a wall thickness of 13.84 mm was carried out. Manufactured. “-” In Table 1 indicates that the content of the corresponding element is at the impurity level.

The seamless steel pipe produced by hot working was subjected to in-line quenching by heating it to 950 ° C. in a heating furnace and then cooling it with water, and then tempering it at 550 ° C. for 30 minutes. After that, the heat treatments shown in Table 2 were further carried out.

As shown in Table 2, No. In 1-4, 8 to 15, and 18, after in-line quenching and tempering, one more reheating quenching and tempering were performed. No. In 5 to 7, 16 and 17, after in-line quenching and tempering, two additional reheating quenching and tempering were performed. In the column of "TP" in Table 2, the value of the tempering parameter TP of the final tempering is described.

[Old austenite particle size measurement]
After the final quenching, the Behcet-Beaujard method described in the embodiment is applied to each seamless steel pipe before the final tempering to reveal the old austenite grains, and the particle size number is investigated according to ASTM E112-10. did.

[Tensile test]
From each seamless steel pipe after the final tempering, an API round bar test piece (test piece size: parallel part diameter 8.9 mm, GL 36.5 mm) was collected in accordance with the specifications of API 5CT, and a tensile test was performed. Yield strength (YS) and tensile strength (TS) were determined. The yield strength was defined as 0.2% offset proof stress.

[HIC test]
The hydrogen embrittlement resistance characteristics of each seamless steel pipe after the final tempering were evaluated by performing a HIC test based on NACE-TM0284. Specifically, test pieces having a thickness of 13.84 mm (same as the material wall thickness), a width (circumferential direction) of 20 mm, and a length of 100 mm were collected from each seamless steel pipe after the final tempering. The specimen 4 under a temperature environment of ° C., Solution B solution _{(5.0wt% NaCl + 2.5wt% CH} 3 COOH + 0.41wt% CH 3 COONa, ph: 3.5) to the partial pressure 0.1 bar _{H 2} S gas Was immersed in the saturated test solution for 96 hours (4 days).

Ultrasonic flaw detection (UT) was performed on the test piece after the test to identify the maximum cracked portion, and the portion was cut. The cross section was the wall thickness x width (circumferential direction) of the test piece. Using the cut test piece, the crack length ratio CLR (= crack length (mm) / width of the test piece (mm)) was determined. The goal was to have a CLR of 5% or less.

In the HI C test, the lower the environmental temperature, the higher the hydrogen concentration in the steel. Therefore, the HIC test of this example carried out in a temperature environment of 4 ° C. is a test in a harsher environment than the HIC test at room temperature.

The results are shown in Table 3.

As shown in Table 3, all of the manufactured seamless steel pipes had a structure in which the tempered martensite phase was the main phase. In the column of "old γ grain size" in Table 3, the crystal grain size numbers of the old austenite grains are described. If the "CLR" column is "-", it means that the HIC test was not performed.

As shown in Table 3, No. The seamless steel pipes of 2, 3, 6, 9 and 12 had a yield strength of 1103 MPa or more and a CLR of 5% or less.

No. The seamless steel pipes 1, 5, 8 and 11 had a CLR of more than 5%. It is considered that this is because the value of the tempering parameter TP was too small and the yield strength was too high.

No. The yield strength of the seamless steel pipes of 4, 7, 10, and 13 was less than 1103 MPa. It is considered that this is because the value of the tempering parameter TP was too large.

No. For 14 to 17, the yield strength was less than 1103 MPa, or the CLR exceeded 5%. Since the chemical composition of these steel pipes was not appropriate, there was no tempering condition that could achieve both yield strength and hydrogen embrittlement resistance. No. Since 18 (steel G) did not contain V, the yield strength could not be 1103 MPa or more unless the tempering parameter TP was significantly lowered as compared with other seamless steel pipes.

FIG. 2 is a scatter diagram showing the relationship between the tempering parameter TP and the yield strength YS. The white squares (□) are for seamless steel pipes manufactured from steels A to D, and the solid circles (●) are for seamless steel pipes manufactured from steels E and F. From FIG. 2, it can be seen that the yield strength can be increased to 1103 to 1206 MPa by tempering the seamless steel pipes manufactured from steels A to D under the condition that the tempering parameter TP is 18200 to 18400. Further, the seamless steel pipes manufactured from steels A to D have higher yield strengths as compared with the seamless steel pipes manufactured from steels E and F, even if they are treated with the same level of tempering parameters. It can be seen that is obtained.

FIG. 3 is a scatter diagram showing the relationship between the yield strength YS and the crack length ratio CLR. The white circles (○) are for seamless steel pipes manufactured from steels A to D, and the solid squares (■) are for seamless steel pipes manufactured from steels E and F. From FIG. 3, it can be seen that the CLR can be reduced to 5% or less by setting the yield strength of the seamless steel pipes manufactured from steels A to D to 1206 MPa or less. Further, the seamless steel pipes manufactured from steels A to D have better hydrogen embrittlement resistance than the seamless steel pipes manufactured from steels E and F, even if the yield strength is the same level. It can be seen that it has been obtained.

The embodiments of the present invention have been described above. The above-described embodiment is merely an example for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and the above-described embodiment can be appropriately modified and implemented within a range that does not deviate from the gist thereof.

Claims (10)

The chemical composition is mass%,
C: 0.25 to 0.50%,
Si: 0.05 to 1.00%,
Mn: 0.1 to 1.0%,
P: 0.030% or less,
S: 0.0050% or less,
Al: 0.005-0.1%,
Cr: 0.10-1.20%,
N: 0.01% or less,
Mo: 1.1-2.5%,
Ti: 0.002-0.020%,
Nb: 0.050% or less,
V: 0.01 to 0.30%,
B: 0.0050% or less,
O: 0.0050% or less,
Ca: 0 to 0.0050%,
Mg: 0 to 0.0050%,
REM: 0-0.0050%,
Cu: 0-1.0%,
Ni: 0-1.0%,
Co: 0-1.0%,
W: 0-1.0%,
Remaining: Fe and impurities,
It has a yield strength of 1103-1206 MPa and has a yield strength of 1103-1206 MPa.
The chemical composition satisfies the following formulas (1) and (2).
A seamless steel pipe for high-strength low-alloy oil wells with excellent hydrogen embrittlement resistance, having austenite grains with an austenite grain size number of 8.5 or more in accordance with ASTM E112-10.
Vc90 ≤ 1.0 (1)
Mo / Cr ≧ 2.0 (2)
However, logVc90 = 2.94-0.75 (2.7C + 0.4Si + Mn + 0.45Ni + 0.45Cu + 0.8Cr + 2Mo)
The content of the corresponding element is substituted in mass% for the element symbol in the above formula. log is the common logarithm. The seamless steel pipe for high-strength low-alloy oil wells according to claim 1.
When the chemical composition is mass%,
Ca: 0.0010 to 0.0050%,
Mg: 0.0010 to 0.0050%, and REM: 0.0010 to 0.0050%,
A seamless steel pipe for high-strength low-alloy oil wells containing one or more elements selected from the group consisting of. The seamless steel pipe for high-strength low-alloy oil wells according to claim 1 or 2.
When the chemical composition is mass%,
Cu: 0.01-1.0%, and Ni: 0.01-1.0%
A seamless steel pipe for high-strength low-alloy oil wells containing 1 or 2 elements selected from the group consisting of. The seamless steel pipe for a high-strength low-alloy oil well according to any one of claims 1 to 3.
When the chemical composition is mass%,
Co: 0.01 to 1.0%, and W: 0.01 to 1.0%
A seamless steel pipe for high-strength low-alloy oil wells containing 1 or 2 elements selected from the group consisting of. The method for manufacturing a seamless steel pipe for a high-strength low-alloy oil well according to any one of claims 1 to 4.
The chemical composition is C: 0.25 to 0.50%, Si: 0.05 to 1.00%, Mn: 0.1 to 1.0%, P: 0.030% or less, S in mass%. : 0.0050% or less, Al: 0.005 to 0.1%, Cr: 0.10 to 1.20%, N: 0.01% or less, Mo: 1.1 to 2.5%, Ti: 0.002 to 0.020%, Nb: 0.050% or less, V: 0.01 to 0.30%, B: 0.0050% or less, O: 0.0050% or less, Ca: 0 to 0. 0050%, Mg: 0 to 0.0050%, REM: 0 to 0.0050%, Cu: 0 to 1.0%, Ni: 0 to 1.0%, Co: 0 to 1.0%, W: 0-1.0%, balance: Fe and the process of preparing steel materials that are impurities,
The process of hot-working the steel material to make a steel pipe,
The process of cooling the steel pipe to room temperature at a speed higher than air cooling,
The process of tempering the cooled steel pipe and
A step of reheating the tempered steel tube to a temperature of Ac ₃ transformation point or more and 1000 ° C. or less and quenching.
It is provided with a step of tempering the hardened steel pipe again.
The chemical composition satisfies the following formulas (1) and (2).
A method for producing a seamless steel pipe for a high-strength low-alloy oil well having excellent hydrogen embrittlement resistance and having a tempering parameter TP of 18200 to 18400 defined by the following formula (3) in the step of tempering again.
Vc90 ≤ 1.0 (1)
Mo / Cr ≧ 2.0 (2)
TP = (T + 273) (log (t) +20) (3)
However, logVc90 = 2.94-0.75 (2.7C + 0.4Si + Mn + 0.45Ni + 0.45Cu + 0.8Cr + 2Mo)
The content of the corresponding element is substituted in mass% for the element symbol in the above formula. log is the common logarithm. In the formula (3), T is the tempering temperature and t is the tempering holding time. The unit of T is ° C. and the unit of t is time. The method for manufacturing a seamless steel pipe for a high-strength low-alloy oil well according to claim 5.
When the chemical composition is mass%,
Ca: 0.0010 to 0.0050%,
Mg: 0.0010 to 0.0050%, and REM: 0.0010 to 0.0050%,
A method for producing a seamless steel pipe for a high-strength low-alloy oil well, which contains one or more elements selected from the group consisting of. The method for manufacturing a seamless steel pipe for a high-strength low-alloy oil well according to claim 5 or 6.
When the chemical composition is mass%,
Cu: 0.01-1.0%, and Ni: 0.01-1.0%
A method for producing a seamless steel pipe for a high-strength low-alloy oil well, which contains 1 or 2 elements selected from the group consisting of. The method for manufacturing a seamless steel pipe for a high-strength low-alloy oil well according to any one of claims 5 to 7.
When the chemical composition is mass%,
Co: 0.01 to 1.0%, and W: 0.01 to 1.0%
A method for producing a seamless steel pipe for a high-strength low-alloy oil well, which contains 1 or 2 elements selected from the group consisting of. The method for manufacturing a seamless steel pipe for a high-strength low-alloy oil well according to any one of claims 5 to 8.
A method for producing a seamless steel pipe for a high-strength low-alloy oil well, further comprising a step of heating the steel pipe to a temperature equal to or higher than the Ac ₃ transformation point after the hot working and before the step of cooling to room temperature. The method for manufacturing a seamless steel pipe for a high-strength low-alloy oil well according to any one of claims 5 to 9.
A method for producing a seamless steel pipe for a high-strength low-alloy oil well, further comprising a step of quenching and tempering the steel base pipe after the step of tempering and before the step of reheating and quenching.

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