JP2017039998A - Seamless stainless steel pipe for oil well and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a seamless stainless steel pipe for an oil well having high yield strength, high toughness and excellent corrosion resistance, and a method for producing the same.SOLUTION: A billet having a prescribed composition is heated at 1,150 to 1,300°C for 100 to 400 min, is heated, and is further subjected to hot working into a seamless steel pipe, the seamless steel pipe is cooled to a room temperature at a cooling rate of air cooling or more, thereafter, the seamless steel pipe is heated at 850°C or higher, is successively cooled to 100°C or lower at a cooling rate of air cooling or more to a room temperature, is quenched, and is next tempered at 700°C or lower so as to be a seamless steel pipe having a structure composed of a ferritic phase of 10 to 60 vol.%, an austenitic phase of 0 to 20 vol.%, and the balance martensitic phase, in which the density of the precipitates with a grain size of 0.5 μ or higher precipitated into the structure is 6×10pieces/mmor lower, in which yield strength is 758 MPa or higher, and absorption energyEis 40 J or higher by a Charpy impact test at a test temperature of -10°C.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a seamless stainless steel pipe having excellent corrosion resistance, high strength and high toughness suitable for use as an oil well pipe, and a production method capable of stably obtaining the same.

In recent years, fossil fuels that can be easily mined have been depleted, and oil fields are being developed in areas where mining is difficult. In particular, oil wells with high strength and toughness are required for oil wells in a deep and high temperature environment. Moreover, CO ₂ and Cl ^- in the oil field for mine oil resources containing such, because it is easy corrosion proceeds, OCTG corrosion resistance is required.

As an oil well pipe used in such a severe environment, a martensitic stainless steel (so-called 13Cr steel) seamless steel pipe containing about 13% by mass of Cr has been put into practical use. However, Cl ^- and containing a large amount, moreover in the mining of hot oil resources exceeding 100 ° C., there is a problem that sufficient strength and corrosion resistance can not be obtained even by using a seamless steel pipe 13Cr steel.

  As an oil well pipe that can be used in a high-temperature corrosive environment, a seamless stainless steel pipe of a duplex stainless steel has been developed. However, duplex stainless steel adds a large amount of alloying elements, which not only increases the raw material cost, but also deteriorates the hot workability in the manufacturing process of the seamless steel pipe, resulting in decreased productivity. There exists a problem that the manufacturing cost of a seamless steel pipe rises.

  Therefore, it can be easily and stably manufactured through a series of processes from a tilt rolling piercing mill, which is a general manufacturing technology for seamless steel pipes, and has excellent corrosion resistance even at high temperatures, and also has high strength and high toughness. Techniques for obtaining seamless stainless steel pipes are being studied.

  For example, in Patent Document 1, a billet having a predetermined component is used to make a seamless steel pipe by hot working, and further, by quenching and tempering, it is excellent in corrosion resistance and has both high strength and high toughness. A technique for obtaining a stainless steel pipe is disclosed. However, since the strength and toughness of the seamless stainless steel pipe varies depending on the heating temperature and holding time in tempering, there is still room for improvement in this technique from the viewpoint of stably obtaining a desired strength.

  In Patent Document 2, a billet having a predetermined component is used to perform a hot working of a predetermined reduction amount to make a seamless steel pipe, and further, quenching and tempering are performed to provide excellent corrosion resistance, and high strength and high toughness. A technique for obtaining a seamless stainless steel pipe having both of the above is disclosed. However, since the strength of the seamless stainless steel pipe fluctuates according to the cooling stop temperature in quenching, there is still room for improvement from the viewpoint of stably obtaining a desired strength.

JP 2005-336595 A JP 2013-249516 A

The present invention solves the problems of the prior art, and the absorbed energy _V E _-10 is 40 J or more in a Charpy impact test with a yield strength of 110 ksi (= 758 MPa) or higher and a test temperature of −10 ° C. It is an object of the present invention to provide a seamless stainless steel pipe for oil wells having high toughness and excellent corrosion resistance, and a production method capable of stably obtaining the same. The excellent corrosion resistance here refers to a corrosion test conducted by immersion for 14 days in a 20 mass% NaCl aqueous solution (liquid temperature 230 ° C, saturated with 3.0 MPa CO ₂ gas) held in an autoclave. The case where the speed is 0.127 mm / year or less and there is no pitting corrosion.

  The inventor examined the influence of quenching and tempering on the toughness of seamless stainless steel pipes, and as a result, it was found that when a large amount of Al and N are contained, the toughness is likely to deteriorate. Then, when the structure of the seamless stainless steel pipe having a large content of Al and N and deteriorated toughness was investigated, it was found that a large amount of precipitates were precipitated in the structure.

Furthermore, the observed deposits were investigated in detail. As a result, about the precipitate,
(a) Precipitates mainly found are Cr ₂ N, Cr ₂₃ C ₆ , Fe ₃ C, Nb (C, N), AlN, and most of the precipitates having a particle size of 0.5 μm or more are particularly AlN. ,
(b) The number of precipitates having a particle size of 0.5 μm or more greatly affects the toughness of the seamless stainless steel pipe.
(c) The number of precipitates with a particle size of 0.5μm or more per unit area (mm ² ) (hereinafter referred to as density) varies depending on the concentration product [% Al] x [% N] of the seamless stainless steel pipe The knowledge to do was acquired (refer FIG. 1).

That is, in order to solve the problems of the present invention and obtain a seamless stainless steel pipe for oil wells having desired strength, toughness, and corrosion resistance, a precipitate having a particle size of 0.5 μm or more is 6 × 10 ³ pieces / mm ² or less. In order to obtain its structure, the density product of Al content and N content of oil well seamless stainless steel pipe: [% Al] x [% N] is 0.0020 or less, It is necessary to adjust the composition of seamless stainless steel pipes for oil wells. In order to stably produce the oil well seamless stainless steel pipe, it is necessary to design the components in order to improve the hot workability.

The present invention has been made based on such knowledge.
That is, the present invention is C: 0.005-0.06 mass%, Si: 0.05-0.5 mass%, Mn: 0.2-1.8 mass%, P: 0.03 mass% or less, S: 0.005 mass% or less, Cr: 15.5-18.0 mass% Ni: 1.5-5.0 mass%, V: 0.02-0.2 mass%, Al: 0.002-0.05 mass%, N: 0.01-0.15 mass%, O: 0.006 mass% or less, and Mo: 1.0-3.5 Contains one or more selected from mass%, W: 0.5 to 3.0 mass%, Cu: 0.5 to 3.5 mass%, with the balance being Fe and inevitable impurities, and C, Si, Mn , Cr, Ni, Mo, W, Cu, Al, N is a billet having a composition satisfying the following formulas (1), (2) and (3) at a temperature of 1150 to 1300 ° C. Heat for ~ 400 minutes, further hot work to make seamless steel pipe, cool the seamless steel pipe to room temperature at a cooling rate higher than air cooling, then heat the seamless steel pipe to 850 ° C or higher and continue to 100 ° C or lower Cool down to room temperature at a cooling rate higher than air cooling. By cooling and quenching, and then tempering at 700 ° C. or lower, 10 to 60% by volume of ferrite phase, 0 to 20% by volume of austenite phase, and the balance consisted of martensite phase and precipitated in the structure It has a structure in which the density of precipitates with a particle size of 0.5 μm or more is 6 × 10 ³ particles / mm ² or less, the yield strength is 758 MPa or more, and the absorbed energy in a Charpy impact test with a test temperature of −10 ° C. This is a method for producing seamless stainless steel pipes for oil wells having a _V E _-10 of 40 J or more.
[% Cr] +0.65 [% Ni] +0.6 [% Mo] +0.3 [% W] +0.55 [% Cu] -20 [% C] ≧ 19.5 (1)
[% Cr] + [% Mo] + [% W] +0.3 [% Si] −43.5 [% C] −0.4 [% Mn] − [% Ni]
−0.3 [Cu] −9 [% N] ≧ 11.5 (2)
[% Al] × [% N] ≦ 0.0020 (3)
[% C], [% Si], [% Mn], [% Cr], [% Ni], [% Mo], [% W], [% Cu], [% Al], [% N]: This is the content (% by mass) of each element.

  In the method for producing a seamless stainless steel pipe for oil wells of the present invention, billet is added to the above composition, Nb: 0.2 mass% or less, Ti: 0.3 mass% or less, Zr: 0.2 mass% or less, B: 0.01 mass% It is preferable to contain 1 type (s) or 2 or more types chosen from the following, and also it is preferable to contain Ca: 0.01 mass% or less.

In the present invention, C: 0.005 to 0.06 mass%, Si: 0.05 to 0.5 mass%, Mn: 0.2 to 1.8 mass%, P: 0.03 mass% or less, S: 0.005 mass% or less, Cr: 15.5 to 18.0 mass% Ni: 1.5-5.0 mass%, V: 0.02-0.2 mass%, Al: 0.002-0.05 mass%, N: 0.01-0.15 mass%, O: 0.006 mass% or less, and Mo: 1.0-3.5 Contains one or more selected from mass%, W: 0.5 to 3.0 mass%, Cu: 0.5 to 3.5 mass%, with the balance being Fe and inevitable impurities, and C, Si, Mn , Cr, Ni, Mo, W, Cu, Al, N content of ferrite satisfying the following formulas (1), (2) and (3), and 10-60% by volume of ferrite Phase, 0 to 20% by volume of austenite phase, and the balance is martensite phase, and the density of precipitates with a grain size of 0.5μm or more deposited in the structure is 6 × 10 ³ pieces / mm ² or less. Yield strength is 758 MPa or more and the test temperature Absorbed energy _V E _-10 at -10 ° C. Charpy impact test is oil well seamless stainless steel pipe is at least 40 J.
[% Cr] +0.65 [% Ni] +0.6 [% Mo] +0.3 [% W] +0.55 [% Cu] -20 [% C] ≧ 19.5 (1)
[% Cr] + [% Mo] + [% W] +0.3 [% Si] −43.5 [% C] −0.4 [% Mn] − [% Ni]
−0.3 [Cu] −9 [% N] ≧ 11.5 (2)
[% Al] × [% N] ≦ 0.0020 (3)
[% C], [% Si], [% Mn], [% Cr], [% Ni], [% Mo], [% W], [% Cu], [% Al], [% N]: This is the content (% by mass) of each element.

  In the seamless stainless steel pipe for oil well of the present invention, in addition to the above-mentioned composition, Nb: 0.2% by mass or less, Ti: 0.3% by mass or less, Zr: 0.2% by mass or less, B: 0.01% by mass or less It is preferable to contain 1 type (s) or 2 or more types, and also it is preferable to contain Ca: 0.01 mass% or less.

  According to the present invention, a seamless stainless steel pipe for oil wells having high strength and high toughness and having excellent corrosion resistance can be stably obtained, so that it has a remarkable industrial effect.

It is a graph which shows the relationship between the density | concentration product of Al and N, and the density of a precipitate.

  First, the composition of the seamless stainless steel pipe for oil wells of the present invention will be described. Since the seamless steel pipe is generally manufactured by hot working a billet, the composition of the oil well seamless stainless steel pipe of the present invention is the same as the composition of the billet as the material.

C: 0.005-0.06 mass%
C is an element having an action of increasing the strength of a seamless stainless steel pipe for oil wells, and if less than 0.005% by mass, the effect cannot be obtained. On the other hand, when it exceeds 0.06 mass%, corrosion resistance will fall remarkably. Therefore, C is 0.005 to 0.06 mass%. Preferably it is 0.01-0.04 mass%.

Si: 0.05-0.5% by mass
Si is an element that functions as a deoxidizer in the process of producing molten steel, which is a raw material of seamless stainless steel pipes for oil wells, and its effect cannot be obtained at less than 0.05% by mass. On the other hand, if it exceeds 0.5 mass%, not only corrosion due to CO ₂ is likely to proceed, but also hot workability in the production process of oil well seamless stainless steel pipes is deteriorated. Therefore, Si is 0.05 to 0.5 mass%. Preferably it is 0.1-0.4 mass%.

Mn: 0.2-1.8% by mass
Mn is an element that has an effect of increasing the strength of a seamless stainless steel pipe for oil wells, and if it is less than 0.2% by mass, a desired strength cannot be obtained. On the other hand, when it exceeds 1.8 mass%, toughness will deteriorate. Therefore, Mn is 0.2 to 1.8% by mass. Preferably it is 0.2-0.8 mass%.

P: 0.03 mass% or less
P is an element that lowers the corrosion resistance of seamless stainless steel pipes for oil wells, and when it exceeds 0.03% by mass, the corrosion resistance significantly decreases. Therefore, P is set to 0.03% by mass or less. However, in order to reduce P to less than 0.005% by mass, it takes a long time for de-P treatment in the process of melting molten steel, leading to an increase in the manufacturing cost of seamless stainless steel pipes for oil wells. Therefore, P is preferably 0.005% by mass or more.

S: 0.005 mass% or less
S is an element that decreases the hot workability in the production process of oil well seamless stainless steel pipes, and if it exceeds 0.005% by mass, it interferes with the production of oil well seamless stainless steel pipes. Therefore, S is 0.005 mass% or less. However, in order to reduce S to less than 0.0005 mass%, it takes a long time to remove S in the process of melting molten steel, leading to an increase in the production cost of seamless stainless steel pipes for oil wells. Therefore, S is preferably 0.0005% by mass or more.

Cr: 15.5-18.0% by mass
Cr is an element having an effect of improving the corrosion resistance of seamless stainless steel pipes for oil wells, and contributes to prevention of corrosion caused by CO ₂ in a high temperature environment. If Cr is less than 15.5% by mass, the effect cannot be obtained. On the other hand, if it exceeds 18.0% by mass, the hot workability in the production process of the oil well seamless stainless steel pipe decreases, and the strength of the oil well seamless stainless steel pipe decreases. Therefore, Cr is 15.5 to 18.0% by mass. Preferably it is 16.0-17.5 mass%, More preferably, it is 16.5-17.0 mass%.

Ni: 1.5-5.0 mass%
Ni is an element that has the effect of improving the corrosion resistance and increasing the strength of seamless stainless steel pipes for oil wells. If Ni is less than 1.5% by mass, the effect cannot be obtained. On the other hand, if it exceeds 5.0% by mass, a martensite phase is hardly generated, and the strength of a seamless stainless steel pipe for oil wells is lowered. Therefore, Ni is 1.5 to 5.0% by mass. Preferably it is 3.0-4.5 mass%.

V: 0.02-0.2 mass%
V is an element that has the effects of improving the corrosion resistance and increasing the strength of seamless stainless steel pipes for oil wells. If V is less than 0.02% by mass, the effect cannot be obtained. On the other hand, when it exceeds 0.2 mass%, toughness will deteriorate. Therefore, V is 0.02 to 0.2 mass%. Preferably it is 0.03-0.08 mass%.

Al: 0.002 to 0.05 mass%
Al is an element that functions as a deoxidizer in the process of melting molten steel, which is a raw material for seamless wells for oil wells, and the effect cannot be obtained at less than 0.002% by mass. On the other hand, if it exceeds 0.05% by mass, alumina inclusions are likely to precipitate, the hot workability in the production process of the oil well seamless stainless steel pipe is lowered, and the toughness is deteriorated. Therefore, Al is 0.002 to 0.05 mass%. Preferably it is 0.01-0.04 mass%.

N: 0.01-0.15 mass%
N is an element having an action of improving the corrosion resistance of seamless stainless steel pipes for oil wells, and if less than 0.01% by mass, the effect cannot be obtained. On the other hand, if it exceeds 0.15% by mass, it binds to various elements and precipitates nitrides, so that the toughness of seamless stainless steel pipes for oil wells deteriorates. Therefore, N is set to 0.01 to 0.15 mass%.

Even if the contents of Al and N are within the above ranges, as already explained, when precipitates are precipitated in the structure, the toughness of the oil well seamless stainless steel pipe deteriorates. Therefore, the contents of Al and N in the oil well seamless stainless steel pipe are adjusted so that the concentration product satisfies the following formula (3).
[% Al] × [% N] ≦ 0.0020 (3)
[% Al], [% N]: Content of each element (% by mass), and zero if not contained.

O: 0.006% by mass or less
O exists as an oxide in seamless stainless steel pipes for oil wells and is an element that adversely affects not only hot workability but also toughness and corrosion resistance. If it exceeds 0.006% by mass, hot workability decreases and toughness Cause deterioration of corrosion resistance. Therefore, O is 0.006 mass% or less. Preferably it is 0.004 mass% or less.

Ca: 0.01% by mass or less
Ca is an element having an action of spheroidizing sulfide inclusions. By spheroidizing sulfide inclusions, the lattice strain around them is reduced, so that trapping of hydrogen atoms is suppressed, which in turn contributes to the improvement of the corrosion resistance of oil well seamless stainless steel pipes. However, when Ca exceeds 0.01 mass%, oxide inclusions increase and corrosion resistance decreases. Therefore, Ca is preferably 0.01% by mass or less. On the other hand, if it is less than 0.0005% by mass, the effect of improving the corrosion resistance cannot be obtained, so 0.0005 to 0.01% by mass is more preferable.

One or two selected from Mo: 1.0 to 3.5% by mass, W: 0.5 to 3.0% by mass, Cu: 0.5 to 3.5% by mass
Mo is an element having an effect of improving the corrosion resistance of the oil well seamless stainless steel pipe, especially Cl ^- contributing to due to the prevention of pitting. If Mo is less than 1.0% by mass, the effect cannot be obtained. On the other hand, if it exceeds 3.5% by mass, the strength decreases and the toughness deteriorates. Moreover, the manufacturing cost of the seamless stainless steel pipe for oil wells is increased. Therefore, when it contains Mo, it is 1.0-3.5 mass%. Preferably it is 1.5-3.0 mass%.

  W, like Mo, is an element that has the effect of improving the corrosion resistance of seamless stainless steel pipes for oil wells, and contributes particularly to the prevention of pitting corrosion caused by Cl. However, if W exceeds 3.0% by mass, the toughness of seamless stainless steel pipes for oil wells deteriorates. Therefore, when it contains W, it is 3.0 mass% or less. On the other hand, if it is less than 0.5% by mass, the effect of improving the corrosion resistance cannot be obtained. Preferably it is 0.5-2.5 mass%.

  Cu is an element that has an action of suppressing the penetration of hydrogen into a seamless stainless steel pipe for oil wells, and contributes to an improvement in corrosion resistance. However, when Cu exceeds 3.5 mass%, the hot workability in the production process of a seamless stainless steel pipe for oil wells is lowered. Therefore, when it contains Cu, it is 3.5 mass% or less. On the other hand, if it is less than 0.5% by mass, the effect of improving the corrosion resistance cannot be obtained. Preferably it is 0.5-2.5 mass%.

  In addition to these elements, Nb: 0.2% by mass or less, Ti: 0.3% by mass or less, Zr: 0.2% by mass or less, B: 0.01% by mass or less Also good.

Nb, Ti, Zr, and B are all elements that have the effect of increasing the strength of seamless stainless steel pipes for oil wells, and may be added as necessary. However, if the content is too large, the toughness of the oil well seamless stainless steel pipe deteriorates. Therefore, Nb: 0.2% by mass or less, Ti: 0.3% by mass or less, Zr: 0.2% by mass or less, and B: 0.01% by mass or less are preferable. On the other hand, if the content is too small, the effect of increasing the strength cannot be obtained. Therefore, Nb: 0.02 mass% or more, Ti: 0.04 mass% or more, Zr: 0.02 mass% or more, and B: 0.001 mass% or more are more preferable.
The balance other than the above components is Fe and inevitable impurities.

Then, the content of each element is within the above range so that the content of C, Si, Mn, Cr, Ni, Mo, W, Cu satisfies the following formulas (1) and (2): adjust. In the formulas (1) and (2), [% C], [% Si], [% Mn], [% Cr], [% Ni], [% Mo], [% W], [% Cu] is the content (% by mass) of each element, and is zero when not contained.
[% Cr] +0.65 [% Ni] +0.6 [% Mo] +0.3 [% W] +0.55 [% Cu] -20 [% C] ≧ 19.5 (1)
[% Cr] + [% Mo] + [% W] +0.3 [% Si] −43.5 [% C] −0.4 [% Mn] − [% Ni]
−0.3 [% Cu] −9 [% N] ≧ 11.5 (2)
Cr, Ni, Mo, W, Cu, to satisfy the content of C is a (1), CO ₂ and Cl ^- in corrosive environments containing, oil well seamless stainless steel pipe having a sufficient corrosion resistance to 230 ° C. Can be obtained. From the viewpoint of further improving the corrosion resistance, it is preferable to adjust the components so as to satisfy the following formula (4).
[% Cr] +0.65 [% Ni] +0.6 [% Mo] +0.3 [% W] +0.55 [% Cu] -20 [% C] ≧ 20.0 (4)
Also, if the content of Cr, Mo, W, Si, C, Mn, Ni, Cu, N satisfies the formula (2), sufficient hot workability can be achieved in the production process of seamless stainless steel pipes for oil wells. Can be obtained. From the viewpoint of further improving the hot workability, it is preferable to adjust the components so as to satisfy the following formula (5).
[% Cr] + [% Mo] + [% W] +0.3 [% Si] −43.5 [% C] −0.4 [% Mn] − [% Ni]
−0.3 [% Cu] −9 [% N] ≧ 12.5 (5)
Next, a procedure for producing a seamless stainless steel pipe for an oil well will be described.
The means for melting the molten steel, which is a raw material of the oil well seamless stainless steel pipe, is not particularly limited, and ordinary techniques (for example, a converter, an electric furnace, etc.) are used. Furthermore, you may perform a degassing process as needed.

  And after melting the molten steel which has already demonstrated the component, the steel ingot cast | cast with the usual technique (for example, continuous casting, ingot-making, etc.) is rolled, and it is set as a billet. The billet is the material of the oil well seamless stainless steel pipe.

  The process of producing a seamless stainless steel pipe for oil wells from a billet is performed after conventionally known hot working (eg, Mannesmann-plug mill hot rolling, Mannesmann-mandrel mill hot rolling, etc.) An oil well seamless stainless steel pipe is obtained by cooling to room temperature at a cooling rate higher than air cooling. Note that cooling at a cooling rate equal to or higher than air cooling means forced cooling (for example, immersion, jetting, etc.) using a refrigerant (for example, cooling water) or air cooling. Specifically, it is set to 0.1 ° C./second or more. More preferably, it is 0.3 ° C./second or more.

  If the heating temperature of the billet prior to hot working is too low, the deformation resistance of the billet increases, and an excessive load is applied to the piercing mill (for example, a piercer mill), causing equipment failure. On the other hand, if the heating temperature is too high, the billet crystal grains become coarse, so that the oil well seamless stainless steel pipe obtained through the subsequent steps also becomes coarse and the toughness deteriorates. In addition, the scale loss increases, resulting in a decrease in yield. Therefore, the heating temperature of the billet is 1150 to 1300 ° C. More preferably, it is 1200-1280 degreeC. Also, if the heating time is less than 100 minutes, the precipitate that existed before the heating does not dissolve sufficiently, and a lot of precipitate remains in the seamless stainless steel pipe for oil wells obtained through the subsequent steps, and the toughness Deteriorates. In addition, when the heating time exceeds 400 minutes, the billet crystal grains become coarse, so that the crystal grains of the oil well seamless stainless steel pipe obtained through the subsequent steps also become coarse and the toughness deteriorates. Therefore, the billet heating time is 100 to 400 minutes.

  The billet thus heated is subjected to hot working including piercing and rolling. As hot working, Mannesmann-plug mill type hot rolling to obtain seamless stainless steel pipe for oil well through piercer mill, elongator mill, plug mill, reeler mill and sizing mill, or oil well seam from piercer mill through mandrel mill and reducer mill. The present invention can be applied to Mannesmann-Mandmill type hot rolling or the like to obtain a stainless steel pipe.

The oil well seamless stainless steel pipe obtained by hot working is immediately cooled to room temperature at a cooling rate higher than air cooling. As a result, a martensite phase is formed in the seamless stainless steel pipe for oil wells.
The oil well seamless stainless steel pipe cooled to room temperature is re-quenched to further increase the martensite phase.

  If the heating temperature prior to quenching (hereinafter referred to as quenching temperature) is too low, the martensite phase is not sufficiently formed, so that a seamless stainless steel pipe for oil wells having a desired strength cannot be obtained. Accordingly, the quenching temperature is 850 ° C. or higher. On the other hand, if the quenching temperature is too high, the martensite phase is excessively generated, so that a seamless stainless steel pipe for oil wells having desired toughness cannot be obtained. Therefore, the quenching temperature is preferably 850 to 1000 ° C. More preferably, it is 920-980 degreeC.

  Then, quenching is performed by cooling the oil well seamless stainless steel pipe heated to a predetermined quenching temperature to 100 ° C. or less at a cooling rate of air cooling or higher. When the temperature at which quenching is stopped exceeds 100 ° C., the austenite phase is not sufficiently transformed into a martensite phase, and a desired strength cannot be obtained, which is not preferable.

After quenching, the oil well seamless stainless steel pipe is tempered. If the heating temperature in tempering (hereinafter referred to as tempering temperature) is too high, the austenite phase fraction increases and the desired strength cannot be obtained. Accordingly, the tempering temperature is 700 ° C. or lower. On the other hand, if the tempering temperature is too low, the toughness deteriorated by quenching cannot be sufficiently recovered. Therefore, the tempering temperature is preferably 400 to 700 ° C. After the heating is completed, it is cooled to room temperature at a cooling rate that is equal to or higher than air cooling.
The oil well seamless stainless steel pipe obtained by hot working may be cooled to room temperature at a cooling rate equal to or higher than air cooling, and then tempering may be performed without performing the quenching.

  The oil well seamless stainless steel pipe thus produced has a structure consisting of 10 to 60% by volume of a ferrite phase, 0 to 20% by volume of an austenite phase, and the balance consisting of a martensite phase. If the ferrite phase is less than 10% by volume, the hot workability of the oil well seamless stainless steel pipe decreases, and if it exceeds 60% by volume, the strength decreases and the toughness deteriorates. If the austenite phase exceeds 20% by volume, an oil well seamless stainless steel pipe having a desired strength cannot be obtained.

In the structure, intermetallic compounds having a particle size of 0.5 μm or more are precipitated at a density of 6 × 10 ³ pieces / mm ² or less. By controlling the precipitation of such precipitates, an oil well seamless stainless steel pipe having desired strength, toughness, and corrosion resistance can be obtained. Moreover, since the billet used as a material has the composition defined by the formula (2) in order to improve hot workability, the control of the billet heating temperature prior to hot working stabilizes the seamless stainless steel pipe for oil wells. Can be obtained.

  Molten steel having the composition shown in Table 1 was melted, further degassed, billet (100 kg steel ingot) was subsequently produced by the ingot forming method, and air-cooled to room temperature. Next, the billet was heated in a heating furnace, then pierced and rolled with a Piercer experimental machine, and further water-cooled to obtain a seamless steel pipe (outer diameter 83.8 mm = 3.3 inch, wall thickness 12.7 mm = 0.5 inch).

  The relationship between the components of the molten steel (steel symbols A to L, N to X) shown in Table 1 and the formulas (1), (2), and (3) is as shown in Table 2.

  The inner and outer surfaces of the obtained seamless steel pipe were visually observed to evaluate hot workability. The results are shown in Table 4. In Table 4, those having cracks of 5 mm or more in length of seamless steel pipes are indicated as “present”, and those other than that are indicated as “absent”.

  Moreover, after quenching and tempering the obtained seamless steel pipe, specimens were collected, and the structure, tensile properties, toughness, and corrosion resistance were investigated. The investigation method is as follows. The conditions for quenching and tempering are as shown in Table 3.

(A) Microstructure Samples for microstructure observation were collected from the center of the thickness of the seamless steel pipe that had been quenched and tempered, and after polishing the cross section in the thickness direction, Virella etching solution (picric acid, hydrochloric acid, and ethanol were added respectively. 2g, mixed at a rate of 10ml and 100ml), and a tissue photograph was taken using an optical microscope (100 to 1000 times). The martensite phase (hereinafter referred to as M phase), ferrite phase (hereinafter referred to as α phase), and austenite phase (hereinafter referred to as γ phase) are judged from the structure photograph, and the fraction (volume fraction) of α phase. Was calculated by image analysis. The results are shown in Table 4.
The fraction (volume fraction) of the γ phase was determined by measuring the diffraction X-ray integrated intensity of the (220) plane of the γ phase and the (211) plane of the α phase using the X-ray diffraction method. Calculated. The results are shown in Table 4.
Volume ratio of γ phase (%) = 100 / <1 + {(Iα × Rγ) / (Iγ × Rα)}>
Iα: Integral intensity of α phase Iγ: Integral intensity of γ phase Rα: Crystallographic theoretical calculation value of α phase Rγ: Crystallographic theoretical calculation value of γ phase The fraction (volume fraction) of M phase is The remainder other than the γ phase is shown in Table 4.

Next, a structure photograph of the test piece corroded with the Virella etching solution was taken with an optical microscope (2000 times). From the structure photograph, the number of intermetallic compounds having a particle size of 0.5 μm or more precipitated in the structure was measured. The results are shown in Table 4 as the number per unit area (mm ² ).

(B) Tensile properties API arc-shaped tensile test specimens were collected from the center of the thickness of the seamless steel pipe that had been quenched and tempered so that the tensile direction was the pipe axis direction in accordance with the API-5CT standard. A tensile test was performed in accordance with the standard, and the yield strength YS (MPa) and the tensile strength TS (MPa) were measured as tensile properties. The results are shown in Table 4.

(C) Toughness V-notch specimens (thickness 10 mm) are taken from the thickness center of seamless steel pipes that have been quenched and tempered in accordance with ISO-11960 standard so that the circumferential direction is the specimen length. Further, a Charpy impact test was performed at a test temperature of −10 ° C., and the absorbed energy _V E ₋₁₀ (J) was measured. The results are shown in Table 4. Note that the number of test pieces is three, and the arithmetic average values are shown in Table 4.

(D) Corrosion resistance Corrosion test pieces (thickness 3 mm, width 25 mm, length 50 mm) were sampled from the center of the thickness of the seamless steel pipe that had been quenched and tempered, and the weight was measured. Further, the test piece was immersed in a 20 mass% NaCl aqueous solution (liquid temperature 230 ° C., saturated with 3.0 MPa CO ₂ gas) held in an autoclave for 14 days for a corrosion test. After the corrosion test was completed, the weight of the corrosion test piece was measured, and the weight loss before and after the corrosion test was converted to a thickness to obtain the corrosion rate (mm / year). The results are shown in Table 4.

  Next, the surface of the corrosion test piece after the corrosion test was observed with a magnifying glass (50 times) to investigate the presence or absence of pitting corrosion. The results are shown in Table 4. In Table 4, the case where pitting corrosion with a diameter of 0.2 mm or more was observed on the surface of the seamless steel pipe is indicated as “present”, and the other cases are indicated as “absent”.

As is clear from Table 4, invention examples (Steel Tube Nanba1～7,16～20) are both high strength of YS is 758 MPa (= 110 ksi) or more, _V E _-10 and the more high toughness 40J Furthermore, even in a severe environment containing high-temperature CO ₂ and Cl ⁻ , the corrosion rate is 0.127 mm / year or less and excellent corrosion resistance without pitting corrosion. In addition, it can be seen that no cracks are generated on the surface, and that it has excellent hot workability.

On the other hand, steel pipe Nos. 8 to 11, 14, and 23, which are comparative examples, do not satisfy the formula (3), and the number of intermetallic compounds having a particle size of 0.5 μm or more precipitated in the M phase is 6 × 10 ³ pieces / mm. Since it exceeds ² , _V E _-10 is below 40J. Steel tube No. 15 has an intermetallic compound with a grain size of 0.5 μm or more deposited in the M phase that does not exceed 6 × 10 ³ pieces / mm ² , but the heating time before rolling is too long, so the crystal grains are coarse. It turned into _V E _-10 and is lower than the 40J.

  Steel pipe Nos. 8, 12, and 21 did not satisfy the formula (1), and the amount of solid solution of elements effective for improving corrosion resistance was insufficient, so that the corrosion rate exceeded 0.127 mm / year and pitting corrosion occurred. Steel pipe No. 24 did not contain V, which is an element effective in improving corrosion resistance, so pitting corrosion occurred while the corrosion rate exceeded 0.127 mm / year. Steel pipe No. 25 does not contain any of Mo, Cu, and W, which are effective elements for improving corrosion resistance. Therefore, the corrosion rate exceeded 0.127 mm / year and pitting corrosion occurred.

Steel pipe No. 26 has a yield strength lower than 758 MPa because the tempering temperature is too high and the austenite fraction exceeds 20%.
Further, Steel Pipe Nos. 9, 13, and 22 did not satisfy the formula (2), and the surface was cracked because the ferrite phase fraction was insufficient during hot rolling.

Claims (6)

C: 0.005-0.06 mass%, Si: 0.05-0.5 mass%, Mn: 0.2-1.8 mass%, P: 0.03 mass% or less, S: 0.005 mass% or less, Cr: 15.5-18.0 mass%, Ni: 1.5- 5.0% by mass, V: 0.02 to 0.2% by mass, Al: 0.002 to 0.05% by mass, N: 0.01 to 0.15% by mass, O: 0.006% by mass or less, Mo: 1.0 to 3.5% by mass, W: It contains one or more selected from 0.5 to 3.0% by mass, Cu: 0.5 to 3.5% by mass, the balance is Fe and inevitable impurities, and the C, Si, Mn, Cr, Ni Billet having a composition satisfying the following formulas (1), (2) and (3) for Mo, W, Cu, Al and N is heated at a temperature of 1150 to 1300 ° C. for 100 to 400 minutes. Further, it is subjected to hot working to obtain a seamless steel pipe, and after cooling the seamless steel pipe to room temperature at a cooling rate of air cooling or higher, the seamless steel pipe is heated to 850 ° C. or higher, and subsequently air cooled to 100 ° C. or lower. Cool to room temperature with a cooling rate of By carrying out quenching and then tempering at 700 ° C. or less, the particle size of 0.5 to 10 vol% ferrite phase, 0 to 20 vol% austenite phase, and the balance consisting of martensite phase precipitated in the structure. It has a structure in which the density of precipitates of μm or more is 6 × 10 ³ particles / mm ² or less, the absorbed energy _V E _{− in} a Charpy impact test with a yield strength of 758 MPa or more and a test temperature of −10 ° C. _A method for producing a seamless stainless steel pipe for oil wells, characterized in that ₁₀ is a seamless steel pipe of 40J or more.
[% Cr] +0.65 [% Ni] +0.6 [% Mo] +0.3 [% W] +0.55 [% Cu] −20 [% C] ≧ 19.5 (1)
[% Cr] + [% Mo] + [% W] +0.3 [% Si] −43.5 [% C] −0.4 [% Mn] − [% Ni]
−0.3 [Cu] −9 [% N] ≧ 11.5 (2)
[% Al] × [% N] ≦ 0.0020 (3)
[% C], [% Si], [% Mn], [% Cr], [% Ni], [% Mo], [% W], [% Cu], [% Al], [% N]: This is the content (% by mass) of each element.   In addition to the above composition, the billet comprises one or more selected from Nb: 0.2% by mass or less, Ti: 0.3% by mass or less, Zr: 0.2% by mass or less, and B: 0.01% by mass or less. The method for producing a seamless stainless steel pipe for oil wells according to claim 1, comprising:   The said billet contains Ca: 0.01 mass% or less in addition to the said composition, The manufacturing method of the seamless stainless steel pipe for oil wells of Claim 1 or 2 characterized by the above-mentioned. C: 0.005-0.06 mass%, Si: 0.05-0.5 mass%, Mn: 0.2-1.8 mass%, P: 0.03 mass% or less, S: 0.005 mass% or less, Cr: 15.5-18.0 mass%, Ni: 1.5- 5.0% by mass, V: 0.02 to 0.2% by mass, Al: 0.002 to 0.05% by mass, N: 0.01 to 0.15% by mass, O: 0.006% by mass or less, Mo: 1.0 to 3.5% by mass, W: It contains one or more selected from 0.5 to 3.0% by mass, Cu: 0.5 to 3.5% by mass, the balance is Fe and inevitable impurities, and the C, Si, Mn, Cr, Ni , Mo, W, Cu, Al, and N have a composition satisfying the following formulas (1), (2), and (3), and 10 to 60% by volume of a ferrite phase, 0 to It has a 20% by volume austenite phase and the balance is a martensite phase, and has a structure in which the density of precipitates with a grain size of 0.5 μm or more deposited in the structure is 6 × 10 ³ pieces / mm ² or less, and yield strength Is 758MPa or more, and the test temperature is -10 ℃ Oil well seamless stainless steel tube absorbed energy _V E _-10 in Charpy impact test is characterized in that at least 40 J.
[% Cr] +0.65 [% Ni] +0.6 [% Mo] +0.3 [% W] +0.55 [% Cu] -20 [% C] ≧ 19.5 (1)
[% Cr] + [% Mo] + [% W] +0.3 [% Si] −43.5 [% C] −0.4 [% Mn] − [% Ni]
−0.3 [Cu] −9 [% N] ≧ 11.5 (2)
[% Al] × [% N] ≦ 0.0020 (3)
[% C], [% Si], [% Mn], [% Cr], [% Ni], [% Mo], [% W], [% Cu], [% Al], [% N]: This is the content (% by mass) of each element.   In addition to the above composition, Nb: 0.2% by mass or less, Ti: 0.3% by mass or less, Zr: 0.2% by mass or less, B: 0.01% by mass or less The seamless stainless steel pipe for oil wells according to claim 4,   In addition to the said composition, Ca: 0.01 mass% or less is contained, The seamless stainless steel pipe for oil wells of Claim 4 or 5 characterized by the above-mentioned.

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