JP2018031027A - High strength seamless oil well tube and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength seamless oil well tube hardly generating deformation even with mist hardening and excellent in productivity by using steel having C content which is increased to 0.40 mass% or more, for securing yield strength of 1103 MPa or more.SOLUTION: There is provided a high strength seamless oil well tube containing, by mass%, C:0.40 to 1.00%, Si:0.05 to 1.00%, Mn:0.05 to 1.00%, P≤0.050%, S≤0.010%, Al:0.01 to 0.10%, N≤0.010%, Cr:0.10 to 2.50%, Mo:0.30 to 3.00, Ni:0.02 to 4.00%, Cu:0 to 0.50%, Ti:0 to 0.030%, Nb:0 to 0.150%, V:0 to 0.500%, B:0 to 0.0030%, Ca:0 to 0.0040%, Mg:0 to 0.0040%, Zr:0 to 0.0040% and the balance:Fe with impurities and having 25×C+Ni≥12.4. There is provided a high strength seamless oil well tube having Fn1 represented by Fn1=25×C+Ni of 12.4 or more.SELECTED DRAWING: Figure 3

Description

  The present invention relates to a high-strength seamless oil-well pipe and a method for producing the same. Specifically, the present invention is made of steel having a high C content, and is deformed even when subjected to mist quenching in which air and water are mixed in order to suppress cracking during quenching (hereinafter referred to as “quenching”). The present invention relates to a high-strength seamless oil-well pipe excellent in productivity and a method for producing the same.

  In recent years, in the energy-related field, for example, oil wells and gas wells (hereinafter, oil wells and gas wells are collectively referred to simply as “oil wells”) have become deep wells. For this reason, high strength is required for “seamless well pipes” such as casings and tubing used in such oil wells.

As a technique related to a high-strength seamless well pipe, for example, Patent Documents 1 to 3 describe that “high-strength seamless steel pipe for oil wells” has a yield strength (hereinafter sometimes referred to as “YS”) of 862 MPa (125 ksi) or more. In addition, Patent Documents 4 and 5 disclose “high-strength seamless steel pipes for oil wells excellent in sulfide stress cracking resistance and their manufacturing methods” with YS of 758 MPa (110 ksi) or more. Has been. Furthermore, as high-strength low alloy steel for oil wells, for example, both of Patent Documents 6 and 7 are excellent in low-temperature toughness and sulfide stress cracking resistance with YS of 618-853 MPa (63-87 kgf / mm ² ). “High-strength low-alloy oil tempered steel” and “high-strength low-alloy oil well steel excellent in low temperature toughness and sulfide stress cracking resistance” are disclosed. On the other hand, as a technique related to cooling of various steel materials, Patent Literature 8 discloses a “steel material cooling control method”, and Patent Literature 9 discloses a “steel plate temperature prediction method considering transformation heat generation”. Yes.

Japanese Patent No. 5943165 Japanese Patent No. 5943164 Japanese Patent No. 5930140 JP 2013-129879 A International Publication No. 2010/150915 JP 7-011384 A Japanese Patent Laid-Open No. 4-066665 JP-A-1-162508 JP 2011-212743 A

  Since the structures of the high-strength seamless oil-well pipe and the high-strength low-alloy steel for oil well disclosed in Patent Documents 1 to 7 are both tempered martensite, the quenching-tempering treatment (so-called “tempering” Processing "). In addition, although various cooling media are used for quenching according to the chemical composition of the steel, when quenching by water cooling, the C content of the steel is 0.40% by mass or more, particularly 0.50% by mass or more. May cause a significant reduction in productivity due to burning cracks. On the other hand, when steel with a high C content of 0.40% by mass or more is used, if the cooling rate at the time of quenching is slowed, even if cracking can be prevented, deformation (bending) as shown in FIG. ) May result in a decrease in productivity. However, none of the above Patent Documents 1 to 7 has been studied at all about “bending” when quenching a steel having a high C content at a low cooling rate in order to prevent quench cracking. In addition, although each of the above patent documents is said to be “high strength”, the specific value of YS is at most 986 MPa in the case of “examples of the present invention” of each patent document (steel pipe No. 7 in Table 3 of Patent Document 1). Furthermore, even in the case of the “comparative example”, it is at most 1098 MPa (see steel pipe No. 16 in Table 3 of Patent Document 2).

  In Patent Documents 8 and 9, no consideration is given to “bending” when a seamless steel pipe having a high C content is quenched at a low cooling rate in order to prevent burning cracks.

  In the present invention, in order to stably secure a high strength of 1103 MPa (160 ksi) or more in YS, steel with a C content increased to 0.40 mass% or more is used as a raw material, and mist quenching from the viewpoint of suppressing cracking. It is an object of the present invention to provide a high-strength seamless oil-free pipe that is not easily deformed and has excellent productivity, and a method for manufacturing the same.

  The present invention has been completed in order to solve the above-described problems, and the gist of the present invention is a high-strength seamless oil-well pipe and a method for producing the same as described below.

(1) In mass%,
C: 0.40 to 1.00%,
Si: 0.05-1.00%,
Mn: 0.05 to 1.00%,
P: 0.050% or less,
S: 0.010% or less,
Al: 0.01 to 0.10%,
N: 0.010% or less,
Cr: 0.10 to 2.50%,
Mo: 0.30 to 3.00,
Ni: 0.02 to 4.00%,
Cu: 0 to 0.50%,
Ti: 0 to 0.030%,
Nb: 0 to 0.150%,
V: 0 to 0.500%,
B: 0 to 0.0030%,
Ca: 0 to 0.0040%,
Mg: 0 to 0.0040%,
Zr: 0 to 0.0040%,
Balance: Fe and impurities,
Fn1 represented by the following formula [1] is 12.4 or more,
Having a chemical composition,
High strength seamless oil well pipe.
Fn1 = 25 × C + Ni ... [1]
However, C and Ni in the formula [1] mean the content (mass%) of each element in steel.

(2) The chemical composition is mass%,
Cu: 0.02 to 0.50%,
Ti: 0.003-0.030%,
Nb: 0.003 to 0.150%,
V: 0.005-0.500%,
B: 0.0003 to 0.0030%,
Ca: 0.0003 to 0.0040%,
Mg: 0.0003 to 0.0040%, and
Zr: 0.0003 to 0.0040%,
The high-strength seamless oil-well pipe according to (1) above, which contains one or more selected from:

(3) The yield strength is 1103 MPa or more.
The high-strength seamless oil-free pipe according to (1) or (2) above.

(4) The following steps from [i] to [iii] are sequentially performed on a seamless steel pipe finished by Mannesmann pipe having the chemical composition described in (1) or (2) above. ) To (3), the method for producing a high-strength seamless oil-well pipe.
[i]: Heating step of heating to 850 to 1200 ° C. and holding for 10 minutes to 3 hours
[ii]: A quenching process in which the cooling rate at 800 to 500 ° C. is 2 to 60 ° C./second and mist cooling is performed at a cooling rate from the start of cooling to room temperature of 1 to 30 ° C./second.
[iii]: Tempering step of heating to 100 to 750 ° C. and holding for 10 minutes to 3 hours, and then cooling to room temperature

  According to the present invention, steel having a high C content, suitable for oil well pipes such as casings for deep wells and tubing that require high YS, is used as a raw material, and even if mist quenching is performed to suppress sinter cracking. It is possible to obtain a high-strength seamless oil-free pipe that is not easily deformed and has excellent productivity. The above-described high-strength seamless oil country tubular goods can be stably obtained by the production method of the present invention.

It is a figure which shows typically the generation | occurrence | production state of a deformation | transformation (bending) after tempering a seamless steel pipe. “L” indicates the length (full length) of the seamless steel pipe, and “M” indicates the amount of deformation. Schematic method for obtaining the latent heat of martensitic transformation (hereinafter simply referred to as “transformation latent heat”) from the calorific value during cooling measured using a differential scanning calorimeter (hereinafter referred to as “DSC”). FIG. For each test number, the relationship between the rate of occurrence of bending and the latent heat of transformation determined from the DSC measurement results in an example in which 20 steel pipes each having an outer diameter of 245 mm, a wall thickness of 13.8 mm and a length of 12 m were mist-quenched was arranged FIG. It should be noted that “bending” is defined when “M / L”, which is the ratio of M and L shown in FIG. 1 in the same length unit, is 0.02 or more. Examples of steels 2 to 5 and steel 26 in Table 1 in which the C content is 0.48 to 0.50% by mass. The relationship between the transformation latent heat obtained from the DSC measurement results and the Ni content is organized. FIG. It is a figure which rearranges and shows the relationship between the transformation latent heat calculated | required from the DSC measurement result and "25 * C + Ni (= Fn1)" about 26 steel types used in the Example.

  The present inventors have studied various chemical compositions of low alloy steel that is suitable as a material steel for seamless oil well pipes such as casings for deep wells and tubing that require high YS, and relatively inexpensive, The production method of seamless oil well pipes using the low alloy steel was also examined.

  As a result, first, it was concluded that the C content should be 0.40% by mass or more in order to obtain high strength (high YS) with a low-alloy comparatively inexpensive steel. On the other hand, in the case of a high-C low alloy steel having a C content of 0.40% by mass or more, if water quenching is performed to increase the strength, there is a very high possibility of causing cracking.

  For this reason, various slow cooling methods were investigated from the viewpoint of preventing cracking. When slow cooling, particularly mist quenching, was performed, the thickness direction and length of the seamless steel pipe (hereinafter sometimes referred to as “steel material”) were determined. In the vertical direction, it became clear that various characteristics such as mechanical characteristics may vary. Furthermore, it was also confirmed that the steel material may be deformed and the dimensional accuracy may be lowered.

  Thus, as a result of further investigation on mist quenching, it was found that variations and deformations of various properties such as the above-described mechanical properties are phenomena based on temperature variations in the steel material during mist quenching.

  The present inventors conducted further detailed investigation on the temperature variation during mist quenching. As a result, a new finding was obtained that the main cause of temperature variation during mist quenching is heat generation due to martensitic transformation (in other words, transformation latent heat). That is, when a part of the steel material undergoes martensitic transformation, the vicinity thereof is warmed by the heat generated by the transformation, resulting in temperature variations inside the steel material. Therefore, the smaller the latent heat of transformation, the smaller the temperature variation within the steel material, and the smaller the variation and deformation of various properties such as mechanical properties.

  Then, the present inventors examined the influence of the alloy element on the transformation latent heat by changing the chemical composition of the steel variously in order to reduce the temperature variation inside the steel material at the time of mist quenching. As a result, it has been clarified that an increase in the content of C in steel is extremely effective in reducing transformation latent heat, and that Ni is also effective in reducing transformation latent heat.

Furthermore, based on the above findings, the present inventors examined the transformation latent heat and the bending of the seamless steel pipe during mist quenching. As a result, it was found that when the transformation latent heat is 68 J / g or less, the deformation of the seamless steel pipe during mist quenching is greatly reduced. It has also been found that the transformation latent heat is 68 J / g or less if the condition that Fn1 represented by the following formula [1] is 12.4 or more is satisfied.
Fn1 = 25 × C + Ni ... [1]
However, C and Ni in the formula [1] mean the content (mass%) of each element in steel.

  Furthermore, in order to suppress the bending of the seamless steel pipe at the time of mist quenching, and to provide the desired high YS stably and without variation in the seamless steel pipe, in particular, a cooling rate between 800 to 500 ° C. at the time of mist quenching, It was also found that the cooling rate from the start of cooling to room temperature has a significant effect.

  The present invention has been completed based on the above contents. Hereinafter, each requirement of the present invention will be described in detail.

1. Chemical composition:
The reasons for limiting the chemical composition of the high-strength seamless oil country tubular goods according to the present invention are as follows. In the following description, “%” for the content of each element means “mass%”.

C: 0.40 to 1.00%
C is an extremely effective element for reducing the latent heat of transformation. C also has the effect of increasing the strength of the steel. However, if the C content is less than 0.40%, the desired strength may not be ensured by mist quenching. On the other hand, if C exceeds 1.00%, quench cracking may not be prevented even by mist quenching. Therefore, the C content is set to 0.40 to 1.00%. The minimum with preferable C content is 0.48%, and it is more preferable to make it contain exceeding 0.50%. The upper limit with preferable C content is 0.80%, and a more preferable upper limit is 0.62%.

Si: 0.05-1.00%
Si is an element effective for deoxidation of steel. In order to obtain a deoxidation effect, the Si content needs to be 0.05% or more. On the other hand, when Si content exceeds 1.00%, hot workability will be reduced. For this reason, content of Si shall be 0.05-1.00%. A preferable lower limit of the Si content is 0.15%, and a more preferable lower limit is 0.20%. The upper limit with preferable Si content is 0.55%, and a more preferable upper limit is 0.50%.

Mn: 0.05-1.00%
Mn is an element effective for improving the hardenability of steel. In order to obtain this effect, it is necessary to contain at least 0.05% of Mn. On the other hand, when the Mn content exceeds 1.00%, the hot workability is lowered. For this reason, content of Mn shall be 0.05-1.00%. The minimum with preferable Mn content is 0.25%, and a more preferable minimum is 0.30%. The upper limit with preferable Mn content is 0.80%, and a more preferable upper limit is 0.70%.

P: 0.050% or less P segregates at the grain boundary as an impurity and causes grain boundary fracture. For this reason, P content needs to be limited to 0.050% or less. The upper limit with preferable P content is 0.020%.

S: 0.010% or less S, like P, segregates as an impurity at the grain boundary and causes grain boundary fracture. For this reason, S content needs to be 0.010% or less. The upper limit with preferable S content is 0.005%.

Al: 0.01-0.10%
Al is an element effective for deoxidation of steel. The effect cannot be obtained when the Al content is less than 0.01%. On the other hand, when Al is contained exceeding 0.10%, inclusions are coarsened and mechanical properties are deteriorated. For this reason, the content of Al is set to 0.01 to 0.10%. The minimum with preferable Al content is 0.015%, and a more preferable minimum is 0.020%. The upper limit with preferable Al content is 0.060%, and a more preferable upper limit is 0.055%. In addition, Al content of this invention points out content in acid-soluble Al ("Sol.Al").

N: 0.010% or less N is present in the steel as an impurity. When the content exceeds 0.010%, coarse nitrides are formed, and mechanical properties such as toughness are deteriorated. For this reason, N content shall be 0.010% or less. The upper limit with preferable N content is 0.005%.

Cr: 0.10 to 2.50%
Cr is an element effective for improving the hardenability of steel. In order to obtain this effect, it is necessary to contain at least 0.10% Cr. On the other hand, if the Cr content exceeds 2.50%, the carbide is coarsened and the mechanical properties are deteriorated. For this reason, content of Cr shall be 0.10-2.50%. The minimum with preferable Cr content is 0.25%, and a more preferable minimum is 0.30%. The upper limit with preferable Cr content is 1.50%, and a more preferable upper limit is 1.30%.

Mo: 0.30 to 3.00%
Mo is an element effective for improving the hardenability of steel. In order to obtain this effect, it is necessary to contain at least 0.30% Mo. However, if the Mo content exceeds 3.00%, the material cost increases. For this reason, the Mo content is set to 0.30 to 3.00%. The minimum with preferable Mo content is 0.50%, and a more preferable minimum is 0.55%. The upper limit with preferable Mo content is 2.00%, and a more preferable upper limit is 1.70%.

Ni: 0.02 to 4.00%
Ni is an element effective for reducing the latent heat of transformation. In order to obtain this effect, it is necessary to contain at least 0.02% Ni. However, if the Ni content exceeds 4.00%, the material cost increases. For this reason, the Ni content is 0.02 to 4.00%. A preferable lower limit of the Ni content is 0.03%, and a more preferable lower limit is 0.06%. The upper limit with preferable Ni content is 3.00%, and a more preferable upper limit is 2.90%.

Cu: 0 to 0.50%
Cu is an element that improves the hardenability of steel. For this reason, you may contain Cu as needed. However, if the Cu content exceeds 0.50%, the material cost increases. For this reason, the upper limit of Cu content in the case of making it contain shall be 0.50%. The upper limit of the Cu content is preferably 0.35%, and more preferably 0.30%. In addition, in order to acquire the said effect stably, it is preferable that the minimum of Cu content is 0.02%, and it is further more preferable that it is 0.05%.

Ti: 0 to 0.030%
Ti has the effect of refining crystal grains and improving mechanical properties. For this reason, you may contain Ti as needed. However, if the Ti content exceeds 0.030%, the nitride becomes coarse and the mechanical properties are deteriorated. Therefore, the upper limit of the Ti content when contained is 0.030%. The upper limit of the Ti content is preferably 0.015%, more preferably 0.012%. In addition, in order to acquire the said effect stably, it is preferable that the minimum of Ti content is 0.003%, and it is further more preferable that it is 0.005%.

Nb: 0 to 0.150%
Nb has the effect of refining crystal grains and improving mechanical properties. For this reason, you may contain Nb as needed. However, if the content of Nb exceeds 0.150%, the nitride becomes coarse and the mechanical properties are deteriorated. Therefore, the upper limit of the Nb content when contained is 0.150%. The upper limit of the Nb content is preferably 0.050%, and more preferably 0.035%. In addition, in order to acquire the said effect stably, it is preferable that the minimum of Nb content is 0.003%, and it is further more preferable that it is 0.007%.

  The total amount when Ti and Nb are combined and contained is preferably 0.150% or less, and more preferably 0.050% or less.

V: 0 to 0.500%
V has the effect | action which raises the intensity | strength of steel. For this reason, you may contain V as needed. However, even if V is contained in an amount exceeding 0.500%, the above effect is saturated, and further toughness is reduced. Therefore, the upper limit of the V content when contained is 0.500%. The upper limit of the V content is preferably 0.150%, and more preferably 0.110%. In addition, in order to acquire the said effect stably, it is preferable that the minimum of V content is 0.005%, and it is further more preferable that it is 0.015%.

B: 0 to 0.0030%
B has the effect of enhancing the hardenability of steel with a small amount. For this reason, you may contain B as needed. However, even if B is contained in an amount exceeding 0.0030%, the above effect is saturated, and the nitride (BN) becomes coarse. Therefore, the upper limit of the B content when contained is 0.0030%. The upper limit of the B content is preferably 0.0015%, and more preferably 0.0012%. In addition, in order to acquire the said effect stably, it is preferable that the minimum of B content is 0.0003%, and it is further more preferable that it is 0.0007%.

Ca: 0 to 0.0040%
Ca combines with S in steel to refine sulfides and has the effect of improving mechanical properties. For this reason, you may contain Ca as needed. However, when the Ca content exceeds 0.0040%, the oxide is coarsened. For this reason, the upper limit of Ca content in the case of making it contain shall be 0.0040%. The upper limit of the Ca content is preferably 0.0025%, and more preferably 0.0020%. In addition, in order to acquire the said effect stably, it is preferable that the minimum of Ca content is 0.0003%, and it is further more preferable that it is 0.0006%.

Mg: 0 to 0.0040%
Mg combines with S in steel to refine sulfides and has the effect of improving mechanical properties. For this reason, you may contain Mg as needed. However, when the Mg content exceeds 0.0040%, the oxide is coarsened. For this reason, when making it contain, the upper limit of Mg content shall be 0.0040%. The upper limit of the Mg content is preferably 0.0025%, and more preferably 0.0020%. In order to obtain the above effect stably, the lower limit of the Mg content is preferably 0.0003%, and more preferably 0.0006%.

Zr: 0 to 0.0040%
Zr also combines with S in the steel to refine sulfides and has the effect of improving mechanical properties. For this reason, you may contain Zr as needed. However, if the Zr content exceeds 0.0040%, the oxide is coarsened. For this reason, the upper limit of Zr content in the case of making it contain shall be 0.0040%. The upper limit of the Zr content is preferably 0.0025%, and more preferably 0.0020%. In order to obtain the above effect stably, the lower limit of the Zr content is preferably 0.0003%, and more preferably 0.0006%.

  The total amount when two or more selected from the above-mentioned Ca, Mg and Zr are combined is preferably 0.0040% or less.

  The high-strength seamless oil country tubular good according to the present invention has a chemical composition in which each of the above-described elements, the balance is Fe and impurities, and Fn1 represented by the formula [1] is 12.4 or more.

  Here, “impurities” are components mixed in due to various factors of raw materials such as ores and scraps and manufacturing processes when industrially producing steel materials, and are permitted within a range that does not adversely affect the present invention. Means what will be done.

Fn1: 12.4 or higher The high-strength seamless oil-free pipe according to the present invention has Fn1 expressed by the following formula [1] of 12.4 or higher.
Fn1 = 25 × C + Ni ... [1]
However, C and Ni in the formula [1] mean the content (mass%) of each element in steel.

  Fn1 is an index of transformation latent heat. If the value of Fn1 is 12.4 or more, the latent heat of transformation is 68 J / g or less. When the latent heat of transformation is 68 J / g or less, the deformation of the seamless steel pipe at the time of mist quenching (refers to the ratio of M to L (“M / L”) shown in FIG. 1 in the same length unit) is 0. It can be suppressed to less than 0.02. The lower limit of Fn1 is preferably 12.5, more preferably 12.55. The upper limit of Fn1 is preferably 26.0, and more preferably 23.0.

2. Yield strength:
The high-strength seamless oil country tubular good of the present invention preferably has a YS of 1103 MPa or more. This YS can be applied as oil well pipes such as casings and tubing even when deep wells are advanced. The upper limit of YS is about 1240 MPa from the viewpoint of securing sufficient delayed fracture resistance.

3. Production method:
The high-strength seamless oil country tubular goods of this invention can be manufactured with the following method, for example.

  After melting the low-alloy steel having the chemical composition described above, it is cast into an ingot or slab, and the cast ingot or slab is batch-rolled into a circular billet shape, and then Mannesmann Pipes are produced to finish seamless steel pipes. When a slab having a circular billet shape for Mannesmann pipe is formed by the so-called “round CC” method, the slab having the circular billet shape may be directly manufactured by Mannesmann and finished into a seamless steel pipe.

  The seamless steel pipe finished by the Mannesmann pipe is subjected to the following steps [i] to [iii] in order to produce the high-strength seamless oil well pipe of the present invention.

[i]: Heating process at 850 to 1200 ° C. and holding for 10 minutes to 3 hours. The seamless steel pipe finished to a predetermined shape by Mannesmann tube is heated to 850 to 1200 ° C. for 10 minutes to 3 hours. Retained and austenitized. When the heating and holding temperature is lower than 850 ° C., the mist treatment may not be sufficiently quenched and a desired high strength may not be obtained. On the other hand, when the heating and holding temperature exceeds 1200 ° C., the prior austenite crystal grains are coarsened and the mechanical properties may be deteriorated. Even when heating at 850 to 1200 ° C., if the holding time is less than 10 minutes, quenching may be insufficient and a desired high strength may not be obtained. When the time is exceeded, the prior austenite crystal grains may be coarsened and the mechanical properties may be deteriorated. Therefore, the seamless steel pipe was heated to 850 to 1200 ° C. and held for 10 minutes to 3 hours. In addition, it is preferable that the minimum of heating temperature shall be 900 degreeC. Further, the upper limit of the heating temperature is preferably 1050 ° C, and more preferably 1000 ° C. Furthermore, the lower limit of the holding time is preferably 20 minutes, and the upper limit is preferably 2 hours. The heating temperature in the step [i] refers to the temperature on the outer surface of the seamless steel pipe.

[ii]: Quenching step in which the cooling rate at 800 to 500 ° C. is 2 to 60 ° C./second and the cooling rate from the start of cooling to room temperature is 1 to 30 ° C./second, after the heating step, The seamless steel pipe is mist-cooled under the conditions that the cooling rate at 800 to 500 ° C is 2 to 60 ° C / sec and the cooling rate from the start of cooling to room temperature is 1 to 30 ° C / sec. When the cooling rate at 800 to 500 ° C. during mist cooling is less than 2 ° C./second, quenching may be insufficient and desired high strength may not be obtained. On the other hand, when the cooling rate at 800 to 500 ° C. exceeds 60 ° C./second, occurrence of burning cracks may be unavoidable. In addition, even if the cooling rate at 800 to 500 ° C. at the time of mist cooling is 2 to 60 ° C./second, quenching becomes insufficient when the cooling rate from the start of cooling to room temperature is less than 1 ° C./second. In some cases, the desired high strength cannot be obtained, and when the cooling rate from the start of cooling to room temperature exceeds 30 ° C./second, the occurrence of burning cracks cannot be avoided. Therefore, the seamless steel pipe after the heating step is mist-cooled under the conditions that the cooling rate at 800 to 500 ° C. is 2 to 60 ° C./sec and the cooling rate from the start of cooling to room temperature is 1 to 30 ° C./sec. did. The lower limit of the cooling rate at 800 to 500 ° C. is preferably 3 ° C./second, and the upper limit is preferably 40 ° C./second. Furthermore, the lower limit of the cooling rate from the start of cooling to room temperature is preferably 2 ° C./second, and the upper limit is preferably 20 ° C./second. The temperature in the step [ii] indicates the temperature on the outer surface of the seamless steel pipe, and the cooling rate also indicates the cooling rate on the outer surface of the seamless steel pipe. The above two cooling rates during mist cooling are:
“Cooling rate at 800 to 500 ° C.” = 300 ° C./Time required for cooling (seconds)
“Cooling rate from start of cooling to room temperature” = (cooling start temperature (° C.) − Room temperature (25 ° C.)) / Time required for cooling (seconds)
Point to.

[iii]: Heating to 100 to 750 ° C. and holding for 10 minutes to 3 hours, then cooling to room temperature, tempering process The seamless steel pipe quenched by mist cooling to room temperature is then brought to the desired YS level. In order to adjust, after heating to 100-750 degreeC and hold | maintaining for 10 minutes-3 hours, the tempering process which cools to room temperature is given. When the heating holding temperature is lower than 100 ° C., a desired high strength (especially, high YS) may not be obtained or mechanical properties such as poor toughness may be deteriorated. The desired high strength may not be obtained. In addition, even if it is heating at 100 to 750 ° C., if the holding time is less than 10 minutes, mechanical properties such as toughness may deteriorate, and if the holding time exceeds 3 hours The desired high strength may not be obtained. Therefore, the mist-quenched seamless steel pipe was heated to 100 to 750 ° C. and held for 10 minutes to 3 hours, and then cooled to room temperature. The lower limit of the heating temperature is preferably 400 ° C, and more preferably 450 ° C. The upper limit of the heating temperature is preferably 700 ° C., and more preferably 680 ° C. Furthermore, the lower limit of the holding time is preferably 20 minutes, and the upper limit is preferably 2 hours. The heating temperature in the step [iii] refers to the temperature on the outer surface of the seamless steel pipe. In addition, there is no restriction | limiting in particular about the cooling rate at the time of cooling to room temperature by this tempering process.

  EXAMPLES Hereinafter, although an Example demonstrates this invention more concretely, this invention is not limited to these Examples.

  Low alloy steels 1 to 26 having the chemical composition shown in Table 1 are melted by an ordinary method, then rolled into a round billet shape, and then Mannesmann pipe is manufactured to have an outer diameter of 245 mm and a wall thickness of 13. The finished steel pipe was 8 mm and 12 m long.

  Steels 1 to 21 in Table 1 are steels whose chemical compositions are within the range defined by the present invention, while steels 22 to 26 are steels whose chemical compositions deviate from the conditions defined by the present invention.

  For each steel, first, from a position 1 m from the longitudinal pipe end of the seamless steel pipe of the above size, the diameter is 5 mm and the thickness is 1 mm in the longitudinal direction (the longitudinal direction of the steel pipe and the thickness direction of the test piece are parallel). Disc test pieces were collected and the calorific value during cooling was measured using DSC to determine the latent heat of transformation and the Ms point.

Specifically, using a DSC, the above disk specimen was heated from room temperature to 950 ° C. at 20 ° C./min, held at 950 ° C. for 10 minutes, and then cooled at 60 ° C./min (1 ° C./second). ) To room temperature (25 ° C.). The rising temperature of the exothermic peak due to martensitic transformation under the above cooling conditions measured using DSC was determined as the Ms point. Moreover, the transformation latent heat is as shown in FIG.
<i> Extend the baseline before and after the exothermic peak due to martensitic transformation during cooling,
Then
<ii> Integrate the range surrounded by the exothermic peak and the baseline and divide by the cooling rate of 60 ° C / min.
Was determined by When DSC is used, the rate of temperature increase (20 ° C./min) and the rate of cooling (60 ° C./min) are constant in any temperature range.

  For each steel, 20 seamless steel pipes each having an outer diameter of 245 mm, a wall thickness of 13.8 mm, and a length of 12 m were used, heated to 950 ° C. and held for 1 hour, and then rotated at room temperature ( After quenching by mist cooling to 25 ° C.), the deformation of the seamless steel pipe was investigated. At this time, if “M / L”, which is the ratio of M (deformation amount) and L (full length) shown in FIG. 1 in the same length unit, is less than 0.02, the deformation (bending) is within an allowable range. Assuming that “M / L” is equal to or greater than 0.02, “curvature occurrence” is assumed, and a curb occurrence rate of 0 (zero) is targeted. In addition, the cooling rate in 800-500 degreeC at the time of mist cooling was 4-10 degrees C / sec, and the cooling rate from 950 to room temperature (25 degreeC) was 2-5 degrees C / sec.

  Next, for each steel, one seamless steel pipe was randomly selected from each of the 20 mist-quenched seamless steel pipes, held at the temperature shown in Table 2 for 1 hour, and then cooled to room temperature (25 ° C.). . Then, two round bar tensile test pieces each having a parallel part diameter of 6 mm were cut out in the longitudinal direction from the three positions in the longitudinal direction of both ends and the center of the seamless steel pipe after tempering. Tensile test is performed to measure the yield strength (YS) and tensile strength (hereinafter referred to as “TS”), and the arithmetic average value and variation (maximum value-minimum value) of six specimens for each steel. Asked. The average value of YS was targeted to be 1103 MPa or more.

  Among the test results, Table 1 shows the Ms point together, and Table 2 shows the other test results excluding the Ms point. Fig. 3 summarizes the relationship between the rate of occurrence of bending by mist quenching 20 steel pipes each having an outer diameter of 245 mm, a wall thickness of 13.8 mm and a length of 12 m for each steel, and the transformation latent heat obtained from the DSC measurement results. Show. FIG. 4 shows the relationship between the transformation latent heat obtained from the DSC measurement results and the Ni content for steels 2 to 5 and steel 26 having a C content of 0.48 to 0.50%. . Further, FIG. 5 shows the relationship between the transformation latent heat obtained from the DSC measurement results for Steels 1 to 26 and “25 × C + Ni (= Fn1)”.

  As is apparent from Table 2 and FIG. 3, in the case of test numbers 1 to 21 of the present invention examples using steels 1 to 21 in which the chemical composition of the steel satisfies the conditions specified in the present invention, the bending according to the above-described criteria is performed. Has not occurred. Furthermore, the average value of YS of the test numbers of the above-described inventive examples far exceeds 1103 MPa.

  On the other hand, in the case of the test numbers 22 to 26 of the comparative examples using 22 to 26 whose chemical composition deviated from the conditions specified in the present invention, the bending based on the above-described criteria occurred. Moreover, the variation between YS and TS is also quite large.

  From FIG. 4 where the C content is the same level, it is clear that even if the Ni content is very small, the transformation latent heat is greatly reduced, and the transformation latent heat is decreased as the Ni content is increased.

Furthermore, from FIG. 5, the transformation latent heat is
(Fn1 =) 25 × C + Ni
It is clear that the transformation latent heat is 68 J / g or less if the value of this formula is 12.4 or more.

According to the present invention, steel having a high C content, suitable for oil well pipes such as casings for deep wells and tubing that require high YS, is used as a raw material, and even if mist quenching is performed to suppress sinter cracking. It is possible to obtain a high-strength seamless oil-free pipe that is not easily deformed and has excellent productivity. The above-described high-strength seamless oil country tubular goods can be stably obtained by the production method of the present invention.

Claims (4)

% By mass
C: 0.40 to 1.00%,
Si: 0.05-1.00%,
Mn: 0.05 to 1.00%,
P: 0.050% or less,
S: 0.010% or less,
Al: 0.01 to 0.10%,
N: 0.010% or less,
Cr: 0.10 to 2.50%,
Mo: 0.30 to 3.00,
Ni: 0.02 to 4.00%,
Cu: 0 to 0.50%,
Ti: 0 to 0.030%,
Nb: 0 to 0.150%,
V: 0 to 0.500%,
B: 0 to 0.0030%,
Ca: 0 to 0.0040%,
Mg: 0 to 0.0040%,
Zr: 0 to 0.0040%,
Balance: Fe and impurities,
Fn1 represented by the following formula [1] is 12.4 or more,
Having a chemical composition,
High strength seamless oil well pipe.
Fn1 = 25 × C + Ni ... [1]
However, C and Ni in the formula [1] mean the content (mass%) of each element in steel. The chemical composition is mass%,
Cu: 0.02 to 0.50%,
Ti: 0.003-0.030%,
Nb: 0.003 to 0.150%,
V: 0.005-0.500%,
B: 0.0003 to 0.0030%,
Ca: 0.0003 to 0.0040%,
Mg: 0.0003 to 0.0040%, and
Zr: 0.0003 to 0.0040%,
The high-strength seamless oil country tubular good of Claim 1 containing 1 or more types selected from these. Yield strength is 1103 MPa or more,
The high-strength seamless oil well pipe according to claim 1 or 2. The following steps [i] to [iii] are sequentially processed on the seamless steel pipe finished by Mannesmann pipe having the chemical composition according to claim 1 or 2: The manufacturing method of the high intensity | strength seamless oil well pipe described in 1.
[i]: Heating step of heating to 850 to 1200 ° C. and holding for 10 minutes to 3 hours
[ii]: A quenching process in which the cooling rate at 800 to 500 ° C. is 2 to 60 ° C./second and mist cooling is performed at a cooling rate from the start of cooling to room temperature of 1 to 30 ° C./second.
[iii]: Tempering step of heating to 100 to 750 ° C. and holding for 10 minutes to 3 hours, and then cooling to room temperature

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