JP2006312772A - Martensitic stainless steel for oil well and method for manufacturing martensitic stainless steel pipe for oil well - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a martensitic stainless steel for an oil well, which has adequate SSC resistance under coexistence of Cl<SP>-</SP>, wet CO<SB>2</SB>and a very small amount of H<SB>2</SB>S, and has superior low-temperature toughness as well. <P>SOLUTION: The martensitic stainless steel for the oil well comprises 0.16-0.22% C, 0.1-0.8% Si, 0.25-1.00% Mn, 0.025% or less P, 0.010% or less S, 12.0-13.5% Cr, 0.010% or less Al, 0-0.2% Ni, 0-0.10% Cu, 0-0.20% Mo, 0-0.050% Ti, 0.01-0.1% N, and the balance Fe with impurities. The steel may include a specific amount of one or more elements of Nb and V and/or one or more elements among Ca, Mg, La and Ce. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

The present invention relates to a method for producing a martensitic stainless steel for oil wells and a martensitic stainless steel pipe for oil wells, and more specifically, hydrogen sulfide (H ₂ S), carbon dioxide (CO ₂ ), chloride ions (Cl ⁻ ), and the like. The present invention relates to a method for producing a martensitic stainless steel for oil wells and a martensitic stainless steel pipe for oil wells that are suitable for use in cold regions with severe corrosive environments containing corrosive substances. More specifically, in cold regions, such as oil and natural gas production facilities, carbon dioxide removal facilities and geothermal power generation facilities, and seamless steel pipes, ERW steel pipes and laser welded steel pipes used in oil wells and natural gas wells. The present invention relates to a method for manufacturing a martensitic stainless steel for oil wells and a martensitic stainless steel pipe for oil wells suitable for use in oil well pipes.

  In recent years, the environment of wells for extracting oil or natural gas has become increasingly severe, and the development of oil wells and gas wells containing corrosive gases such as hydrogen sulfide and carbon dioxide has been actively promoted. It has become.

  For this reason, in oil well pipes used for the extraction of oil and natural gas in the harsh environment as described above, material deterioration due to corrosion has become a major problem.

  That is, as steel pipes for oil wells and gas wells that do not contain carbon dioxide, those made of carbon steel or low alloy steel are generally used, but steel pipes for oil wells or gas wells that contain a large amount of carbon dioxide are used. When carbon steel or low alloy steel is used as a raw material, sufficient corrosion resistance cannot be ensured. For this reason, steel pipes made of steel with a high content of alloy elements have been used. .

  Specifically, a 13Cr martensitic stainless steel represented by SUS420J1 defined in JIS is often used as a material for oil well steel pipes containing a large amount of carbon dioxide.

  However, 13Cr martensitic stainless steel represented by SUS420J1 has poor corrosion resistance against hydrogen sulfide, and sulfide stress cracking (hereinafter referred to as sulfide stress) in an environment containing carbon dioxide and hydrogen sulfide at the same time. In fact, cracks are referred to as “SSC”) and their use is limited.

  On the other hand, the development of oil wells and gas wells that simultaneously contain chloride ions, wet carbon dioxide gas and a small amount of hydrogen sulfide has also been actively developed in cold regions. There is an increasing demand for steels that not only have good SSC resistance in the presence of C, but also have excellent low-temperature toughness.

  For this reason, Patent Document 1 discloses an oil well pipe steel containing 12 to 13.5% Cr by mass%.

JP-A-60-52525

  The object of the present invention is to have good SSC resistance in the coexistence of chloride ions, wet carbon dioxide gas and a small amount of hydrogen sulfide, and excellent low temperature toughness, including hydrogen sulfide, carbon dioxide gas and chloride ions in cold regions. Another object of the present invention is to provide a method for producing a martensitic stainless steel for oil wells and a martensitic stainless steel pipe for oil wells having high toughness and high SSC resistance that can be sufficiently used even in an environment.

  The oil well pipe steel disclosed in Patent Document 1 described above cannot always ensure excellent low temperature toughness.

  That is, in the oil well pipe steel disclosed in Patent Document 1, the Ni content is limited to 0.10% or less by mass% in order to improve SSC resistance and pitting resistance. Therefore, although the oil well tubular steel has good SSC resistance in the presence of chloride ions, wet carbon dioxide gas and a small amount of hydrogen sulfide, it cannot secure sufficient low temperature toughness.

  For this reason, the present inventors have made various martensites containing 12.5% or more of Cr by mass% and yield strength (YS) of 558 to 655 MPa in order to ensure carbon dioxide corrosion resistance. For site-based stainless steel, various investigations and studies were conducted on SSC susceptibility in an environment where chloride ions, wet carbon dioxide and a small amount of hydrogen sulfide coexist, and low-temperature toughness was also investigated and studied. As a result, first, the following findings (a) to (c) were obtained.

  (A) In the martensitic stainless steel containing 12.5% or more by mass% of the Cr described above, good SSC resistance can be obtained even when the Ni content is mass% and exceeds 0.10%. Sometimes.

  (B) In the martensitic stainless steel, good low temperature toughness may be obtained even when the Ni content is mass% and is 0.10% or less.

  (C) The low temperature toughness of the martensitic stainless steel is not necessarily improved only by increasing the Ni content by mass%, exceeding 0.10%.

  Therefore, a more detailed investigation was then conducted on the chemical composition of steel and the structure after tempering, particularly the precipitates, which affect the SSC resistance and low temperature toughness in the environment. As a result, the following findings (d) to (g) were obtained.

(D) If the content of Al in the martensitic stainless steel is mass% and not more than 0.010%, the main precipitates present in the tempered structure are M ₂₃ C ₆ and MC (however, “ “M” means a metal element.)

  (E) When the contents of Al and Ni in the martensitic stainless steel are mass%, 0.010% or less, and more than 0.10% and 0.2% or less, Good SSC resistance is obtained in the environment. Moreover, its low temperature toughness is very good.

  (F) When the contents of Al and Ni in the martensitic stainless steel are 0.010% or less and 0.10% or less, respectively, in mass%, good SSC resistance in the environment Is obtained. Moreover, the low temperature toughness is good despite the low Ni content.

(G) When the content of Al in the martensitic stainless steel is mass% and exceeds 0.010%, the main precipitates present in the tempered structure are M ₂₃ C ₆ and AlN. In this case, even if the Ni content is mass% and exceeds 0.10% and is 0.2% or less, the low temperature toughness is inferior.

  The present invention has been completed based on the above findings.

  The gist of the present invention resides in a method for producing a martensitic stainless steel for oil wells shown in the following (1) to (4) and a martensitic stainless steel pipe for oil wells shown in (5).

  (1) By mass%, C: 0.16-0.22%, Si: 0.1-0.8%, Mn: 0.25-1.00%, P: 0.025% or less, S: 0.010% or less, Cr: 12.0 to 13.5%, Al: 0.010% or less, Ni: 0 to 0.2%, Cu: 0 to 0.10%, Mo: 0 to 0.20 %, Ti: 0 to 0.050% and N: 0.01 to 0.1%, the balance being made of Fe and impurities, martensitic stainless steel for oil wells.

  (2) The oil well according to the above (1), which contains at least one selected from Nb: 0.020 to 0.045% and V: 0.01 to 0.2% instead of part of Fe For martensitic stainless steel.

  (3) Instead of a part of Fe, Ca: 0.0002 to 0.005%, Mg: 0.0002 to 0.005%, La: 0.0002 to 0.005%, and Ce: 0.0002 to The martensitic stainless steel for oil wells according to the above (1), which contains one or more selected from 0.005%.

  (4) Instead of part of Fe, one or more selected from Nb: 0.020 to 0.045% and V: 0.01 to 0.2%, and Ca: 0.0002 to 0.005 %, Mg: 0.0002 to 0.005%, La: 0.0002 to 0.005% and Ce: one or more selected from 0.0002 to 0.005% Martensitic stainless steel for oil wells.

(5) The martensitic stainless steel pipe having the chemical composition according to any one of (1) to (4) above is heated to a temperature range of 920 to 1050 ° C. and then air-cooled, and then at least 625 ° C. Ac ₁ A method for producing a martensitic stainless steel pipe for oil wells, characterized by tempering at a temperature below the point.

  The “air cooling” includes not only so-called “forced air cooling” but also “cooling” in the atmosphere.

  Hereinafter, the inventions according to the above (1) to (4) “martensitic stainless steel for oil wells” and the invention related to “manufacturing method of martensitic stainless steel pipe for oil wells” are respectively referred to as “present invention”. (1) ”to“ present invention (5) ”. Also, it may be collectively referred to as “the present invention”.

  Since the martensitic stainless steel for oil wells of the present invention has good SSC resistance and excellent low-temperature toughness in the presence of chloride ions, wet carbon dioxide and a small amount of hydrogen sulfide, hydrogen sulfide in a cold region, It can be used in an environment containing carbon dioxide and chloride ions. Further, according to the method for producing a martensitic stainless steel pipe for oil wells of the present invention, a steel pipe that can sufficiently withstand use in the above-mentioned severe environment can be easily produced.

  Hereinafter, each requirement of the present invention will be described in detail. In addition, “%” of the content of the chemical component means “mass%”.

(A) Chemical composition C: 0.16-0.22%
C is an element necessary for imparting desired strength to steel. C also has the effect of improving the SSC resistance by easily converting the steel structure into martensite by air cooling. In order to improve the SSC resistance with a high-strength material, it is desirable that the steel has a martensite structure (preferably 95% or more). However, since C is an austenite forming element, the content of C is increased. By doing so, the ratio of martensite can be easily increased. However, when the C content is less than 0.16%, it is difficult to obtain the above effects. On the other hand, if the content exceeds 0.22%, the toughness is significantly reduced. Therefore, the content of C is set to 0.16 to 0.22%.

Si: 0.1 to 0.8%
Si has a deoxidizing action. However, if the content is less than 0.1%, the effect of addition is poor. On the other hand, when the content exceeds 0.8%, toughness is lowered and hot workability is lowered. Therefore, the Si content is set to 0.1 to 0.8%. The upper limit is more preferably 0.4%.

Mn: 0.25 to 1.00%
Mn has the effect of expanding the austenite region and improving strength and toughness. However, if the content is less than 0.25%, a desired effect cannot be obtained in the above action. On the other hand, when the Mn content exceeds 1.00%, the carbon dioxide corrosion resistance is remarkably increased, and the carbon dioxide corrosion resistance is lowered. Therefore, the content of Mn is set to 0.25 to 1.00%.

P: 0.025% or less P is an impurity of steel, resulting in a decrease in toughness and SSC resistance. In particular, when its content exceeds 0.025%, the decrease in toughness and SSC resistance becomes significant. . Therefore, the content of P is set to 0.025% or less. It is desirable to reduce the P content as much as possible.

S: 0.010% or less S is an impurity of steel and causes a decrease in toughness and SSC resistance. In particular, when its content exceeds 0.010%, the decrease in toughness and SSC resistance becomes significant. . Therefore, the content of S is set to 0.010% or less. It is desirable to reduce the S content as much as possible.

Cr: 12.0 to 13.5%
Cr has the effect of increasing the carbon dioxide gas corrosion resistance. However, if the content is less than 12.0%, sufficient carbon dioxide gas corrosion resistance cannot be ensured. On the other hand, if the Cr content exceeds 13.5%, the rate of formation of δ-ferrite increases, and therefore a structure having a high ratio of martensite preferable for SSC resistance by air cooling, particularly a martensite structure of 95% or more. You will not be able to get. Therefore, the content of Cr is set to 12.0 to 13.5%. The lower limit of the Cr content is more preferably 12.3%, and the upper limit is more preferably 13.2%.

Al: 0.010% or less When the Al content increases, the main precipitates present in the tempered structure become M ₂₃ C ₆ and AlN, and the SSC resistance decreases. In particular, when the Al content exceeds 0.010%, the SSC resistance is significantly lowered. In this case, even if the Ni content is more than 0.10% and 0.2% or less, the low-temperature toughness is inferior. On the other hand, if the Al content is 0.010% or less, the main precipitates present in the tempered structure are M ₂₃ C ₆ and MC. When the Al content is 0.010% or less and the Ni content is more than 0.10% and 0.2% or less, good SSC resistance and very good low temperature toughness are obtained. can get. When the Al content is 0.010% or less, good low temperature toughness can be obtained even if the Ni content is 0.10% or less.

  Therefore, the Al content is preferably suppressed to the impurity level and is set to 0.010% or less. The Al content is preferably 0.005% or less, and more preferably 0.003% or less.

Ni: 0 to 0.2%
The addition of Ni is optional. If added, it has the effect of increasing strength and low temperature toughness. However, when the Ni content increases, the SSC sensitivity increases. In particular, when the Ni content exceeds 0.2%, the SSC sensitivity increases remarkably, so that the SSC resistance decreases. Therefore, the content of Ni is set to 0 to 0.2%. In addition, in order to acquire the said effect reliably, it is preferable to contain Ni exceeding 0.10%.

Cu: 0 to 0.10%
Addition of Cu is optional. If added, it has the effect of enhancing the general corrosion resistance in a high temperature region. However, if the Cu content increases, and particularly exceeds 0.10%, the sensitivity to local corrosion (pitting sensitivity) increases. Therefore, the Cu content is set to 0 to 0.10%.

Mo: 0 to 0.20%
The addition of Mo is optional. If added, it has the effect of improving the general corrosion resistance in a high temperature region. However, when the Mo content increases, especially when it exceeds 0.20%, δ-ferrite is generated by heating before air cooling, resulting in a decrease in strength and high susceptibility to local corrosion (pitting sensitivity). Become. Therefore, the content of Mo is set to 0 to 0.20%.

Ti: 0 to 0.050%
The addition of Ti is optional. If added, N in the steel is fixed as TiN and has the effect of reducing free N. However, when the Ti content is increased, especially when it exceeds 0.050%, the toughness decreases. Therefore, the content of Ti is set to 0 to 0.050%.

N: 0.01 to 0.1%
Since N is an austenite forming element, the ratio of martensite can be easily increased by increasing the content of N, thereby having the effect of increasing the SSC resistance. However, if the content is less than 0.01%, it is difficult to obtain the effect. On the other hand, if the N content is excessively large, a large amount of Cr nitride is formed, so that the carbon dioxide corrosion resistance is lowered. In particular, if the N content exceeds 0.1%, the carbon dioxide corrosion resistance is reduced. The reduction of the becomes remarkable. Therefore, the N content is set to 0.01 to 0.1%. The upper limit of the N content is more preferably 0.05%, and even more preferably 0.04%.

  For the reasons described above, the oil well martensitic stainless steel according to the present invention (1) is defined to contain the elements from C to N in the above-mentioned range, with the balance being Fe and impurities.

  In addition, the martensitic stainless steel for oil wells according to the present invention is one type selected from at least one of a first group and a second group, which will be described later, instead of a part of Fe if necessary. The above elements may be added and included as optional additional elements.

  Hereinafter, the optional additive element will be described.

First group: Nb: 0.020-0.045% and V: 0.01-0.2%
Nb is an element effective for increasing the strength of steel. However, if the content is less than 0.020%, the above effect is insufficient. On the other hand, if the content of Nb exceeds 0.045%, the above effect is saturated or the strength is lowered due to the formation of δ-ferrite. Therefore, when Nb is added, the content of Nb is set to 0.020 to 0.045%.

  V is an element effective for increasing the strength of steel. However, if the content is less than 0.01%, the above effect is insufficient. On the other hand, if the content of V exceeds 0.2%, the above effects are saturated, or the strength is lowered due to the formation of δ-ferrite. Therefore, when V is added, the content of V is set to 0.01 to 0.2%.

  In addition, said Nb and V can be added only in any 1 type or 2 types of composite.

Second group: Ca: 0.0002 to 0.005%, Mg: 0.0002 to 0.005%, La: 0.0002 to 0.005%, and Ce: 0.0002 to 0.005%
Ca is an element effective for enhancing the hot workability of steel. However, if the content is less than 0.0002%, the above effect cannot be obtained. On the other hand, if the Ca content exceeds 0.005%, a coarse oxide is generated and the SSC resistance is lowered. Therefore, when Ca is added, the content of Ca is set to 0.0002 to 0.005%.

  Mg is an element effective for enhancing the hot workability of steel. However, if the content is less than 0.0002%, the above effect cannot be obtained. On the other hand, if the Mg content exceeds 0.005%, a coarse oxide is generated and the SSC resistance is lowered. Therefore, the content of Mg when added is set to 0.0002 to 0.005%.

  La is an element effective for improving the hot workability of steel. However, if the content is less than 0.0002%, the above effect cannot be obtained. On the other hand, when the content of La exceeds 0.005%, a coarse oxide is generated and the SSC resistance is lowered. Therefore, the La content when added is set to 0.0002 to 0.005%.

  Ce is also an effective element for enhancing the hot workability of steel. However, if the content is less than 0.0002%, the above effect cannot be obtained. On the other hand, when the content of Ce exceeds 0.005%, a coarse oxide is generated and the SSC resistance is lowered. Therefore, the Ce content when added is 0.0002 to 0.005%.

  Said Ca, Mg, La, and Ce can be added only by any 1 type or 2 or more types of composite. Of the above elements, elements that are particularly preferably added are Ca and La.

  For the above reason, the martensitic stainless steel for oil well according to the present invention (2) is replaced with a part of Fe of the martensitic stainless steel for oil well according to the present invention (1), Nb: 0.020-0 0.045% and V: specified to contain one or more selected from 0.01 to 0.2%.

  Further, in the martensitic stainless steel for oil wells according to the present invention (3), Ca: 0.0002 to 0.005% instead of a part of Fe of the martensitic stainless steel for oil wells in the present invention (1) Mg: 0.0002 to 0.005%, La: 0.0002 to 0.005%, and Ce: One or more selected from 0.0002 to 0.005%.

  Furthermore, the martensitic stainless steel for oil wells according to the present invention (4) is replaced with a part of Fe of the martensitic stainless steel for oil wells according to the present invention (1), Nb: 0.020 to 0.045% And V: one or more selected from 0.01 to 0.2%, Ca: 0.0002 to 0.005%, Mg: 0.0002 to 0.005%, La: 0.0002 to 0.00. 005% and Ce: specified to contain one or more selected from 0.0002 to 0.005%.

(B) Heat treatment A martensitic stainless steel pipe for an oil well that can be sufficiently used even in an environment containing hydrogen sulfide, carbon dioxide gas, and chloride ions in a cold region has, for example, the chemical composition described in the above section (A). The above-mentioned book characterized in that a steel pipe made of martensitic stainless steel is “heated to a temperature range of 920 to 1050 ° C. and then air-cooled and then tempered at a temperature of 625 ° C. to Ac ₁ point”. According to the invention (5), it can be manufactured relatively easily.

(B-1) Heating temperature before air cooling The heating temperature before air cooling is preferably 920 to 1050 ° C. When the heating temperature is less than 920 ° C., the undissolved carbide may exist and the strength variation may increase. On the other hand, when it exceeds 1050 ° C., the structure becomes coarse and low-temperature toughness may decrease. Therefore, in this invention (5), the heating temperature before air cooling was 920-1050 degreeC.

(B-2) was cooled after heating to a temperature range of 920 to 1,050 ° C. tempering temperature above (B-1) is better to carry out the tempering at a temperature of less than ₁ point 625 ° C. or higher Ac. Tempering at a high temperature removes the internal stress of martensite generated by air cooling, and improves the steel material performance. In particular, good steel performance can be obtained by tempering at 625 ° C. or higher. However, when the tempering temperature exceeds the Ac ₁ point, the strength may fluctuate significantly. Therefore, in the present invention (5), the tempering temperature after air cooling is set to 625 ° C. or more and Ac ₁ point or less.

  Hereinafter, the present invention will be described in more detail with reference to examples.

  Steels 1 to 29 having the chemical composition shown in Table 1 were melted and continuously cast. Steels 1 to 23 in Table 1 are steels of the present invention examples whose chemical compositions are within the range defined by the present invention. On the other hand, steels 24 to 29 are steels of comparative examples that deviate from the conditions defined in the present invention.

  Subsequently, the steel slab was subjected to hot forging and hot rolling to produce a steel pipe having an outer diameter of 130 mm and a wall thickness of 18.24 mm.

  The steel pipe thus obtained was heated to the temperature shown in Table 2 and air-cooled and tempered, and then various test pieces were cut out and examined for tensile properties, toughness and SSC resistance.

1. Tensile properties A round bar tensile test piece with a diameter of 6.35 mm and a parallel part length of 25.4 mm is taken from the center of the thickness of the steel pipe, and a tensile test is performed at room temperature to obtain the yield strength (YS). It was measured.

2. Toughness V-notch specimens with a width of 10 mm as defined in JIS Z 2202 (1998) are collected in the longitudinal direction from the thickness center of the steel pipe, and the Charpy impact test is conducted at -40 ° C to absorb the absorbed energy (vE _{− 40} ).

3. SSC resistance A C method test piece specified in NACE TM0177-96 was collected from a steel pipe, and the partial pressure of hydrogen sulfide was 30397.5 Pa (0.3 atm) and the partial pressure of nitrogen gas was 70927.5 Pa (0. The NACE C method test was conducted in an environment in which a mixed gas of 7 atm) was blown into a 5% NaCl aqueous solution at 30 ° C. In addition, 100% of YS measured previously was made into test stress. The SSC resistance was evaluated based on whether or not cracking occurred in the test piece when the test time was 720 hours.

  Table 2 also shows the results of the above tests. In the “SSC resistance” column, “◯” indicates that no crack occurred in the test piece, and “X” indicates that the crack occurred.

From Table 2, in the case of Test Nos. 1 to 25 and Test Nos. 32 to 35 that satisfy the conditions specified in the present invention in the chemical composition, the strength-toughness balance is good, and NACE method C under the above conditions It is clear that cracks do not occur when the test is carried out and that the SSC resistance is excellent. Among them, in the case of test numbers 1 to 25 satisfying the production conditions defined in the present invention (5), the strength-toughness balance and the SSC resistance are good, and the toughness is vE ₋₄₀ ≧ 70 J and much more. It is clear that it is good.

  On the other hand, in the case of test numbers 26 to 31 where the chemical composition deviates from the conditions specified in the present invention, cracks occurred even when the NACE C method test was performed under the above conditions, It is clear that the SSC property is inferior.

Since the martensitic stainless steel for oil wells of the present invention has good SSC resistance and excellent low-temperature toughness in the presence of chloride ions, wet carbon dioxide and a small amount of hydrogen sulfide, hydrogen sulfide in a cold region, It can be used in an environment containing carbon dioxide and chloride ions. Further, according to the method for producing a martensitic stainless steel pipe for oil wells of the present invention, a steel pipe that can sufficiently withstand use in the above-mentioned severe environment can be easily produced.

Claims (5)

  In mass%, C: 0.16-0.22%, Si: 0.1-0.8%, Mn: 0.25-1.00%, P: 0.025% or less, S: 0.010 %: Cr: 12.0 to 13.5%, Al: 0.010% or less, Ni: 0 to 0.2%, Cu: 0 to 0.10%, Mo: 0 to 0.20%, Ti : 0 to 0.050% and N: 0.01 to 0.1%, the balance being composed of Fe and impurities, martensitic stainless steel for oil wells.   The martensite system for oil wells according to claim 1, comprising at least one selected from Nb: 0.020 to 0.045% and V: 0.01 to 0.2% in place of a part of Fe. Stainless steel.   Instead of a part of Fe, Ca: 0.0002 to 0.005%, Mg: 0.0002 to 0.005%, La: 0.0002 to 0.005%, and Ce: 0.0002 to 0.005 The martensitic stainless steel for oil wells according to claim 1, comprising one or more selected from%.   Instead of a part of Fe, one or more selected from Nb: 0.020 to 0.045% and V: 0.01 to 0.2%, and Ca: 0.0002 to 0.005%, Mg The marten for oil wells according to claim 1, comprising at least one selected from: 0.0002 to 0.005%, La: 0.0002 to 0.005%, and Ce: 0.0002 to 0.005%. Site-based stainless steel. A martensitic stainless steel pipe having the chemical composition according to any one of claims 1 to 4 is heated to a temperature range of 920 to 1050 ° C and then air-cooled, and then tempered at a temperature of 625 ° C or more and Ac ₁ point or less. A method for producing a martensitic stainless steel pipe for oil wells.