JP2579094B2 - Manufacturing method of oil well steel pipe with excellent sulfide stress cracking resistance - 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 producing a steel pipe for oil wells having excellent sulfide stress cracking resistance, particularly a steel pipe for oil wells having a yield strength of 60 kgf / mm ² or more.

[0002]

2. Description of the Related Art As a method for producing a steel pipe for an oil well having excellent resistance to sulfide stress cracking, there is known a method of quenching and tempering steel having a specific chemical component as disclosed in Japanese Patent Application Laid-Open No. 62-253720. Have been.

[0003]

However, the above-described quenching / tempering method is inferior in thermal efficiency and contradicts the technical trend of saving steps, and the surface quality is deteriorated by heating for quenching. There are drawbacks such as. Conventionally, quenching and tempering were indispensable because sulfide stress cracking resistance required fine grains, whereas direct quenching after hot seamless rolling could produce fine-grained austenite. In addition, there is a problem that a steel pipe for oil wells having excellent sulfide stress cracking resistance cannot be obtained due to large variation in particle size.

[0004] The present invention solves the above-mentioned problems, and the steel composition and the hot rolling conditions are combined to make the austenite grains relatively fine, and to increase the sulfide stress cracking resistance at the obtained grain size. An object of the present invention is to provide a steel pipe for oil wells having high strength and excellent sulfide stress cracking resistance.

[0005]

Means for Solving the Problems The inventors of the present invention have conducted many experimental studies to achieve the above object, and as a result, by adjusting the steel composition and the hot rolling conditions, the austenite grains can be relatively reduced. It has been found that a steel pipe for an oil well of a low alloy steel excellent in sulfide stress cracking resistance can be manufactured by controlling the steel structure with fine grains and controlling the steel structure with the grain size. That is, the present invention has been constructed based on this finding, and the gist thereof is as follows: C: 0.10 to 0.35%, Si: 0.50% or less, and Mn: 0.10 to 1. 2%, S: 0.005% or less, P: 0.015% or less, Mo: 0.10 to 0.80%, Al: 0.1% or less, Nb: 0.005 to 0.1%, Ti : 0.005 to 0.1% and Ti ≧ 3.4N, N: 0.008% or less, B: 0.0005 to 0.0030%, and if necessary, Cr: 0.1 to 1 0.5%, V: 0.01 to 0.1%, Co: 0.05 to 0.5%, Zr: 0.001 to 0.1%, and further, if necessary, Rare earth element: 0.001 to 0.05%, Ca: 0.001 to 0.02% 1 or 2 or more types, with the balance being substantial After heating the slab consisting of Fe above 1150 ° C., by heating the plain tube dropped to a 900 ° C. to Ar ₁ point Seikan the hollow shell by hot piercing continuous rolling in 900 to 1000 ° C., Hot finishing rolling is performed at a finishing temperature of Ar ₃ points + 50 ° C. or higher, and then the finished steel pipe is quenched to rapidly cool from a temperature of Ar ₃ points or higher to have a martensite structure of 90% or more. This is a method for producing a steel pipe for an oil well having excellent resistance to sulfide stress cracking in which a tempering treatment is performed at a temperature of ₁ point or less.

[0006]

The production method of the present invention will be described below in detail. First, the reason why the present invention is limited to the above steel components will be described. C is a basic component of steel that has the effect of increasing the strength of the steel and increasing the hardenability, but if it is too small, it has no effect, and if it is too large, it causes quenching cracking. 0.35%. Si
Is the one in which the deoxidizing agent remains, but if it is too much, the sulfide stress cracking resistance is reduced. Mn is indispensable for detoxifying S as a basic component of steel, and is an effective element in terms of enhancing hardenability. However, Mn itself significantly reduces sulfide stress cracking resistance. The addition amount was 0.1 to 1.2%.

S is an inevitable impurity in steel, but MnS
Is harmful to sulfide stress cracking resistance and 0.00
5% or less. P segregates at the grain boundaries and greatly reduces sulfide stress cracking resistance.
% Or less.

Mo has the effect of increasing the resistance to sulfide stress cracking and has the effect of improving the hardenability.
If it is 0.1% or less, there is no effect. If it exceeds 0.8%, the effect is not only saturated, but also Mo ₂ C precipitates and the toughness is deteriorated. 0.8%.
Al has a deoxidizing agent remaining like Si. If the content is too large, inclusions increase and the sulfide stress cracking resistance deteriorates, so the upper limit was made 0.1%.

[0009] Nb is the most important element in the components of the present invention. After making a hollow shell by hot piercing continuous rolling, the hollow shell was cooled to a temperature of 900 ° C. to Ar ₁ point.
The austenite grains when heated to ~ 1000 ° C become abnormally coarse at the usual reheating temperature (the temperature required to secure the quenching temperature after the final finish rolling). Nb
Is an important element that suppresses the coarsening of austenite grains having such a rolling history. If the amount is too small, the effect is not obtained, and if the amount is too large, the effect is saturated and the cost is extremely high.

[0010] Ti is an essential element in the direct quenching step in order to fix N, which is a component that cannot be completely removed from steel, as TiN and to exert the effect of improving the hardenability of B. Further, it has the effect of forming Ti oxide to allow P to be present in the grains, thereby reducing the amount of segregation at the grain boundaries. If the amount is too small, this effect is not exhibited, and addition of 3.4N or more in the same atomic molar ratio is necessary for fixing N. However, if it is too large, a large amount of TiC precipitates during tempering and significantly impairs toughness.
≧ 3.4N. N is inevitably contained in steel,
If the amount is too large, the amount of Ti added will increase as a result.

B is an element which significantly improves the hardenability in a very small amount and does not lower the sulfide stress cracking resistance. On the other hand, the hardenability improving element except Mo usually has an effect of reducing sulfide stress cracking resistance. Therefore, B, which is extremely effective for turning the structure after quenching into martensite and does not reduce the resistance to sulfide stress cracking, is an essential element in the present invention. If the amount is too small, the effect is not sufficient. If the amount is too large, a B composite precipitate is formed and the sulfide stress cracking resistance is reduced.
5 to 0.0030%.

[0012] The present invention, in addition to the above component composition, Cr, V,
Co, Zr, rare earth elements, and Ca are selectively added as needed. Cr has the effect of improving the hardenability and forming martensite, but when added in large amounts, it reduces the sulfide stress cracking resistance. If it is less than 0.1%, there is no effect of addition, and if it exceeds 1.5%, the sulfide stress cracking resistance is significantly reduced.
1.5%. V is effective as a strengthening element, but has no effect in a very small amount, and if added in a large amount, the toughness is deteriorated. Therefore, V was added in an amount of 0.01 to 0.1%. Co has the effect of improving the sulfide stress cracking resistance by forming a robust film on the steel surface in a wet hydrogen sulfide environment and reducing the amount of hydrogen penetration. The effect is not remarkable in a small amount, and the effect is saturated even if added in a large amount.
0.5% was added. Zr has the effect of forming a Zr oxide to cause P to be present in the grains, thereby reducing the amount of segregation at the grain boundaries. If the amount is too small, this effect is not exhibited, and if it is added in a large amount, it is feared that a large amount of Zr oxide is formed and becomes a starting point of cracking, so 0.001 to 0.1%
Was added. Rare earth elements and Ca are effective elements that make the form of inclusions spherical and harmless. If the amount is too small, the effect is not obtained. If the amount is too large, inclusions are increased and the sulfide stress cracking resistance is reduced.
-0.05%, 0.001-0.02%.

Next, the rolling conditions of the hot piercing continuous rolling will be described. The steel having the above composition is melted in a melting furnace such as a converter or an electric furnace or further subjected to a vacuum degassing process, and a steel slab is manufactured by a continuous casting method or an ingot lump method. The slab is heated immediately or after cooling once to a temperature of 1150 ° C. or higher. The heating temperature depends on most components, especially Nb
Must be high enough to form a solid solution. 1
At a temperature of 150 ° C. or higher, a solid solution is almost formed and no problem occurs in hot working.

The slab heated to a high temperature is conveyed to a continuous hot piercing mill, rolled to a desired outer diameter and thickness, and roughly formed into a hollow shell. Then the plain tube is finished steel pipe finishing temperature was heated to 900 to 1000 ° C. is obtained by performing hot finish rolling at Ar ₃ point + 50 ℃ or more drops to a temperature of 900 ° C. to Ar ₁ point, Ar ₃ A quenching process of rapidly cooling from a temperature above the point is performed. FIG. 1 shows the effect of Nb on the austenite grain size after the direct quenching treatment of the steel pipe manufactured by this rolling. That is, the austenite grain size after direct quenching is remarkably coarse when Nb is not added, or when the added amount is less than 0.005%, the ASTM
No. However, if it exceeds 0.005%, ASTM No. 6 to 7. Therefore, 0.005 to 0.1% of Nb is required to prevent the austenite grains from becoming coarse. The present inventors presume the influence of Nb as follows. That is, if Nb is not added, or if the addition is 0.005% or less, the raw tube falls to a temperature of 900 ° C. to Ar ₁ point after the hot piercing continuous rolling step, and then is heated to a temperature of Ac ₃ points or more. In the case where the final step in the continuous tube rolling process is relatively low temperature and under small pressure, grain boundary migration occurs due to non-uniform strain between austenite grains in the reheating process, so-called abnormal grain growth and Become. Addition of 0.005% or more of Nb has an important function of suppressing the growth and coarsening of austenite grains having such a rolling history. That is, Nb precipitates as Nb (CN) before and during the reheating process, and has an important effect of suppressing the coarsening of austenite grains.

Further, as shown in FIG. 2, when the austenite grain size is 6 or more, the sulfide stress cracking resistance is at a high level. Heating such a hollow shell dropped to a temperature of 900 ° C. to Ar ₁ point to 900 to 1000 ° C.
If it is lower than 900C, the quenching temperature after the hot final finish rolling cannot be secured, and if it is higher than 1000C, the grain growth may occur in some cases, so the temperature was limited to 900 to 1000C.

[0016] The quenching starting temperature to the martensite structure after quenching needs to Ar ₃ point or more, and the Ar ₃ point + 50 ℃ Since this condition is not satisfied when the finish rolling temperature is lowered. Finish rolling is usually performed to adjust the dimensions of the steel pipe, and the method is not particularly specified.However, scale formed during reheating in this step is removed, and almost no scale is formed in the tempering step. It is also an advantage of the present method that a steel pipe having a good surface texture can be obtained as compared with the ordinary reheating quenching method.

Further, in the present invention, the steel structure after quenching needs to be 90% or more of martensite. The factor that has the greatest effect on sulfide stress cracking resistance is the structure, which is one of the important requirements of the present invention. FIG. 3 shows the relationship between the sulfide stress cracking resistance and the martensite ratio.
When the martensite ratio is lower than 90%, the sulfide stress cracking resistance sharply decreases. On the other hand, the strength also drops sharply. Therefore, the structure after quenching was limited to 90% or more of martensite. The martensite ratio can be known from the C content and the hardness. Generally, in order to form martensite, it is necessary to increase the cooling rate during quenching or to enhance the hardenability of steel. Generally, it is necessary to obtain martensite by increasing the hardenability of steel on the premise of a cooling rate determined by the cooling capacity of the quenching device and the wall thickness of the steel pipe. As described in the section on the reasons for limiting the steel composition, elements other than Mo and C, such as Mn and Cr, which enhance the hardenability lower the sulfide stress cracking resistance. Therefore, N is fixed by adding Ti, and B is added. M
It is important to reduce the amount of n.

After the quenching process is completed, tempering is performed to adjust the strength to a desired value. Tempering temperature since reaustenitization portion lowers the resistance to sulfide stress cracking becomes low-temperature transformation product exceeds Ac ₁ points Ac ₁
Points or less.

The steel pipe for oil well obtained under the above manufacturing conditions is high in strength, excellent in sulfide stress cracking resistance and inexpensive, and can be greatly useful for oil well development under severe conditions.

[0020]

Next, an embodiment of the present invention will be described. The steel slab produced through continuous casting and smelting in a converter is hot-pierced and continuously rolled, then reheated, then subjected to hot final finishing rolling, directly quenched, and tempered to produce a steel pipe. The sulfide stress cracking properties were evaluated. Sulfide stress cracking resistance is NACE
Perform a constant load test based on TM0177-90A,
Determine the maximum stress that does not break within the test time of 720 hours,
The value was divided by the yield strength of the material, and the value was represented by the Rs value.
Table 1 shows the chemical composition of the steel used, and Table 2 shows the manufacturing conditions, yield strength, and Rs value. From the results in Table 2, the Rs of the steel pipe for oil well manufactured according to the present invention is 0.4 to 0.4 in the comparative method.
It can be seen that the sulfide stress cracking resistance is extremely excellent when the ratio is 0.9 or more, while it is decreased to 0.7. Note that the ratio
Comparative steel 5 was oil quenched (other steels were quenched in water)
Therefore, the martensite ratio is as low as 80%.

[0021]

[Table 1]

[Table 2]

[0022]

As described above, according to the present invention, the steel composition is specified, the hot rolling conditions are adjusted, and the steel structure is improved, whereby high strength and sulfide corrosion cracking resistance can be obtained. An excellent oil well steel pipe can be manufactured.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the amount of Nb added and austenite grain size.

FIG. 2 is a graph showing the relationship between austenite grain size and sulfide stress corrosion cracking resistance.

FIG. 3 is a graph showing a relationship between a martensite ratio and sulfide stress corrosion cracking resistance.

Claims (4)

(57) [Claims] C: 0.10 to 0.35%, Si: 0.50% or less, Mn: 0.10 to 1.2%, S: 0.005% or less, P: 0. 015% or less, Mo: 0.10 to 0.80%, Al: 0.1% or less, Nb: 0.005 to 0.1%, Ti: 0.005 to 0.1% and Ti ≧ 3. A steel slab containing 4N, N: 0.008% or less, B: 0.0005 to 0.0030%, and the balance substantially consisting of Fe is 1150.
After heating above ° C., the plain tube dropped to a 900 ° C. to Ar ₁ point Seikan the hollow shell by hot piercing continuous rolling, 900
Heat to ~ 1000 ° C, finishing temperature is Ar ₃ points + 50 ° C
The hot finish rolling is performed as described above.
r Quenching from ₃ or more temperatures to 9
Martensite structure of 0% or more, and then A
c A method for producing a steel pipe for an oil well having excellent sulfide stress cracking resistance, wherein a tempering treatment is performed at a temperature of ₁ point or less. 2. In mass%, C: 0.10 to 0.35%, Si: 0.50% or less, Mn: 0.10 to 1.2%, S: 0.005% or less, P: 0. 015% or less, Mo: 0.10 to 0.80%, Al: 0.1% or less, Nb: 0.005 to 0.1%, Ti: 0.005 to 0.1% and Ti ≧ 3. 4N, N: 0.008% or less, B: 0.0005 to 0.0030%, Cr: 0.1 to 1.5%, V: 0.01 to 0.1%, Co: 0 0.05-0.5%, Zr: 0.001-0.1% A steel slab containing one or more of the following and substantially consisting of Fe is heated to 1150 ° C. or more, The hollow tube was formed into a hollow shell by continuous piercing and then heated to 900-1000 ° C. which had dropped to 900 ° C.-Ar ₁ point, and the finishing temperature was ₃ points Ar + 50 ° C. or less. Hot finish rolling above, followed by quenching the finished steel tube from a temperature of ₃ points or more to a martensite structure of 90% or more, and then at a temperature of ₁ point or less of Ac. A method for producing a steel pipe for oil wells having excellent resistance to sulfide stress cracking, characterized by performing a tempering treatment. 3. In mass%, C: 0.10 to 0.35%, Si: 0.50% or less, Mn: 0.10 to 1.2%, S: 0.005% or less, P: 0. 015% or less, Mo: 0.10 to 0.80%, Al: 0.1% or less, Nb: 0.005 to 0.1%, Ti: 0.005 to 0.1% and Ti ≧ 3. 4N, N: 0.008% or less, B: 0.0005 to 0.0030%, rare earth element: 0.001 to 0.05%, Ca: 0.001 to 0.02% Alternatively, after heating a steel slab containing at least two kinds and substantially consisting of Fe to 1150 ° C. or higher, it is formed into a hollow shell by hot piercing continuous rolling and dropped to 900 ° C. to Ar ₁ point. The raw tube is heated to 900 to 1000 ° C., subjected to hot finishing rolling at a finishing temperature of ₃ points of Ar + 50 ° C. or higher, and then the finishing is performed. A quenching treatment of rapidly raising the raised steel pipe from a temperature of ₃ points or more to a martensite structure of 90% or more, followed by a tempering treatment at a temperature of ₁ point or less of Ac. A method for manufacturing oil well steel pipes with excellent material stress cracking properties. 4. C: 0.10 to 0.35%, Si: 0.50% or less, Mn: 0.10 to 1.2%, S: 0.005% or less, P: 0. 015% or less, Mo: 0.10 to 0.80%, Al: 0.1% or less, Nb: 0.005 to 0.1%, Ti: 0.005 to 0.1% and Ti ≧ 3. 4N, N: 0.008% or less, B: 0.0005 to 0.0030%, Cr: 0.1 to 1.5%, V: 0.01 to 0.1%, Co: 0 0.05 to 0.5%, Zr: 0.001 to 0.1%, or two or more rare earth elements: 0.001 to 0.05%, Ca: 0.001 to 0.02% After heating a steel slab containing one or more kinds and substantially the remainder of which is made of Fe to 1150 ° C. or more, it is formed into a hollow shell by hot piercing continuous rolling. Heating the 0 ° C. to Ar the plain tube drops to _a point to 900 to 1000 ° C., the finishing temperature is subjected to hot finish rolling at Ar ₃ point + 50 ℃ or higher, followed by the finishing steel pipe of _three or more Ar An oil well with excellent sulfide stress cracking resistance, characterized by quenching from a temperature to a martensitic structure of 90% or more by applying a quenching treatment and then performing a tempering treatment at a temperature of ₁ point or less of Ac. Method of manufacturing steel pipes.