JP6680142B2 - High-strength seamless oil country tubular good and method for manufacturing the same - Google Patents

Description

  The present invention relates to a high-strength seamless oil country tubular good and a method for manufacturing the same. More specifically, the present invention uses steel with a high C content as a raw material, and in order to prevent cracks during quenching (hereinafter referred to as "quenching cracks"), it is deformed even if mist quenching is performed by mixing air and water. TECHNICAL FIELD The present invention relates to a high-strength seamless oil country tubular good that is less likely to cause rusting and has excellent productivity, and a method for manufacturing the same.

  In recent years, for example, in the field of energy, oil wells and gas wells (hereinafter, oil wells and gas wells are collectively referred to simply as “oil wells”) are becoming deeper. For this reason, the "seamless oil well pipe" such as casing and tubing used in such oil wells is required to have high strength.

As a technique related to a high-strength seamless oil country tubular good, for example, Patent Documents 1 to 3 have a yield strength (hereinafter sometimes referred to as “YS”) of 862 MPa (125 ksi) or more “high-strength seamless steel pipe for oil well” And its manufacturing method ", and Patent Documents 4 and 5 disclose" High strength seamless steel pipe for oil well excellent in sulfide stress cracking resistance and YS of 758 MPa (110 ksi) or more and manufacturing method thereof ". Has been done. Further, as a high-strength low-alloy steel for oil wells, for example, in Patent Documents 6 and 7, YS of 618 to 853 MPa (63 to 87 kgf / mm ² ) is "excellent in low temperature toughness and sulfide stress cracking resistance. "High-strength low alloy oil well 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 relating to cooling of various steel materials, Patent Document 8 discloses “a steel material cooling control method”, and Patent Document 9 discloses “a steel plate temperature prediction method in consideration of transformation heat value”. There is.

Patent No. 5943165 Patent No. 5943164 Japanese Patent No. 5930140 JP, 2013-129879, A International Publication No. 2010/150915 JP, 7-011384, A JP-A-4-066645 JP-A-1-162508 JP, 2011-212743, A

  Since the structures of the high-strength seamless oil well pipes and high-strength low-alloy steels for oil wells disclosed in Patent Documents 1 to 7 are mainly composed of tempered martensite, quenching-tempering treatment (so-called "tempering" Processing ”) is performed. Although various cooling media are used for quenching depending on the chemical composition of the steel, when quenching by water cooling is performed when the C content of the steel is 0.40 mass% or more, particularly 0.50 mass% or more. May cause quench cracking, resulting in a significant decrease in productivity. On the other hand, when steel with a high C content of 0.40 mass% or more is used, if the cooling rate during quenching is slowed, even though quenching cracks can be prevented, the deformation (curvature) schematically shown in FIG. ) May occur, resulting in a decrease in productivity. However, none of the above Patent Documents 1 to 7 discusses "bending" when quenching steel having a high C content at a low cooling rate to prevent quench cracking. In addition, although each of the above patent documents says “high strength”, the specific value of YS is at most 986 MPa in the case of the “present invention example” of each patent document (steel pipe No. 7 in Table 3 of Patent document 1). In addition, even in the case of the “comparative example”, it is no more than 1098 MPa (see steel pipe No. 16 in Table 3 of Patent Document 2).

  In Patent Documents 8 and 9 as well, there is no study on "bending" when quenching a seamless steel pipe having a high C content at a slow cooling rate to prevent quench cracking.

  In order to stably secure a high strength of 1103 MPa (160 ksi) or more in YS, the present invention uses steel having a C content increased to 0.40 mass% or more as a material, and mist quenching from the viewpoint of quenching crack suppression. It is an object of the present invention to provide a high-strength seamless oil country tubular good which is hardly deformed even if it is excellent in productivity and a manufacturing method thereof.

  The present invention has been completed to solve the above problems, and the gist thereof is a high-strength seamless oil country tubular good and a method for producing the same.

(1) In mass%,
C: 0.40 to 1.00%,
Si: 0.05 to 1.00%,
Mn: 0.05-1.00%,
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%,
The 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 contents (mass%) of the respective elements in the steel.

(2) The chemical composition is represented by mass%
Cu: 0.02 to 0.50%,
Ti: 0.003 to 0.030%,
Nb: 0.003 to 0.150%,
V: 0.005 to 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 as described in (1) above, which contains at least one selected from

(3) The yield strength is 1103 MPa or more,
The high-strength seamless oil country tubular good according to (1) or (2) above.

(4) A seamless steel pipe manufactured by Mannesmann pipe having the chemical composition as described in (1) or (2) above is subjected to the following steps [i] to [iii] in order. ) To (3), the method for producing a high-strength seamless oil country tubular good.
[i]: Heating step of heating to 850 to 1200 ° C. and holding for 10 minutes to 3 hours
[ii]: A quenching step in which the cooling rate at 800 to 500 ° C. is 2 to 60 ° C./second, and the mist is cooled 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., holding for 10 minutes to 3 hours, and then cooling to room temperature

  According to the present invention, a steel having a high C content, which is suitable as a casing for deep wells requiring a high YS, an oil country tubular good such as tubing, is used as a raw material, and is subjected to mist quenching to prevent quench cracking. It is possible to obtain a high-strength seamless oil country tubular good that is resistant to deformation and has excellent productivity. The high-strength seamless oil country tubular good can be stably obtained by the production method of the present invention.

It is a figure which shows typically the deformation | transformation (bending) generation | occurrence | production state after mist hardening of a seamless steel pipe. “L” indicates the length (total length) of the seamless steel pipe, and “M” indicates the amount of deformation. A method of 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 occurrence rate of bending and the transformation latent heat obtained from the DSC measurement results in the 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. In addition, when the ratio "M / L" of M and L shown in FIG. 1 in the same length unit is 0.02 or more, "bending occurs". Regarding the steels 2 to 5 and the steel 26 in the examples in which the C content is 0.48 to 0.50 mass%, the relationship between the transformation latent heat obtained from the DSC measurement results and the Ni content is summarized. FIG. It is a figure which rearranges and shows the relation between the transformation latent heat calculated | required from the DSC measurement result, and "25xC + Ni (= Fn1)" about the 26 steel types used in the Example.

  The present inventors have made various studies on the chemical composition of low-alloy steel, which is suitable as a raw material steel for seamless oil well pipes such as deep well casings and tubing that require high YS, and , And the manufacturing method of seamless oil country tubular goods using the low alloy steel was also examined.

  As a result, first, it was concluded that the content of C should be 0.40 mass% or more in order to obtain high strength (high YS) in a low-alloy type relatively inexpensive steel. On the other hand, in the case of a high C low alloy steel having a C content of 0.40 mass% or more, if water quenching is performed to increase the strength, it is extremely likely that quench cracking will occur.

  Therefore, various slow cooling methods have been studied from the viewpoint of preventing quenching cracks. When slow cooling, especially mist quenching, the thickness direction and length of seamless steel pipes (hereinafter sometimes referred to as "steel materials") are investigated. It became clear that variations in various properties such as mechanical properties may occur in the vertical direction. Moreover, it was confirmed that the steel material may be deformed and the dimensional accuracy may be degraded.

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

  The present inventors conducted a more detailed investigation on the above-mentioned temperature variation during mist quenching. As a result, a new finding was obtained that the main cause of the temperature variation during mist quenching is the heat generation by the martensitic transformation (in other words, transformation latent heat). That is, when a part of the steel material undergoes martensitic transformation, the vicinity thereof is heated by the heat generated by the transformation, so that temperature variation occurs inside the steel material. Therefore, the smaller the transformation latent heat, the smaller the temperature variation inside the steel material, and the smaller the variation and deformation of various characteristics such as mechanical characteristics.

  Therefore, the present inventors next examined the influence of alloying elements on the latent heat of transformation by changing the chemical composition of the steel in order to reduce the temperature variation inside the steel during mist quenching. As a result, it was clarified that the increase of the content of C in the steel is extremely effective in reducing the transformation latent heat, and that the addition of Ni in the steel is also effective in reducing the transformation latent heat.

Further, based on the above findings, the present inventors have examined transformation latent heat and bending of a seamless steel pipe during mist quenching. As a result, it was clarified that if the transformation latent heat is 68 J / g or less, the deformation of the seamless steel pipe during mist quenching is significantly reduced. It was also found that the transformation latent heat becomes 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 contents (mass%) of the respective elements in the steel.

  Furthermore, in order to suppress the bending of the seamless steel pipe during mist quenching and to provide the seamless steel pipe with a stable and desired high YS, in particular, a cooling rate between 800 and 500 ° C. during mist quenching, It was also clarified that the cooling rate from the start of cooling to room temperature has a great influence.

  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 good according to the present invention are as follows. In the following description, “%” regarding 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 secured by mist quenching. On the other hand, if C is contained in excess of 1.00%, quench cracking may not be prevented even by mist quenching. Therefore, the C content is 0.40 to 1.00%. A preferable lower limit of the C content is 0.48%, and it is more preferable that the C content be more than 0.50%. The preferable upper limit of the C content is 0.80%, and the more preferable upper limit is 0.62%.

Si: 0.05-1.00%
Si is an element effective for deoxidizing steel. To obtain the deoxidizing effect, the Si content needs to be 0.05% or more. On the other hand, if the Si content exceeds 1.00%, the hot workability is reduced. Therefore, the Si content is set to 0.05 to 1.00%. A preferable lower limit of the Si content is 0.15%, and a more preferable lower limit thereof is 0.20%. The preferable upper limit of the Si content is 0.55%, and the more preferable upper limit is 0.50%.

Mn: 0.05-1.00%
Mn is an element effective in improving the hardenability of steel. To obtain this effect, it is necessary to contain at least 0.05% Mn. On the other hand, if the Mn content exceeds 1.00%, the hot workability is deteriorated. Therefore, the Mn content is set to 0.05 to 1.00%. The preferable lower limit of the Mn content is 0.25%, and the more preferable lower limit thereof is 0.30%. The preferable upper limit of the Mn content is 0.80%, and the more preferable upper limit is 0.70%.

P: 0.050% or less P segregates at the crystal grain boundaries as an impurity and causes grain boundary destruction. Therefore, the P content needs to be limited to 0.050% or less. The preferable upper limit of the P content is 0.020%.

S: 0.010% or less S, like P, segregates as an impurity at the grain boundaries and causes grain boundary destruction. Therefore, the S content needs to be 0.010% or less. The preferable upper limit of the S content is 0.005%.

Al: 0.01 to 0.10%
Al is an element effective for deoxidizing steel. The effect cannot be obtained when the Al content is less than 0.01%. On the other hand, when Al is contained in excess of 0.10%, the inclusions are coarsened and the mechanical properties are degraded. Therefore, the content of Al is set to 0.01 to 0.10%. The preferable lower limit of the Al content is 0.015%, and the more preferable lower limit is 0.020%. The preferable upper limit of the Al content is 0.060%, and the more preferable upper limit is 0.055%. The Al content in the present invention refers to the content in acid-soluble Al (“Sol. Al”).

N: 0.010% or less N is present in steel as an impurity, and if the content exceeds 0.010%, coarse nitrides are formed and mechanical properties such as toughness are deteriorated. Therefore, the N content is 0.010% or less. The preferable upper limit of the N content is 0.005%.

Cr: 0.10 to 2.50%
Cr is an element effective in improving the hardenability of steel. To obtain this effect, it is necessary to contain at least 0.10% Cr. On the other hand, when the Cr content exceeds 2.50%, the carbides are coarsened and the mechanical properties are deteriorated. Therefore, the content of Cr is set to 0.10 to 2.50%. The preferable lower limit of the Cr content is 0.25%, and the more preferable lower limit thereof is 0.30%. The preferable upper limit of the Cr content is 1.50%, and the more preferable upper limit is 1.30%.

Mo: 0.30 to 3.00%
Mo is an element effective in improving the hardenability of steel. 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 is increased. Therefore, the Mo content is set to 0.30 to 3.00%. A preferable lower limit of the Mo content is 0.50%, and a more preferable lower limit thereof is 0.55%. The preferable upper limit of the Mo content is 2.00%, and the more preferable upper limit is 1.70%.

Ni: 0.02 to 4.00%
Ni is an element effective in reducing the latent heat of transformation. 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 is increased. Therefore, 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 thereof is 0.06%. The preferable upper limit of the Ni content is 3.00%, and the more preferable upper limit is 2.90%.

Cu: 0 to 0.50%
Cu is an element that improves the hardenability of steel. Therefore, Cu may be contained if necessary. However, if the Cu content exceeds 0.50%, the material cost is increased. Therefore, the upper limit of the Cu content when contained is set to 0.50%. The upper limit of the Cu content is preferably 0.35%, more preferably 0.30%. In order to obtain the above effect stably, the lower limit of the Cu content is preferably 0.02%, and more preferably 0.05%.

Ti: 0 to 0.030%
Ti has the function of refining the crystal grains and improving the mechanical properties. Therefore, Ti may be contained if necessary. However, when the content of Ti exceeds 0.030%, the nitride becomes coarse and the mechanical properties are rather deteriorated. Therefore, the upper limit of the Ti content when contained is set to 0.030%. The upper limit of the Ti content is preferably 0.015%, more preferably 0.012%. In order to stably obtain the above effects, the lower limit of the Ti content is preferably 0.003%, more preferably 0.005%.

Nb: 0 to 0.150%
Nb has a function of refining crystal grains to improve mechanical properties. Therefore, Nb may be contained if necessary. However, when the content of Nb exceeds 0.150%, the nitride is coarsened and the mechanical properties are rather deteriorated. Therefore, the upper limit of the Nb content when contained is set to 0.150%. The upper limit of the Nb content is preferably 0.050%, more preferably 0.035%. In order to stably obtain the above effects, the lower limit of the Nb content is preferably 0.003%, more preferably 0.007%.

  When the above Ti and Nb are combined and contained, the total amount is preferably 0.150% or less, and more preferably 0.050% or less.

V: 0 to 0.500%
V has the effect of increasing the strength of steel. Therefore, V may be contained if necessary. 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 set to 0.500%. The upper limit of the V content is preferably 0.150%, more preferably 0.110%. In order to obtain the above effect stably, the lower limit of the V content is preferably 0.005%, and more preferably 0.015%.

B: 0 to 0.0030%
B has an effect of enhancing the hardenability of steel when it is contained in a small amount. Therefore, B may be contained if necessary. However, even if B is contained in an amount of more than 0.0030%, the above effect is saturated and the nitride (BN) is coarsened. Therefore, when B is contained, the upper limit of the B content is set to 0.0030%. The upper limit of the B content is preferably 0.0015%, more preferably 0.0012%. In order to stably obtain the above effects, the lower limit of the B content is preferably 0.0003%, more preferably 0.0007%.

Ca: 0 to 0.0040%
Ca has the effect of combining with S in steel to refine sulfides and improving mechanical properties. Therefore, Ca may be contained if necessary. However, when the content of Ca exceeds 0.0040%, the oxide is coarsened. Therefore, when Ca is contained, the upper limit of the Ca content is set to 0.0040%. The upper limit of the Ca content is preferably 0.0025%, more preferably 0.0020%. In order to stably obtain the above effects, the lower limit of the Ca content is preferably 0.0003%, more preferably 0.0006%.

Mg: 0 to 0.0040%
Mg has the effect of combining with S in steel to refine sulfides and improving mechanical properties. Therefore, Mg may be contained if necessary. However, when the content of Mg exceeds 0.0040%, the oxide is coarsened. Therefore, the upper limit of the Mg content when contained is set to 0.0040%. The upper limit of the Mg content is preferably 0.0025%, more preferably 0.0020%. In order to obtain the above effect stably, the lower limit of the Mg content is preferably 0.0003%, more preferably 0.0006%.

Zr: 0 to 0.0040%
Zr also has the effect of combining with S in steel to refine sulfides and improve mechanical properties. Therefore, Zr may be contained if necessary. However, if the Zr content exceeds 0.0040%, the oxide is coarsened. Therefore, the upper limit of the Zr content when contained is set to 0.0040%. The upper limit of the Zr content is preferably 0.0025%, more preferably 0.0020%. In order to stably obtain the above effects, the lower limit of the Zr content is preferably 0.0003%, more preferably 0.0006%.

  When two or more kinds selected from the above-mentioned Ca, Mg and Zr are compounded and contained, the total amount 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 the above elements, the balance are Fe and impurities, and Fn1 represented by the above formula [1] is 12.4 or more.

  Here, the "impurities" are components that are mixed due to various factors such as ores, raw materials such as scraps, and manufacturing processes when industrially manufacturing steel materials, and are allowed within a range that does not adversely affect the present invention. Means what is done.

Fn1: 12.4 or more The high-strength seamless oil country tubular good according to the present invention has Fn1 represented by the following formula [1] of 12.4 or more.
Fn1 = 25 × C + Ni ... [1]
However, C and Ni in the formula [1] mean the contents (mass%) of the respective elements in the steel.

  Fn1 is an index of transformation latent heat. When the value of Fn1 is 12.4 or more, the transformation latent heat 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 during mist quenching (refers to the ratio of M and 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, and 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 an oil well pipe such as a casing or tubing even if the deep well is advanced. The upper limit of YS is about 1240 MPa from the viewpoint of ensuring sufficient delayed fracture resistance.

3. Production method:
The high-strength seamless oil country tubular good of the present invention can be produced, for example, by the following method.

  A low alloy steel having the chemical composition described above is melted, and then cast into an ingot or a slab, and the cast ingot or slab is slab-rolled into a round billet shape, and then Mannesmann Pipes are made to make seamless steel pipes. When a cast slab having a circular billet shape for a Mannesmann pipe is produced by a so-called "round CC" method, the cast slab having the circular billet shape may be directly produced by a Mannesmann pipe to finish a seamless steel pipe.

  The high-strength seamless oil country tubular good of the present invention is manufactured by sequentially performing the following steps [i] to [iii] on the seamless steel pipe manufactured by the Mannesmann pipe and finished.

[i]: Heating step of heating to 850 to 1200 ° C. and holding for 10 minutes to 3 hours The seamless steel pipe finished in a predetermined shape by Mannesmann pipe heating is heated to 850 to 1200 ° C. for 10 minutes to 3 hours. Hold and austenitized. If the heating and holding temperature is lower than 850 ° C, quenching may be insufficient in the mist treatment, and desired high strength may not be obtained. On the other hand, if the heating and holding temperature exceeds 1200 ° C., the austenite crystal grains may become coarse and the mechanical properties may deteriorate. Even when heating at 850 to 1200 ° C., if the holding time is less than 10 minutes, quenching may be insufficient and desired high strength may not be obtained, and the holding time may be 3 If the time is exceeded, the austenite crystal grains may become coarse and the mechanical properties may deteriorate. Therefore, the seamless steel pipe is heated to 850 to 1200 ° C. and held for 10 minutes to 3 hours. The lower limit of the heating temperature is preferably 900 ° C. The upper limit of the heating temperature is preferably 1050 ° C, 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 mist is cooled at a cooling rate from the start of cooling to room temperature of 1 to 30 ° C./second. The seamless steel pipe is mist-cooled at a cooling rate of 2 to 60 ° C / sec at 800 to 500 ° C and a cooling rate of 1 to 30 ° C / sec from the start of cooling to room temperature. If the cooling rate at 800 to 500 ° C. during mist cooling is less than 2 ° C./sec, quenching may be insufficient and desired high strength may not be obtained. On the other hand, if the cooling rate at 800 to 500 ° C. exceeds 60 ° C./second, the occurrence of quench cracking may be unavoidable. Even if the cooling rate at 800 to 500 ° C. during mist cooling is 2 to 60 ° C./second, quenching becomes insufficient if the cooling rate from the start of cooling to room temperature is less than 1 ° C./second. Therefore, the desired high strength may not be obtained, and if the cooling rate from the start of cooling to room temperature exceeds 30 ° C./sec, the occurrence of quench cracking may be unavoidable. Therefore, the seamless steel pipe after the heating step is subjected to mist cooling 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 / sec, and the upper limit is preferably 40 ° C / sec. Furthermore, the lower limit of the cooling rate from the start of cooling to room temperature is preferably 2 ° C./sec, and the upper limit is preferably 20 ° C./sec. The temperature in the step [ii] refers to the temperature on the outer surface of the seamless steel pipe, and the cooling rate also refers to the cooling rate on the outer surface of the seamless steel pipe. The cooling rates of the above two types during mist cooling are
"Cooling rate at 800 to 500 ° C" = 300 ° C / time required for cooling (seconds)
"Cooling rate from the start of cooling to room temperature" = (cooling start temperature (° C) -room temperature (25 ° C)) / time required for cooling (seconds)
Refers to.

[iii]: Tempering step of heating to 100 to 750 ° C. and holding for 10 minutes to 3 hours, and then cooling to room temperature. The seamless steel pipe that was mist cooled to room temperature and quenched was then brought to a desired YS level. For adjustment, a tempering treatment is performed in which the temperature is heated to 100 to 750 ° C. and kept for 10 minutes to 3 hours, and then cooled to room temperature. If the heating and holding temperature is lower than 100 ° C., the desired high strength (in particular, high YS) may not be obtained, or the mechanical properties such as poor toughness may be deteriorated, while if it exceeds 750 ° C. The desired high strength may not be obtained. Even when heated at 100 to 750 ° C., mechanical properties such as toughness may deteriorate when the holding time is less than 10 minutes, and when the holding time exceeds 3 hours. However, the desired high strength may not be obtained. Therefore, the mist-quenched seamless steel pipe was heated to 100 to 750 ° C., 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. The cooling rate when cooling to room temperature in this tempering process is not particularly limited.

  Hereinafter, the present invention will be described more specifically with reference to Examples, but the present invention is not limited to these Examples.

  After low melting alloy steels 1 to 26 having the chemical composition shown in Table 1 are melted by a usual method, they are slab-rolled into a round billet shape, then manufactured by Mannesmann pipe, and have an outer diameter of 245 mm and a wall thickness of 13. A seamless steel pipe having a length of 8 mm and a length of 12 m was finished.

  Steels 1 to 21 in Table 1 are steels having a chemical composition within the range specified by the present invention, while steels 22 to 26 and steels having chemical compositions outside the conditions specified by the present invention.

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

Specifically, using a DSC, the above disc test piece was heated from room temperature to 950 ° C. at 20 ° C./min and held at 950 ° C. for 10 minutes, and then cooled at 60 ° C./min (1 ° C./sec. ) To room temperature (25 ° C.). The rising temperature of the exothermic peak due to martensitic transformation under the cooling conditions measured by DSC was determined as the Ms point. Further, 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 exothermic peak and the range surrounded by the baseline, and divide by the cooling rate of 60 ° C / min.
Sought by. When DSC is used, the temperature rising rate (20 ° C / min) and the cooling rate (60 ° C / min) are constant in any temperature range.

  For each steel, 20 seamless steel pipes each having the above-mentioned outer diameter of 245 mm, wall thickness of 13.8 mm and length of 12 m were used, heated to 950 ° C. and held for 1 hour, and then rotated at room temperature ( After mist cooling to 25 ° C. and quenching, deformation of the seamless steel pipe was investigated. At this time, if “M / L”, which is the ratio of M (deformation amount) and L (overall length) shown in FIG. 1 in the same length unit, is less than 0.02, the deformation (bending) is within the allowable range. Assuming that there is any, the case where the above "M / L" is 0.02 or more is defined as "bending", and a bending occurrence rate of 0 (zero) is set as a target. The cooling rate at 800 to 500 ° C during mist cooling was 4 to 10 ° C / sec, and the cooling rate from 950 to room temperature (25 ° C) was 2 to 5 ° C / sec.

  Next, for each steel, one seamless steel pipe was randomly selected from the 20 mist-quenched seamless steel pipes, held at the temperature shown in Table 2 for 1 hour, and then air-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 two positions in the longitudinal direction at both ends and the center of the seamless steel pipe after the tempering, respectively, and in an atmosphere at room temperature. Yield strength (YS) and tensile strength (hereinafter referred to as "TS") are measured by a tensile test with, and arithmetic mean value and variation (maximum value-minimum value) of 6 test pieces for each steel are measured. I asked. The average value of YS was set to 1103 MPa or more.

  Among the test results, Table 1 also shows the Ms points, and Table 2 also shows the other test results except the above Ms points. FIG. 3 summarizes the relationship between the incidence of bending when 20 steel pipes each having an outer diameter of 245 mm, a wall thickness of 13.8 mm, and a length of 12 m are mist-quenched, and the transformation latent heat obtained from the DSC measurement results. Show. Further, 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%. . Furthermore, 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)” in a organized manner.

  As is clear from Table 2 and FIG. 3, in the case of the test numbers 1 to 21 of the examples of the present invention using the steels 1 to 21 having the chemical composition of steel satisfying the conditions specified in the present invention, the bending based on the above-mentioned criteria. Has not occurred. Further, the average value of YS in the test number of the above-mentioned inventive example is far more than 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 compositions deviated from the conditions defined by the present invention, bending occurred on the basis of the above-mentioned criteria. Moreover, the variation between YS and TS is quite large.

  It is clear from FIG. 4 that the C content is at the same level, even if the Ni content is very small, the transformation latent heat significantly decreases, and the transformation latent heat decreases as the Ni content increases.

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

According to the present invention, a steel having a high C content, which is suitable as a casing for deep wells requiring high YS, oil well pipes such as tubing, is used as a raw material, and mist quenching is performed to prevent quench cracking. It is possible to obtain a high-strength seamless oil country tubular good that is resistant to deformation and has excellent productivity. The high-strength seamless oil country tubular good can be stably obtained by the production method of the present invention.

Claims (4)

In mass%,
C: 0.40 to 1.00%,
Si: 0.05 to 1.00%,
Mn: 0.05-1.00%,
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: 0.06 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%,
The 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 contents (mass%) of the respective elements in the steel. The chemical composition is% by mass,
Cu: 0.02 to 0.50%,
Ti: 0.003 to 0.030%,
Nb: 0.003 to 0.150%,
V: 0.005 to 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 according to claim 1, containing at least one selected from the group consisting of: The yield strength is 1103 MPa or more,
The high-strength seamless oil country tubular good according to claim 1 or 2. A seamless steel pipe manufactured by Mannesmann pipe having the chemical composition according to claim 1 or 2 and subjected to the following steps [i] to [iii] in order. The method for producing a high-strength seamless oil country tubular goods according to.
[i]: Heating step of heating to 850 to 1200 ° C. and holding for 10 minutes to 3 hours
[ii]: A quenching step in which the cooling rate at 800 to 500 ° C. is 2 to 60 ° C./second, and the mist is cooled 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., holding for 10 minutes to 3 hours, and then cooling to room temperature

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