JP6519025B2 - Low alloy high strength seamless steel pipe for oil well - Google Patents

Description

  The present invention relates to a low alloy high strength seamless steel pipe suitable for oil wells, particularly for oil wells, which is excellent in low temperature toughness.

  In recent years, severe corrosion in so-called sour environments, including deep oil fields and hydrogen sulfide, which were not previously considered, in view of soaring crude oil prices and the near-future depletion of petroleum resources Development in environmental oil fields and gas fields has become popular. A steel pipe for oil well used under such an environment is required to have both high strength and resistance under sour environment, that is, resistance to sulfide stress cracking (SSC resistance). In addition, with regard to the excavated area, examinations are widely conducted to extremely cold areas where transportation of steel pipes is difficult. Heretofore, in such a region, there is a risk that the steel pipe is exposed to a low temperature during transportation and is broken by an impact generated during transportation, and there is a need for a steel pipe having excellent low temperature toughness.

  For such a demand, for example, in Patent Document 1, C: 0.05 to 0.35%, Si: 0.01 to 0.5%, Mn: 0.75 to 2.5 in weight%. %, S: 0.01% or less, P: 0.02% or less, Al: 0.005 to 0.1%, Ti: 0.005 to 0.020%, Nb: 0.005 to 0.1% , B: 0.0003 to 0.003%, N: 0.0030 to 0.0050%, and ΔTi (%) −-0.005% (however, ΔTi (%) = Ti content (%) There is disclosed a method for producing a low temperature seamless steel pipe having improved low temperature toughness which prescribes quenching and tempering after hot forming and rolling that is a steel piece having -3.4N content (%)).

  In Patent Document 2, C: 0.15 to 0.50%, Si: 0.8% or less, Mn: 0.3 to 1.0%, P: 0.012% or less, S: in mass%. 0.0020% or less, Al: 0.01 to 0.10%, N: 0.01% or less, Cr: 0.1 to 1.7%, Mo: 0.4 to 1.2%, V: 0 .01 to 0.10%, Nb: 0.01 to 0.08%, Ti: 0.005 to 0.03%, B: 0.0005 to 0.0030%, with the balance being Fe and unavoidable impurities The steel material having the following composition is heated, piped by a hot working process to form a seamless steel pipe, and then subjected to a quenching treatment and a tempering treatment to the seamless steel pipe. Disclosed is a method of producing a low alloy high strength seamless steel pipe for oil well with improved properties.

JP-A-9-20961 JP, 2015-183197, A

  However, the technology described in Patent Document 1 improves only the low temperature toughness, and it is unclear about the improvement of the high strength and the SSC resistance. The technique described in Patent Document 2 has a problem that the improvement in low temperature toughness is still insufficient. Therefore, in patent documents 1 and 2, it has not come to make high strength and low temperature toughness compatible. In recent years, the properties required for oil well seamless steel pipes have become severe from the viewpoint of environmental protection, and not only SSC resistance but also properties other than SSC resistance are regarded as important. There are methods of adding various alloys in order to improve SSC resistance, but there is a problem that adverse effects may occur on characteristics other than SSC resistance as a side effect of the added elements. Therefore, even if the SSC resistance is improved, the addition of an alloy whose other properties are deteriorated should be avoided as much as possible.

  Conventionally, addition of Mo is considered effective for improving SSC resistance. However, since Mo is an expensive alloy, using Mo having high purity leads to an increase in cost, and there is a problem of reducing competitiveness in terms of manufacturing cost. Therefore, in recent years, the use of inexpensive alloy materials is increasing from the viewpoint of reducing the manufacturing cost of steel pipes. However, since cheap alloy materials contain impurities, it is difficult to improve SSC resistance.

  In view of the above problems, the present invention is an oil well low temperature oil excellent in low temperature toughness, particularly for improving impact absorption energy under conditions of -20 ° C or less while maintaining excellent SSC resistance suitable for oil wells An object of the present invention is to provide an alloy high strength seamless steel pipe.

  As a result of conducting earnest research to achieve the above-mentioned problems, the present inventors obtained the following findings.

  From the viewpoint of reducing the manufacturing cost of the steel pipe, on the premise of using the low cost alloy material containing impurities, the influence was investigated thoroughly. As a result, it was found that the low cost alloy material has an adverse effect on the SSC resistance because it is inherently rich in impurities. However, by defining the component composition centering on the minor additive element, it is possible to improve the toughness without adversely affecting the SSC resistance. In particular, it was newly found that this can be achieved by adding 0.01 to 0.06% by mass of Co.

The present invention has been made based on the above-described findings, and the gist of the present invention is as follows.
[1] Component composition is, in mass%, C: 0.15 to 0.50%, Si: 0.80% or less, Mn: 0.3 to 1.0%, P: 0.012% or less, S : 0.0050% or less, Al: 0.01 to 0.10%, N: 0.01% or less, Cr: 0.1 to 1.7%, Mo: 0.2 to 2.0%, V: 0.10% or less, Nb: 0.01 to 0.08%, Ti: 0.04% or less, B: 0.0005 to 0.0030%, Cu: 0.03 to 0.07%, Co: 0 A low alloy high strength seamless steel pipe for oil well containing 01 to 0.06%, Ca: 0.0001 to 0.0050, and the balance being iron and unavoidable impurities.

In the present invention, high strength refers to one having a strength of yield strength 650 MPa or more (95 ksi or more). Moreover, the sulfide stress corrosion cracking resistance (SSC resistance) excellent in the present invention is a test based on NACE TM0177 Method A, and is a 0.5% acetic acid + 5.0% saline solution saturated with H ₂ S. A constant load test is conducted by using (liquid temperature: 24 ° C.) and applying an applied stress of 90% of the yield strength for 720 hours, which means that no cracking occurs. It is desirable to carry out three or more tests in the same test sample, in consideration of variations. Furthermore, in the present invention, low temperature toughness means impact absorption energy under conditions of -20 ° C or less. The steels of the present invention were evaluated on the characteristics of the steel of the present invention with a Co content of 0% by mass as the comparative steel and the impact absorption energy of the comparative steel was 1.00. It means that the ratio of impact absorption energy of is 1.00 or more.

  According to the present invention, while having high strength with a yield strength of 650 MPa or more, while maintaining excellent sulfide stress corrosion cracking resistance (SSC resistance) under a hydrogen sulfide gas saturated environment (sour environment), the temperature is further lowered. In particular, an oil well low alloy high strength seamless steel pipe excellent in toughness under the condition of -20 ° C or less can be obtained. In addition, since the steel pipe of the present invention can use a Mo alloy containing a small amount of Co, the use of an inexpensive Mo source can greatly contribute to cost reduction.

FIG. 1 is a diagram showing the relationship between the Co content (% by mass), Charpy impact test temperature (° C.) and the ratio of impact energy absorption of the invention steel to the comparative steel according to the present invention. The temperature is used, and (B) is the one in which the horizontal axis is the Co content.

  Hereinafter, the present invention will be described in detail.

  First, the composition of the seamless steel pipe of the present invention and the reason for limitation will be described. Unless otherwise indicated,% which represents the following component composition shall mean the mass%.

C: 0.15 to 0.50%
C has the effect of increasing the strength of the steel and is an important element to ensure the desired high strength. Further, C is an element improving the hardenability, and contributes to the formation of a structure having a tempered martensite phase as a main phase. In order to achieve high strength with a yield strength of 650 MPa or more, in order to obtain these effects, C needs to contain 0.15% or more. On the other hand, when the content of C exceeds 0.50%, a large amount of carbides acting as trap sites for hydrogen are precipitated at the time of tempering, and it becomes impossible to prevent excessive penetration of diffusible hydrogen into steel. Moreover, it becomes impossible to suppress the crack at the time of hardening. Therefore, C is set to 0.15 to 0.50%. Preferably, it is 0.20 to 0.30% or less.

Si: 0.80% or less Si acts as a deoxidizing agent, and forms a solid solution in the steel to increase the strength of the steel and has an action to suppress rapid softening during tempering. In order to acquire these effects, it is preferable to contain Si 0.05% or more. More preferably, it is 0.10% or more. On the other hand, when the content of Si exceeds 0.80%, coarse oxide inclusions are formed to act as strong hydrogen trap sites. In addition, the solid solution amount of Si that is effective for solid solution strengthening decreases. Therefore, the Si content is 0.80% or less. Preferably it is 0.40% or less.

Mn: 0.3 to 1.0%
Mn increases the strength of the steel through the improvement of the hardenability, and combines with S to fix S as MnS, and has the effect of preventing intergranular embrittlement due to S. In order to secure a high strength having a yield strength of 650 MPa or more, the Mn needs to be 0.3% or more. On the other hand, when the content of Mn exceeds 1.0%, cementite precipitated at grain boundaries is coarsened to reduce sulfide stress corrosion cracking resistance. Therefore, the Mn content is 0.3 to 1.0%. Preferably, it is 0.5 to 0.9% or less.

P: 0.012% or less P tends to segregate in grain boundaries etc. and cause intergranular embrittlement cracking etc. in the solid solution state, and it is desirable to reduce as much as possible in the present invention, but up to 0.012% acceptable. From such a thing, P is made into 0.012% or less. In addition, Preferably it is 0.010% or less.

S: 0.0050% or less S mostly exists as sulfide inclusions in steel, and lowers the ductility, toughness, and corrosion resistance such as sulfide stress corrosion cracking resistance. A part of S may be present in a solid solution state, but in that case, it tends to segregate at grain boundaries and the like and cause intergranular brittleness cracking and the like. For this reason, although it is desirable to reduce S as much as possible in the present invention, S is made 0.0050% or less at which the adverse effect is acceptable. However, excessive S reduction raises the refining cost. From such a thing, in the present invention, it is preferable to make S into 0.0004% a minimum.

Al: 0.01 to 0.10%
Al acts as a deoxidizer, and combines with N to form AlN, which contributes to the refinement of austenite grains. In order to obtain these effects, Al needs to be contained 0.01% or more. On the other hand, if the Al content exceeds 0.10%, oxide inclusions increase and the toughness decreases. For this reason, Al is made 0.01 to 0.10%. Preferably, it is 0.02 to 0.07%.

N: 0.01% or less N combines with a nitride-forming element such as Ti, Nb, or Al to form a MN-type precipitate. However, these precipitates become coarse precipitates and reduce sulfide stress corrosion cracking resistance. Therefore, it is preferable to reduce N as much as possible, but up to 0.01% is acceptable. Therefore, N is 0.01% or less. In addition, a small amount of MN-type precipitates has the effect of suppressing the coarsening of crystal grains when the steel material or the like is heated. Therefore, N preferably contains about 0.003% or more. More preferably, it is 0.003 to 0.005%.

Cr: 0.1 to 1.7%
Cr improves the hardenability and as a result contributes to the improvement of the strength of the steel and also improves the corrosion resistance. In addition, Cr combines with C during tempering to form carbides such as M ₃ C, M ₇ C ₃ and M ₂₃ C ₆ series, and in particular M ₃ C carbide improves temper softening resistance. To reduce the intensity change due to tempering and to facilitate the strength adjustment. In order to obtain such an effect, Cr needs to be contained 0.1% or more. On the other hand, when Cr is contained in excess of 1.7%, a large amount of M ₇ C ₃ carbides and M ₂₃ C ₆ carbides are formed. These carbides act as trap sites for hydrogen, thereby reducing the resistance to sulfide stress corrosion cracking. For this reason, Cr is made into 0.1 to 1.7%. Preferably, it is 0.5 to 1.5%. More preferably, it is 0.9 to 1.5%.

Mo: 0.2 to 2.0%
Mo forms carbides and contributes to an increase in strength by precipitation strengthening, and forms a solid solution and segregates in the prior austenite grain boundaries to further contribute to the improvement of the resistance to sulfide stress corrosion cracking. In addition, Mo densifies the corrosion product, and further suppresses the formation and growth of pits and the like that become the starting point of cracking. In order to obtain such an effect, Mo needs to be contained 0.2% or more. On the other hand, when the content of Mo exceeds 2.0%, needle-like M ₂ C type precipitates and Laves phase (Fe ₂ Mo) are formed to reduce sulfide stress corrosion cracking resistance. Therefore, Mo is set to 0.2 to 2.0%. Preferably, it is 0.6 to 1.5%.

V: 0.10% or less V forms carbides or nitrides and contributes to the improvement of the strength of the steel by precipitation strengthening. In order to acquire such an effect, it is preferable to contain 0.01% or more of V. On the other hand, even if V is contained in excess of 0.10%, the above-mentioned effect is saturated, and an effect commensurate with the content can not be expected. It is also economically disadvantageous. Therefore, V is 0.10% or less. Preferably, it is 0.02 to 0.08%.

Nb: 0.01 to 0.08%
Nb retards recrystallization in the austenite (γ) temperature region, contributes to the refinement of γ grains, and works extremely effectively for the refinement of martensite substructure (eg, packets, blocks, laths). Nb also forms carbides and has the effect of contributing to the improvement of the strength of the steel by precipitation strengthening. In order to obtain such an effect, Nb needs to be contained 0.01% or more. On the other hand, even if Nb is contained in excess of 0.08%, the precipitation of coarse precipitates (NbC, NbN) is promoted, and the resistance to sulfide stress corrosion cracking is lowered. Therefore, Nb is set to 0.01 to 0.08%. Preferably, it is 0.02 to 0.06%.

Ti: 0.04% or less Ti forms carbides or nitrides and contributes to the improvement of the strength of the steel by precipitation strengthening. In order to obtain such an effect, the content of Ti is preferably 0.001% or more. On the other hand, if the content of Ti exceeds 0.03%, the formation of coarse MC-type nitride (TiN) is promoted at the time of casting and it does not form a solid solution even after the subsequent heating, so toughness and sulfide stress corrosion cracking resistance Cause a decline. Therefore, the Ti content is 0.04% or less. Preferably, it is 0.01 to 0.03%.

B: 0.0005 to 0.0030%
B is an element which contributes to the improvement of the hardenability with a slight content. In order to obtain such an effect, B needs to contain 0.0005% or more. On the other hand, even if B is contained in a large amount exceeding 0.0030%, the above effect is saturated, or the formation of Fe boride (Fe-B) makes it impossible to expect the desired effect conversely. Be a disadvantage. In addition, a large B content exceeding 0.0030% promotes the formation of coarse borides such as Mo ₂ B, Fe ₂ B, etc., and makes it easy to generate cracks during hot rolling. Therefore, B is set to 0.0005 to 0.0030%. Preferably, it is 0.0010 to 0.0030%.

Cu: 0.03 to 0.07%
Cu is an element which has the effect | action which improves toughness and corrosion resistance while increasing the intensity | strength of steel. In particular, when severe sulfide stress corrosion cracking resistance is required, it becomes an extremely important element. Cu forms a precise corrosion product, and further suppresses the formation and growth of pits that are the starting point of cracking, thereby significantly improving the resistance to sulfide stress corrosion cracking. In order to acquire such an effect, it is necessary for Cu to contain 0.03% or more. On the other hand, even if the content is more than 0.07%, the above-mentioned effect is saturated, and an effect commensurate with the content can not be expected. For this reason, Cu is made into 0.07% or less. Preferably, it is 0.04 to 0.05%.

Co: 0.01 to 0.06%
Co is an element that contributes to the improvement of temper resistance and the increase in hardness at high temperatures. In order to obtain the above effects, it is generally effective to contain 0.05% or more. However, since Co is an expensive alloy, containing a large amount is economically disadvantageous. Co is also an element that contributes to the improvement of impact absorption energy at low temperatures, particularly at -20 ° C. or less. In order to acquire such an effect, Co needs to contain 0.01% or more. On the other hand, when the content of Co exceeds 0.06%, the impact absorption energy is lowered. It is also economically disadvantageous. For this reason, Co is made 0.01 to 0.06%. Preferably, it is 0.01 to 0.05%. More preferably, it is 0.01 to 0.04%.

Ca: 0.0001 to 0.0050%
Ca is useful as an element having a function of controlling the form of inclusions, and makes the spread sulfide-based inclusions as particulate inclusions. It has the effect | action which improves ductility, toughness, and a sulfide stress-corrosion-corrosion crackability through the form control of this inclusion, and it can contain it as needed. In order to exhibit such an effect effectively, it is preferable to contain Ca 0.0001% or more. On the other hand, if the content is more than 0.0050%, the amount of non-metallic inclusions may increase, and the ductility, the toughness and the sulfide stress corrosion cracking resistance may decrease. In addition, it may be economically disadvantageous. Therefore, when it contains Ca, it makes it 0.0050% or less. Preferably, it is 0.0001 to 0.0030%.

  The balance is iron and unavoidable impurities.

  With the above essential elements, the steel sheet of the present invention can obtain the desired characteristics.

  Next, the method for producing a seamless steel pipe of the present invention will be described.

  The seamless steel pipe of the present invention is heated, for example, after heating a steel pipe material having the above-described component composition to form a seamless steel pipe by hot forming, and then, the seamless steel pipe is quenched and tempered It can be manufactured by applying.

  In the present invention, as a method for producing a steel pipe material having the above-described component composition, for example, molten steel having the above-described composition is melted by a generally known melting method such as a converter, electric furnace, vacuum melting furnace, It is preferable to use as a cast material which is a steel pipe material by a generally known continuous casting method, ingot-lumping method, or the like. In addition, although the slab obtained may be a bloom, billet, any shape, if the shape cast by a continuous casting method is a billet of a round section, the hot rolling to the billet of the round section mentioned later can be omitted. Therefore, it is more preferable. Furthermore, after heating the obtained slab, it may be hot-rolled to make a slab which is a steel pipe material.

  It is preferable to shape | mold the obtained steel pipe raw material into a seamless steel pipe by hot forming, after heating to the temperature of the range of 1100-1300 degreeC with a heating furnace. In the hot forming method, after piercing, piercing to a predetermined thickness using either plug mill rolling or mandrel mill rolling is followed by hot reduction to appropriate diameter reduction rolling. In addition, it is good also as a seamless steel pipe by the hot extrusion by a press system.

  After seamless steel pipe forming, in order to achieve a target yield strength of 650 MPa or more, it is preferable to apply a quenching treatment (Q) and a tempering treatment (T). The hardening treatment and the tempering treatment can be effective even once, but may be performed twice or more. By repeating the quenching treatment twice or more, the steel structure is refined, and a steel pipe having high strength, high toughness, and excellent sulfide stress corrosion cracking resistance can be obtained. When the quenching process is repeated twice or more, either the QQ process in which the quenching process is repeated continuously or the QTQT process in which the quenching process and the tempering process are alternately repeated may be used.

As quenching treatment, after reheating to a quenching temperature below 1000 ° C. or higher Ac ₃ transformation point, from the Ac ₃ transformation point or higher 1000 ° C. or less of the temperature range below Ms transformation point, preferably to a temperature range of 100 ° C. or less, 2 ° C. / It is preferable to cool at an average cooling rate of s or more. If the quenching temperature is less than the Ac ₃ transformation point, heating to the austenite single phase region can not be performed, and a sufficient martensitic structure can not be secured by subsequent cooling. Therefore, the desired high strength can not be secured. On the other hand, when the quenching temperature exceeds 1000 ° C., coarsening of crystal grains is caused, and the toughness and the resistance to sulfide stress corrosion cracking decrease. For this reason, it is preferable to set the quenching temperature to the Ac ₃ transformation point or more and 1000 ° C. or less. In the present invention, the holding time at the quenching temperature is preferably 5 minutes or more. By setting the average cooling rate to 2 ° C./s or more, a sufficient quenched structure can be obtained. Specifically, the main phase can be a martensitic phase having a fine substructure which is transformed from a fine austenite (γ) phase. Here, the main phase refers to a phase that occupies 90% or more by volume. The upper limit of the average cooling rate need not be particularly limited. However, in view of securing the shape of the pipe, it is preferable to set the temperature to 25 ° C./s or less.

  As tempering treatment, after holding for 10 minutes or more at tempering temperature of a temperature range of 640 to 720 ° C., it is preferable to cool from a temperature range of 640 to 720 ° C. to room temperature at an average cooling rate of air cooling or more. The tempering treatment is performed to reduce excessive dislocations and stabilize the structure, and to combine the desired high strength with further excellent resistance to sulfide stress corrosion cracking. When the tempering temperature is less than 640 ° C., hydrogen trap sites such as dislocations increase and sulfide stress corrosion cracking resistance decreases. On the other hand, if the tempering temperature exceeds 730 ° C., the softening of the structure becomes remarkable, and the target strength can not be secured. For this reason, it is preferable to make tempering temperature into a temperature range of 640-720 degreeC. More preferably, it is a temperature range of 670 to 710 ° C. The holding time at the tempering temperature is preferably 10 minutes or more. If it is less than 10 minutes, desired tissue homogenization can not be achieved. More preferably, it is 20 minutes or more.

  Next, the characteristics of the present invention will be described.

  The high strength seamless steel pipe of the present invention is particularly excellent in low temperature toughness. Hereinafter, description will be made with reference to FIG.

  FIG. 1 is a graph showing the relationship between the Co content (% by mass), the Charpy impact test temperature (° C.), and the ratio of the impact absorption energy ratio of the invention steel to a comparative steel. In FIG. 1 (A), the horizontal axis is a Charpy impact test temperature, and in FIG. 1 (B), the horizontal axis is a Co content. In the present invention, a steel having a Co content of 0% by mass is used as a comparative steel, and the characteristics of the steel of the present invention are evaluated. The impact absorption energy ratio is the ratio of the impact absorption energy of the present invention steel to the comparison steel when the impact absorption energy of the comparison steel is 1.00. In addition, the impact absorption energy of a Charpy impact test is measured by the method as described in the below-mentioned Example.

  From FIG. 1, in the steel pipe material (the invention steel) having the above-described component composition, the Charpy impact test temperature is -20, particularly when the Co content is 0.04% by mass with respect to 0% by mass. It turns out that it becomes a high impact energy absorption ratio also in any of -40 degreeC, -60 degreeC. On the other hand, when the Co content is 0.06% by mass, the impact absorption energy ratio decreases at Charpy impact test temperatures of −20 ° C., −40 ° C., and −60 ° C., and the improvement margin decreases from 0.04% by mass You also know what to do.

  As mentioned above, in this invention, the low temperature toughness, ie, the impact absorption energy in -20 degrees C or less, improved remarkably by adding Co predetermined amount.

  Hereinafter, the present invention will be described in detail by way of examples. In addition, impact absorption energy changes with the product dimensions of a steel pipe, ie, the processing degree from a raw material, a lot. Here, it evaluated by plate rolling which simulated steel pipe rolling. The invention is not limited to the following examples.

  After melting the steel having the component composition shown in Table 1 in a vacuum melting furnace, a slab was produced from a 135 mm thick 50 kg square ingot. After heating the ingot at 1100 ° C. for 1 hour, rough rolling was performed to a plate thickness of 45 mm, and then heated to 1250 ° C., and finish rolling was performed to a plate thickness of 15 mm. The obtained sample was held at 920 ° C. for 5 minutes and then subjected to a water quenching treatment, and then held at 685 ° C. for 30 minutes and subjected to a tempering treatment.

Test pieces were collected from the finally obtained samples and subjected to a tensile test, a Charpy impact test, and a sulfide stress corrosion cracking test. The test method was as follows.
(1) Tensile test The yield strength (YS) and tensile strength (TS) are the tensile test pieces of a round bar (parallel part 6 mmφ × GLL 24 mm) collected so that the tensile direction is the rolling direction (longitudinal direction of sample). It calculated | required by the tension test based on JISZ2241 using In addition, the yield strength was taken as the strength at 0.5% elongation. In the present invention, the yield strength evaluated that 650 MPa or more was a pass.
(2) Charpy impact test From the obtained sample, a Charpy impact test piece (2 mm V-notch test piece) is taken so that the direction perpendicular to the longitudinal direction is the test piece longitudinal direction, and it conforms to JIS Z 2242 Charpy impact test was conducted. The test was performed three times and the average of impact absorption energy (J) was determined. Charpy test temperature implemented at 0 degreeC, -20 degreeC, -40 degreeC, and -60 degreeC. Here, steel No. 1 in Table 1 which is a steel having a Co content of 0% by mass. A was used as a comparative steel, and the shock absorption energy ratio was determined.
(3) Sulfide Stress Corrosion Cracking Test Three test pieces were taken from the obtained seamless steel pipe, and a sulfide stress corrosion cracking test was conducted by a method in accordance with NACE TM0177 Method A. In the test, using a 0.5% acetic acid + 5.0% saline solution (liquid temperature: 24 ° C) saturated with H ₂ S, a constant load test was carried out by applying 90% load stress of the yield strength for 720 hours did. The number of tests was three for each sample. After completion of the test, the occurrence of SSC was examined, and the SSC occurrence rate 0%, that is, the case where no cracking occurred was regarded as pass.

  The results obtained by the above are shown in Table 2.

  All of the inventive examples are high strength seamless steel pipes having a yield strength of 650 MPa or more, a cracking incidence rate of 0% or more in the sulfide stress corrosion cracking test, and excellent low temperature toughness.

Claims (1)

The component composition is in mass%,
C: 0.15 to 0.50%,
Si: 0.80% or less,
Mn: 0.3 to 1.0%,
P: 0.012% or less,
S: less than 0.0050%,
Al: 0.01 to 0.10%,
N: 0.01% or less,
Cr: 0.1 to 1.7%,
Mo: 0.2 to 2.0%,
V: 0.10% or less,
Nb: 0.01 to 0.08%,
Ti: 0.04% or less,
B: 0.0005 to 0.0030%,
Cu: 0.03 to 0.07%,
Co: 0.01 to 0.06%,
Ca: 0.0001 to 0.0050%
A low alloy high strength seamless steel pipe for oil well, characterized in that the balance is composed of iron and unavoidable impurities.