JP4428237B2 - High strength martensitic stainless steel with excellent carbon dioxide corrosion resistance and sulfide stress corrosion cracking resistance - Google Patents

Description

  The present invention relates to a steel material suitable for use in a severe corrosive environment containing corrosive substances such as carbon dioxide, hydrogen sulfide, and chlorine ions. More specifically, steel materials for seam welded steel pipes such as seamless steel pipes for oil and natural gas production equipment, decarbonation gas removal equipment, geothermal power generation equipment, etc., ERW steel pipes, laser welded steel pipes, spiral welded pipes, Alternatively, the present invention relates to a steel material constituting a tank or the like for a carbon dioxide-containing liquid, particularly a steel material for an oil well pipe used in oil or natural gas wells.

  In recent years, oil wells under harsh environments, that is, deep wells, sour gas fields, and the like have been actively developed from the viewpoint of depleting oil resources expected in the near future. Therefore, oil well steel pipes used for such applications are required to have high strength and excellent corrosion resistance and resistance to sulfide stress corrosion cracking.

  Conventionally, carbon steel and low alloy steel were generally used as steel materials for oil well pipes, etc., but steels with increased alloy content have been used as the environment of the well becomes severe. It was. For example, 13Cr martensitic stainless steel represented by SUS420 is used for oil well steel materials containing a large amount of carbon dioxide gas.

  However, although the above SUS420 steel has excellent corrosion resistance to carbon dioxide gas, it has poor corrosion resistance to hydrogen sulfide, and sulfide stress corrosion cracking (SSCC) easily occurs in an environment containing carbon dioxide gas and hydrogen sulfide at the same time. . Therefore, various steel materials that replace this steel have been proposed.

  Japanese Patent No. 2861024, Japanese Patent Application Laid-Open No. 5-287455, and Japanese Patent Application Laid-Open No. 7-62499 disclose steels whose corrosion resistance is improved by reducing the carbon content of the SUS420. However, steels having a low carbon content as described in these publications may not have the strength required for use in deep wells, that is, the yield strength of 860 MPa or more.

  Japanese Patent Application Laid-Open No. 2000-192196 discloses a martensite single phase containing Co: 0.5 to 7% and Mo: 3.1 to 7% as steel having high strength and good resistance to sulfide stress cracking. Tissue steel is disclosed. In the invention described in the publication, by containing Co in the above range, the formation of retained austenite during cooling is suppressed, and the structure becomes a martensite single phase. However, since Co is an expensive element, it is desirable not to use it as much as possible.

  The present invention has been made in view of the above situation, has sufficient strength for use in oil well pipes for deep wells, that is, has a high strength of 860 MPa or more, and carbon dioxide, hydrogen sulfide, It is an object of the present invention to provide martensitic stainless steel having excellent carbon dioxide gas corrosion resistance and sulfide stress corrosion cracking resistance that can be used even in an environment containing chlorine ions or two or more of these.

  For this reason, the gist of the present invention is the high-strength martensitic stainless steel for oil country tubular goods described in (a) and (b) below. In the following description, “%” regarding the component content means “mass%”.

(A) In mass%, C: 0.005 to 0.04%, Si: 0.5% or less, Mn: 0.1 to 3.0%, P: 0.04% or less, S: 0.01 %: Cr: 10-15%, Ni: 5.85-8% , Mo: 2.8-5.0%, Al: 0.001-0.10 %, N : 0.07% or less, and Ti : Containing 0.005 to 0.25% , the balance being Fe and impurities, and satisfying the following formula (1), the metal structure is mainly tempered martensite, carbide precipitated during tempering and fine during tempering High strength martensitic stainless steel with 0.2% proof stress of 860 MPa or more, excellent in carbon dioxide gas corrosion resistance and sulfide stress corrosion cracking resistance, characterized by comprising precipitated intermetallic compounds such as Laves phase and σ phase It is steel. However, each element symbol in the formula (1) indicates the content (% by mass) of each element.
Mo ≧ 2.3-0.89Si + 32.2C (1)

The present invention further includes martensite containing at least one alloy component selected from at least one of the following first group, second group and third group in addition to the alloy component described in (a) above. The gist is stainless steel. This steel also satisfies the formula (1), and the metal structure is as described above.
First group : V : 0.005 to 0.25%, Nb: 0.005 to 0.25%, and Zr: 0.005 to 0.25%
Second group: Cu: 0.05 to 1%
Third group: Ca: 0.0002 to 0.005%, Mg: 0.0002 to 0.005%, La: 0.0002 to 0.005%, and Ce: 0.0002 to 0.005%

(B) After quenching a steel having the composition defined in any of the above (a) and satisfying the above formula (1) at a quenching temperature of 880 ° C. to 1000 ° C., the tempering temperature range is 450 to 620. When the tempering temperature T (° C.) and the tempering time t (hours) are set, the tempering process in which (20 + logt) (T + 273) satisfies 13500 to 17700 is performed, so that the metal structure is mainly tempered martensite and tempered. 0.2% proof stress of 860 MPa or more excellent in carbon dioxide gas corrosion resistance and sulfide stress corrosion cracking resistance, characterized by comprising precipitated carbides and intermetallic compounds such as Laves phase and σ phase finely precipitated during tempering High strength martensitic stainless steel.

  FIG. 1 is a diagram showing the relationship between the Mo content of various steels tested in Examples and the right side of the formula (1), that is, “2.3-0.89Si + 32.2C” (IM value). .

  FIG. 2 is a diagram for explaining the tempering conditions defined in the present invention. After tempering at 920 ° C., the tempering temperature is changed from 460 to 650, and the value of (20 + logt) (T + 273) is changed. The relationship between 2% yield strength and (20 + logt) (T + 273) is shown.

  Hereinafter, the reason for limiting the content of each element defined in the present invention will be described. In addition,% of each content means the mass%.

C: 0.005-0.04%
C is an element effective for improving the strength of steel, but is preferably as small as possible from the viewpoint of corrosion resistance. However, if it is less than 0.005%, the yield strength does not become 860 MPa or more, so the lower limit of its content was made 0.005%. On the other hand, if the content exceeds 0.04%, the hardness after tempering becomes too high, and the sensitivity to sulfide stress corrosion cracking becomes high. Therefore, the content of C is set to 0.005 to 0.04%.

Si: 0.5% or less Si is an element necessary as a deoxidizer. The residual amount in steel may be an impurity level. However, in order to obtain a greater deoxidation effect, the content is desirably 0.01% or more. On the other hand, when the content exceeds 0.5%, toughness is lowered and hot workability is lowered. Therefore, the Si content is set to 0.5% or less.

Mn: 0.1 to 3.0%
Mn is an element effective for improving hot workability. In order to obtain this effect, the content needs to be 0.1% or more. On the other hand, when the content exceeds 3.0%, the effect is saturated and the cost is increased. Therefore, the Mn content is set to 0.1 to 3.0%.

P: 0.04% or less P is an impurity contained in steel, and its content is preferably as small as possible. In particular, when the content exceeds 0.04%, the resistance to sulfide stress corrosion cracking is significantly reduced. Therefore, the content of P is set to 0.04% or less.

S: 0.01% or less S is also an impurity contained in steel, and its content should be as small as possible. In particular, when the content exceeds 0.01%, hot workability, corrosion resistance, and toughness are remarkably lowered. Therefore, the S content is set to 0.01% or less.

Cr: 10-15%
Cr is an element effective for improving the carbon dioxide gas corrosion resistance. In order to obtain this effect, the content needs to be 10% or more. On the other hand, when the content exceeds 15%, it becomes difficult to make the structure after tempering mainly a martensite phase. Therefore, the Cr content is set to 10 to 15%.

Ni: 5.85-8%
Ni is an element necessary for making the structure after tempering mainly a martensite phase. However, when the content is less than 5.85%, a large amount of ferrite phase precipitates in the structure after tempering, and the structure after tempering does not mainly become a martensite phase. On the other hand, when the content exceeds 8%, the structure after tempering mainly becomes an austenite phase. Therefore, the Ni content is set to 5.85 to 8% .

Mo: 2.8 to 5.0%
Mo is an element effective for improving the resistance to sulfide stress corrosion cracking of a high-strength material. In order to acquire this effect, the content needs to be 2.8% or more. However, when the content exceeds 5.0%, this effect is saturated and the cost is increased. Therefore, the Mo content is set to 2.8 to 5.0%.

Al: 0.001 to 0.10%
Al is an element used as a deoxidizer in the melting process. In order to obtain this effect, the content needs to be 0.001% or more. However, when the content exceeds 0.10%, inclusions increase and corrosion resistance is impaired. Therefore, the Al content is set to 0.001 to 0.10%.

N: 0.07% or less N is an impurity contained in steel, and its content is preferably as small as possible. In particular, when the content exceeds 0.07%, the inclusions increase and the corrosion resistance deteriorates. Therefore, the N content is set to 0.07% or less.

Ti: 0.005-0.25%
Ti is an element necessary for fixing C and reducing variation in strength. If the content is less than 0.005%, the above effect cannot be obtained. On the other hand, if the content exceeds 0.25%, the structure after tempering cannot be mainly made into the martensite phase, and the strength cannot be increased to 860 MPa or more. Therefore, the Ti content is set to 0.005 to 0.25% .

  One of the martensitic stainless steels of the present invention is composed of Fe and inevitable impurities in the balance in addition to the above components. The other one contains at least one alloy component selected from at least one of the following first group, second group and third group in addition to the above components. Hereinafter, the components of each group will be described.

First group (V 1, Nb, Zr: 0.005 to 0.25% each)
V, Nb, and Zr all have the effect of fixing C and reducing the variation in strength, and therefore may contain one or more selected from these, if necessary. However, the above effects cannot be obtained when the content of any element is less than 0.005%. On the other hand, if the content of any element exceeds 0.25%, the structure after tempering cannot be mainly made into the martensite phase, and the strength cannot be increased to 860 MPa or more. Therefore, the contents when these elements are selectively contained are 0.005 to 0.25%, respectively.

Second group (Cu: 0.05 to 1%)
Cu, like Ni, is an element effective for making the structure after tempering mainly a martensite phase. In order to obtain this effect by adding Cu, the content may be 0.05% or more. However, when the content exceeds 1%, the hot workability of the steel decreases. Therefore, when Cu is contained, the content is set to 0.05 to 1%.

Third group (Ca, Mg, La, Ce: 0.0002 to 0.005% each)
Since Ca, Mg, La, and Ce are all effective elements for improving the hot workability of steel, they may contain one or more selected from these as necessary. . However, the above effects cannot be obtained when the content of any element is less than 0.0002%. On the other hand, if the content of any element exceeds 0.005%, a coarse oxide is generated, and the corrosion resistance of the steel decreases. Therefore, the content when these elements are selectively contained is set to 0.0002 to 0.005%. In particular, it is desirable to contain Ca and / or La.

The steel of the present invention has the above chemical composition and must satisfy the following formula (1). This is because, when the above formula (1) is satisfied, the strength of the steel can be improved to 860 MPa or more without deteriorating the resistance to sulfide stress corrosion cracking.
Mo ≧ 2.3-0.89Si + 32.2C (1)
However, each element symbol in the formula (1) indicates the content (% by mass) of each element.

  FIG. 1 shows the relationship between the Mo content of various steels tested in Examples described later and the right side of the formula (1), that is, “2.3-0.89Si + 32.2C” (this is called the IM value). FIG. Specifically, it is based on the results of the steel of the present invention and the comparative steel (Test Nos. 18 to 21). In the figure, “◯” indicates that the sample was not broken in the sulfide stress corrosion cracking test, and “X” indicates that it was broken. Even if the Mo content is 2.8% or more, the stress corrosion cracking resistance is poor unless the above formula (1) is satisfied.

  When the Mo content is outside the range defined by the present invention (ie, less than 2.8%), the 0.2% proof stress is less than 860 MPa, and the Mo content is within the range defined by the present invention. Even in the case of (ie, 2.8 to 5%), the 0.2% proof stress is less than 860 MPa unless the above formula (1) is satisfied.

  However, if the steel satisfies the condition of the above expression (1), the 0.2% proof stress is 860 MPa or more, and a sufficient strength that can be used as an oil well steel is obtained. Therefore, the steel of the present invention must satisfy the above formula (1) within the range of the chemical composition.

  The present inventors further examined the influence of the metal structure. For example, the present inventors have found that the strength of steel can be improved without reducing the resistance to sulfide stress corrosion cracking.

  The “mainly tempered martensite” means that 70 vol% or more of the metal structure is a tempered martensite structure, and a residual austenite structure or a ferrite structure may exist in addition to the tempered martensite structure.

  The “intermetallic compounds such as Laves phase and σ phase” may include intermetallic compounds such as μ phase and χ phase in addition to Laves phase such as Fe 2 Mo and σ phase.

  The metal structure of the steel of the present invention contains carbides precipitated during tempering. Carbide is an effective metal structure for ensuring the strength of steel, but high strength of 860 MPa or more cannot be achieved simply by including carbide in steel. Therefore, in the present invention, it is necessary that the carbide precipitates and the intermetallic compounds such as the Laves phase and the σ phase are finely precipitated.

  The heat treatment of the steel of the present invention is ordinary quenching-tempering. In order to precipitate a fine intermetallic compound during tempering, it is necessary to sufficiently dissolve the intermetallic compound during quenching. The quenching temperature is desirably 880 to 1,000 ° C.

  Furthermore, fine intermetallic compounds such as Laves phase and σ phase are precipitated, and the 0.2% proof stress can be 860 MPa or more. The tempering temperature range is 450 to 620 ° C., and the tempering temperature T (° C.). When time t (time) is satisfied, (20 + logt) (T + 273): 13500-17700 can be satisfied.

  FIG. 2 is a diagram for explaining the tempering conditions defined in the present invention. In the same figure, test No. The relationship between 0.2% proof stress and (20 + logt) (T + 273) when the value of (20 + logt) (T + 273) was changed by changing the tempering temperature from 400 to 650 after quenching at 920 ° C. using the material No. 1 Is shown.

As shown in FIG. 2, the 0.2% proof stress is 860 MPa or more when (20 + logt) (T + 273) is in the range of 13500-17700.
When tempering is performed under a condition where (20 + logt) (T + 273) exceeds 17700, the dislocation density decreases, or the intermetallic compound is solid-solved in the metal structure, and the 0.2% yield strength is increased to 860 MPa or more. Can not. On the other hand, when tempering under a condition of less than 13500, intermetallic compounds and carbides do not precipitate, so the 0.2% proof stress cannot achieve 860 MPa or more.

  From the above principle, the steel of the present invention has the above-described chemical composition, satisfies the formula (1), and its metal structure is mainly tempered martensite, carbide precipitated during tempering, and fine precipitation during tempering. It is necessary to be an intermetallic compound such as Laves phase or σ phase.

  The steel having the chemical composition shown in Table 1 is melted and cast, and the resulting slab is hot forged and hot rolled to have a thickness of 15 mm, a width of 120 mm, and a length of 1,000 mm. Steel plates were prepared and subjected to various tests as test steel plates that were quenched (920 ° C. water cooling) and tempered (equal cooling at 550 ° C. for 30 minutes and then air cooling: (20 + logt) (T + 273) = 16212). did.

First, a round bar test piece having a diameter of 6.35 mm and a parallel part length of 25.4 mm was taken from each test steel plate, and a tensile test was performed at room temperature. Table 2 shows the 0.2% yield strength at this time.
Next, a test piece having a thickness of 3 mm, a width of 20 mm, and a length of 50 mm was taken from each test steel plate, and this test piece was polished with No. 600 emery paper, then degreased and dried. It was immersed in a 25% NaCl aqueous solution (temperature: 165 ° C.) saturated with 973 MPa of CO ₂ gas and 0.0014 MPa of H ₂ S gas for 720 hours.

  After immersion, the corrosion weight loss of the test piece [(mass before test) − (mass after test)] was measured, and the presence or absence of local corrosion on the test piece surface was visually confirmed. As a result, the corrosion rate of the steel of the present invention was 0.5 mm / year or less, and no local corrosion was observed on the surface.

Subsequently, according to NACE TM0177-96 Method A, a constant load test was performed using a spring type (proof ring type) tester. Specifically, a round bar test piece having a diameter of 6.3 mm and a parallel part length of 25.4 mm was taken from each test steel plate, and 0.003 MPa of H ₂ S gas (CO ₂ bal.) Was saturated. In addition, an 85% constant load test was performed using a 25% NaCl solution (pH 4.0) at a test temperature of 25 ° C. for 720 hours and a test stress of 0.2% yield strength.

  As a result, the steel of the present invention having a 0.2% proof stress of 860 MPa or more did not break in all the test pieces. The metal structure was also observed with an optical microscope and an extraction replica. The results are also shown in Table 2.

As shown in Table 2, Examples 4 , 5, 10-13 , and 16 of the present invention have a 0.2% proof stress of 860 MPa or more, and have excellent carbon dioxide gas corrosion resistance and sulfide stress corrosion cracking resistance. ing. On the other hand, Comparative Examples 22 to 25 outside the range defined by the present invention and the content range of each component is within the range defined by the present invention, but the content of Cr and / or Mo does not satisfy the formula (1) In all of Examples 18 to 21, the carbon dioxide gas performance or the stress cracking resistance performance was not sufficient.

  According to the martensitic stainless steel of the present invention, the steel composition is limited for specific components, the Mo content in the steel is defined in relation to the IM value, and the metal structure is mainly tempered martensite and precipitated during tempering. It is composed of carbonized carbides and intermetallic compounds such as Laves phase and σ phase finely precipitated during tempering, so that 0.2% proof stress is high strength of 860 MPa or more, and excellent carbon dioxide corrosion resistance and sulfide stress corrosion resistance. It can have cracking properties. Thereby, it can be applied in a wide field as a highly practical steel that can be widely used for oil well pipes and other uses in an environment containing carbon dioxide, hydrogen sulfide, chlorine ions, or two or more thereof.

Claims (5)

In mass%, C: 0.005 to 0.04%, Si: 0.5% or less, Mn: 0.1 to 3.0%, P: 0.04% or less, S: 0.01% or less, Cr: 10-15%, Ni: 5.85-8% , Mo: 2.8-5.0%, Al: 0.001-0.10 %, N : 0.07% or less and Ti: 0.00 . 005 to 0.25% , the balance being Fe and impurities, and satisfying the following formula (1) , 70 vol% or more of the metal structure is a tempered martensite structure, and carbide precipitated during tempering And at least one intermetallic compound phase of Laves phase, σ phase, μ phase, or χ phase finely precipitated during tempering, which is excellent in carbon dioxide gas corrosion resistance and sulfide stress corrosion cracking resistance. High strength martensitic stainless steel with 0.2% proof stress steel.
Mo ≧ 2.3-0.89Si + 32.2C (1)
However, each element symbol in the formula (1) indicates the content (% by mass) of each element.   In place of a part of Fe, one or more selected from V: 0.005 to 0.25%, Nb: 0.005 to 0.25% and Zr: 0.005 to 0.25% The high-strength martensitic stainless steel according to claim 1, comprising:   The high-strength martensitic stainless steel according to claim 1 or 2, further comprising Cu: 0.05 to 1% instead of a part of Fe.   In place of part of Fe, Ca: 0.0002 to 0.005%, Mg: 0.0002 to 0.005%, La: 0.0002 to 0.005%, and Ce: 0.0002 to 0 The high-strength martensitic stainless steel according to any one of claims 1 to 3, comprising one or more selected from 0.005%. A steel having the composition defined in any one of claims 1 to 4 and satisfying the following formula (1) is quenched at a quenching temperature of 880 ° C to 1000 ° C, and a tempering temperature region is set to 450 to 620 ° C. When a tempering temperature T (° C.) and a tempering time t (hours) are used, a tempering process in which (20 + logt) (T + 273) satisfies 13500 to 17700 is performed, whereby 70 vol% or more of the metal structure is tempered martensite structure. Carbon dioxide corrosion resistance and sulfidation resistance characterized by containing carbide precipitated during tempering and at least one intermetallic compound phase of Laves phase, σ phase, μ phase or χ phase finely precipitated during tempering High-strength martensitic stainless steel with 0.2% proof stress of 860 MPa or more with excellent physical stress corrosion cracking properties. Mo ≧ 2.3-0.89Si + 32.2C (1)
However, each element symbol in the formula (1) indicates the content (% by mass) of each element.

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