NO337858B1 - High-strength martensitic stainless steel excellent for corrosion resistance to carbon dioxide gas and sulphide stress corrosion crack resistance. - Google Patents

Description

The present invention relates to a steel material which is suitable for use in a severe corrosion environment containing corrosive materials such as carbon dioxide gas, hydrogen sulphide, chlorine ions and the like. Specifically, the present invention relates to a steel material for a fully drawn steel pipe and a seam-welded steel pipe such as a resistance-welded steel pipe, a laser-welded steel pipe, a spiral-welded pipe or the like, which is used in applications for petroleum or natural gas production facilities, facilities for eliminating carbon dioxide gas, or for geothermal power generation, or for a tank for liquid containing carbon dioxide gas, especially for a steel material for oil well pipes for oil wells or gas wells.

BACKGROUND TECHNOLOGY

From the point of view of release of petroleum resources, which is expected in the near future, development of an oil well under strict environment that is an oil well in a deeper layer, of a sour gas field or the like, has often been carried out. Therefore, high strength and excellent corrosion resistance and sulphide stress corrosion cracking resistance are required for oil well steel pipes, which are used in such applications.

As a steel material for oil well pipe or the like, carbon steel or low-alloy steel has generally been used. However, as the environment in the well becomes severe, steels containing increasing amounts of alloying elements have been used. For example, as oil wells for steel material containing a large amount of carbon dioxide gas, 13 Cr series martensitic stainless steel such as typically SUS 420 and the like have been used.

However, although the SUS 420 steel has excellent corrosion resistance to carbon dioxide gas, it has poor corrosion resistance to hydrogen sulfide. Therefore, the SUS 420 steel is prone to generate sulfide stress corrosion cracking (SSCC) under the environment containing carbon dioxide gas and hydrogen sulfide simultaneously. Therefore, different materials instead of the SUS 420 steel have been proposed.

Japanese patent no. 2861024, Japanese Patent Application Publication No. 05-287455 and Japanese Patent Application Publication No. 07-62499 show steels that have improved corrosion resistance by reducing the carbon content of SUS 420. However, such low carbon steel types described in these publications may not have enough strength required for application in a deep well, i.e. conventional yield strength of 860 MPa or more.

Alternatively, Japanese Patent Application Publication No. 2000-192196 discloses steel with a martensitic single-phase structure containing Co: 0.5-7% and Mo: 3.1-7% which has high strength and excellent sulfide stress corrosion cracking resistance. The invention described in the publication is a steel containing Co in the above range to suppress the formation of retained austenite during cooling so that the structure is made to be a martensitic single phase. Since Co is an expensive element, however, it is desired not to use it.

Japanese patent applications JP05156409 A and JP10130787 A describe high strength martensitic stainless steel suitable for use in corrosive environments.

SUMMARY OF THE INVENTION

The present invention was made by considering the above-mentioned circumstances. The purpose of the present invention is to provide a martensitic stainless steel having sufficient strength for use in oil well tubing for a deep well, that is, high strength of a conventional yield strength of 860 MPa or more, and excellent carbon dioxide gas corrosion resistance and sulphide stress corrosion cracking resistance whereby it can be used even under the environment containing carbon dioxide gas, hydrogen sulfide or chlorine ions or two or more of them. The symbols for the respective elements in the following expression show the content (% by weight) of each element.

Accordingly, the present invention provides a high-strength martensitic stainless steel having excellent carbon dioxide gas corrosion resistance and sulfide stress corrosion cracking resistance and having 0.2% conventional yield strength of 860 MPa or more, including, by weight %, C: 0.005-0.04%, Si : 0.5% or less, Mn: 0.1-3.0%, P: 0.04% or less, S: 0.01% or less, Cr: 10-15%, Ni: 4.0- 8%, Mo: 2.8-5.0%, Al: 0.001-0.10% and N: 0.07% or less, Ti: 0.005-0.25%, and optionally further contains Cu: 0.05 -1%, and/or one or more selected from V: 0.005-0.25%, Nb: 0.005-0.25% and Zr: 0.005-0.25%, and/or one or more selected from Ca: 0 .0002-0.005%, Mg: 0.0002-0.005%, La: 0.0002-0.005% and Ce: 0.0002-0.005%, and the balance is Fe and impurities, and also satisfies the expression (1) given under which the steel has a microstructure that includes tempered martensite, carbide precipitated during tempering, and intermetallic compounds, such as Lowe's phase, a-phase and the like, finely precipitated during tempering opening.

in which the symbols of the respective elements in the expression (1) show the content (% by weight) of each element.

The high-strength martensitic stainless steel can be obtained by subjecting a steel having the composition as set forth in claims 1 to 4 to tempering in which (20 + log t) (T + 273) satisfies 13500-17700 when, after quenching the steel at a quenching temperature of 880-1000°C, a range of a tempering temperature is set to 450°C-620°C, a tempering temperature is set to T (°C) and tempering time is set to t (hours).

BRIEF DESCRIPTION OF THE FIGURES

Fig. 1 is a picture showing the relationship between Mo content in different types of steel tested in examples and the right-hand side of the expression (1), that is, "2.3 - 0.89 Si + 32.2 C" (IM value). Fig. 2 is a picture to explain tempering conditions defined in the present invention, showing the relationship between 0.2% conventional yield strength obtained by changing the value of (20 + log t) (T + 273) while changing tempering temperatures in 400 -650°C after quenching of steel at 920°C, and (20 + log t) (T + 273).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The reasons for limiting the contents of various elements defined by the present inventors will be described below."%" of the respective contents means % by weight.

C: 0.005-0.04%

Although C (carbon) is an effective alloying element for improving the strength of steel, from the point of view of corrosion resistance, low C content is preferred. If the content of C is less than 0.005%, however, the conventional yield strength does not reach 860 MPa or more. Therefore, the lower limit of the C content was set at 0.005%. On the other hand, if the C content exceeds 0.04%, the hardness of the tempered steel becomes excessively hard, the steel had high sulfide

stress cracking sensitivity. Accordingly, the C content was set at 0.005-0.04%.

Say: 0.5% or less

Si (silicon) is an alloying element that is needed as a deoxidizer. An amount of Si retained in the steel can be a level of impurities. In order to achieve a large deoxidation effect, however, it is preferred that the Si content be set to 0.01% or more. On the other hand, if the Si content exceeds 0.5%, the toughness of the steel is reduced and the machinability of the steel is also reduced. Accordingly, the Si content was set at 0.5% or less.

Mn: 0.1-3.0%

Mn (manganese) is an effective alloying element to improve hot workability. To achieve this effect, Mn content of 0.1% or more is necessary. On the other hand, if the Mn content exceeds 3.0%, the effect is saturated resulting in a cost increase. Accordingly, the Mn content was set at 0.1-3.0%.

P: 0.04% or less

P (phosphorus) is a polluting element contained in the steel and the P content is better as low as possible. In particular, if the P content exceeds 0.04%, the sulfide stress cracking resistance is noticeably reduced. Accordingly, the P content was set at 0.04% or less.

S: 0.01% or less

S (sulphur) is a contaminant contained in the steel and the S content is better as low as possible. In particular, if the S content exceeds 0.01%, the hot workability, corrosion resistance and toughness are noticeably reduced. Accordingly, the S content was set at 0.01% or less.

Cr: 10-15%

Cr (chromium) is an effective alloying element to improve carbon dioxide gas corrosion resistance. To achieve this effect, a Cr content of 10% or more is necessary. On the other hand, if the Cr content exceeds 15%, it becomes difficult to make the microstructure of tempered steel mainly a martensitic phase. Accordingly, the Cr content was set at 10-15%.

Nine: 4.0-8%

Ni (nickel) is an alloying element, which is necessary to make the microstructure of tempered steel mainly a martensitic phase. However, if the Ni content is 4.0% or less, a large number of ferrite phases were precipitated in the microstructure of the tempered steel and the microstructure of the tempered steel does not become a mainly martensitic phase. On the other hand, if the Ni content exceeds 8%, the microstructure of tempered steel becomes mainly an austenite phase. Accordingly, the Ni content was set at 4.0-8%. More preferably, the Ni content was set to 4-7%.

Mo: 2.8-5.0%

Mo (molybdenum) is an effective alloying element to improve the sulfide stress cracking resistance of a high strength material. To achieve this effect, Mo content of 2.8% or more is necessary. If the Mo content exceeds 5.0%, however, this effect is saturated, resulting in an increase in cost. Accordingly, the Mo content was set at 2.8-5.0%.

Al: 0.001-0.10%

Al (aluminium) is an alloy element, which is used as a deoxidizer in a smelting process. To achieve this effect, Al content of 0.001% or more is necessary. If the Al content exceeds 0.10%, however, many inclusions are formed in the steel, so that corrosion resistance is lost. Accordingly, the Al content was set at 0.001-0.10%.

N: 0.07% or less

N (nitrogen) is a polluting element contained in the steel and the N content is better as low as possible. In particular, if the N content exceeds 0.07%, many inclusions are formed in the steel, so that the corrosion resistance is lost. Accordingly, the N content was set to 0.07% or less.

Ti: 0.005-0.25%

Ti has the effect of fixing C to reduce variations in strength. If the amount of Ti is less than 0.005%, the above effect cannot be achieved. On the other hand, if the amount of Ti exceeds 0.25%, the microstructure of the steel cannot become a mainly martensitic phase so that strong strengthening of the steel with a conventional yield strength of 860 MPa or more cannot be achieved. Ti was set at 0.005-0.25%.

The martensitic stainless steel of the present invention consists of the above chemical composition as well as the balance of Fe and indispensable impurities. The martensitic stainless steel according to the present invention may further contain, in addition to the above-mentioned components, at least one alloying element selected from at least one group consisting of a first group, a second group and a third group as follows. The components (elements) of the respective groups will be described below.

First group (V, Nb, Zr: 0.005-0.25% respectively)

Since V, Nb and Zr have the effect of fixing C to reduce variations in strength, one or more selected from these elements may optionally be contained. However, if any of the elements is less than 0.005%, the above effect cannot be obtained. On the other hand, if any of the elements exceeds 0.25%, the microstructure of the steel cannot become a mainly martensitic phase so that strong strengthening of the steel with a conventional yield strength of 860 MPa or more cannot be achieved. Accordingly, the respective contents to selectively contain these elements were set at 0.005-0.25%.

Second group (Cu: 0.05-1%)

Cu is an effective element to make the microstructure of tempered steel mainly a martensitic phase like Ni. To achieve the effect of the addition of Cu, the Cu content can be 0.05% or more. If the content of Cu exceeds 1%, however, the hot workability of the steel is reduced. Accordingly, when Cu is contained in the steel, the Cu content was set at 0.05-1%.

Third group (Ca, Mg, La, Ce: 0.0002-0.005% respectively)

Since Ca, Mg, La and Ce are effective elements for improving the hot workability of the steel, one or more selected from these elements can optionally be contained. However, if any of the elements is less than 0.0002%, the above effect cannot be obtained. On the other hand, if any of the elements exceeds 0.005%, coarse oxides are formed in the steel, thereby reducing the corrosion resistance of the steel. Accordingly, the respective contents of selectively containing these elements were set at 0.0002-0.005%. In particular, it is preferred to contain Ca and/or La in the steel.

The steel according to the present invention should have the above-mentioned chemical composition and satisfy the following expression (1). This is because, if the steel satisfies the expression (1), the strength of the steel can be improved to conventional yield strength of 860 MPa or more without sulphide weakening

stress corrosion cracking resistance.

in which the symbols of the respective elements in the expression (1) show the content (% by weight) of each element.

Fig. 1 is a picture showing the relationship between Mo content in different types of steel tested in examples, which will be described later, and the right side of the expression (1), that is, "2.3 - 8.9 Si + 32.2 C" (IM value). Specifically, the results shown in Fig. 1 are based on steel according to the present invention and comparative steel (tests no. 18-21). The mark "o" shows an example that did not crack in a sulphide stress corrosion cracking test, and the mark "x" shows an example that cracked therein.

Even if the Mo content exceeds 2.8%, if the Mo content does not satisfy the expression (1), the steel has poor sulfide stress corrosion cracking resistance.

When the Mo content is outside a range (which is less than 2.8%) defined in the present invention, the 0.2% conventional yield strength of the steel is less than 860 MPa. Furthermore, even if the Mo content is in a range (which is 2.8-5%) defined in the present invention, if the MO content does not satisfy the above expression (1), the 0.2% conventional yield strength of the steel is less than 860 MPa.

However, if the steel satisfies the above expression (1), 0.2% conventional yield strength reaches 860 MPa or more and the steel can withstand the application of an oil well steel material due to its sufficient strength. Consequently, the steel according to the present invention should be in a range of said chemical composition and satisfy the above-mentioned expression (1).

Furthermore, the present inventors have investigated the effects of the microstructure. As a result, the present inventors have found that if the microstructure is a structure mainly comprising tempered martensite, carbide precipitated during tempering, and intermetallic compounds, such as Lowe's phase, a-phase and the like finely precipitated during tempering, the strength of the steel can be improved without damaging sulphide stress cracking resistance.

It is noted that "predominantly extensive tempered martensite" means that 70 vol% or more of the microstructure of the steel is a tempered martensitic structure, and a retained austenitic structure and/or a ferritic structure other than a tempered martensitic structure may be present.

Furthermore, "intermetallic compounds, such as Lowe's phase, a-phase and the like" may contain intermetallic compounds such as n-phase and X-phase other than Lowe's phase such as Fe2Mo and the like and a-phase.

The microstructure of the steel according to the present invention contains carbide precipitated during tempering. Although carbide is an effective microstructure to ensure the strength of the steel, high strength of conventional yield strength of 860 MPa or more cannot be realized by only carbide contained in the steel. Accordingly, in the present invention it is necessary to precipitate carbide as well as fine precipitation of intermetallic compounds such as the above-mentioned Lowe's phase, a-phase and the like.

Heat treatment for the steel according to the present invention is typically quenching. In order to precipitate fine intermetallic compounds during tempering, it is necessary to sufficiently dissolve the intermetallic compounds during quenching. The quenching temperature is preferably 880-1000°C.

Further conditions in which intermetallic compounds, such as Lowe's phase, a-phase and the like are precipitated and 0.2% conventional yield strength of 860 MPa or more can be achieved is in a case when a tempering temperature range is 450-620°C, then well as the tempering temperature is set to T(°C) and the tempering time is set to t (hours), (20 + log t) (T + 273) can satisfy 13500-17700.

Fig. 2 is a picture to explain the tanning conditions defined in the present invention. Fig. 2 shows the relationship between the 0.2% conventional yield strength obtained by changing the values for (20 + log t) (T + 273) while changing the tempering temperatures in 400-650°C after quenching of steel at 920°C and (20 + log t) (T + 273).

As shown in Fig.2, when (20 + log t) (T + 273) is in a range of 13500-17700, 0.2% conventional yield strength achieves 860 MPa or more.

When tempering is carried out at a condition that (20 + log t) (T + 273) exceeds 17700, dislocation density is reduced or intermetallic compounds are dissolved in the microstructure of the steel, whereby strong strengthening of 0.2% conventional yield strength of 860 MPa or more cannot is achieved. On the other hand, when the steel is tempered at a condition of less than 13500, intermetallic compounds and carbide are not precipitated. Consequently, 0.2% conventional yield strength of 860 MPa or more cannot be achieved.

From the above-mentioned principle, the steel according to the present invention should have the above-mentioned chemical compositions and satisfy the expression (1) and the microstructure of the steel should mainly include tempered martensite, carbide precipitated during tempering, and intermetallic compounds such as Lowe's phase, a-phase and similar fine precipitated during tempering. (Examples)

Steels having chemical compositions shown in Tables 1(1) and 1(2) were melted and cast, and the obtained cast metal ingots were forged and hot-rolled to produce steel plates each having a thickness of 15 mm, a width of 120 mm, and a length of 1,000 mm. These steel plates were subjected to quenching (water cooling at 920°C) and tempering [air cooling after soaking at 550°C for 30 min, ((20 + log t) (T + 273) = 16212], and the obtained steel plates were provided in various tests as test steel plates.

First, round bar test pieces each having a diameter of 6.35 mm and a length of the parallel portion of 25.4 mm were taken from the respective test steel plates and subjected to tensile tests at normal temperatures. The 0.2% conventional yield strength obtained is shown in Table 2.

Then, test pieces each having a thickness of 3 mm, a width of 20 mm and a length of 50 mm were taken from the respective test steel plates and these test pieces were polished with a No. 600 emery paper and degreased and dried. Then the obtained test pieces were immersed in 25% NaCl water solution saturated with 0.973 MPa CO2 gas and 0.0014 MPa H2S gas (temperature: 165°C) for 720 hours.

After the immersion, the weight reductions of the test pieces by corrosion [(mass before testing)-(mass after testing)] were measured and the presence and absence of local corrosion on surfaces of the test pieces was confirmed by a visual test. As a result, the corrosion rate of the steel of the present invention is 0.5 mm/year or less, and no localized corrosion on its surface could be found.

Samples in which the 0.2% conventional yield strength was 860 MPa or more in the tensile tests were then subjected to fixed load tests using a spring type (proven ring type) testing machine according to TM0177-96 method A according to NACE. Specifically, round bar test pieces each having a diameter of 6.3 mm and a length of the parallel portion of 25.4 mm were taken from the respective test steel plates and subjected to 0.2% conventional yield strength -85% (test load) fixed load tests at a test temperature of 25°C for 720 hours using 0.003 MPa H2S gas (CO2bal.) saturated 25% NaCI aqueous solution (pH 4.0). As a result, all the test pieces were not cracked.

The microstructure of the test pieces was observed by an optical microscope and an extraction replica. These results are also shown in Table 2.

Note 2) In the SSC test, a steel that did not generate a crack is shown with "o" and a steel

which generated cracks are shown with "x".

Note 3) In microstructure, tempered martensite is shown with "M", ferrite is shown with "F",

intermetallic compounds are shown with "IM" and carbide is shown with "C".

As shown in Table 2, the examples of the present invention each have 0.2% conventional yield strength of 860 MPa or more and excellent carbon dioxide gas corrosion resistance and sulfide stress corrosion cracking resistance. On the other side, comparative examples Nos. 22 to 25, which have Cr and/or Mo contents outside the range defined in the present invention, and comparative examples Nos. 18 to 21, which have the content ranges of the respective components in the range defined in the present invention but the expression (1) previously described was not satisfied, were not sufficient in carbon dioxide gas resistance and/or stress cracking resistance.

INDUSTRIAL APPLICABILITY

The martensitic stainless steel according to the present invention can have high strength of 0.2% conventional yield strength of 860 MPa or more and excellent carbon dioxide gas corrosion resistance and sulfide stress corrosion cracking resistance by limiting the steel composition of specified elements and defining the Mo content of the steel in relation to IM values as well as by forming the microstructure of the steel with mainly tempered martensite, carbide precipitated during tempering, and intermetallic compounds such as Lowe's phase, an a-phase and the like. As a result, the martensitic stainless steels according to the present invention can be used for practical steels, which can be widely used in oil well pipes, and the like under environments that include carbon dioxide gas, hydrogen sulfide, chlorine ions or two or more of them within wide fields.

Claims (5)

1. High-strength martensitic stainless steel that has excellent carbon dioxide gas corrosion resistance and sulfide stress corrosion cracking resistance and has 0.2% conventional yield strength of 860 MPa or more, characterized in that it includes, in % by weight, C: 0.005-0.04%, Si: 0.5% or less, Mn: 0.1-3.0%, P: 0.04% or less, S: 0.01% or less, Cr: 10-15%, Ni: 4.0-8%, Mo: 2.8-5.0%, Al: 0.001-0.10% and N: 0.07% or less, Ten: 0.005-0.25%, and optionally further contains Cu: 0.05-1%, and/or one or more selected from V: 0.005-0.25%, Nb: 0.005-0.25% and Zr: 0.005- 0.25%, and/or one or more selected from Ca: 0.0002-0.005%, Mg: 0.0002-0.005%, La: 0.0002-0.005% and Ce: 0.0002-0.005% and the balance is Fe and impurities, and also satisfies the expression (1) given below in which the steel has a microstructure comprising tempered martensite, carbide precipitated during tempering, and intermetallic compounds finely precipitated during tempering: in which the symbols of the respective elements in expression (1) show the content (wt%) of each element. 2. High-strength martensitic stainless steel according to claim 1, which further contains in % by weight one or more selected V: 0.005-0.25%, Nb: 0.005-0.25% and Zr: 0.005-0.25%. 3. High-strength martensitic stainless steel according to claim 1 or 2, which contains Cu: 0.05-1% by weight. 4. High strength martensitic stainless steel according to any one of claims 1 to 3, further containing, in % by weight, one or more selected from Ca: 0.0002-0.005%, Mg: 0.0002-0.005%, La : 0.0002-0.005% and Ce: 0.0002-0.005%. 5. High-strength martensitic stainless steel according to any one of claims 1 to 4, characterized in that the steel can be obtained by subjecting a steel with the composition as defined in any one of claims 1 to 4 to tempering in which (20 + log t) (T + 273) satisfies 13500 - 17700 when, after quenching the steel at a quenching temperature of 880-1000°C, a range of a tempering temperature is set to 450°C - 620°C, a tempering temperature is set to T(°C ) and run-in time is set to t (hours).