JP2002339038A - Method for producing low-alloyed high strength steel having excellent fracture strength and sulfide stress corrosion cracking resistance, and steel tube consisting of the steel - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To provide both excellent fracture toughness and sulfide stress corrosion cracking resistance in a mild sour environment, with a yield strength of 75.
A low-alloy high-strength steel for use in a seamless steel pipe for oil or gas wells having a high strength of 8 MPa or more and 861 MPa or less and a steel pipe made of this steel are manufactured. SOLUTION: C: 0.15 to 0.4%, Si: 0.1
-0.3%, Mn: 0.1-0.3%, P: 0.01
5% or less, S: 0.005% or less, Cr: 0.5 to l.
5%, Mo: 0. 1-1.0%, B: 0.0005-
0.003%, Al: 0.01 to 0.1%, Nb: 0.
0.01 to 0.05%, N: 0.003 to 0.01% (or more, mass%), and the balance: a billet having a component composition of substantially Fe and having a diameter of 340 mm or less is continuously cast. After that, without going through a rolling step, a Mannesmann perforation is performed to prepare a raw tube, and after quenching the raw tube, it is tempered at a temperature not higher than the Ac ₁ transformation point.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a seamless steel pipe for an oil or gas well having excellent fracture toughness and sulfide stress corrosion cracking resistance in a mild sour environment, and having a high yield strength of 758 to 861 MPa. Method for producing low-alloy high-strength steel for steel and a steel pipe made of this steel,
In particular, a low alloy alloy with excellent fracture toughness and sulfide stress corrosion cracking resistance is produced by directly drilling Mannesmann without going through a round billet rolling process by continuous casting, and then making pipes with a rolling mill such as a mandrel mill or plug mill. The present invention relates to a method for producing a high strength steel and a steel pipe made of the steel.

[0002]

2. Description of the Related Art Due to the tight energy situation, oil wells and gas wells that have not been developed so far and contain hydrogen sulfide are also to be developed, and high-strength oil well pipes used in such a sour environment are being developed. It has been strongly desired to produce a seamless steel pipe having high resistance to sulfide stress corrosion cracking.

[0003] Steel pipes used as oil country tubular goods have been subjected to ordinary quenching and tempering heat treatment. However, in order to prevent sulfide stress corrosion cracking, it is effective to regulate them to 22 or less on the Rockwell C scale. It had been.

[0004] However, according to the regulation of the hardness, the yield strength is about 700 MPa, and it has been difficult to manufacture a seamless steel pipe having higher strength and higher resistance to sulfide stress corrosion cracking.

For this reason, JP-A-59-232220 and JP-A-6-172859 disclose a method in which steel containing a specific chemical component is rolled seamlessly and then quenched twice or more. JP-A-59-119324 discloses a method in which the rate of temperature rise before quenching is significantly increased.

As a method for evaluating the resistance to sulfide stress corrosion cracking, a method of performing a uniaxial tensile test in an extremely accelerated environment is used as specified in NACE TM0177. Was a typical way. That is, this evaluation method is 5% NaCl + 0.5% CH ₃ C
The material is subjected to stress for 720 hours in an environment in which hydrogen sulfide gas is saturated in an OOH aqueous solution (pH: 2.7), and the presence or absence of sulfide stress corrosion cracking is examined.

However, there is a question whether the pH is too low to simulate the real environment, which is a mild sour environment, and that the performance of the material in the real environment is well evaluated.
Note that a mild sour environment has a pH of 4 or more and H ₂
This refers to an environment where the S partial pressure is 0.1 atm or less. In the uniaxial tensile test, a design stress is required, but it is difficult to evaluate the performance of an actual pipe. Therefore, it is important to evaluate the fracture toughness of the material under such an environment.

Accordingly, an object of the present invention is to provide a seamless steel pipe for oil or gas wells having excellent fracture toughness and sulfide stress corrosion cracking resistance in a mild sour environment and having a high yield strength of 758 MPa to 861 MPa. Method for producing low-alloy high-strength steel for use and a steel pipe made of this steel, in particular, directly through a Mannesmann perforation without going through a round billet rolling step by continuous casting, and then producing the pipe by a rolling machine such as a mandrel mill, plug mill or the like. Another object of the present invention is to provide a low-alloy high-strength steel excellent in fracture toughness and sulfide stress corrosion cracking resistance, and a method for producing a steel pipe made of this steel.

[0009]

Means for Solving the Problems In order to solve the above problems, the present inventors first examined various laboratory methods for evaluating the performance of materials in a real environment, which is a mild sour environment. As a result, pH: about 0.1 at 0.1
It was concluded that it was appropriate to evaluate the fracture toughness value and the sulfide stress corrosion cracking resistance in an environment containing hydrogen sulfide of atm or less. Furthermore, various effects of the chemical composition and hardness of the low alloy steel on the fracture toughness value and sulfide stress corrosion cracking resistance of the above environment are examined, and by reducing Si and Mn, good fracture toughness in a mild sour environment is achieved. Values and sulfide stress corrosion cracking resistance were obtained. The present invention has been made based on this finding, and has the following features.

The invention according to claim 1 is characterized in that C: 0.15 to 0.15.
0.4%, Si: 0.1-0.3%, Mn: 0.1-
0.3%, P: 0.015% or less, S: 0.005% or less, Cr: 0.5 to l. 5%, Mo: 0. l to 1.0
%, B: 0.0005 to 0.003%, Al: 0.01
0.1%, Nb: 0.01 to 0.05%, N: 0.0
0.3-0.01%, and further, Ti: 0.01-
It is characterized by having at least one of 0.03% and V: 0.01 to 0.05% (above, mass%) and the balance: substantially Fe.

The invention according to claim 2 is characterized in that C: 0.15 to
0.4%, Si: 0.1-0.3%, Mn: 0.1-
0.3%, P: 0.015% or less, S: 0.005% or less, Cr: 0.5 to l. 5%, Mo: 0. l to 1.0
%, B: 0.0005 to 0.003%, Al: 0.01
0.1%, Nb: 0.01 to 0.05%, N: 0.0
0.3-0.01%, and further, Ti: 0.01-
0.03%, V: at least one of 0.01 to 0.05% (or more, mass%), and the balance: a component composition substantially containing Fe, and having a diameter of 3
A billet of 40 mm or less was cast by a continuous casting method, and thereafter, without going through a rolling step, a Mannesmann perforation was performed to prepare a raw tube, and after the raw tube was quenched, at a temperature not higher than the Ac ₁ transformation point. Tempering, strength upper limit is 86
It is characterized by having a strength of 1 MPa.

According to a third aspect of the present invention, C: 0.15
0.4%, Si: 0.1-0.3%, Mn: 0.1-
0.3%, P: 0.015% or less, S: 0.005% or less, Cr: 0.5 to l. 5%, Mo: 0. l to 1.0
%, B: 0.0005 to 0.003%, Al: 0.01
0.1%, Nb: 0.01 to 0.05%, N: 0.0
0.3-0.01%, and further, Ti: 0.01-
0.03%, V: at least one of 0.01 to 0.05% (or more, mass%), and the balance: a component composition substantially containing Fe, and having a diameter of 3
The following billet 40mm was cast by continuous casting, then without a rolling process, the blank pipe was prepared by performing Mannesmann piercing, then after quenching the blank tube, Ac ₃ transformation from Ac ₃ transformation point Heated to a temperature in the range of the point plus 100 ° C., then quenched again and
It is characterized in that it is tempered at a temperature lower than the c ₁ transformation point and has an upper limit of 861 MPa in strength.

According to a fourth aspect of the present invention, C: 0.15
0.4%, Si: 0.1-0.3%, Mn: 0.1-
0.3%, P: 0.015% or less, S: 0.005% or less, Cr: 0.5 to l. 5%, Mo: 0. l to 1.0
%, B: 0.0005 to 0.003%, Al: 0.01
0.1%, Nb: 0.01 to 0.05%, N: 0.0
0.3-0.01%, and further, Ti: 0.01-
0.03%, V: at least one of 0.01 to 0.05% (or more, mass%), and the balance: a component composition substantially containing Fe, and having a diameter of 3
A billet of 40 mm or less is cast by a continuous casting method, and thereafter, without going through a rolling step, a Mannesmann perforation is performed to prepare a raw tube, and then, after quenching the raw tube, at a temperature not higher than the Ac ₁ transformation point. Tempering, heating again from the Ac ₃ transformation point to the Ac ₃ transformation point plus 100 ° C., then quenching from the heating temperature range, and
Tempering at a temperature below the Ac ₁ transformation point, with an upper limit of 861 MPa
It is characterized by having a strength of.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The reasons for limiting the component composition in the present invention will be described.

C: 0.15 to 0.4 mass% C is an essential element for improving the hardenability and the tempering resistance in addition to the effect of securing the strength of the low alloy steel. .15 mass%. 0.4 ma
If it exceeds ss%, cracking will occur if the cooling rate is high during quenching. Therefore, the C content is 0.15 to 0.4 mass
%.

Si: 0. 1 to 0.3 mass% Si has an effect of improving the strength of the steel material in addition to the function as a deoxidizing agent, so it needs to be added in an amount of 0.1 mass% or more. It has an adverse effect on fracture toughness, sulfide stress corrosion cracking resistance, toughness, and grain boundary strength in mild sour environments. Therefore, the content of Si is 0.1. 1 to 0.3 mass%.

Mn: 0.1 to 0.3 mass% Mn fixes sulfur as a sulfide,
If it is less than ss%, there is a high possibility that rolling cracks will occur.
Requires addition of 0.1% or more. On the other hand, 0.3mas
If it exceeds s%, it has an adverse effect on the fracture toughness and sulfide stress corrosion cracking resistance in a mild sour environment, so the upper limit was made 0.3 mass%.

P: 0.015 mass% or less P is a harmful element that lowers the grain boundary strength.
If the content exceeds 0.015 mass%, the sulfide stress corrosion cracking resistance is adversely affected.
ass% or less.

S: 0.005 mass% or less S is an unavoidable impurity in steel, and when contained in a large amount, forms sulfide and becomes a starting point of cracking.
s% or less.

Cr: 0.5 to 1.5 mass% Cr is an element which has a remarkable effect on the improvement of hardenability and has an effect of increasing the strength of steel. However, the effect can be expected if it is less than 0.5 mass%. On the other hand, if it is 1.5 mass% or more, it has an adverse effect on fracture toughness and sulfide stress corrosion cracking resistance. Therefore, the Cr content is 0.5 to 1.5 mass%.
Within the range.

Mo: 0.1 to 1.0 mass% Mo is an element that enhances the tempering resistance of steel.
If the content is less than ass%, the effect is small, and 1.0 mass%
Above, the steel becomes brittle and the toughness deteriorates. Therefore, the Mo content is in the range of 0.1 to 1.0 mass%.

B: 0.0005 to 0.003 mass% B is an element that improves the hardenability of steel.
The effect appears when the content is 05 mass% or more. on the other hand,
Even if it is added in a large amount exceeding 0.003 mass%, not only the effect is saturated, but also cracks occur during hot working. Therefore, the B content is 0.0005 to 0.003 mass%.
Within the range.

Al: 0.01 to 0.1 mass% Al is an element effective as a deoxidizing agent for steel, and combines with N in steel together with Ti to form a nitride, thereby exerting the action of B. Make it noticeable. In particular, the effect is 0.01 mass%
It is remarkable above. On the other hand, if the content exceeds 0.1 mass%, the amount of oxides increases, which adversely affects sulfide stress corrosion cracking resistance. Therefore, the Al content is 0.01 to 0.1 mass
%.

N: 0.003 to 0.01 mass% N forms a nitride with Ti and Al.
Has the effect of suppressing the grain growth of steel and making it finer. Therefore, the added Ti can be effectively used to reduce the crystal grain size.
In order to precipitate N in steel, the content is made 0.003 mass% or more from the stoichiometric consideration. 0.01mas
When the content exceeds s%, the effect of B addition is reduced,
Sufficient hardenability cannot be obtained. Therefore, N
The content was in the range of 0.003 to 0.01 mass%.

Nb and V: 0.01 to 0.05 mass% Nb and V have an effect of making recrystallized grains smaller during rolling. Therefore, Nb and V are elements to be added when finer crystal grains are desired. , Less than 0.01 mass% is not sufficient, and if it exceeds 0.05 mass%, not only the effect is saturated, but also the toughness is reduced. Therefore, the respective contents of Nb and V are set in the range of 0.01 to 0.05 mass%.

Ti: 0.01 to 0.03 mass% Ti fixes N as TiN like Al, improves the hardenability of B, and is finely dispersed in a billet cast by a continuous casting method. Precipitation has the effect of suppressing coarsening of crystal grains during billet heating. However, if the content is less than 0.01% by mass, the desired effect cannot be obtained. On the other hand, if the content exceeds 0.03% by mass, the effect of suppressing the growth of crystal grains due to the coarsening of TiN is increased. Not only does it cause a deterioration in toughness. Therefore, T
The i content was set to 0.01 and 0.03 mass%.

The reason for limiting the billet diameter to 340 mm or less is as follows.

When a billet is continuously cast, the properties of the central portion are governed by the cooling rate. If the billet diameter is too large, the cooling rate is reduced, segregation and shrinkage of the center are liable to occur, and defects on the inner surface are induced. According to the study of the present invention and the like, the billet diameter during continuous casting is 340
It was clarified that when the diameter was equal to or less than mm, the generation of defects was suppressed even when the Mannesmann perforation was performed. Therefore, in the present invention, the billet diameter is limited to 340 mm or less.

[0029]

Next, the present invention will be further described with reference to examples.

Steel having the chemical components shown in Table 1 was smelted using a vacuum melting furnace, and was subjected to slab rolling to form a steel sheet having a thickness of 40 mm, and further hot-rolled to prepare a steel sheet having a thickness of 12 mm. Then, the steel sheet prepared in this manner is quenched with a target yield strength of 750 to 900 MPa.
Tempering heat treatment (heat treatment method 1), quenching, quenching and tempering again (heat treatment method 2), quenching and tempering, and quenching and tempering again (heat treatment method 3) were performed.

The quenching heat treatment is 920 ° C., the tempering heat treatment is 680 ° C., and the quenching heat treatment is 880 ° C.
° C and the tempering heat treatment again were performed at 680 ° C. However, steel A (steel type No.) with yield strength deviated
(Example No.)-2 (heat treatment method No.), B-2-3, C
-2-2, D-2-3, E-2-1 and E-2-2) were performed at 640 ° C.

[0032]

[Table 1]

The yield strength in each heat treatment method,
The SSC resistance and the fracture toughness were examined and comprehensively evaluated.

When the yield strength was 758 to 861 MPa, the mark was ○, and otherwise, the mark was x. An evaluation test of sulfide stress corrosion cracking resistance (SSC resistance) in a mild sour environment was performed using NACE TM0177-96 Meth.
In accordance with od A, a test piece loaded with a stress of 90% yield strength was exposed to a 3.5% aqueous NaCl solution (pH = 4.0) saturated with 0.07 atm of hydrogen sulfide at room temperature for 720 hours. In the case of no break, it was marked with ○, and in the case of break, it was marked with x. An evaluation test for fracture toughness in a mild sour environment was made according to NACE TM0177-96 Method D.
In accordance with the test method described above, four DCB test pieces were
3.5% NaC saturated with 0.07 atm of hydrogen sulfide
l + CH ₃ in COOH solution (pH: 4.0) to measure the 14 days pickled K _{1 SSC} value and an average value of 37MPa · m ⁰ · ⁵ or more case ○ mark, was × indicia otherwise. Table 2 shows the results and overall evaluation.

[0035]

[Table 2]

As is clear from Table 2, the method of the present invention, A-1-1, A-1-2, A-1-3, and B-1-
1, B-1-2 and B-1-3 all passed the yield strength, the fracture toughness and the sulfide stress corrosion cracking resistance.

On the other hand, the comparison methods A-2-2 and B-
2-3, C-1-1, C-2-2, D-1-1 and D
-2-3, the yield strength was too high, and the fracture toughness and sulfide stress corrosion cracking resistance were rejected. Also,
Comparative steels C-1 and C-2 both have high Mn and Si contents, and even if the yield strength is acceptable (in the case of C-1),
Both fracture toughness and sulfide stress corrosion cracking resistance were rejected. Comparative steels D-1 and D-2 both had a high Cr content, and even if the yield strength was acceptable (in the case of D-1), both the fracture toughness and the sulfide stress corrosion cracking resistance were rejected. Was.

Further, steel having the chemical composition shown in Table 3 was melted in a converter and formed into a billet having a diameter of 230 mm by a continuous casting machine. Then, a steel pipe having an outer diameter of 273 mm was prepared by a plug mill and then quenched and tempered. (Heat treatment method 1), quenching, and quenching and tempering again (heat treatment method 2) were performed to adjust the strength. Then, an evaluation test for fracture toughness and sulfide stress corrosion cracking resistance in a mild sour environment was performed. The evaluation test method at this time was the same as the case described above. Table 4 shows the test results. In Table 4, the method of evaluating the test results is the same as in Table 2.

[0039]

[Table 3]

[0040]

[Table 4]

As is clear from Table 4, E-1-1 and E-1-2, which are the methods of the present invention, passed all of the yield strength, fracture toughness and sulfide stress corrosion cracking resistance.

On the other hand, the comparison methods E-2-1 and E-2-1
2-2 was too high in yield strength, and failed in all of yield strength, fracture toughness and sulfide stress corrosion cracking resistance.

[0043]

As described above, according to the present invention, both the fracture toughness and the sulfide stress corrosion cracking resistance are excellent in a mild sour environment, and the yield strength is 758 MPa.
This has industrially useful effects such as obtaining a low-alloy high-strength steel for a seamless steel pipe for an oil well or gas well having a high strength of not less than a and not more than 861 MPa and a steel pipe made of this low-alloy high-strength steel.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yusuke Minami 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Inside Nihon Kokan Co., Ltd. (72) Katsumi Masamura 1-2-1, Marunouchi, Chiyoda-ku, Tokyo No. Nippon Kokan Co., Ltd. (72) Inventor Takashi Osako 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Japan Nippon Kokan Co., Ltd. 4K042 AA06 BA01 BA02 BA03 DA01 DA02 DC02

Claims (4)

[Claims] C: 0.15 to 0.4%; Si: 0.1 to 0.3%; Mn: 0.1 to 0.3%; P: 0.015% or less; 005% or less, Cr: 0.5 to l. 5%, Mo: 0. 1 to 1.0%, B: 0.0005 to 0.003%, Al: 0.01 to 0.1%, Nb: 0.01 to 0.05%, N: 0.003 to 0.01% And at least one of Ti: 0.01 to 0.03% and V: 0.01 to 0.05% (or more, mass%), and the balance: substantially Fe Low alloy high strength steel with excellent fracture toughness and sulfide stress corrosion cracking resistance. 2. C: 0.15 to 0.4%; Si: 0.1 to 0.3%; Mn: 0.1 to 0.3%; P: 0.015% or less; 005% or less, Cr: 0.5 to l. 5%, Mo: 0. 1 to 1.0%, B: 0.0005 to 0.003%, Al: 0.01 to 0.1%, Nb: 0.01 to 0.05%, N: 0.003 to 0.01% And at least one of Ti: 0.01 to 0.03% and V: 0.01 to 0.05% (or more, mass%), and the balance: substantially Fe A billet having a component composition having the following formula and having a diameter of 340 mm or less was cast by a continuous casting method. Thereafter, tempering is performed at a temperature equal to or lower than the Ac ₁ transformation point.
A method for producing a low-alloy high-strength steel pipe excellent in fracture toughness of MPa and resistance to sulfide stress corrosion cracking. 3. C: 0.15 to 0.4%; Si: 0.1 to 0.3%; Mn: 0.1 to 0.3%; P: 0.015% or less; 005% or less, Cr: 0.5 to l. 5%, Mo: 0. 1 to 1.0%, B: 0.0005 to 0.003%, Al: 0.01 to 0.1%, Nb: 0.01 to 0.05%, N: 0.003 to 0.01% And at least one of Ti: 0.01 to 0.03% and V: 0.01 to 0.05% (or more, mass%), and the balance: substantially Fe A billet having a component composition having the following formula and having a diameter of 340 mm or less was cast by a continuous casting method. after heating to a temperature in the range of from Ac ₃ transformation point Ac ₃ transformation point plus 100 ° C., then quenched once again, and, characterized in that the tempering at a temperature of less than Ac ₁ transformation point, the intensity upper limit 861M
A method for producing a low-alloy, high-strength steel pipe having excellent fracture toughness of Pa and resistance to sulfide stress corrosion cracking. 4. C: 0.15 to 0.4%, Si: 0.1 to 0.3%, Mn: 0.1 to 0.3%, P: 0.015% or less, S: 0. 005% or less, Cr: 0.5 to l. 5%, Mo: 0. 1 to 1.0%, B: 0.0005 to 0.003%, Al: 0.01 to 0.1%, Nb: 0.01 to 0.05%, N: 0.003 to 0.01% And at least one of Ti: 0.01 to 0.03% and V: 0.01 to 0.05% (or more, mass%), and the balance: substantially Fe A billet having a component composition having the following formula and having a diameter of 340 mm or less was cast by a continuous casting method. after tempering at a temperature of less than Ac ₁ transformation point, and heated again from Ac ₃ transformation point to a temperature in the range of Ac ₃ transformation point plus 100 ° C., then quenched from the heating temperature range and, Ac ₁ transformation point The upper limit of strength is characterized by tempering at the following temperature. 61MP
A method for producing a low-alloy, high-strength steel pipe having excellent fracture toughness and sulfide stress corrosion cracking resistance.