JP2008189945A - METHOD FOR MANUFACTURING THICK-WALL 13Cr-BASE STAINLESS STEEL PIPE - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a thick-wall seamless pipe of a 13Cr martensitic stainless steel which can stably secure desired strength and has high toughness. <P>SOLUTION: The seamless steel pipe comprises, by mass%, 0.15-0.22% C, 1.0% or less Si, 1.0% or less Mn, 0.020% or less P, 0.010% or less S, and 12-14% Cr. The manufacturing method comprises the steps of: heating the above seamless steel pipe to a heating temperature (T) which is 900°C or higher and satisfies T≥2.5×CR+1,015 (wherein (T) is a heating temperature (°C), and CR is an average cooling rate (°C/min) in between 700 to 600°C); subjecting the pipe to quenching treatment which cools it to a temperature of 100°C or lower at a cooling rate of CR that is equal to or higher than that in radiational cooling; and subsequently subjecting the pipe to tempering treatment which heats it to the Ac<SB>1</SB>transformation temperature or lower. Thus manufactured thick-wall seamless pipe of the 13Cr martensitic stainless steel stably has a yield strength of a 551-MPa (80 ksi) grade, and has high toughness. The seamless steel pipe may also contain one or more elements of Cu and Ni, one or more elements of V and Nb, or Al. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to oil wells for crude oil or natural gas, and 13Cr martensitic stainless steel pipes for oil wells used for gas wells, and more particularly to a method for stably producing thick-walled seamless steel pipes.

  In recent years, in order to cope with soaring crude oil prices and the depletion of oil resources expected in the near future, the development of drilling technology has been remarkable, and deep oil fields that have not been excluded in the past, or once abandoned development The development of oil fields with highly corrosive gas has been thriving on a global scale. Such oil fields are generally very deep, so the atmosphere is a high temperature, high pressure, and a harsh environment with corrosive gas. With such an increase in oil fields, steel pipes that are particularly excellent in corrosion resistance have been demanded as oil well steel pipes.

  Such a steel pipe excellent in corrosion resistance is used as a production casing which is arranged in the center in an oil well, in which a production fluid (crude oil) flows inside, and outside the tubing. In particular, the casing mainly plays a role of resisting the pressure of the formation and preventing the collapse of the anti-wall, and generally a high-strength and thick-walled steel pipe is used.

In recent years, oil fields have been actively developed in cold regions such as Russia and Canada, and the steel pipes used for oil wells are required to have further excellent low temperature toughness.
For this reason, there is a demand for a thick steel pipe having excellent corrosion resistance, high strength and high toughness, particularly for production casings.
Since the 1980s, 13% Cr martensitic stainless steel pipes with excellent resistance to CO ₂ corrosion have been used as oil well pipes in oil and gas fields that contain CO ₂ and other environments. However, normal 13% Cr martensitic stainless steel pipes have excellent resistance to CO ₂ corrosion, but have low low-temperature toughness, which limits their use in cold regions.

For such a problem, for example, Patent Document 1 includes C: 0.15 to 0.22%, Si, Mn, S, Al, and N are adjusted to an appropriate range, and P is reduced to 0.008% or less. A high strength martensitic stainless steel pipe excellent in low temperature toughness containing B: 0.0005 to 0.02% and Cr: 11 to 14% has been proposed. The technique described in Patent Document 1 refines the crystal grains by heat treatment at a low austenitizing temperature of 950 ° C. or lower, yield strength: high strength of 650 MPa or more, and excellent low temperature toughness. It is said that the steel pipe which has is obtained.
JP 2001-323339 A

  13% Cr martensitic stainless steel has high hardenability and can obtain a martensitic structure (quenched structure) even when air-cooled after austenitizing. However, when the austenitizing temperature is lowered as in the technique described in Patent Document 1, particularly in a thick steel pipe having a thickness of 10 mm or more, an extreme strength reduction may occur, and a process such as reheat treatment is required. Therefore, there is a problem that productivity is greatly hindered.

The present invention proposes a method for manufacturing a thick 13Cr martensitic stainless steel seamless pipe having a wall thickness of 10 mm or more, which can solve the above-described problems of the prior art and can stably secure desired strength and has high toughness. The purpose is to do. The term “high toughness” as used herein refers to the case where the absorbed energy vE- _{20 at} the test temperature of the Charpy impact test: −20 ° C. is 50 J or more.

In order to achieve the above-mentioned object, the present inventors diligently studied the influence of quenching conditions on the mechanical properties of 13Cr martensitic stainless steel pipe.
First, experimental results conducted by the present inventors will be described.
A 13Cr martensitic stainless steel pipe (outer diameter φ87 mm × thickness 14.8 mm) having the composition shown in Table 1 was prepared. From these steel pipes, test pieces (φ4mm × 10mm) are collected, heated to each temperature in the range of 900-1000 ° C (holding: 5 min), and then the steel pipe with a wall thickness of 8-30mm is allowed to cool. The cooling (cooling stop temperature: 50 ° C. or less) was applied, and the change in thermal expansion during cooling was measured, and after completion of cooling, the hardness (Vickers hardness HV: test force: 98 N) was measured. In addition, cooling at the time of standing_to_cool was based on the cooling curve calculated | required by heat transfer calculation.

得られた結果を、加熱温度Ｔと冷却速度CRとの関係で図１に示す。○印または□印中の数字は冷却後の試験片の硬さHVである。なお、冷却中の熱膨張変化でフェライト変態が明瞭に認められた場合を□印で示している。○印はフェライト変態が認められない場合である。図１から、13Crマルテンサイト系ステンレス鋼管においては、フェライト変態が生じると、強度の急激な低下が生じるため、安定して所望の強度を確保するためには、加熱温度Ｔと冷却速度CRとの関係は、次（１）式
Ｔ ≧ 2.5CR＋1015 ……（１）
（ここで、Ｔ：焼入れ加熱温度（℃）、CR：700〜600℃の平均冷却速度（℃／min））
を満足する必要があることを見いだした。焼入れ時の冷却速度CRは鋼管の肉厚でほぼ決定されるため、肉厚に応じて焼入れ加熱温度を上記した（１）式を満足するように調整すれば、フェライトの生成による極端な強度低下を生じることなく、安定して所望の強度を有する厚肉鋼管の製造が可能となることを新規に見いだした。

The obtained results are shown in FIG. 1 in relation to the heating temperature T and the cooling rate CR. The numbers in the circles or squares are the hardness HV of the specimen after cooling. The case where the ferrite transformation is clearly recognized by the thermal expansion change during cooling is indicated by □. A circle indicates a case where no ferrite transformation is observed. From FIG. 1, in the 13Cr martensitic stainless steel pipe, when ferrite transformation occurs, the strength sharply decreases. Therefore, in order to ensure a desired strength stably, the heating temperature T and the cooling rate CR are The relationship is the following formula (1) T ≧ 2.5CR + 1015 (1)
(Here, T: quenching heating temperature (° C), CR: average cooling rate of 700-600 ° C (° C / min))
I found it necessary to satisfy. Since the cooling rate CR during quenching is almost determined by the thickness of the steel pipe, if the quenching heating temperature is adjusted so as to satisfy the above equation (1) according to the thickness, an extreme decrease in strength due to the formation of ferrite. It has been newly found that it is possible to produce a thick-walled steel pipe having a desired strength stably without causing any problems.

The desired strength is assumed to be a strength level of 5CT, L80 class, that is, yield strength 80 ksi (551 MPa) class, or higher, planned by the American Petroleum Institute (API).
The present invention has been completed based on the above findings and further studies. That is, the gist of the present invention is as follows.
(1) In mass%, C: 0.15-0.22%, Si: 1.0% or less, Mn: 1.0% or less, P: 0.020% or less, S: 0.010% or less, Cr: 12-14%, the balance a seamless steel pipe having a composition consisting of Fe and unavoidable impurities, was heated to a temperature above 930 ° C., subjected to a quenching treatment to cool to a temperature of 100 ° C. or less cool than the cooling rate CR, then Ac ₁ transformation point In performing the tempering process for heating to the following temperature, the relationship between the heating temperature T of the quenching process and the cooling rate CR is expressed by the following equation (1).
T ≧ 2.5CR + 1015 (1)
(Where T: heating temperature (° C.), CR: average cooling rate of 700 to 600 ° C. (° C./min))
A method for producing a thick-walled 13Cr martensitic stainless steel seamless pipe characterized by satisfying the above requirements.

(2) In (1), in addition to the above-mentioned composition, the thickness further includes a composition containing one or two of Cu: 0.25% or less and Ni: 0.5% or less in mass%. Manufacturing method of 13Cr martensitic stainless steel seamless steel pipe.
(3) In (1) or (2), in addition to the above-mentioned composition, the composition further includes one or two of mass: V: 0.10% or less and Nb: 0.10% or less. A method for producing a featured thick 13Cr martensitic stainless steel seamless pipe.

(4) In any one of (1) to (3), in addition to the above-described composition, the thick 13Cr martensitic stainless steel seam is characterized by having a composition containing, in mass%, Al: 0.10% or less. Manufacturing method of steelless pipe.
(5) The method for producing a thick 13Cr martensitic stainless steel seamless pipe according to any one of (1) to (4), wherein the heating temperature T is 1000 ° C. or less.

  According to the present invention, a thick 13Cr martensite system that can easily and stably secure yield strength: 551 MPa (80 ksi) grade or higher without impairing productivity, and also has high toughness. Stainless steel seamless steel pipes can be manufactured, and there are significant industrial effects.

  Although the manufacturing method of the 13Cr martensitic stainless steel seamless pipe used in the present invention is not particularly limited, it is preferably manufactured by the following steps. Molten steel having the composition shown below is melted by a generally known melting method such as a converter, electric furnace, vacuum melting furnace, etc. It is preferable to use a steel pipe material. Subsequently, these steel pipe materials are heated and hot-worked and formed using a normal Mannesmann-plug mill system or Mannesmann-Mandrel mill process to obtain seamless steel pipes of desired dimensions. After the pipe making, the seamless steel pipe is preferably cooled to room temperature at a cooling rate equal to or higher than air cooling. Thereby, the structure | tissue of a steel pipe can be made into the structure | tissue which makes a martensite phase a base phase. In addition, you may manufacture a seamless steel pipe by the hot extrusion by a press system.

First, the reasons for limiting the composition of the 13Cr martensitic stainless steel seamless pipe used in the present invention will be described. Hereinafter, unless otherwise specified, mass% is simply written as%.
C: 0.15-0.22%
C is an important element related to the strength of martensitic stainless steel, and in the present invention, it is necessary to contain 0.15% or more in order to ensure desired strength and excellent hot workability. On the other hand, if the content exceeds 0.22%, the toughness is lowered and the sensitivity to fire cracking, cracking and the like increases. For this reason, C was limited to the range of 0.15-0.22%. In addition, Preferably it is 0.18 to 0.21%.

Si: 1.0% or less
Si is an element that acts as a deoxidizing agent in the normal steelmaking process, and it is desirable to contain 0.01% or more, but if it exceeds 1.0%, a large amount of δ-ferrite is formed and the desired strength can be secured. It becomes difficult, and hot workability also decreases. For this reason, Si was limited to 1.0% or less. In addition, Preferably it is 0.05 to 0.5%.

Mn: 1.0% or less
Mn is an austenite-forming element, is an element that has the effect of suppressing the formation of δ-ferrite and improving hot workability, and is preferably contained in an amount of 0.1% or more. On the other hand, the content exceeding 1.0% adversely affects toughness. For this reason, Mn was limited to 1.0% or less. In addition, Preferably it is 0.2 to 0.8%.

P: 0.020% or less P is an element that easily segregates. In particular, when concentrated in the central segregation part of seamless steel pipe material, hot workability is remarkably lowered, and it causes a major hindrance to the production of seamless steel pipe, and toughness In order to reduce the corrosion resistance, it is desirable to reduce as much as possible, but extreme reduction leads to an increase in manufacturing cost. P is limited to 0.020% or less as a range that can be industrially implemented at a relatively low cost and does not deteriorate hot workability, toughness, and corrosion resistance. In addition, Preferably it is 0.015% or less.

S: 0.010% or less S is an element that significantly deteriorates hot workability in the pipe manufacturing process, and it is desirable that it be as small as possible, but if it is reduced to 0.010% or less, pipes can be manufactured in the normal process Therefore, S is limited to 0.010% or less. In addition, Preferably it is 0.003% or less.
Cr: 12-14%
Cr is an element that improves the corrosion resistance by forming a protective film. In order to obtain such an effect, the content of 12% or more is required. On the other hand, since Cr is a ferrite-forming element, if it exceeds 14%, δ-ferrite is remarkably produced, hot workability is lowered, and it is difficult to secure a desired strength. For this reason, Cr was limited to the range of 12-14%. In addition, Preferably it is 12.5 to 13.5%.

The above-mentioned components are basic components. In addition to these basic compositions, Cu: 0.25% or less, Ni: 0.5% or less, and / or V: 0.10% or less, Nb: One or two of 0.10% or less and / or Al: 0.10% or less can be selected and contained.
One or two of Cu: 0.25% or less, Ni: 0.5% or less
Cu and Ni are both elements that strengthen the protective film and improve the corrosion resistance, and can be selected and contained as necessary.

In order to obtain such an effect, it is preferable to contain Cu: 0.1% or more and Ni: 0.1% or more. On the other hand, when Cu exceeds 0.25%, the hot workability decreases, and when Ni exceeds 0.5%, the Ac ₁ transformation point decreases and tempering takes a long time, which is efficient. Strength adjustment becomes difficult. For this reason, when it contains, it is preferable to limit to Cu: 0.25% or less and Ni: 0.5% or less, respectively.

One or two of V: 0.10% or less and Nb: 0.10% or less V and Nb are elements having an action of increasing the strength, and can be selected and contained as necessary. In order to acquire such an effect, it is desirable to contain V: 0.01% or more and Nb: 0.005% or more, but inclusion exceeding V: 0.10% and Nb: 0.10% reduces toughness. For this reason, when it contains, it is preferable to limit to V: 0.10% or less and Nb: 0.10% or less, respectively.

Al: 0.10% or less
Al is an element having a strong deoxidizing action, and in order to obtain such an effect, it is desirable to contain 0.001% or more, but inclusion exceeding 0.10% adversely affects cleanliness and toughness. For this reason, it is preferable to limit Al to 0.10% or less. In addition, Preferably it is 0.05% or less.

The balance other than the components described above consists of Fe and inevitable impurities. Inevitable impurities include N: 0.04% or less and O: 0.01% or less.
In the present invention, a seamless steel pipe having the above-described composition is heated to a temperature T (° C.) of 930 ° C. or higher, and is cooled to a temperature of 100 ° C. or lower, preferably room temperature, at a cooling rate CR (° C./min) of standing or higher. Apply a quenching treatment to cool.

In the present invention, the heating temperature T in the quenching process is 930 ° C. or higher and the following equation (1) in relation to the cooling rate CR:
T ≧ 2.5CR + 1015 (1)
(Where T: heating temperature (° C.), CR: average cooling rate of 700 to 600 ° C. (° C./min))
Adjust to satisfy.

  When the heating temperature T of the quenching treatment is less than 930 ° C., even if the austenite single phase region is heated, ferrite is likely to be generated upon cooling, and the quenched structure cannot be made into a desired martensite structure. In addition, when the heating temperature T in the quenching process does not satisfy the formula (1), ferrite is generated during the cooling of the quenching process, and the strength is significantly reduced. For this reason, the heating temperature T in the quenching process is adjusted to be 930 ° C. or higher and satisfy the above expression (1) in relation to the cooling rate CR. The heating temperature T in the quenching treatment is preferably set to 1050 ° C. or lower from the viewpoint of coarsening crystal grains and ensuring high toughness. In addition, More preferably, it is 1000 degrees C or less.

  In addition, the cooling after heating to the heating temperature T is performed (quenching) at a cooling rate CR higher than that of cooling, such as cooling or water cooling. Here, the cooling rate CR to be used depends on the thickness of the steel pipe, and an average cooling rate of 700 to 600 ° C. on the inner surface of the pipe is used. The cooling is performed to a temperature of 100 ° C. or lower, preferably to room temperature. If the cooling stop temperature exceeds 100 ° C., the desired martensite structure may not be obtained, and the desired strength may not be ensured.

In addition, even if the heating temperature T in the quenching process is selected according to the wall thickness that is the controlling factor of the cooling rate so as to satisfy the above-described formula (1), The temperature T may be set to a low temperature, and forced cooling with a large cooling rate CR may be selected so as to satisfy the formula (1).
Note that the forced cooling may be performed by appropriately selecting fan blowing, mist spraying, or the like so as to ensure a desired cooling rate.

The seamless steel pipe that has been subjected to the quenching treatment is then subjected to a tempering treatment in which the steel pipe is heated to a temperature below the Ac ₁ transformation point. By this tempering treatment, a seamless steel pipe having desired strength, further desired high toughness, and desired excellent corrosion resistance is obtained. If the heating temperature of the tempering treatment exceeds the Ac ₁ transformation point, austenite is generated and transformed into martensite during cooling, so that the toughness is lowered and it is difficult to adjust to the desired strength. The heating temperature for the tempering treatment is preferably 600 ° C. or higher. If the heating temperature in the tempering treatment is less than 600 ° C., the desired high toughness cannot be ensured.

After the steel pipe material with the composition shown in Table 2 is heated to 1250 ° C, it is formed into a seamless steel pipe by the Mannesmann plug mill method or hot working with a model seamless steel pipe rolling mill, and then air-cooled after the pipe making. A seamless steel pipe having an outer diameter (mm) × thickness (mm) shown in FIG.
A specimen material was cut out from the obtained seamless steel pipe, heated under the heating conditions shown in Table 3, and then subjected to a quenching treatment for cooling under the cooling conditions shown in Table 3. Further, a tempering treatment was carried out under the conditions shown in Table 3. In addition, it cooled after the tempering process.

In addition, a hardness test piece (size: thickness x 10 mm width x 10 mm length) is taken from the test piece material after quenching, and the cross section perpendicular to the tube axis direction is polished to obtain a Vickers hardness tester (test force). : 98N), and Vickers hardness HV was measured in accordance with the provisions of JIS Z 2244. The measurement points were three points at the center of the wall thickness, and these were arithmetically averaged to be the hardness after quenching of the steel pipe.
In addition, JIS No. 12 tensile test specimens were collected from the specimen material after quenching and tempering so that the tensile direction coincides with the pipe axis direction in accordance with the provisions of JIS Z 2201. A tensile test was performed according to the regulations, the yield strength YS, the tensile strength TS, and the elongation El were measured, and the yield ratio YR was calculated.

In addition, a V-notch test piece was sampled from the specimen material after quenching and tempering so that the length direction of the test specimen coincided with the pipe axis direction in accordance with the provisions of JIS Z 2242. A Charpy impact test was conducted at 0 ° C., the absorbed energy (J) at −20 ° C. was determined, and the toughness was evaluated. In addition, 3 each was performed and the arithmetic average was made into the absorbed energy vE- ₂₀ (J) in _-20 degreeC of the steel pipe.

Furthermore, a corrosion test piece having a thickness of 3 mm, a width of 30 mm, and a length of 40 mm was produced from the specimen material after quenching and tempering by machining, and a corrosion test was performed.
The corrosion test was performed by immersing the corrosion test piece in a test solution retained in an autoclave: 20% NaCl aqueous solution (liquid temperature: 80 ° C., CO ₂ gas atmosphere at 30 atm) and the immersion period was 2 weeks. . The test piece after the corrosion test was weighed, and the corrosion rate calculated from the weight loss before and after the corrosion test was obtained.

  The results obtained are shown in Table 4.

本発明例はいずれも、10mm厚を超える厚肉の鋼管であるにもかかわらず、降伏強さYS：551MPa以上の高強度と、vＥ₋₂₀：50Ｊ以上の高靭性とを安定して確保でき、しかも腐食量も少なく、油井用として好適な優れた耐食性を有している鋼管となっている。一方、本発明の範囲を外れる比較例は、顕著な強度低下が見られ、所望のYS：551MPa以上を確保できなかった。

All of the examples of the present invention can stably secure a high strength of yield strength YS: 551 MPa or more and a high toughness of vE- ₂₀ : 50 J or more despite being a thick steel pipe exceeding 10 mm in thickness. In addition, the steel pipe has a low corrosion amount and excellent corrosion resistance suitable for oil wells. On the other hand, in the comparative example outside the scope of the present invention, a significant decrease in strength was observed, and the desired YS: 551 MPa or more could not be secured.

It is a graph which shows the relationship between the heating temperature T and the cooling rate CR of the quenching process which affects Vickers hardness HV.

Claims (5)

mass%
C: 0.15-0.22%, Si: 1.0% or less,
Mn: 1.0% or less, P: 0.020% or less,
S: 0.010% or less, Cr: 12-14%,
A steel pipe having a composition comprising the balance Fe and inevitable impurities is heated to a temperature of 930 ° C. or higher, and is quenched to a temperature of 100 ° C. or lower at a cooling rate CR that is not lower than that of cooling. _In performing a tempering process for heating to a temperature equal to or lower than ₁ transformation point, the relationship between the heating temperature T of the quenching process and the cooling rate CR is adjusted so as to satisfy the following expression (1). Manufacturing method for thick 13Cr martensitic stainless steel pipe.
Record
T ≧ 2.5CR + 1015 (1)
Here, T: heating temperature (° C.), CR: average cooling rate of 700 to 600 ° C. (° C./min)   In addition to the said composition, it is set as the composition containing 1 type or 2 types of Cu: 0.25% or less and Ni: 0.5% or less by mass%, The thick 13Cr of Claim 1 characterized by the above-mentioned. Manufacturing method of martensitic stainless steel pipe.   3. The thickness according to claim 1, wherein in addition to the composition, the composition further includes one or two of mass%, V: 0.10% or less, and Nb: 0.10% or less. Manufacturing method for meat 13Cr martensitic stainless steel pipe.   The method for producing a thick 13Cr martensitic stainless steel pipe according to any one of claims 1 to 3, wherein in addition to the composition, the composition further includes mass: Al: 0.10% or less.   The method for producing a thick 13Cr martensitic stainless steel pipe according to any one of claims 1 to 4, wherein the heating temperature T is 1000 ° C or lower.

Images