JP2727865B2 - Manufacturing method of high strength and high corrosion resistance seamless steel pipe - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a high-strength, high-corrosion-resistant seamless steel pipe suitable for use as an oil country tubular good having high strength and high corrosion resistance.

[0002]

2. Description of the Related Art Since the oil crisis, demand for oil and natural gas has increased, and oil and gas fields have been developed around the world. The proportion of oil wells and gas wells in corrosive atmospheres that are difficult to dig deep and that contain wet hydrogen sulfide and carbon dioxide in their products is increasing. In a deep oil well or a gas well, a long oil well pipe or a gas well pipe (hereinafter, referred to as an oil well pipe) having a length of several thousand meters is suspended, and a large load is applied to an oil well pipe near the ground, so that high strength is required. In a corrosive environment such as hydrogen sulfide and carbon dioxide, corrosion resistance, especially sulfide stress corrosion cracking resistance (hereinafter referred to as SSCC resistance)
An oil well tube with excellent performance is required. For this reason, seamless steel pipes used as oil well pipes for drilling of oil wells and gas wells, oil extraction, gas extraction, and the like have been required to have higher strength and higher corrosion resistance than ever before.

Conventionally, strength design of oil country tubular goods is usually performed by yield stress (YS), and the hardness affecting corrosion resistance is equivalent to tensile strength (TS). The points of oil well pipe production include high yield stress (YS) and low tensile strength (TS), that is, high yield ratio (YR = YS / TS). To achieve a high yield ratio, conventionally, C-Si-Mn-C
r A method of adding Mo to steel and using a material having good quenchability, a method of refining the structure by repeating quenching and tempering, and a method of refining the structure by performing cold working such as drawing. Or a combination was employed.

As the SSCC resistance evaluation method, there are known three types: a shell test method, a NACE test method (constant load method), and an SSRT method (low strain rate tensile test method). These all show the same tendency, but the severity is greatest in the shell test method. In the shell test method, a test piece having a thickness of 1.7 mm and a width of 4.5 mm was 0.7 mm in diameter at the center in the longitudinal direction.
In a state where room temperature, 0.5% CH ₃ COOH, 1 atm, H ₂
Crack limit stress is evaluated by Sc value (corrosion resistance index) by keeping in an environment of S saturation for 200 to 500 hours.
FIG. 7 shows the relationship between the Sc value and the Rockwell hardness HRC.
The following relationship exists, and in order to increase the Sc value, it is necessary to decrease the hardness HRC, and the strength decreases.

[0005] The conventional C-Si-Mn-Cr steel is Mo
Is combined with the method of refining the structure by repeating quenching and tempering, the strength is 0.2%.
100 ksi (70 kgf / mm ² ) class, yield strength 110
Although ksi (77 kgf / mm ² ) -class seamless steel pipes can be mass-produced, Mo is added to C-Si-Mn-Cr steel in order to obtain more stable corrosion resistance or to obtain higher corrosion resistance. In addition, it is necessary to combine a method of performing cold working such as drawing to refine the structure, and the cost increases accordingly, resulting in an expensive product and cannot be realized as an industrial product.

As a countermeasure against this, C: 0.15% by weight.
~ 0.45%, Si: 0.1 ~ 1%, Mn: 0.3 ~
1.8%, Sol. Al: 0.01% or less, Ti: 0.
One or two kinds of 005 to 0.1% and Zr: 0.01 to 0.2%, N: ｛0.002+ [Ti (%) + Zr
(%)] / 8% or less, AlN: 0.005% or less, with the balance being 8% with respect to the low alloy steel pipe substantially made of Fe.
After quenching from 80 to 980 ° C, 600 to 730
And tempering at 800 to 950 ° C., and one or more times of plastic working in a temperature range of 600 to 730 ° C. so that the total strain amount is 1 to 20%, followed by quenching from 800 to 950 ° C. and 600 to 730 ° C. (Japanese Unexamined Patent Publication No. 1-283322) has been proposed.

[0007]

SUMMARY OF THE INVENTION The above-mentioned Japanese Patent Application Laid-Open No. 1-283 is disclosed.
No. 322 discloses a method in which mild warm plastic working is introduced into twice quenching of an oil country tubular good and ferrite recrystallization is performed by applying mild warm plastic working to martensite obtained by quenching. 110 k by obtaining fine-grained crystals that are promoted and prevent coarsening of recrystallized grains in the second quench, thereby obtaining fine grains that cannot be obtained by the second quench alone.
Si (77 kgf / mm ² ) or higher strength and SSCC resistance required for it are secured. After quenching, mild warm plastic working is required, and quenching and tempering are required again, which is complicated. Has the disadvantage that

An object of this invention is to seamless steel tube subjected to quenching and tempering, the hardness of the steel pipe in the outer peripheral surface that is exposed to by the warm working Rot <br/> corrosion atmosphere at a specific temperature range residual strain Reduced
Reduced by small is to provide a method of producing a high strength and high corrosion resistance seamless steel pipes that can improve corrosion resistance.

[0009]

Means for Solving the Problems The present inventors have conducted various tests and studies to achieve the above object. As a result, when to steel pipe was quenched and tempered, Ru added warm mild plastic strain <br/> Thus the inner and outer surface vicinity of the wall thickness in the processing of a particular temperature range, low
What gives work hardening if plastic working is given at temperature,
Conversely, warming in a specific temperature range without causing structural changes
The residual strain near the inner and outer surfaces during pipe production that remained in the material due to the construction
The hardness of the inner and outer peripheral surfaces of the steel pipe exposed to corrosive atmosphere decreases, the corrosion resistance improves, and an effect that cannot be considered with the conventional common sense that a high strength and high corrosion resistance seamless steel pipe can be obtained is obtained.
And reached this invention.

That is, according to the present invention, C ≦ 0.30%, S
i: 0.05 to 1.00%, Mn: 0.30 to 1.20
%, S ≦ 0.03%, Cr: 0.50 to 1.50%, M
o: 0.10 to 2.00%, Ni ≦ 0.50%, Cu ≦
After quenching and tempering a steel pipe containing 0.10%, with the balance being Fe and unavoidable impurities,
A plastic strain having a cross-sectional plasticity of 10 to 90% is applied to the steel pipe at a temperature of not more than the final tempering temperature .

[0011]

The reasons for limiting the chemical composition of the steel pipe, the temperature in the warm working after quenching and tempering, and the sectional plastic modulus in the present invention will be described in detail. C is an indispensable element for securing the strength toughness, but if it is too much, quenching cracks will occur during high-temperature quenching, so the upper limit was made 0.30%. S
i is necessary as a deoxidizing element and for securing strength, but if it is less than 0.05%, deoxidation is not sufficient, and if it exceeds 1%, toughness and surface properties are impaired.
%. Mn is an element necessary for deoxidation and securing strength, but if it is less than 0.30%, deoxidation is not sufficient, and
If it exceeds 20%, the cleanliness of the steel is impaired.
1.20%. Since S impairs the cleanliness of the steel and deteriorates the toughness and ductility, the upper limit is set to 0.03%. Cr
Is an element necessary for ensuring quenchability and corrosion resistance, but if less than 0.50%, the effect is not sufficient,
When the content exceeds 1.50%, the hardenability is further improved, but the toughness is reduced. Therefore, the content is set to 0.50 to 1.50%. Mo is an indispensable element for improving hardenability, but if its content is less than 0.10%, its effect is not sufficient, and if it exceeds 2.00%, quenching cracks occur. did. C
If u is too large, deterioration of corrosion resistance such as pitting corrosion and red-hot embrittlement at high temperatures occur, so the upper limit was made 0.10%. Ni is an element effective for improving the corrosion resistance, but if it is too much, corrosion resistance such as pitting corrosion is deteriorated. Therefore, the upper limit is set to 0.50%.

The temperature in the warm working is 400 to 750 ° C.
The reason why the temperature is set to be equal to or lower than the final tempering temperature is that when the temperature is lowered to less than 400 ° C., the material is not softened, but is hardened at the same time.
Above the final tempering temperature , the entire steel pipe is annealed and softened.
And the desired strength cannot be obtained.
The temperature was 750 ° C. and lower than the final tempering temperature . The section plasticity in warm working was set to 10 to 90% because if it is less than 10%, the strain is not applied beyond the elastic range, and if it is 90% or more, the crush amount and the offset amount are considerably large. Which is not effective in terms of deformation and workability of the steel pipe .

[0013] The quenching and tempering of the steel pipe in the present invention comprises:
Martensite structure by quenching and tempering, yield stress YS is 90-125 ksi, tensile strength TS is 100-
The test is performed once or more than once so that the hardness becomes 140 or less and the hardness HRC is 30 or less according to the Rockwell hardness test method specified in JIS Z 2245. The quenched and tempered steel pipe thus obtained is at 400 to 750 ° C and
The cross-section plasticity is 1 by warm working below the final tempering temperature.
Since the plastic strain of 0 to 90% is applied, the hardness of the inner and outer peripheral surfaces of the steel pipe in contact with the corrosive atmosphere is reduced without reducing the structure and the strength of the central part of the wall thickness, and the corrosion resistance represented by SSCC resistance is reduced. It will improve. The reason is not clear
However, the plasticity given by the warm working in the specific temperature range
Residue during pipe production that remained on the inner and outer surfaces of steel pipe due to sexual strain
Distortion is reduced, and hardening caused by residual strain is reduced.
It is considered that the hardness of the peripheral surface decreases. As shown in FIG. 2, the plastic strain in the present invention is applied so that an appropriate stress slightly larger than the yield stress at the working temperature is applied.
The plastic deformation amount is controlled by setting the roll opening (K) (crash amount) shown in FIG. 4 shows the plastic deformation region δ ₀ due to the offset, and FIG. 5 shows the plastic deformation region δc due to the crush. At this time, the section plasticity 塑₀ and ξ _c are the pipe diameter d and the pipe wall thickness t, respectively. When used, ξ ₀ = δ ₀ / d and ξ _c = δc / t. FIG.
Is a graph showing an example of the relationship between the offset, the amount of crush and the section plasticity ξ ₀ , ξc.

[0014]

【Example】

Example 1 C: 0.27%, Si: 0.16%, Mn: 0.46
%, S: 0.001%, Cr: 0.98%, Mo: 0.
Outer diameter 244.5 mm, wall thickness 15.11 mm, length 12 containing 70%, Ni: 0.02%, Cu: 0.02%
m, quenched at 920 ° C. for 5 minutes, tempered at 600 ° C. for 30 minutes, and subsequently
After a second quenching at 90 ° C. for 5 minutes, 690 ° C.
For 30 minutes. Then, at a temperature of 640 to 660 ° C., a cross-section plasticity ratio of 5 was obtained by crushing and offset processing using an inclined 6-roll type straightening machine shown in FIG.
Plastic working was performed at 0 to 60%. In addition, 1 in FIG. 1 shows a seamless steel pipe, 2 shows a straightening roll. The yield stress, tensile strength, Rockwell hardness and Sc value of the outer peripheral portion, the thickness center, and the inner peripheral portion of the manufactured steel pipe were measured. For comparison, the same measurement was performed on a steel pipe obtained by quenching and tempering under the same conditions except that crash or offset processing was not performed by a warm straightener as a conventional method. Table 1 shows the results.

[0015]

[Table 1]

As shown in Table 1, in the conventional method, the tempering and warm working conditions were such that the hardness was uniform in the thickness direction of the steel pipe. Therefore, when the strength was increased, the hardness also increased and the corrosion resistance was increased. In particular, this method had an adverse effect on the SSCC resistance. However, in the method of the present invention, the hardness can be reduced only on the inner and outer surface layers of the steel pipe in contact with the corrosive atmosphere with the same strength, and the Sc value showing the SSCC resistance is significantly increased. Can be improved.

Example 2 Heat having an outer diameter of 244.5 mm and a wall thickness of 11.99 mm comprising steels of the present invention indicated by a to e in Table 2 and steels not indicated by the present invention indicated by f to h in the same table. As shown in Table 3, the first quenching and tempering was performed on the cold-worked pipe, and then the second quenching and tempering was performed. Then, the pipe was crashed at a temperature of 640 to 660 ° C. by an inclined 6-roll type straightening machine. Alternatively, plastic working was performed by offset processing. Further, for comparison, normal quenching and tempering were further performed twice. The yield stress, tensile strength, Rockwell hardness and Sc value of the inner and outer peripheral portions and the central portion of the thickness of each manufactured steel pipe were measured. Table 4 shows the results.

[0018]

[Table 2]

[0019]

[Table 3]

[0020]

[Table 4]

As shown in Table 4, according to the method of the present invention, the hardness of the inner and outer peripheral portions of the steel pipe is reduced by 2 to 3 as compared with the central portion, and accordingly, the Sc values of the outer and inner peripheral portions become 1 to 2. Has improved. On the other hand, in the comparative example, the strength is high but the Sc value is low, or one of the strength, hardness and Sc value is inferior.

[0022]

As described above, according to the method of the present invention, the Sc value is increased by imparting plastic strain to only the inner and outer surface layers of the steel pipe in contact with the corrosive atmosphere, thereby increasing the Sc value and increasing the strength and strength. It is possible to mass-produce high-strength, high-corrosion-resistant oil well pipes without satisfying both corrosion resistance and cold drawing, etc., which is extremely advantageous in terms of production equipment, production efficiency and production cost, and reduces the use of high-grade oil well pipes. It can be manufactured industrially at low cost.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram of an inclined roll straightener used in an embodiment.

FIG. 2 is an explanatory diagram of an offset amount.

FIG. 3 is an explanatory diagram of a crash amount.

FIG. 4 is a conceptual diagram showing a plastic deformation region δ ₀ due to offset.

FIG. 5 is a conceptual diagram showing a plastic deformation region δc due to a crash.

FIG. 6 shows offset, crush amount and section plasticity 断面₀ ,
It is a graph which shows an example of change of (DELTA) c and plastic deformation area (delta) ₀ , (delta) c.

FIG. 7 is a graph showing the relationship between the Sc value and the Rockwell hardness HRC.

[Explanation of symbols]

 1 Seamless steel pipe 2 Straightening roll

Claims (1)

(57) [Claims] 1. C ≦ 0.30%, Si: 0.05-1.
00%, Mn: 0.30 to 1.20%, S ≦ 0.03
%, Cr: 0.50 to 1.50%, Mo: 0.10 to
A steel pipe containing 2.00%, Ni ≦ 0.50%, Cu ≦ 0.10%, and the balance being Fe and unavoidable impurities, after quenching and tempering, at 400 to 750 ° C. and final
Sectional plasticity of 10 to 90% for steel pipes at temperatures below tempering temperature
A method for producing a high-strength, high-corrosion-resistant seamless steel pipe, characterized by applying plastic strain.