JPH0633194A - High strength steel for oil well pipe excellent in corrosion resistance - Google Patents

Abstract

PURPOSE:To obtain a steel for oil well pipe having high toughness and high strength and also having superior corrosion resistance, e.g., under the environment containing carbon dioxide gas by specifying a composition consisting of C, Si, Mn, Cr, Mo, Al, Nb, V, N, P, S, and Fe. CONSTITUTION:This high strength steel for oil well pipe excellent in corrosion resistance can be obtained by providing a composition which consists of, by weight, 0.08-0.15% C, 0.01-<1.2% Si, 0.02-3.0% Mn, 8.0-9.9% Cr, 0.82-3.0% Mo, 0.005-1.0% Al, 0.05-0.50% Nb, 0.16-0.60% V, 0.02-0.07% N, P and S reduced, respectively, to <=0.03% and <=0.015%, and the balance Fe with inevitable impurities and further contains, if necessary, <=0.2% each of one or more elements among W, Ni, Cu, and Co as additive components and further contains one or more kinds among <=0.05% rare earth elements and <=0.03% Ca and where the MA value represented by (MA value)=80+420C+440N+30(Mn+Ni+Cu+ Co)-12(Si+Cr+Mo+Nb+V)-6W is regulated to >=0.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength steel for oil country tubular goods having excellent corrosion resistance, and more specifically, for use in oil country tubular goods used for drilling and production of petroleum and natural gas, for example, wet carbon dioxide gas and a trace amount of wet sulfurization. The present invention relates to steel having excellent corrosion resistance in an environment containing hydrogen.

[0002]

2. Description of the Related Art Recently, petroleum and natural gas produced have been increasing in amount containing moist carbon dioxide and hydrogen sulfide. It is a well-known fact that carbon steel and low alloy steel are significantly corroded in such an environment. Therefore, when transporting such corrosive oil and natural gas, it has been common to add a corrosion inhibitor as a corrosion protection measure for the oil country tubular good. However, in an ocean oil well, the cost required for adding and recovering a corrosion inhibitor becomes enormous, and due to the problem of marine pollution, its use is becoming difficult. Therefore, the need for corrosion resistant materials that do not require the addition of corrosion inhibitors has increased recently.

As a corrosion resistant material for petroleum and natural gas containing a large amount of carbon dioxide gas, the application of stainless steel has been first examined. J. Klein, Corrosio
n) '84, paper number 211, about 0.2% C and 12 to 13% C as represented by AISI 420 steel, which has high strength and is relatively inexpensive.
Martensitic stainless steel containing r is widely used. However, in order to obtain high strength of such a steel as an oil country tubular good, it cannot be tempered at a too high temperature, and there is a drawback that the impact toughness is inferior.

[0004]

SUMMARY OF THE INVENTION In view of these circumstances, an object of the present invention is to provide a high strength steel for oil country tubular goods having excellent corrosion resistance in an environment containing carbon dioxide gas.

[0005]

As a result of various studies on the composition of steel in order to achieve the above object, the present inventors have found that Cr
Content of 8.0 to 9.9%, Mo content of 0.82 to 3.0%, C content of 0.08 to 0.15%, and N content of 0.02 to 0.0.
Steel containing 7% contains 0.05% or more of Nb and 0.1% of V.
When 6% or more is added in combination, extremely high strength can be obtained as an oil country tubular goods, and therefore high strength can be obtained even by tempering at high temperature, so that impact toughness can be improved, and corrosion resistance in a wet carbon dioxide environment can also be obtained. I found it to be excellent. Also,
The inventors of the present invention further studied optimization of each element and found that the addition amount of each element is the MA value ([% X] is% by weight) of the following equation.
It was also found that excellent impact toughness can be obtained when the content of the element X represented by (4) is 0 or more. MA value = 80 + 420 [% C] +440 [% N] +30 ([% Mn] + [% Ni] + [% Cu] + [% Co])-12 ([% Si] + [% Cr] + [% Mo] + [% Nb] + [% V]) −6 [% W] Furthermore, the present inventors applied W, Ni, Cu, C to such steel.
It was found that the corrosion resistance is further improved by adding o, a rare earth element or Ca.

The present invention was made mainly on the basis of the above findings. The gist of the first invention of the present invention is C: 0.08 to 0.15% by weight, Si: 0.
01% or more and less than 1.2%, Mn: 0.02 to 3.0%,
Cr: 8.0-9.9%, Mo: 0.82-3.0%,
Al: 0.005-1.0%, Nb: 0.05-0.5
0%, V: 0.16 to 0.60%, N: 0.02 to 0.
07%, P is 0.03% or less, S is 0.015
%, And the MA value consisting of the balance Fe and unavoidable impurities and given by the following formula ([% X] is the content of the element X expressed in% by weight) MA value = 80 + 420 [% C] +440 [% N] +30 ([% Mn] + [% Ni] + [% Cu] + [% Co])-12 ([% Si] + [% Cr] + [% Mo] + [% Nb] + [% V] ]) -6 [% W] is 0 or more, which is a high-strength steel for oil country tubular goods having excellent corrosion resistance, and the gist of the second invention is the first aspect.
In addition to the inventive steel as an additional component, in wt%, W: 2.
0% or less, Ni: 2.0% or less, Cu: 2.0% or less,
Co: 2.0% or less, a steel containing one or more kinds, and the gist of the third invention is that the steel of the first invention or the second invention further contains, as an additional component, wt%,
Rare earth element: 0.05% or less, Ca: 0.03% or less,
Steel containing one or two of.

[0007]

The reason why the range of each component is limited in the present invention will be described below. C: C has the effect of forming a compound with Nb and V to increase the strength, but if its content is less than 0.08%, it is difficult to obtain sufficient impact toughness because low temperature tempering is required. However, if it is contained in excess of 0.15%, carbides are generated in the prior austenite grain boundaries to lower the impact toughness, so the content of C is made 0.08 to 0.15%.

Si: Si is effective to be added as a deoxidizing agent and a strengthening element to steel containing Cr in an amount of 8.0% or more, but if the content is less than 0.01%, the deoxidizing effect is not sufficient. If the content is 1.2% or more, not only the effect is already saturated but also the impact toughness is lowered.
The content range of i is limited to 0.01% or more and less than 1.2%.

Mn: Mn is necessary as a deoxidizing agent for steel, and it is necessary to contain 0.02% or more. However, even if it is contained in excess of 3.0%, its effect is already saturated, Excessive Mn content causes manufacturing difficulties, so the upper limit content is 3.0%. Cr: Cr is required to be contained in an amount of 8.0% or more in order to secure the corrosion resistance and high strength in the wet carbon dioxide environment targeted by the present invention, but even if it is contained in excess of 9.9%. Since the cost is unnecessarily increased, the upper limit content is set to 9.9%.

Mo: Mo should be added to steel containing 8.0% or more of Cr in an amount of 0.82% or more in order to ensure corrosion resistance in a wet carbon dioxide environment, but 3.0% is required. Even if added in excess, the effect will no longer be saturated, but it will only increase the possibility that ferrite will be easily formed to reduce hot workability and strength, so the upper limit content is 3.0%. And

Al: Al is 0.00 as a deoxidizing agent for steel.
It is necessary to add 5% or more, but if added over 1.0%, coarse oxide inclusions are formed and the stress corrosion cracking resistance is lowered, so the content is 0.005 to 1. Limited to 0%. Nb: Nb contains Cr in an amount of 8.0% or more and Mo in an amount of 0.
When it is added together with V to steel containing 82% or more, carbonitride is generated and the effect of increasing the strength is remarkable, but the content is less than 0.
If the content is less than 05%, the effect is insufficient. On the other hand, if the content exceeds 0.50%, coarse carbides are formed and the toughness and stress corrosion cracking resistance are impaired, so the content range is 0.0
5 to 0.50%.

V: V also contains Cr in an amount of 8.0% or more,
Addition of Mo to steel containing 0.82% or more together with Nb produces carbonitrides and has a remarkable effect of increasing strength.
If the content is less than 0.16%, the effect is insufficient,
On the other hand, if the content exceeds 0.60%, coarse carbides are generated and the toughness and stress corrosion cracking resistance are impaired, so the content is made 0.16 to 0.60%.

N: N is solid solution strengthening and Nb and V
However, if the content is less than 0.02%, the margin for improving strength is insufficient, and 0.0
If the content exceeds 7%, the impact toughness decreases, so the content is made 0.02 to 0.07%. P: If a large amount of P is present, the toughness is lowered, so it is preferable that the P content be small, and it is necessary to reduce it to 0.03% or less.

S: If S is present in a large amount, hot workability and ductility are deteriorated, so S is preferable to be small, and
It is necessary to reduce it to 015% or less. Furthermore, in the present invention, it is necessary that the MA value defined by the following formula is 0 or more as a combination of the contents of the respective elements. MA value = 80 + 420 [% C] +440 [% N] +30 ([% Mn] + [% Ni] + [% Cu] + [% Co])-12 ([% Si] + [% Cr] + [% Mo] + [% Nb] + [% V]) −6 [% W] Here, [% X] represents the content of the element X when expressed in weight%. The MA value is a finding based on the results of the present inventors' detailed examination of the characteristics as an oil country tubular good, particularly the optimum component composition and combination for obtaining strength and impact toughness, and when the MA value is less than 0, heat treatment conditions This is because it is difficult to make the structure essentially a structure mainly composed of martensite, and the strength and toughness, which are important properties as an oil country tubular good, are reduced, no matter how controlled the In order to obtain an excellent oil country tubular good, it is important to set the MA value to 0 or more.

The above are the basic components of the high-strength steel for oil country tubular goods to which the present invention is directed. In the present invention, however, steels whose properties are further improved by further adding the following elements as required are also targeted. There is. W: W contains 8.0% or more of Cr and 0.8 of Mo
Addition to steel containing 2% or more has the effect of further increasing the corrosion resistance required for oil country tubular goods, but even if added in excess of 2.0% the effect will no longer saturate, but the cost will be unnecessarily increased. The upper limit content is set to 2.0% because it is merely increased.

Ni: When Ni is added to steel containing Cr in an amount of 8.0% or more and Mo in an amount of 0.82% or more, it is effective in further improving the corrosion resistance required as an oil country tubular good and improving the toughness. However, even if added over 2.0%, the effect is already saturated, but the cost only unnecessarily increases, so the upper limit content is made 2.0%.

Cu: Cu contains 8.0% or more of Cr, and when added to steel containing 0.82% or more of Mo, it has the effect of further increasing the corrosion resistance required for an oil country tubular good, but 2.0%. If it is added in excess, the effect will be saturated but the cost will be unnecessarily increased. Therefore, the upper limit content is 2.0%. Co: Co contains 8.0% or more of Cr and contains Mo of less than 0.8%.
When it is added to steel containing 82% or more, it is effective in further improving the corrosion resistance required for oil country tubular goods and improving toughness, but even if it is added in excess of 2.0%, the effect is already saturated. On the other hand, the cost is merely increased unnecessarily, so the upper limit content is 2.0%.

Rare Earth Elements (REM), Ca: Rare earth elements and Ca are elements which are effective in improving hot workability and impact toughness, but rare earth elements exceed 0.05% and Ca is 0%. If added in excess of 0.03%, coarse non-metallic inclusions are generated and conversely deteriorate hot workability and corrosion resistance. Therefore, the upper limit content of rare earth elements is 0.
It should be 05% and Ca should be 0.03%. In the present invention, the rare earth elements refer to elements having atomic numbers 57 to 71 and 89 to 103 and Y.

In the steel of the present invention, in addition to the above-mentioned components, Ti, B, etc. can be contained as impurities mixed from scraps or the like, or for the purpose of adjusting toughness, workability, etc. M defined in the claims
It goes without saying that the A value must be 0 or more. Further, although the oxygen content is not particularly limited in the present invention, it is a matter of course that the smaller the oxygen content, the better since it is an impurity that is a source of forming oxide-based nonmetallic inclusions.

When the steel proposed in the present invention is used as an oil country tubular good, a seamless steel tube may be formed by a normal seamless rolling process or a hot extrusion process, or a steel strip may be formed by hot rolling and then a normal electric resistance welding may be performed. It is also possible to manufacture as a steel pipe according to the present invention, and any steel having a composition and a combination of elements defined in the present invention including other processes is a target of the present invention, and the cost and the existing manufacturing equipment The optimum oil country tubular goods production process can be selected due to restrictions such as above, and whichever oil country tubular goods production process is selected does not depart from the scope of the present invention.

[0021]

EXAMPLES Examples of the present invention will be described below. Steels having the components shown in Table 1 are melted and made into a steel plate having a wall thickness of 12 mm by a normal hot rolling process, quenched at 980 ° C. and then tempered to obtain a high strength steel having a yield strength of 551 N / mm ² or more. did.

Next, test pieces were taken from each steel and subjected to a corrosion test in a wet carbon dioxide gas environment. For a corrosion test in a wet carbon dioxide environment, a thickness of 3 mm, a width of 15 mm,
Using a test piece of 50 mm in length, 5% NaCl under the condition of carbon dioxide gas of 20 atm in an autoclave at a test temperature of 100 ° C.
After being immersed in the aqueous solution for 30 days, the corrosion rate was calculated from the weight change before and after the test. The unit of the corrosion rate is expressed in mm / y. Generally, if the corrosion rate of a material in an environment is less than 0.1 mm / y, it is considered that the material is sufficiently corrosion resistant in that environment and can be used. Has been.
A JIS No. 4 impact test piece (full size) was taken from each steel and a Charpy impact test was performed.

The test results are also shown in Table 1. In the corrosion test results of Table 1, ◯ indicates that the corrosion rate was less than 0.1 mm / y, and x indicates the corrosion rate was 0.1 mm / y.
Above 0.5mm / y, XX has a corrosion rate of 0.5mm
/ Y or more, respectively. Also,
In the Charpy impact test results, ○ indicates that the fracture surface transition temperature is −
Less than 20 ° C., x indicates that the fracture surface transition temperature is -20 ° C. or more and less than 0 ° C., and xx indicates that the fracture surface transition temperature is 0 ° C. or more.

As is clear from Table 1, No. 1 which is an example of the present invention. 1 to 8 are excellent in impact toughness, have a corrosion rate smaller than 0.1 mm / y which is a practically usable corrosion rate even in a high temperature of 100 ° C. in a wet carbon dioxide environment, and have excellent corrosion resistance and high strength. It can be seen that the sample has impact toughness. On the other hand, No. 9 and No. No. 12 has a poor impact toughness because the MA value is less than 0. No. 10 has a poor corrosion resistance and impact toughness due to excessive C, and No. 10 No. 11 is inferior in corrosion resistance and impact toughness because the contents of Cr, Nb and V are insufficient.

[0025]

[Table 1]

[0026]

INDUSTRIAL APPLICABILITY As described above, the present invention makes it possible to provide a steel for oil country tubular goods having excellent corrosion resistance, which is excellent in corrosion resistance in a high temperature wet carbon dioxide environment and has high strength and toughness. , It is extremely large that it contributes to the development of the industry.

Claims (3)

[Claims] 1. By weight%, C: 0.08 to 0.15%, Si: 0.01% to less than 1.2%, Mn: 0.02 to 3.0%, Cr: 8.0 to 9.9%, Mo: 0.82-3.0%, Al: 0.005-1.0%, Nb: 0.05-0.50%, V: 0.16-0.60%, N : 0.02 to 0.07%, P to 0.03% or less, S to 0.015% or less, and the balance Fe and unavoidable impurities, and the MA value ([[ %
X] is the content of the element X expressed in% by weight) MA value = 80 + 420 [% C] +440 [% N] +30 ([% Mn] + [% Ni] + [% Cu] + [% Co]) − 12 ([% Si] + [% Cr] + [% Mo] + [% Nb] + [% V])-6 [% W] is 0 or more, and is a high strength oil well with excellent corrosion resistance. Steel for pipes. 2. One or two of W: 2.0% or less, Ni: 2.0% or less, Cu: 2.0% or less, Co: 2.0% or less, as an additional component. The high strength steel for oil country tubular goods according to claim 1, which contains at least one kind. 3. The excellent corrosion resistance according to claim 1 or 2, further comprising, as an additional component, one or two of a rare earth element: 0.05% or less and Ca: 0.03% or less in weight%. High strength steel for oil country tubular goods.