JP3680628B2 - Manufacturing method of high strength oil well steel pipe with excellent resistance to sulfide cracking - Google Patents

Description

【００４０】
【表２】

【００４１】
【発明の効果】
本発明によれば、降伏強度７５８ＭＰａ以上の高強度と優れた耐ＳＳＣ性を両立させた油井用鋼管をエネルギー効率に優れる直接焼入法によって得ることが可能となる。
【図面の簡単な説明】
【図１】焼入ままの状態における表層直下部硬さに対する肉厚中心部硬さの比と鋼管の降伏強度を指標として、耐ＳＳＣ性を示したものである。
【図２】焼入性が良好な鋼を対象として、耐ＳＳＣ性をＭｏ含有量とＹＳの関係で示したものである。
【図３】焼入性が良好でＭｏ含有量がほぼ一定の鋼を対象として、耐ＳＳＣ性をＹＳとＣｒ含有量の関係で示したものである。
【図４】熱間穿孔連続圧延の工程を温度と時間を軸として模式的に示したものである。
【図５】マンネスマン方式の熱間穿孔連続圧延方法によって継目無鋼管を得る場合の、最下点温度と再加熱温度による耐ＳＳＣ性への影響を示したものである。[0001]
BACKGROUND OF THE INVENTION
The present invention, the casing used in oil and gas wells, a method for producing a high strength oil well steel pipe which is used when exposed to sulphide environment say tubing or drill pipe. In particular , the present invention relates to a method for producing a high-strength oil well steel pipe having a yield strength YS of 758 MPa or more, which is excellent in resistance to sulfide cracking (hereinafter abbreviated as SSC).
[0002]
[Prior art]
Steel pipes used in oil and gas wells containing hydrogen sulfide are required to have SSC resistance. The essence of SSC is hydrogen embrittlement, and it becomes easier to occur as the strength of the steel pipe increases. In order to increase the strength while preventing this, it is essential to optimize the alloy element balance and the metal structure. For example, a technique as disclosed in Japanese Patent Publication No. 6-104849 is disclosed, and the yield strength is 827 MPa. High strength materials to the extent have been obtained. However, in actually utilizing such technology, it is essential to perform reheating and quenching and tempering treatment after performing hot working, but such reheating heat treatment has poor thermal efficiency, Contrary to technological trends in process and energy saving.
[0003]
From this point of view, techniques for simplifying heat treatment have also been studied. For example, Japanese Patent Application Laid-Open No. 5-271773 discloses a technique for directly quenching after finish rolling after optimizing steel components and process conditions. ing. However, when the direct quenching method is used, there is a problem that the SSC resistance decreases because the crystal grains become coarse. For this reason, it was difficult to produce a high strength steel pipe having a yield strength exceeding 758 MPa by the direct quenching method.
[0004]
[Problems to be solved by the invention]
In view of such circumstances, the present invention is optimal for producing a steel pipe having a high yield strength of 758 MPa or more and capable of exhibiting sufficient SSC resistance by utilizing a direct quenching method excellent in energy efficiency. The purpose is to obtain a method.
[0005]
[Means for Solving the Problems]
The present inventor has made researches to solve the above problems. As a result, the inventors have obtained knowledge necessary and sufficient for configuring the present invention. The contents are described below. The SSC of high-strength steel pipe arises from the grain boundaries. In order to suppress such destruction, first, the metal structure must be uniform. In order to obtain high strength of 758 MPa or more and high SSC resistance, there is no use other than using tempered martensite as the metal structure, but this structure needs to be as uniform as possible. If the metal structure includes a different phase having different characteristics, this boundary or the different phase itself becomes a fracture starting point, and sufficient SSC resistance cannot be obtained. The uniformity of the structure is almost determined by the quenching state. In other words, whether or not the heterogeneous phase appears depends on whether or not a sufficiently hardened martensite structure can be obtained in the entire steel pipe . Needless to say, a perfect martensite structure is desirable, but considering the limitation of the content of elements that can contribute to hardenability, which will be described later, in the case of thick materials, the present invention aims at a high strength level of 758 MPa or more. FIG. 1 shows the result of studying the essential conditions.
[0006]
Figure 1 is a 25mm wall thickness of the seamless steel pipe, the entire pipe outer surface from the austenite region temperature after the end of hot rolling is performed directly quenching treatment for water cooling, the hardness of the hardness and the surface portion of the thick central portion of the steel pipe The hardness is measured, and the hardenability is evaluated by the ratio of both measurement results, and the SSC resistance is evaluated by using a test piece taken from the center of the thickness. From FIG. 1, the knowledge that sufficient SSC resistance can be obtained even at a high strength exceeding 758 MPa if the hardness of the thickness center portion is 95% or more with respect to the hardness of the portion immediately below the surface. It was.
[0007]
As a result of studying the relationship between the strength governing the SSC characteristics and the alloy element content on the premise of a uniform metal structure not containing a heterogeneous phase, FIG. 2 was obtained. That is, when the contents of Mn and P that affect the SSC characteristics are controlled to a low level that can be industrially achieved, the Mo content and the yield strength YS of the steel pipe after tempering are plotted as indices. Can be organized well. That is, in order to ensure sufficient SSC resistance in a high strength steel pipe of YS ≧ 758 MPa that has been directly quenched, the Mo content needs to be 0.82 % or more, and in relation to the yield strength YS. It was found that α = Mo ( mass% ) − 0.0218 YS ( MPa ), which is described, needs to be −18.1 or more.
[0008]
However, there were cases where the target could not be achieved even after obtaining the above-mentioned uniform structure and also benefiting from the above-mentioned effective element Mo. As a result of analyzing the cause, as shown in FIG. 3, it was found that the large amount of Cr content has a great influence on the high-strength material targeted by the present invention. That is, the structure at the time of quenching is made uniform, SSC characteristic deterioration elements such as P and Mn are made sufficiently low and constant, and the useful Mo content is also made constant, and the relationship between YS and Cr content is set. As a result, as shown in FIG. 3, it was found that the SSC resistance greatly changes with a Cr content of about 0.8% as a boundary.
[0009]
Conventionally, Cr has been actively used as an element necessary for ensuring hardenability, but as long as the direct quenching method is taken, because the hardenability is improved by reheat quenching only by the coarser grain size, There is no need to actively utilize from the viewpoint of hardenability as in the case of the reheating quenching method. Rather, Cr is interpreted as a harmful effect on SSC as a result of weakening the grain boundary strength as a carbide precipitation element, and its content should be limited.
[0010]
Furthermore, when the direct quenching method is adopted, the austenite grains become coarser than the reheat quenching method, and the hardenability is improved, but the SSC sensitivity is increased. For this reason, it is necessary to consider to suppress this coarsening phenomenon as much as possible. For this purpose, it is the most effective means to utilize the pinning effect of Nb carbide, but Nb is an element having a strong carbide forming ability like Cr. It has been found that it also has the harmful effect of weakening the field strength. Therefore, Nb was contained by keeping the coarsening suppression effect within a range that does not offset the harmful effect of carbide coarsening, and the process conditions necessary to maximize this effect were elucidated.
[0011]
The present invention is configured based on the above knowledge, and the gist thereof is as follows. That is, it is the following (1) or (2) .
( 1 ) By mass%, C: 0.10 to 0.25%, Si ≦ 0.5%, Mn ≦ 0.5%, P ≦ 0.015%, S ≦ 0.0050%, Mo: 0.00. 82 to 2.5%, Al: 0.005 to 0.1%, Ti: 0.005 to 0.1%, 3.4 times or more of N, Nb: 0.01 to 0.1%, N ≦ Steel containing 0.01% and B: 0.0005-0.0050%, and the balance iron and impurities are used as a raw material. After heating the raw material to 1150 ° C. or higher, hot piercing continuous rolling is performed to make it hollow After the raw tube temperature was lowered to 900 ° C. or lower immediately before the reheating step, reheating was performed at 900 to 1050 ° C., and finishing was completed at a temperature of Ar ₃ point + 50 ° C. or higher, and then immediately Ar perform quenching treatment to rapidly cooled from _three or more points of temperature, followed characterized by tempering at a temperature of six hundred and sixty to seven hundred and twenty ° C. and, MPa Method of producing a high strength oil well steel pipe having a yield strength YS and the Mo content relationship following (1) 758 MPa or more yield strength YS excellent in sulfide cracking characteristics satisfying the formula represented.
α = Mo−0.0218YS ≧ −18.1 (1) Formula ( 2 )% by mass, C: 0.10 to 0.25%, Si ≦ 0.5%, Mn ≦ 0. 5%, P ≦ 0.015%, S ≦ 0.0050%, Mo: 0.82-2.5%, Al: 0.005-0.1%, Ti: 0.005-0.1% 3.4 times or more of N, Nb: 0.01 to 0.1%, N ≦ 0.01% and B: 0.0005 to 0.0050%, further Cr ≦ 0.8%, W ≦ 0.5%, V: 0.01 to 0.1%, Zr: 0.001 to 0.010%, Ca: 0.001 to 0.01%, Mg: 0.001 to 0.01%, REM: Containing one or more of 0.001 to 0.01%, steel made of the remaining iron and impurities, and heating the material to 1150 ° C or higher, followed by hot piercing continuous rolling Do Immediately after finishing the hollow tube, the temperature of the tube immediately before the reheating process is lowered to 900 ° C. or lower, then reheating to 900 to 1050 ° C., and finishing processing at a temperature of Ar ₃ point + 50 ° C. or higher. The relationship between the yield strength YS expressed in MPa and the amount of Mo is characterized in that quenching is performed by rapidly cooling from a temperature of Ar ₃ or higher, followed by tempering at a temperature of 660 to 720 ° C. (1) The manufacturing method of the steel pipe for high strength oil wells which has the yield strength YS of 758 Mpa or more excellent in the sulfide cracking-proof characteristic which satisfy | fills Formula.
α = Mo−0.0218YS ≧ −18.1 Equation (1)
DETAILED DESCRIPTION OF THE INVENTION
The present invention is described in detail below. First, the reasons for limiting the alloy components of the present invention will be described. The content of the component is mass%. C: C is an essential element for ensuring the desired high strength and resistance to sulfide cracking at the same time. These strengths and resistance to sulfide cracking depend on hardenability, and if the content is less than 0.10%, incomplete quenching results in lower strength. Temporarily adjusting the tempering conditions will reduce the required strength. Even if it is obtained, sufficient sulfide cracking resistance cannot be obtained. On the other hand, even if the content exceeds 0.25%, the resistance to sulfide cracking is saturated and the problem of burning cracks occurs. For this reason, the appropriate range was made 0.10 to 0.25%.
[0013]
Si: Si is a deoxidizer remaining in the steelmaking process, but if it exceeds 0.5%, the steel becomes brittle and sulfide cracking resistance deteriorates, so the upper limit was made 0.5%. .
[0014]
Mn: Mn is an element harmful to sulfide cracking resistance and should not be added, but it also has the effect of improving hardenability and quenching with low C and Mo contents to improve hardenability. If the performance is insufficient, 0.5% may be included as the upper limit. However, if the content exceeds 0.5%, satisfactory cracking resistance cannot be obtained even when completely quenched, so 0.5% was made the upper limit.
[0015]
P: P is an impurity element that segregates at the grain boundaries and degrades the resistance to sulfide cracking, and should be suppressed to the lowest possible level. The upper limit was set to 0.015% as an acceptable level that enables stable industrial production with the current refining technology taking cost into account.
[0016]
S: S is also an impurity element that segregates at grain boundaries and degrades the resistance to sulfide cracking. In the present invention, since it is fundamental not to contain Mn for fixing S, it should be suppressed to the lowest possible level, but in the range of 0.0050% or less, there is no significant deterioration in SSC resistance. Therefore, 0.0050% was made the upper limit of the content.
[0017]
Mo: Mo is one of the essential elements in the present invention, and is an element that suppresses grain boundary segregation of P, which is harmful to SSC resistance, and is a good element to obtain high strength because it increases the resistance to temper softening. is there. As shown in FIG. 2, it is necessary to contain 0.82 % or more in order to ensure sufficient SSC resistance in a high strength region of YS ≧ 758 MPa on the premise of the direct quenching method. However, even if it is contained in a large amount, the effect is saturated and the degree of freedom in adjusting the strength is narrowed, so 2.5% is made the upper limit.
[0018]
Al: Al is necessary for sufficiently deoxidizing steel in the steel making process, and contains 0.005% or more. However, if it is contained in a large amount, the amount of alumina inclusions increases and there is a risk that the SSC sensitivity will increase, so 0.1% was made the upper limit.
[0019]
Ti: Ti is contained in order to sufficiently achieve the hardenability of B described later. That is, in order to prevent precipitation of BN, it is necessary to fix N as TiN in advance. For this purpose, 0.005% or more and 3.4 times or more of the N content are included. However, a large amount promotes the precipitation of coarse TiN and increases the SSC sensitivity, so the upper limit was made 0.1%.
[0020]
Nb: Nb is an element effective in improving SSC resistance because it reduces the grain boundary segregation of P through its fine graining effect, and is contained in an amount of 0.01% or more. However, even if it is contained in a large amount, the effect of refining is saturated, and the SSC resistance is lowered due to a decrease in grain boundary strength due to coarsening of the carbide, so the upper limit was made 0.1%.
[0021]
N: N is an impurity element that impairs the hardenability of B, and should be suppressed to the lowest possible level. The upper limit is set at 0.01% as the allowable level that enables stable industrial production with the current refining technology taking cost into account.
[0022]
B: B is an element that remarkably improves hardenability, and is an essential element for ensuring hardenability in the present invention. If the content is less than 0.0005%, sufficient hardenability cannot be ensured, so this is set as the lower limit content. Further, even if the content exceeds 0.0050%, the effect of improving hardenability is saturated, and rather the precipitation of carbon boride becomes remarkable and the SSC resistance deteriorates. %.
[0023]
α = Mo− 0.0218 YS: As shown in FIG. 3, when the value of α calculated as a function of the yield strength YS ( MPa ) and the Mo content ( mass% ) is −18.1 or more, it is excellent. SSC resistance is obtained, and if it is less than −18.1, satisfactory SSC resistance cannot be obtained even if each component satisfies the above conditions. Therefore, in addition to the component conditions, α ≧ −18.1 is set as an essential requirement of the present invention.
[0024]
In this invention, in addition to the said element, 1 type (s) or 2 or more types are selectively contained among the following as needed.
Cr: Cr is useful as an element that enhances hardenability, but it is not necessary to contain Cr if other elements such as Mo can ensure sufficient hardenability. When trying to obtain a yield strength of 758 MPa or more by the direct quenching method, the content exceeding 0.8% deteriorates the SSC resistance, so the upper limit was made 0.8% (see FIG. 3).
[0025]
W: W also has the effect of increasing hardenability and increasing temper softening resistance. However, if less than 0.01%, the effect is not sufficient, and if it exceeds 0.5%, the effect is saturated. 0.01 to 0.5% was made the range of proper content.
[0026]
V: V has the effect of increasing temper softening resistance, and if contained in an amount of 0.01% or more, it is advantageous for increasing the strength. However, if contained in a large amount, the SSC resistance deteriorates, so 0.1% is the upper limit. It was.
[0027]
Zr: Zr has the effect of suppressing the grain boundary segregation of P. For that purpose, the content of 0.001% or more is necessary. However, since it is an expensive element and if it is contained in a large amount, there is a risk that the oxide increases and the SSC sensitivity increases, so 0.010% is made the upper limit. did.
[0028]
Ca, Mg, REM: These have the effect of reducing the stress concentration by spheroidizing the shape of inclusions, and reducing S grain boundary segregation by fixing S. In any case, if the content is less than 0.001%, a sufficient effect cannot be obtained, and if it is too much, there is a risk of increasing the oxide and increasing the SSC sensitivity, so 0.010% was made the upper limit.
[0029]
Next, manufacturing conditions in the present invention will be described. In the present invention, a slab obtained by melting and casting steel having the above alloy composition in a converter, an electric furnace, or the like, or a slab obtained by subjecting the slab to ingot rolling as necessary, is heated. Then, normal hot piercing continuous rolling is performed, and after quenching directly from the austenite temperature range immediately after the final finishing process, tempering is performed to adjust to a desired strength.
[0030]
Here, the hot working how to molded seamless steel pipe, although take the hot piercing continuous rolling method as illustrated in FIG. 4 is customary, in the example of FIG. 4, prior to the final finish rolling A reheating step is included, and it is necessary to control the tube temperature in the reheating step and the tube temperature immediately before the reheating furnace insertion, which is referred to as “the lowest point temperature” in FIG. 4. These reheating temperature and lowest point temperature are considered to affect the austenite grain size, which is the SSC resistance controlling factor, through the pinning function of the Nb precipitate. From the viewpoint of SSC resistance, as shown in FIG. It is necessary to set the lowest point temperature to 900 ° C. or lower and the reheating temperature to 1050 ° C. or lower. The lower limit of the reheating temperature needs to be 900 ° C. or higher in order to secure a sufficiently high temperature in the subsequent finish rolling and finally secure a quenching temperature of Ar ₃ or higher.
[0031]
The material temperature for quenching requires Ar ₃ points or more. This is because if it falls below the Ar ₃ point, a uniform martensite structure cannot be obtained, and the SSC resistance decreases. In order to ensure this quenching temperature, the final finishing temperature of hot working is Ar ₃ + 50 ° C. or higher . Incidentally, a typical Ar ₃ point in the present invention is about 830 ° C..
By quenching material temperatures for the Ar ₃ point or more, the component composition of the present invention, can also ensure a predetermined hardenability at 25mm about a thick material, the resulting steel pipes have a tissue uniformity The thickness center hardness is 95% or more of the hardness just below the surface, and the SSC resistance is also good.
[0032]
As the heating temperature of the material, it is necessary to ensure 1150 ° C. or higher. This is a necessary treatment for completely dissolving Nb prior to hot working and making the austenite grains immediately before quenching as fine as possible. The upper limit of the heating temperature is not particularly defined, but 1260 ° C. or lower is desirable in order to suppress scale loss and austenite grain growth during heating.
[0033]
As a temperature condition of tempering, 660-720 degreeC was made into the appropriate range. Under low temperature conditions of less than 660 ° C., YS becomes too high and α becomes low, so that satisfactory SSC resistance cannot be obtained. On the other hand, when it exceeds 720 ° C., there is a risk that the SSC sensitivity is increased by entering the two-phase region and breaking the tissue uniformity. For this reason, 660-720 degreeC was set as appropriate tempering temperature conditions.
[0034]
The above-mentioned hot piercing continuous rolling method is a seamless method represented by the Mannesmann method that pierces and stretches a solid material through a plurality of rolling mills regardless of the rolling mill name such as the plug mill method and the mandrel mill method. It means the whole steel pipe rolling method. Further, in the rolling method that does not include the reheating step, the manufacturing is facilitated by the amount that does not require consideration of the above-mentioned regulations regarding the lowest point temperature and the reheating temperature, and this case can also be included in the present invention. .
[0035]
【Example】
Steel having the chemical composition shown in Table 1 is melted in a converter, and the obtained steel slab is subjected to hot piercing continuous rolling in which the steel slab is processed in the pattern shown in FIG. After the final finish rolling, the entire inner and outer surface of the pipe is quenched with water, and the hardness at the center of the wall and the area directly below the surface is measured while quenching. In addition to the evaluation, a tempering treatment was subsequently performed at a temperature of 630 to 730 ° C., and a tensile test was carried out by collecting a round bar tensile test piece from the thickness center of the plate material.
[0036]
Further, a round bar-shaped sulfide crack test piece having a parallel part length of 25 mm and a diameter of 6.2 mm as defined in NACE-TM0177-A was collected from the thickness center of this tempered material, and 0.5% acetic acid was added. The test was carried out for 720 hours in a corrosive solution at 25 ° C. containing% NaCl and saturated with H ₂ S at a partial pressure of 1 atm with a constant stress of 80% of the yield strength.
[0037]
The hardenability was evaluated as good if the thickness center hardness was 95% or more of the hardness just below the surface. The SSC resistance was evaluated as good if it did not break during the test time of 720 hours.
[0038]
The test results are shown in Table 2. No. in Table 2 Although 1-12 are the test results included in the scope of the present invention, YS can simultaneously satisfy high strength exceeding 758 MPa and excellent SSC resistance. On the other hand, Comparative Example No. Since components 17 to 25 are outside the scope of the present invention, good SSC resistance is not obtained. Further, although the components are within the scope of the present invention, Comparative Example No. Nos. 26 and 29 have α out of the range of the present invention, and Comparative Examples 27 and 28 have tempering temperatures and α out of the range of the present invention. In Nos. 15 and 16, the quenching temperature is inappropriate. Since the reheating temperature and the lowest point temperature of 13 and 14 are outside the range of the present invention, satisfactory SSC resistance is not obtained.
[0039]
[Table 1]

[0040]
[Table 2]

[0041]
【The invention's effect】
According to the present invention, it is possible to obtain an oil well steel pipe having both high strength of yield strength of 758 MPa or more and excellent SSC resistance by a direct quenching method with excellent energy efficiency.
[Brief description of the drawings]
FIG. 1 shows SSC resistance using as an index the ratio of the thickness center hardness to the hardness just below the surface layer in the as-quenched state and the yield strength of the steel pipe .
FIG. 2 shows SSC resistance as a relationship between Mo content and YS for steel with good hardenability.
FIG. 3 shows SSC resistance as a relationship between YS and Cr content for steel with good hardenability and approximately constant Mo content.
FIG. 4 schematically shows a hot piercing continuous rolling process with temperature and time as axes.
FIG. 5 shows the influence of the lowest point temperature and the reheating temperature on the SSC resistance when a seamless steel pipe is obtained by the Mannesmann hot piercing continuous rolling method.

Claims (2)

In mass%, C: 0.10 to 0.25%, Si ≦ 0.5%, Mn ≦ 0.5%, P ≦ 0.015%, S ≦ 0.0050%, Mo: 0.82 to 2 0.5%, Al: 0.005 to 0.1%, Ti: 0.005 to 0.1%, 3.4 times or more of N, Nb: 0.01 to 0.1%, N ≦ 0.01 % And B: 0.0005% to 0.0050%, and the steel consisting of the remaining iron and impurities is used as a raw material, and after heating the raw material to 1150 ° C. or higher, hot piercing continuous rolling is performed to obtain a hollow shell. The tube temperature immediately before the reheating step is lowered to 900 ° C. or lower, then reheated to 900 to 1050 ° C., and after finishing processing at a temperature of Ar ₃ point + 50 ° C. or higher, immediately Ar ₃ point or higher It is expressed in MPa, characterized by performing a quenching process that rapidly cools from the temperature of, and subsequently tempering at a temperature of 660 to 720 ° C. A method for producing a high-strength oil well steel pipe having a yield strength YS of 758 MPa or more excellent in sulfide cracking resistance, wherein the relationship between the yield strength YS and the amount of Mo satisfies the following formula (1).
α = Mo−0.0218YS ≧ −18.1 (1) In mass%, C: 0.10 to 0.25%, Si ≦ 0.5%, Mn ≦ 0.5%, P ≦ 0.015%, S ≦ 0.0050%, Mo: 0.82 to 2 0.5%, Al: 0.005 to 0.1%, Ti: 0.005 to 0.1%, 3.4 times or more of N, Nb: 0.01 to 0.1%, N ≦ 0.01 % And B: 0.0005 to 0.0050%, Cr ≦ 0.8%, W ≦ 0.5%, V: 0.01 to 0.1%, Zr: 0.001 to 0 0.010%, Ca: 0.001-0.01%, Mg: 0.001-0.01%, REM: One or more of 0.001-0.01%, and the balance Steel made of iron and impurities is used as a raw material. After heating the raw material to 1150 ° C or higher, continuous hot rolling is performed to form a hollow shell, and the temperature of the raw tube immediately before the reheating step is lowered to 900 ° C or lower. After subjected to a re-heated to 900 to 1050 ° C., after completing the finishing with Ar ₃ point + 50 ℃ temperatures above performs quenching treatment immediately quenched from Ar ₃ point or higher, followed by 660～ A yield strength YS of 758 MPa or more excellent in sulfide cracking resistance, wherein the relationship between the yield strength YS expressed in MPa and the Mo amount satisfies the following formula (1), characterized by tempering at a temperature of 720 ° C. The manufacturing method of the steel pipe for high strength oil wells which has.
α = Mo−0.0218YS ≧ −18.1 (1)

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