WO2000068450A1 - Steel product for oil well having high strength and being excellent in resistance to sulfide stress cracking - Google Patents

Abstract

A steel product for an oil well which contains 0.10 to 0.40 % of C, 0.5 % or less of Si, 0.5 % or less of Mn, 0.0150 % or less of P, 0.0050 % or less of S, 0.5 to 2.5 % of Mo, 0.005 to 0.1 % of Al, 0.005 to 0.1 % of Ti, 0.01 to 0.1 % of Nb, 0.01 % or less of N, 0.0005 to 0.0050 % of B, with the proviso that the content of Ti is 3.4 times that of N or more, wherein the relationship between the yield strength (YS) thereof in terms of ksi and the Mo content thereof satisfies the formula (1), α = Mo-0.15YS ≥ -18.9 and the balance between the contents of C and Mn and Mo satisfies the formula (2): β = 2.7 C + Mn + 2Mo ≥ 2.0. The steel product is excellent in high strength and in resistance to SSC. A steel product for an oil well which further contains one or more of 0.2 % or less of Cr, 0.5 % or less of W, 0.01 to 0.3 % of V, 0.001 to 0.01 % of Zr, 0.001 to 0.01 % of Ca, 0.001 to 0.01 % of Mg, and 0.001 to 0.01 % of REM, in addition to the above, and has a yield strength of 120 ksi or more is also disclosed.

Description

 Description High-strength steel for oil wells with excellent sulfide cracking resistance and its manufacturing method

 The present invention relates to high-strength and sulfide cracking (SSC), which is used when exposed to a sulfide environment, such as steel pipes used in oil and gas wells, for example, casings, tubing and drill pipes. ) High strength steel for oil wells with excellent resistance to

Background art

 Steel used in oil and gas wells containing hydrogen sulfide must have SSC resistance. The essence of SSC is hydrogen embrittlement, and it becomes more likely to occur as the strength of steel material increases, so it has been difficult to achieve both high strength and SSC resistance.

 Against this background, technology has been developed to improve SSC resistance and increase yield strength. For example, in Japanese Patent Publication No. Hei 6-104849, a technology has been developed to achieve high strength while optimizing the steel composition and metal structure while ensuring SSC resistance. Yield strength of 120 ksi seems to be the limit. In Japanese Patent Application Laid-Open No. 4-66464, there is disclosed a technology that focuses on the interaction between Cr and Ni and controls the content thereof to ensure SSC characteristics and toughness even when the strength is increased. However, with this technology, the attainable strength level seems to be limited to a yield strength of 120 ksi.

On the other hand, according to recent needs of oil well pipe plants, new steel grades that guarantee sufficient SSC resistance and have a yield strength exceeding 125 ksi have been demanded, and demand is expected to increase in the future. Therefore, the conventional achievable yield strength of 120 ksi is not enough to respond to future needs, and new steel grades have to be developed. Disclosure of the invention

 In view of such circumstances, an object of the present invention is to provide a steel material having a yield strength of at least 120 ksi, which was conventionally difficult to reach, and capable of exhibiting excellent SSC resistance. .

 The present inventor has conducted research for solving the above-mentioned problem. As a result, they have obtained necessary and sufficient knowledge for constituting the present invention.

 With increasing steel strength, SSC is generated from grain boundaries. To suppress such breakage, the metal structure must first be uniform. 1 In order to obtain high strength of 20 ksi or more and high SSC resistance, there is no other way than to use tempered martensite as the metal structure, but this structure should be as uniform as possible is necessary.

 If the metal structure contains a heterogeneous phase having different characteristics, this boundary or the heterogeneous phase itself becomes a fracture starting point, so that sufficient SSC resistance cannot be obtained. The uniformity of this structure is substantially determined by the state of quenching. That is, whether or not a different phase appears depends on whether or not a sufficient and uniform quenched martensite structure can be obtained throughout the steel material. Needless to say, a complete martensite structure is desirable. However, in the case of a thick wall material and in consideration of the limitation of the content of elements that can contribute to hardenability described later, the high strength intended by the present invention is considered. Figure 1 shows the results of examining the essential conditions at the level.

Figure 1 shows a quenching process in which a 25 mm thick plate is water-cooled from the austenite temperature range of 900 to 930. The hardness just below the surface of the plate The hardenability was evaluated based on the ratio of the fixed results, and the SSC resistance was evaluated using test pieces taken from the center of the wall thickness.

 Fig. 1 shows that if the hardness at the center of the wall thickness is 95% or more of the hardness just below the surface, sufficient SSC resistance can be obtained even at high strength exceeding 120 ksi. Obtained. In addition, in order to sufficiently quench a steel material with a thickness of about 25 mm, the value of index / 3 (2.7 C + Mn + 2 Mo) calculated from the contents of C, Mn, and Mo must be 2.0 or more. I got the knowledge. Figure 2 was obtained as a result of examining the relationship between the strength governing the SSC characteristics and the content of alloying elements, assuming a homogeneous metal structure that does not contain a heterogeneous phase. In other words, the contents of Mn and P, which affect the SSC characteristics, are controlled to a low level that can be achieved industrially, and the Mo content and the yield strength YS of the tempered steel are used as indices. Plotting makes it very easy to organize. That is, in order to secure sufficient SSC resistance in high-strength materials with YS ≥ 120 ksi, the Mo content must be at least 0.5% or more, and it is described in relation to the yield strength YS. It was found that a = Mo (wt%)-0.15 YS (k si) needed to be -18.9 or more.

 However, in some cases, the target could not be achieved even after obtaining the aforementioned uniform organization and also benefiting from the aforementioned effective element Mo. As a result of analyzing the cause, as shown in FIG. 3, it was found that a slight amount of the Cr content had a great effect on the high-strength material aimed at by the present invention.

In other words, the YS and Cr contents are made uniform after the structure during quenching is made uniform, the elements deteriorating SSC characteristics such as P and Mn are kept at a sufficiently low and constant level, and the beneficial Mo content is also kept constant. When the relationship was examined, as shown in Fig. 3, it was found that the SSC resistance greatly changed at a Cr content of 0.2 to 0.25%. In Fig. 3, limiting the Cr content to 0.2% or less improves the critical strength of SSC occurrence by 10ks. I have.

 Conventionally, Cr has been actively used as an element necessary for ensuring hardenability, but in the high-strength region exceeding 120 ksi, rather as a carbide-precipitating element, the grain boundary strength is weakened. It should be interpreted as harmful to SSC and its content should be limited.

 In addition, the effect of Mn and Mo was investigated in detail after limiting P to a low level and limiting Cr, and as shown in Fig. 4, it was found that both components had an interaction. In other words, in the region where the amount of Mn is less than 0.3%, the SSC resistance depends only on the amount of Mo <.When the amount of Mn is in the range of 0.3 to 0.5%, the amount of Mn must be increased and the amount of Mo must be increased. If the Mn content exceeds 0.5%, the SSC resistance does not improve even if the Mo content is increased. Therefore, to maximize the benefits of Mo, the amount of Mn should be limited to at least 0.5% or less, and preferably to less than 0.3%.

 The present invention has been made based on the above findings, and the gist is as follows.

 (1) In mass%,

 C: 0.10 ~ 0.40%, Si≤0.5%,

 Mn≤ 0.5%, P≤ 0.015%,

 S≤0.0050%. Mo: 0.5 ~ 2.5%,

 Al: 0.005 to 0.1%,

 Ti: 0.005-0.1% and more than 3.4 times ^^%,

 Nb: 0.01-0.1%, N≤0.01%,

 B: 0.0005-0.0050%

And the yield strength, expressed in ksi, between YS and the amount of Mo satisfies the following equation (1), and the balance of the C, Mn, and Mo contents satisfies the following equation (2) For high-strength oil wells with excellent sulfide cracking resistance Steel.

 a = Mo- 0.15YS≥-18.9 (1)

 β = 2.7 C + Mn + 2 Mo≥ 2.0 (2)

 (2) In mass%,

 C: 0.10 ~ 40%, Si≤0.5%,

 Mn ≤ 0.5%, P ≤ 0.015%,

 S ≤ 0.0050%, Mo: 0.5 ~ 2.5%

 Al: 0.005-0.1%,

 Ti: 0.005 to 0.1% and? ^ 3.4 times more than%,

 Nb: 0.01-0.1%, N≤0.01%.

 B: 0.0005 ~ 0.0050%

And the relationship between the yield strength YS and the amount of Mo expressed in ksi satisfies the following equation (1), and the balance of the contents of katsuji, Mn and Mo satisfies the following equation (2).

 Cr≤0.2%, W≤0.5%,

 V: 0.01-0.3%, Zr: 0.001-0.010%,

Ca: 0.001-0.01%, Mg: 0.001-0.01%,

 REM: 0.001-0.01%

High-strength oil well steel with excellent sulfide crack resistance, characterized by containing one or more of these.

 a = Mo- 0.15YS≥ 18.9 (1)

 β = 2.7C + Mn + 2 Mo≥ 2.0 (2)

 (3) In mass%,

 C: 0.10 ~ 40%, Si≤0.5%,

 Μη <0.3%, P ≤ 0.015%,

 S ≤ 0.0050%, Mo: 0.5-2.5%

Al: 0.005-0.1%, Ti: 0.005 to 0.1% and 3.4 times or more of N%,

 Nb: 0.01-0.1%, N≤0.01%,

 B: 0.0005-0.0050%

And the yield strength, expressed in ksi, between YS and the amount of Mo satisfies the following equation (1), and the balance of the contents of katsuji, Mn and Mo satisfies the following equation (2). High strength oil well steel with excellent sulfide crack resistance.

 a = Mo- 0.15YS≥― 18.9-(1)

 β = 2.7C + Mn + 2 Mo≥2.0… (2)

 (4) In mass%,

 C: 0.10 ~ 0.40%, Si≤0.5%,

 Mn <0.3%, P≤ 0.015%,

 S ≤ 0.0050%, Mo: 0.5 ~ 2.5%

 A1: 0.005-0.1%,

 Ti: 0.005-0.1% and 3.4 times more than ^^%,

 Nb: 0.01-1%, N≤0.01%,

 B: 0. 5 to 0.0050%

And the yield strength, expressed in ksi, between YS and the amount of Mo satisfies the following equation (1), and the balance of the content of (:, Mn, Mo) satisfies the following equation (2). ,

 Cr≤0.2%, W≤0.5%,

 V: 0.01-0.3%, Zr: 0.001-0.010

 Ca: 0.001-0.01%, Mg: 0.001-0.01%.

 REM: 0.001 to 0.01%

A high-strength oil well steel with excellent sulfide crack resistance, characterized by containing one or more of these.

a = Mo— 0.15YS≥-18.9-(1) β = 2.7C + Mn + 2 Mo≥ 2.0 (2)

 (5) In mass%,

 C: 0.10 ~ 0.40%, Si≤0.5%,

 Mn ≤ 0.5%, P ≤ 0.015%,

 S ≤ 0.0050%, Mo: 0.5-2.5%,

 Al: 0.005 to 0.1%,

 Ti: 0.005 to 0.1% and 3.4 times or more of N%,

 Nb: 0.0 to 0.1%, N ≤ 0.01%,

 B: 0.0005-0.0050%

, And the yield strength expressed in ksi satisfies the following equation (1), and the balance of the contents of katsuji, Mn and Mo satisfies the following equation (2), and A high-strength oil well steel excellent in sulfide crack resistance, having the above-mentioned yield strength.

 a = Mo- 0.15YS≥ 18.9 (1)

 β = 2.7C + Mn10 2 Mo≥2.0 (2)

 (6) In mass%,

 C: 0.10 ~ 0.40%, Si≤0.5%,

Mn ≤ 0.5%, P ≤ 0.015% _s

 S ≤ 0.0050%, Mo: 0.5-2.5%

 Al: 0.005-0.1%,

 Ti: 0.005-0.1% and 3.4 times or more of N%,

 Nb: 0.01-0.1%, N≤0.01%,

 B: 0.0005—0.0050%

And the relationship between the yield strength YS and the amount of Mo expressed in ksi satisfies the following equation (1), and the balance of the C, Mn, and Mo contents satisfies the following equation (2). ,

Cr≤0.2%, W≤0.5%, V: 0.0 to 0.3%, Zr 0.001 to 0.010%, Ca: 0.001-0.01%, Mg 0.001-0.01%,

 REM: 0.001 to 0.01%

A high-strength oil well steel material with excellent sulfide cracking resistance, characterized by containing one or more of these, and having a yield strength of 120 ksi or more.

 a = Mo- 0.15YS≥-18.9-(1)

 β 2 2.7 C + Μη + 2 Μο≥ 2.0… (2)

 (7) In mass%,

 C: 0.10 ~ 0.40%, Si≤0.5%,

 Μη <0.3%, P ≤ 0.015%,

 S ≤ 0.0050% Mo: 1.0 to 2.5%

 Al: 0.005 to 0.1%,

 Ti: 0.005 to 0.1% and 3.4 times more than 1 ^%,

 Nb: 0.01 ~ 0.1%, Ν≤0.01%,

 Β: 0.0005 to 0.0050%

And the yield strength, expressed in ksi, between the YS and the amount of Mo satisfies the following formula (1), and the balance of the contents of katsuji, Mn and Mo satisfies the following formula (2), and A high-strength oil well steel excellent in sulfide crack resistance, having the above-mentioned yield strength.

 = o- 0.15YS≥ ― 18.9 (1)

 β = 2.7C + Mn + 2 Mo≥ 2.0 (2)

 (8) In mass%,

 C: 0.10 ~ 0.40%, Si≤0.5%,

 Mn <0.3%, P ≤ 0.015%,

 S ≤ 0.0050%, o: 1.0 to 2.5%

 Al: 0.005 to 0.1%,

Ti: 0.005-0.1% and more than 3.4 times ^^%, Nb: 0.0 to 0.1%, N ≤ 0.01%,

 B: 0.0005-0.0050%

And the yield strength, expressed in ksi, between YS and the amount of Mo satisfies the following equation (1), and the balance of the Mn and Mo contents satisfies the following equation (2).

 Cr≤0.2%, W≤0.5%,

 V: 0.01-0.3%, Zr: 0.001-0.010%,

Ca: 0.001-0.01%, Mg: 0.001-0.01%,

 REM: 0.001-0.01%

A high-strength oil well steel material with excellent sulfide cracking resistance, characterized by containing one or more of these, and having a yield strength of 120 ksi or more.

 ヒ 2 Mo— 0.15YS≥ 1 18.9… (1)

 β = 2.7C + Μη + 2 Μο≥ 2.0-(2)

(9) A steel containing the components described in any one of the above (1) to (8) and having a balance of n, Mo content satisfying the following formula (2) was hot worked. After that, it is heated to a temperature range of + 20 ° C or more and 1000 ° C or less at the _three points of Ac and austenitized, then quenched, and the hardness as-quenched at the steel material position farthest from the cooling surface Manufacture of high-strength oil well steel with excellent sulfide cracking resistance characterized by a metal structure with a hardness of 95% or more of the hardness of the cooling surface, followed by tempering at a temperature of 620 to 720 ° C Method.

 β = 2.7C + Μη + 2 Μο≥ 2.0-(2)

(10) The method described in any of (1) to (9) above, wherein the crack initiation limit stress determined by the constant load type sulfide cracking test of NACE TM0177-A is 80% or more of the yield strength. High-strength oil well steel with excellent sulfide cracking resistance. BRIEF DESCRIPTION OF THE FIGURES

 Fig. 1 shows the ratio of the hardness at the center of the thickness to the hardness just below the surface layer in the as-quenched state by the hardenability index 3 of the steel component, and the SSC resistance of the steel material after tempering and 3 It shows the relationship.

 Figure 2 shows the SSC resistance of steel with good hardenability as a function of Mo content and YS.

 Fig. 3 shows the SSC resistance in relation to YS and Cr content for steels with good hardenability and a constant Mo content.

 Figure 4 shows the SSC resistance of steels with the same YS as a function of the Mn and Mo contents.

 Fig. 5 is a diagram showing the SSC resistance when YS is changed, using the ratio of the critical stress at break (thresh) and YS as an indicator of the SSC resistance. BEST MODE FOR CARRYING OUT THE INVENTION

 Hereinafter, the present invention will be described in detail.

 First, the reasons for limiting the alloy components of the present invention will be described. The content of the components is% by weight.

 C: C is an essential element to secure the desired high strength and sulfide cracking resistance at the same time. The strength and sulfide cracking resistance depend on the hardenability.If the content is less than 0.10%, the strength will decrease due to incomplete quenching, and the tempering conditions will need to be adjusted temporarily to adjust the tempering conditions. Even if strength is obtained, sufficient sulfide crack resistance cannot be obtained. On the other hand, if the content exceeds 0.40%, the sulfide cracking resistance is saturated and the susceptibility to burning cracks and standing cracks increases. Therefore, the appropriate range was 0.10 to 0.40%.

S i _: S i is the deoxidizer remaining in the steelmaking process, but if it exceeds 0.5%, the steel becomes brittle and the sulfide cracking resistance is deteriorated, so the upper limit is 0.5%. And Mn: Mn is an element that is harmful to sulfide cracking resistance and should not be added, but it also has the effect of improving hardenability, and has low C and Mo contents that improve hardenability. If the performance is insufficient, 0.5% may be contained as the upper limit. However, if the content exceeds 0.5%, satisfactory sulfide cracking resistance cannot be obtained even after complete quenching, so the upper limit was 0.5%. The preferred content of Mn is less than 0.3%

P: P is an impurity element that deviates at the grain boundaries and degrades sulfide cracking resistance, and should be suppressed to the lowest possible level. The upper limit of 0.015% was set as the permissible level at which stable industrial production is possible with the current refining technology that takes into account costs.

 S: S is also an impurity element that is deflected to grain boundaries and degrades sulfide cracking resistance. In the present invention, since Mn for fixing S is basically not contained, it should be suppressed to as low a level as possible, and in the range of less than 0.0050%, remarkable deterioration of SSC resistance is observed. Therefore, the upper limit of the content was 0.0050%.

 Mo: Mo is one of the essential elements in the present invention. It is an element that suppresses the grain boundary segregation of P, which is harmful to SSC resistance, and also enhances the tempering softening resistance, so it is a good element for obtaining high strength. is there. As shown in Fig. 2, at least 0.5% or more is required to ensure sufficient SSC resistance in the high-strength region of YS ≥ 120 ksi, and the higher the YS, the higher the Mo content The preferred range is at least 1.0%. However, even if it is contained in a large amount, its effect is saturated and the degree of freedom in strength adjustment is narrowed, so the upper limit is 2.5%.

A1: A1 is necessary to sufficiently deoxidize steel in the steelmaking process, and contains at least 0.055%. However, if it is contained in a large amount, the amount of alumina-based inclusions increases and there is a risk that SSC susceptibility may increase. The upper limit was set.

 Ti: Ti is contained in order to sufficiently exert the hardenability of B described later. That is, in order to prevent the precipitation of BN, it is necessary to fix N in advance to ΤίΝ, and therefore, the content of N is set to 0.005% or more and 3.4 times or more of the Ν content. However, a large amount promotes the precipitation of coarse TiN and increases the SSC sensitivity, so the upper limit was 0.1%.

 Nb: Nb is an element effective for improving the SSC resistance to reduce the grain boundary segregation of P through its grain refinement effect, and contains 0.01% or more. However, even if it is contained in a large amount, the effect of refining is saturated, and rather the SSC resistance is lowered due to the reduction of the grain boundary strength due to coarsening of the carbide, so the upper limit was set to 0.1%.

 N: N is an impurity element that inhibits the hardenability of B, and should be suppressed to the lowest possible level. The upper limit of 0.01% was set as the allowable level that enables stable industrial production with current refinement technology that takes into account costs.

 B: B is an element that remarkably improves hardenability and is an essential element for ensuring hardenability in the present invention. If the content is less than 0.0005%, sufficient flammability cannot be ensured, so this was set as the lower limit. Further, even if the content exceeds 0.0050%, the effect of improving hardenability is saturated, and rather, the precipitation of carbohydrate becomes remarkable and the SSC resistance deteriorates. Therefore, the upper limit content is set to 0.0050%.

 = = Mo—0.15YS: As shown in Fig. 2, the yield strength YS (ksi) and the Mo content (wt%) calculated as a function of 18.9 or more, provide excellent resistance. If the SSC property is obtained, and if it is less than 18.9, satisfactory SSC resistance cannot be obtained even if the individual components satisfy the above conditions. For this reason, α ≥ —18.9 was made an essential requirement of the present invention in addition to the above component conditions.

β = 2.7C + Μη + 2Mo: In addition to the above component conditions, sufficient hardenability To ensure and C, Mn, the content of which contributes alloying elements hardenability like Mo _(w t%) an index /? Calculated from a so 2.0 or more on shown in FIG. The upper limit is calculated as 6.11 from the upper limit contents of C, Mn and Mo.

 In the present invention, in addition to the above elements, one or more of the following may be selectively contained as necessary.

 Cr: Cr is useful as an element that enhances hardenability, but need not be contained if other elements such as Mo can secure sufficient hardenability. Rather, when trying to obtain a yield strength of 120 ksi or more, the content exceeding 0.2% deteriorates the SSC resistance, so the upper limit was set to 0.2%.

 W: W also has the effect of increasing the quenchability and increasing the tempering softening resistance, but if it is less than 0.01%, its effect is not sufficient, and if it exceeds 0.5%, the effect is saturated, so it is 0.01 to 0.01%. The appropriate content range was 0.5%.

 V: V has the effect of increasing the tempering softening resistance, and if it is contained in an amount of 0.01% or more, it is advantageous for increasing the strength. However, if it is contained in a large amount, the SSC resistance will deteriorate, so the upper limit is 0.3%.

 Zr: Zr has the effect of suppressing P grain boundary segregation. For this purpose, a content of 0.01% or more is necessary. However, since the element is an expensive element and contains a large amount of oxide, there is a risk of increasing the SSC sensitivity, so the upper limit was set to 0.010%.

 Ca, Mg, REM ··· These elements have the effect of reducing the stress concentration by spheroidizing the shape of inclusions, and also of fixing S to reduce grain boundary segregation of S. In any case, if the content is less than 0.001%, a sufficient effect cannot be obtained. If the content is too large, there is a risk of increasing the oxides and increasing the SSC sensitivity. Therefore, the upper limit was set to 0.010%.

In the present invention, steel having the above alloy composition is converted into a It is melted and formed, formed into a desired shape such as a tube, a plate, a bar, etc. by a normal hot rolling method, and then subjected to a heat treatment such as quenching and tempering to temper to a desired strength.

 The heat treatment conditions in the present invention will be described below.

The austenitizing temperature for quenching shall be between A c ₃ points + 20 ° C and 1000 ° C. If it is less than A c ₃ points + 20 ° C, it is difficult to obtain a uniform martensite structure due to insufficient austenitic toughness of steel. On the other hand, when the temperature exceeds 100 ° C., the grain growth becomes remarkable and the grain boundary area decreases, so that the segregating element such as P exerts the effect of reducing the SSC resistance. Therefore, the range of Ac ₃ + 20 ° C to 1000 was set as the appropriate austenization temperature condition. Note that the representative Ac ₃ point in the steel material of the present invention is 830 ° C.

 Further, as described above, in order to obtain satisfactory SSC resistance, the steel material quenched from this austenitizing temperature must have microstructure uniformity, and the hardness of the part farthest from the quenched end It is necessary to secure a value of 95% or more in comparison with the hardness just below the quenching end. With the component composition of the present invention, a predetermined burning property can be ensured even with a thick material of about 25 mm.

 The appropriate temperature range for tempering was 620 to 720 ° C. Under low temperature conditions of less than 620 ° C, YS becomes too high and α becomes low, so that satisfactory SSC resistance cannot be obtained. On the other hand, if the temperature exceeds 720 ° C, there is a risk that the SSC susceptibility may increase due to the intrusion into the two-phase region and the loss of tissue uniformity. For this reason, 620 to 720 ° C was set as an appropriate tempering temperature condition.

In the above temperature range, for example, by controlling the quenching temperature, the value of YS can be set to a predetermined range, and α = Mo−0.15YS ≧ −18.9 can be satisfied. W As described above, by optimizing the metallographic structure and composition, a high-strength sour-resistant steel material with YS ≥ 120ksi, which was previously difficult to achieve, can be obtained, but it was applied to the strength region where YS is less than 120ksi. In such a case, the SSC resistance is also improved. Fig. 5 shows the results of the NACE TM0177-A constant load SSC test for a steel material that satisfies the conditions of the metallographic structure and components specified in the present invention and has a YS of 117 to 120 ksi, and fractured by changing the applied stress. The results of determining the critical stress (th) are shown. From this, it can be said that the constituent factor of the present invention improves the SSC resistance not only in the region of YS≥120 ksi but also in the steel material with YS of less than 120 ksi in the form of improving ff th. Example

 Steel having the chemical composition shown in Table 1 was melted in a vacuum melting furnace, and the obtained steel ingot was subjected to hot rolling to form a 25 mm thick plate. This plate is heated to 900 to 1025 ° C, austenitized, and then quenched by water cooling.The hardness at the center of the wall thickness and immediately below the surface is measured until quenching, and the ratio between the two is determined. In addition to evaluating the hardenability, a tempering treatment was subsequently performed at a temperature of 630 to 730 ° C, and a tensile test was performed by collecting a round bar tensile test piece from the center of the thickness of the plate.

In addition, a round bar-shaped sulfide crack specimen with a parallel part length of 25 mm and a diameter of 6.2 specified in NACE-TM0177-A was collected from the center of the wall thickness, and 0.5% acetic acid + 5% ^ (A 720 hour test was performed in a corrosion solution at 25 ° C. containing 1 and saturated with H ₂ S at a partial pressure of 1 atm with a constant stress of 80% of the yield strength applied. Accordingly, the additional stress was reduced by 5% YS for those fractured by 80% YS additional stress, and the additional stress was increased by 5% YS every 5% YS for those that did not fracture by 80% YS additional stress. The critical stress for crack initiation ( _CT th) was determined.

Hardenability is good if the thickness center hardness is 95% or more of the hardness just below the surface. We evaluated as good. SSC resistance was evaluated as good if it did not break during the test period of 720 hours. The test results are shown in Tables 2 and 3.

 Nos. 1 to 11 and Nos. 101 to 104 in Table 2 simultaneously satisfy the high strength of YS ≥ 120 ksi, which is the test result included in the scope of the present invention, and the excellent SSC resistance.

 On the other hand, in Comparative Examples Nos. 18 to 30 and Nos. 105 to 107, the components were out of the range of the present invention, so that good SSC resistance was not obtained. In addition, although the components were within the scope of the present invention, in Comparative Examples Nos. 12, 13, 14, 16, and 17 were outside the scope of the present invention, and in Comparative Examples Nos. 15 and 31, However, since the tempering temperature and the austenitizing temperature are out of the ranges of the present invention, sufficient SSC resistance has not been obtained.

In addition, Nos. 108 to 109 in Table 3 show that YS is less than 120 ksi, which is higher than that of Comparative Examples Nos. 113 and 114 of the same YS, and has higher σ th and improved SSC resistance. In Nos. 110 and 111 of the present invention, YS exceeds 120 ksi, and σ th is a high value of 85% of YS. On the other hand, in Comparative Example No. 112, it was only 85% of σ th even though YS was low. In addition, SSC resistance of No. 115 is insufficient.

Table 1 Chemical composition (Wt%, * is |) PH1)

 Category c Si Mn S * Mo Al Ti Nb N * Ti / N Cr W V Zr * Ca * REM * β

X-1 0.20 0.15 0.06 80 15 1.30 0.025 0.021 0.026 39 12 5.38 3.20

X-2 0.20 0.16 0.46 81 15 1.30 0.026 0.022 0.031 41 13 5.37 3.60

X- 3 0.27 0.15 0.15 70 25 0.58 0.029 0.031 0.019 55 13 5.64 2.04

X- 4 0.29 0.15 0.04 60 10 1.51 0.025 0.026 0.026 39 12 6.67 0.21 3.84 Translation A 0.29 0.15 0.45 81 10 0.90 0.040 0.035 0.029 70 15 5.00 0.14 0.25 0.02 21 3.03

 B 0.20 0.15 0.33 70 25 0.62 0.029 0.031 0.019 55 13 5.64 2.11 c 0.20 0.15 0.33 70 15 0.89 0.029 0.031 0.019 55 13 5.64 0.19 2.65

D 0.20 0.20 0.13 81 25 1.40 0.036 0.075 0.021 39 7 19.23 3.47

E 0.19 0.25 0.08 86 16 1.80 0.038 0.021 0.031 41 38 5.12 15 15 4.19

F 0.12 0.15 0.23 57 9 2.21 0.047 0.029 0.041 51 11 5.69 4.97

G 0.19 0.20 0.05 75 10 1.50 0.035 0.041 0.031 45 21 9.11 0.10 21 12 3.56

Y-l 0.23 0.16 0.30 80 16 0.78 0.024 0.023 0.025 44 11 5.23 0.40 2.48

Y 2 0.25 0.15 0.13 80 16 0.42 0.025 0.025 0.025 38 12 6.58 1.65

Y-3 0.21 0.16 0.55 80 16 1.88 0.026 0.024 0.029 41 12 5.85 4.88

H 0.15 0.21 0.30 91 25 1.55 0.026 0.008 0.018 65 13 L90 19 3.81

I 0.28 0.15 0.46 135 21 0.48 0.032 0.040 0.036 39 20 10.30 0.15 2.18 m J 0.17 0.14 0.21 70 23 0.61 0.036 0.025 0.038 46 13 5.43 1.89

K 0.12 0.15 0.20 75 28 0.60 0.046 0.036 0.030 55 12 6.55 1.72

L 0.25 0.16 0.33 51 18 1.40 0.040 0.034 0.034 51 56 6.67 3.81

M 0.19 0.17 0.45 165 14 0.75 0.032 0.033 0.036 55 15 6.00 2.46

N 0.25 0.15 0.33 71 16 0.91 0.031 0.031 0.039 56 13 5.54 0.34 2.83

0 0.18 0.15 0.60 136 13 0.71 0.033 0.035 Γ "θ.029 60 12 5.83 2.51

P 0.19 0.14 0.36 129 55 0.72 0.026 0.029 0.034 78 9 3.71 2.31

Q 0.23 0.15 0.33 71 16 0.91 0.031 0.031 0.039 56 13 5.54 0.25 2.77

R 0.21 0.18 0.31 120 20 1.19 0.0,34 0.021 0.111 40 13 5.25 3.26 Underlines are outside the scope of the present invention

Table 2 Heat treatment conditions and strength and SSC test results

Underlines are outside the scope of the present invention Table 3 Heat treatment conditions, strength, and SSC test results

 Underlines are outside the scope of the present invention

Industrial applicability

 As described above, according to the present invention, it is possible to obtain a steel material for an oil well having both high strength with a yield strength of 120 ksi or more and excellent SSC resistance.

Claims

The scope of the claims 1. In mass%,  C: 0.10 ~ 0.40%,  Si ≤ 0.5%,  Mn ≤ 0.5%,  P ≤ 0.015%,  S ≤ 0.0050%.  o: 0.5 ~ 2.5%,  Al: 0.005-0.1%,  Ti: 0.005-0.1% and 3.4 times or more of N%,  Nb: 0.01-0.1%,  N≤ 0.01%,  B: 0.0005-0.0050% And the yield strength, expressed in ksi, between YS and the amount of Mo satisfies the following equation (1), and the balance of the C, n, and Mo contents satisfies the following equation (2). High strength oil well steel with excellent sulfide cracking resistance.  a-o- 0.15YS≥ 18.9 (1)  β = 2.7 C + Μη + 2 Μο≥ 2.0 (2)  2. In mass%,  C: 0.10 ~ 0.40%,  Si ≤ 0.5%,  Mn≤ 0.5%  P ≤ 0.015%,  S ≤ 0.0050%, Mo: 0.5-2.5%, Al: 0.005-0.1%,  Ti: 0.005 to 0.1% and more than 3, 4 times of N%,  Nb: 0.0 to 0.1%,  N≤ 0.01%,  Β: 0.0005 to 0.0050% And the relationship between the yield strength YS and the amount of Mo expressed in ksi satisfies the following equation (1), and the balance of the C, Mn, and Mo contents satisfies the following equation (2). ,  Cr ≤ 0.2%,  W≤ 0.5%,  V: 0.01-0.3%,  Zr: 0.001 to 0.010%,  Ca: 0.001-0.01%,  Mg: 0.001-0.01%,  REM: 0.001 to 0.01% A high-strength oil well steel with excellent sulfide crack resistance, characterized by containing one or more of these.  = Mo- 0.15YS≥ 1 18.9  β = 2.7C + Mn + 2 o≥ 2.0-(2)  3. In mass%,  C: 0.10 ~ 40%, Si≤0.5%,  Μη <0.3%, P ≤ 0.015%,  S ≤ 0.0050%, Mo: 0.5-2.5%  Al: 0.005 to 0.1%,  Ti: 0.005 to 0.1% and 3.4 times or more of N%,  Nb: 0.01-0.1%, N≤0.01%, B: 0.0005-0.0050% And the yield strength, expressed in ksi, between YS and Mo satisfies the following equation (1), and the C, Mn, Mo content balance satisfies the following equation (2). High strength oil well steel with excellent sulfide cracking resistance.  a = o- 0.15YS≥ ― 18.9 (1)  β 2.7C + Mn + 2 Mo≥ 2.0 (2)  4. In mass%,  C: 0.10-0.40%, Si≤0.5%,  Mn <0.3%, P ≤ 0.015%,  S ≤ 0.0050%, Mo: 0.5-2.5%  Al: 0.005 to 0.1%,  Ti: 0.005-0.1% and 3.4 times or more of N%,  Nb: 0.01-0.1%, N≤0.01%.  B: 0.0005-0.0050% And the relationship between the yield strength YS and the amount of Mo expressed in ksi satisfies the following equation (1), and the balance of the C, Mn, and Mo contents satisfies the following equation (2). ,  Cr≤0.2%, W≤0.5%,  V: 0.01-0.3%, Zr: 0.001-0.010%, Ca: 0.001-0.01%, Mg: 0.001-0.01%,  REM: 0.001 to 0.01% A high-strength oil well steel with excellent sulfide crack resistance, characterized by containing one or more of these.  a = Mo- 0.15YS≥ ― 18.9-(1)  β = 2.7 C + n + 2 Mo≥ 2.0… (2)  5. In mass%, C: 0.10 ~ 0.40%, Si≤0.5%, Mn ≤ 0.5%, P ≤ 0.015%, S ≤ 0.0050%, Mo: 0.5 to 2.5%  Al: 0.005 to 0.1%,  Ti: 0.005-0.1% and 3.4 times more than ^^%,  Nb: 0.01-0.1%, N≤0.01%,  B: 0.0005-0.0050% And the yield strength expressed in ksi satisfies the following equation (1), and the balance of C, Mn, and Mo satisfies the following equation (2). A high-strength oil well steel excellent in sulfide crack resistance, having the above-mentioned yield strength.  a = Mo- 0.15YS≥ one 18.9… (1)  β = 2.7C + Mn + 2 o≥ 2.0-(2)  6. In mass%,  C: 0.10-0.40%, Si≤0.5%,  n ≤ 0.5%, P ≤ 0.015%,  S ≤ 0.0050%, Mo: 0.5-2.5%,  Al: 0.005 to 0.1%,  Ti: 0.005 to 0.1% and 3.4 times or more of N%,  Nb: 0.01-0.1%, N≤0.01%,  B: 0.0005-0.0050% And the yield strength, expressed in ksi, between the YS and the amount of Mo satisfies the following equation (1), and the balance of the contents of katsuji, Mn and Mo satisfies the following equation (2). ,  Cr≤0.2%, W≤0.5%,  V: 0.01-0.3%, Zr: 0.001-0.010%, Ca: 0.001-0.01%. Mg: 0.001-0.01%, REM: 0.001-0.01% A high-strength oil well steel material with excellent sulfide cracking resistance, characterized by containing one or more of these, and having a yield strength of 120 ksi or more.  a = Mo- 0.15YS≥ 1 18.9… (1)  β = 2.7C + Mn + 2 Mo≥ 2.0-(2)  7. In mass%,  C: 0.10 ~ 0.40%, Si≤0.5%,  Mn <0.3%, P ≤ 0.015%  S ≤ 0.0050%, Mo: 1.0 ~ 2.5%,  A1: 0.005 to 0.1%,  Ti: 0.005 to 0.1% and 3.4 times or more of N%,  Nb: 0.01 ~ 0.1%, Ν ≤ 0.01%,  Β: 0.0005-0.0050% And the yield strength expressed in ksi satisfies the following equation (1), and the balance of the Mn and Mo contents satisfies the following equation (2), and is 120 ksi or more. High-strength oil well steel with excellent sulfide crack resistance, characterized by having a high yield strength.  a = Mo- 0.15YS≥-18.9-(1)  β = 2.7C + Mn + 2 o≥ 2.0-(2)  8. In mass%,  C: 0.10 ~ 40%, Si≤0.5%,  η <0.3%, P ≤ 0.015%,  S ≤ 0.0050%, Mo: 1.0-2.5%  Α1: 0.005 to 0.1%,  Ti: 0.005 to 0.1% and 3.4 times or more of N%,  Nb: 0.01-0.1%, N≤0.01%.  B: 0.0005-0.0050% And the yield strength, expressed in ksi, between YS and Mo ), And the balance of the contents of katsuji, n, and Mo satisfies the following equation (2).  Cr≤0.2%, W≤0.5%,  V: 0.01-0.3%, Zr: 0.001-0.010%, Ca: 0.001-0.01%, g: 0.001-0.01%,  REM: 0.00 to 0.01% High-strength oil well steel with excellent sulfide cracking resistance, characterized in that it contains one or more of these, and has a yield strength of 120 ksi or more.  α = Mo- 0.15YS≥― 18.9… (1)  β = 2.7C + Mn + 2 Mo≥2.0… (2) 9. After hot-rolling a steel containing the components described in any one of claims 1 to 8 and having a C, Mn, and Mo content balance satisfying the following expression (2), Ac ₃ points + After heating to a temperature range of 20 ° C or higher and 1000 ° C or lower to form an auto-stainate, quenching treatment is applied, and the hardened steel at the steel material position furthest away from this cooling surface For a high-strength oil well with excellent sulfide cracking resistance, characterized by a metal structure with a hardness <95% of the hardness of the cooling surface, followed by tempering at a temperature of 620 to 720 ° C. A method for producing steel.  β = 2.7C + Μη + 2 ο≥ 2.0-(2)  10. Sulfidation resistance according to any one of claims 1 to 9, wherein the crack initiation limit stress determined by the constant load sulfide cracking test of NACE T 0177-A is 80% or more of the yield strength. High-strength oil well steel with excellent cracking properties.