WO2006054430A1

WO2006054430A1 - Martensitic stainless steel

Info

Publication number: WO2006054430A1
Application number: PCT/JP2005/019685
Authority: WO
Inventors: Hideki Takabe
Original assignee: Sumitomo Metal Industries, Ltd.
Priority date: 2004-11-19
Filing date: 2005-10-26
Publication date: 2006-05-26
Also published as: US20080213120A1; EP1826285A1; CN100549204C; JP2006144069A; EP1826285B1; CN101061245A; EP1826285A4; JP4337712B2

Abstract

A martensitic stainless steel which has a chemical composition, in mass %, that C: 0.001 to 0.01 %, Si: 0.5 % or less, Mn: 0.1 to 3.0 %, P: 0.04 % or less, S: 0.01 % or less, Cr: 10 to 15 %, Ni: 4 to 8 %, Mo: 2.8 to 5.0 %, Al: 0.001 to 0.10 %, N: 0.07 % or less, Ti: 0 to 0.25 %, V: 0 to 0.25 %, Nb: 0 to 0.25 %, Zr: 0 to 0.25 %, Cu: 0 to 1.0 %, Ca: 0 to 0.005 %, Mg: 0 to 0.005 %, La: 0 to 0.005 %, Ce: 0 to 0.005 %, and the balance: Fe and impurities, with a proviso that the following formulae (1) and (2) are satisfied, and exhibits a yield stress of 758 to 860 MPa. 922.6 - 554.5C - 50.9Mn +2944.8P + 1.056Cr - 81.1Ni + 95.8Mo - 125.1Ti - 1584.9Al - 376.1N ≥ 600 (1) 30C + 0.5Mn + Ni + 0.5Cu - 1.5Si - Cr - Mo + 7.9 ≥ 0 (2) The above martensitic stainless steel can achieve a yield stress of 758 to 860 MPa with a range of tempering temperature being broader than that in the case of a conventional martensitic stainless steel.

Description

Specification

Martensitic stainless steel

Technical field

[0001] The present invention relates to martensitic stainless steel, and more particularly to martensitic stainless steel used in a corrosive environment containing corrosive substances such as hydrogen sulfide, carbon dioxide, and chlorine ions.

Background art

[0002] With the deepening of oil wells and gas wells, martensitic stainless steel used as oil well steel materials such as oil well pipes is required to have high strength and high toughness. Therefore, a martensitic stainless steel with a yield stress (0.2% resistance) strength of 758 to 860 MPa (hereinafter referred to as “l lOksi class”) and a martensitic stainless steel with a high strength of 1 lOksi class or higher have been developed. It has been.

[0003] Martensitic stainless steel for oil wells is also required to have high corrosion resistance such as SCC (Stress Corrosion Cracking) resistance and SSC (Sulfide Stress Cracking) resistance. Oil and gas wells are corrosive environments and contain corrosive substances such as hydrogen sulfide, carbon dioxide, and chlorine ions. In other words, martensitic stainless steel for oil wells is required to have high strength, high toughness and high corrosion resistance.

[0004] Japanese Unexamined Patent Publication No. 2003-3243 is disclosed as a martensitic stainless steel having high strength and high corrosion resistance. The martensitic stainless steel disclosed in this document has higher SSC resistance than conventional martensitic stainless steel by setting the Mo content to 1.5% by mass or more.

[0005] By the way, when the Mo content is high, a range of tempering temperatures at which a strength of 1 lOksi class can be obtained.

(Hereinafter referred to as the tempering temperature range) becomes very small. Fig. 1 shows the relationship between the yield stress and the tempering temperature of martensitic stainless steel with a high Mo content (hereinafter referred to as high Mo martensitic stainless steel). High Mo martensitic stainless steel of FIG. 1, the mass ^_0/0 0.016% of. 11.8% Cr, 7.2% Ni 2 2.9% Mo, the balance being Fe and impurities. Referring to Fig. 1, the yield stress ranges from 758 to 860 MPa. Tempering curve in the box The gradient of CIO is large. Therefore, the tempering temperature must be set within the range of about 580 to about 600 ° C in order to increase the strength of high Mo martensitic stainless steel to the lOksi class. In other words, the tempering temperature range ΔΤ for making the strength l lOksi grade is very small.

[0006] If the tempering temperature range ΔT is small, the productivity decreases. Usually, high Mo martensitic stainless steel is produced continuously for several hundred tons. In this case, high Mo martensitic stainless steel is produced from multiple heats (molten steel obtained in a single steelmaking process), but the chemical composition of each heat does not completely match and varies slightly. . When the tempering temperature range ΔΤ is small, the tempering temperature must be changed each time the chemical composition changes to make the steel strength l lOksi grade. In short, it is necessary to change the tempering temperature setting for each heat in order to make the strength l lOksi class. Changing the tempering temperature setting reduces productivity.

[0007] As a patent document related to the present invention, International Publication WO2004Z57050 is cited.

Disclosure of the invention

[0008] An object of the present invention is to provide a martensitic stainless steel having a large tempering temperature range in which a yield stress of 758 to 860 MPa is obtained.

As a result of various experiments and examinations, the present inventor has obtained the following knowledge.

[0010] (A) If the chemical composition is such that the A transformation point of martensitic stainless steel is high

cl

, The tempering temperature range where the yield stress becomes 758 to 860 MPa becomes wide. A Low transformation point

cl

This is because austenite is generated during high-temperature tempering, which reduces strength.

[0011] (B) A Just by raising the transformation point, C content is lowered. This gives the yield stress

cl

The tempering temperature range of 758 to 860 MPa becomes even wider. The higher the C content, the larger the tempering curve gradient within the yield stress range of 758 to 860 MPa.

[0012] (C) If the C content is lowered, δ-flite is likely to be generated, which affects the strength and toughness of steel. 1 lOksi grade martensitic stainless steel is a ring with outside air below 0 ° C. Since it is also used at the border, high strength and high toughness are required. If the chemical composition is such that the steel structure can be martensiticized even if the Acl transformation point is increased and the C content is reduced, the formation of δ ferrite can be suppressed, and l lOksi-class strength can be maintained. However, it is possible to prevent a decrease in toughness.

[0013] As a result of investigation based on the above knowledge, if the C content is not more than 0.01% and the expressions (1) and (2) are satisfied, the tempering yields 758 to 860 MPa. It has been found that the temperature range can be made larger than before.

922. 6- 554. 5C- 50. 9Mn + 2944. 8P + 1. 056Cr— 81. lNi + 95. 8Mo-125. lTi-1584. 9A1— 376. 1N≥600 (1)

30C + 0. 5Mn + Ni + 0.5Cu— 1.5Si— Cr— Mo + 7. 9≥0 (2)

Here, the symbol in the formula is the content (% by mass) of each element.

[0015] The left side of equation (1) (hereinafter, left side of equation (1) = F1) is an equation for predicting the A transformation point of the martensitic stainless steel of the present invention. As mentioned above, if you raise the A transformation point,

cl cl

Since it is possible to suppress the precipitation of retained austenite during tempering, it is possible to prevent the yield stress from rapidly decreasing. In other words, the slope of the tempering curve with a yield stress in the range of 758 to 860 MPa can be reduced.

[0016] The reason why Fl ≥ 600 is that the tempering process is executed at a tempering temperature of 600 ° C or lower. If the tempering temperature is set to 600 ° C or higher, fine carbides and intermetallic compounds in the steel become coarser, which decreases the strength and further reduces the toughness. Since the tempering temperature is 600 ° C or lower, it is sufficient if the F1 value is 600 ° C or higher.

[0017] Formula (2) is a formula for martensiticizing the steel after tempering. If the content of C, Mn, Ni, which are austenite forming elements, and the content of Si, Cr, Mo, which are ferrite forming elements, satisfy the relationship of formula (2), the structure becomes martensite, and δ ferrite Can be prevented. Therefore, strength reduction can be prevented and high toughness can be maintained.

[0018] When martensitic stainless steel does not contain Ti and Cu as selective elements, "Ti" and "Cu" in formulas (1) and (2) are "0".

[0019] If these equations are satisfied, a tempering curve like the curve C1 shown in Fig. 2 can be obtained, and the gradient of the tempering curve in the yield stress range of 758 to 860MPa can be made smaller than before. wear. Therefore, the tempering temperature range ΔΤ1 where the yield stress is 758 to 860 MPa is larger than the tempering temperature range ΔΤ2 of the conventional tempering curve C2. Therefore, it is possible to suppress a decrease in productivity due to a change in the tempering temperature setting during operation.

Based on the above findings, the present inventor has completed the following invention.

[0021] Martensitic stainless steel according to the invention, the mass ^{_{0/0, C: 0.001~0.01%}} , S i: 0.5% or less, Mn:. 0 1~3.0%, P: 0.04% or less, S: 0.01% or less, Cr: 10-15%, Ni: 4-8%, Mo: 2.8-5.0%, A1: 0.001-0.10%, N: 0.07% or less, Ti: 0-0.25%, V: 0 ~ 0.25%, Nb: 0 ~ 0.25%, Zr: 0 ~ 0.25%, Cu: 0 ~ 1.0%, Ca: 0 ~ 0.005%, Mg: 0 ~ 0.005%, La: 0 ~ 0.005%, Ce: The content is 0 to 0.005%, the balance is made of Fe and impurities, satisfies the formulas (1) and (2), and has a yield stress of 758 to 860 MPa.

922.6-554.5C-50.9Mn + 2944.8P + 1.056Cr— 81. lNi + 95.8Mo -125. LTi-1584.9A1— 376.1N≥600 (1)

30C + 0.5Mn + Ni + 0.5Cu— 1.5Si— Cr— Mo + 7.9≥0 (2)

Here, the symbol in the formula represents the content (% by mass) of each element. When Ti and Cu, which are selective elements, are not included, “Ti” and “Cu” in formulas (1) and (2) are “0”. The yield stress is 0.2% proof stress.

[0023] In the martensitic stainless steel according to the present invention, the gradient of the tempering curve can be reduced by making the C content 0.01% or less. Furthermore, by satisfying equation (1), A

The cl transformation point can be made higher than before. Therefore, the slope of the tempering curve is reduced and the yield stress is 7

The tempering temperature range of 58 to 860 MPa is larger than before.

Furthermore, by satisfying the formula (2), the strength can be prevented from becoming less than llOksi and high toughness can be maintained. In addition, it has high corrosion resistance due to its high Mo content.

[0025] Preferably, the martensitic stainless steel according to the present invention has Ti: 0.005 to 0.25%,

V: 0.005 to 0.25%, Nb: 0.005 to 0.25%, Zr: One or more of 0.005 to 0.25%.

[0026] Preferably, the martensitic stainless steel according to the present invention contains Cu: 0.05-1.0%. [0027] Preferably, the martensitic stainless steel according to the present invention includes Ca: 0.0002 to 0.005%, Mg: 0.0002 to 0.005%, La: 0.0002 to 0.005%, Ce : 0.0002 ～ 0.005

Contains one or more of%.

[0028] In this case, the hot workability of the martensitic stainless steel is improved. Note that the inclusion of these elements does not affect the effects of the invention.

Brief Description of Drawings

[0029] FIG. 1 is a graph showing the relationship between yield stress and tempering temperature of conventional high Mo martensitic stainless steel.

FIG. 2 is a graph showing the relationship between yield stress and tempering temperature of Specimen 1 and Specimen 14 in an example of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the drawings, the same or corresponding parts are denoted by the same reference numerals and the description thereof will not be repeated.

[0031] 1. Chemical composition

The martensitic stainless steel according to the embodiment of the present invention has the following composition:

. Hereinafter, “%” related to elements means “% by mass”.

[0032] C: 0.001 to 0.01%

If C is contained excessively, the gradient of the tempering curve becomes steep, and a steel having a yield stress of 758 to 860 MPa cannot be obtained stably. The C content should be limited low. on the other hand,

If the C content is less than 0.001%, the production cost increases. Therefore, the C content is 0.0

01 to 0.01 ^_0/0 [rub. Preferably! / Is a C content ί or 0.001 to 0.008 ^_0/0.

[0033] Si: 0.5% or less

Si is effective as a deoxidizer. On the other hand, Si hardens the steel, so if the Si content is too high, the toughness and cacheability of the steel will deteriorate. In addition, since Si is a ferrite-forming element, it hinders martensitic defects in steel. Therefore, Si content should be 0.5% or less. A preferable Si content is 0.3% or less.

[0034] Mn: 0.1-3.0%

Mn contributes to the improvement of hot workability of steel. Furthermore, Mn is an austenite forming element And contribute to the organization's martensite. However, if Mn is contained excessively, the toughness will decrease. Therefore, the Mn content is set to 0.1 to 3.0%. The preferred Mn content is 0.3 to L 0%.

[0035] P: 0.04% or less

P is an impurity. Since P generates SSC, the P content is limited to a very low level. Specifically, the P content is set to 0.04% or less.

[0036] S: 0.01% or less

S is an impurity. S decreases hot workability. Therefore, the S content is limited to a very low level. Specifically, the S content is set to 0.01% or less.

[0037] Cr: 10-15%

Cr contributes to the improvement of corrosion resistance in a wet carbon dioxide environment. On the other hand, since Cr is a ferrite-forming element, if it is excessively contained, tempered martensite is difficult to form, and strength and toughness are reduced. Therefore, Cr content should be 10-15%. The preferred Cr content is 11-14%.

[0038] Ni: 4-8%

Ni is an austenite-forming element and is necessary to make the structure after tempering martensite. When the Ni content is too low, the structure after tempering contains a large amount of ferrite. On the other hand, when the Ni content is too high, the structure after tempering contains a large amount of austenite. Therefore, the Ni content should be 4-8%. The preferred Ni content is 4-7%.

[0039] Mo: 2. 8 to 5.0%

Mo is an important element that contributes to the improvement of SSC resistance and strength. In the martensitic stainless steel according to the present embodiment, the lower limit of the Mo content is set to 2.8% in order to obtain high SSC resistance. On the other hand, since Mo is a ferrite-forming element, an excessive amount of additive prevents the martensitic structure of the structure. Therefore, the upper limit of Mo content is set to 5.0%. A preferable Mo content is 2.8 to 4.0%.

[0040] A1: 0.001 ~ 0.10%

A1 is effective as a deoxidizer. On the other hand, if the A1 content is too high, a large number of inclusions are produced, and the corrosion resistance is lowered. Therefore, the A1 content should be 0.001-0.10%. preferable The Al content is 0.001 to 0.06%.

[0041] N: 0.07% or less

N forms nitrides and reduces corrosion resistance. Therefore, N content should be 0.07% or less.

[0042] The balance is composed of Fe and impurities. Impurities are included due to various factors in the manufacturing process.

[0043] The martensitic stainless steel according to the present embodiment further includes Ti, V,

Contains one or more of Nb and Zr.

[0044] Ti: 0 ~ 0.25%

V: 0 ~ 0.25%

Nb: 0 ~ 0.25%

Zr: 0 ~ 0.25%

Ti, V, Nb and Zr are selective elements. These elements fix C and reduce strength variations. On the other hand, when these elements are contained excessively, the structure after tempering is prevented from becoming martensite. Therefore, the content of these elements is 0 to 0.25%. A preferable content is 0.005-0.25%, respectively, and a more preferable content is 0.005-0.20% respectively.

[0045] The martensitic stainless steel according to the present embodiment further contains Cu as necessary.

[0046] Cu: 0 to l. 0%

Cu is a selective element. Cu, like Ni, is an austenite-forming element and is effective for martensite formation after tempering. On the other hand, if Cu is contained excessively, the hot workability decreases. Therefore, the Cu content is set to 0 to 1.0%. A preferable Cu content is 0.05 to 1.0%.

[0047] The martensitic stainless steel according to the present embodiment further includes Ca, Mg as necessary.

Contains one or more of La, Ce.

[0048] Ca: 0 to 0.005%

Mg: 0 ~ 0.005% La: 0 ~ 0.005%

Ce: 0 ~ 0.005%

Ca, Mg, La, and Ce are all selective elements. These elements contribute to the improvement of hot workability. On the other hand, if these elements are contained excessively, coarse oxides are formed and the corrosion resistance is lowered. Therefore, the content of these elements is set to 0 to 0.005%. Preferable contents are 0.0002 to 0.005%, respectively. Among these elements, Ca and La are elements that particularly contribute to the improvement of hot workability.

[0049] 2. Manufacturing method

The steel having the above chemical composition is melted and refined by a well-known refinement process. Subsequently, the molten steel is made into a continuous forging material by a continuous forging method. For example, slabs and blooms are billets. Alternatively, the molten steel is made into an ingot by the ingot-making method.

[0050] Slabs, blooms, and ingots are hot-worked into billets. At this time, the billet may be formed by hot rolling, or may be formed by hot forging.

[0051] A billet obtained by continuous forging or hot working is hot worked into an oil well pipe. For example, Mannesmann method is implemented as hot working. As the hot working, hot extrusion such as a Eugene Selgene method may be performed, or a forged pipe manufacturing method such as an Erhardt method may be performed. Quench and temper the oil well pipe after hot working. The quenching process is performed by a well-known method. For example, the quenching temperature is 900 to 950 ° C. However, other temperature ranges may be used.

[0052] For the tempering treatment, the lower limit of the tempering temperature is preferably 500 ° C. On the other hand, if the tempering temperature is too high, retained austenite precipitates and the yield stress cannot be 758 to 860 MPa. Therefore, the preferable upper limit of the tempering temperature is 600 ° C.

Furthermore, the martensitic stainless steel according to the embodiment of the present invention satisfies the following formulas (1) and (2).

30C + 0. 5Mn + Ni + 0.5Cu— 1.5Si— Cr— Mo + 7. 9≥0 (2)

[0054] If Equation (1) is satisfied, the A transformation point becomes high, so the yield stress is in the range of 758 to 860 MPa. The slope of the tempering curve at can be reduced. Also, if the formula (2) is satisfied, the organization can be promoted to martensite. Therefore, if the formulas (1) and (2) are satisfied, the tempering temperature range in which a yield stress of 758 to 860 MPa can be obtained can be made larger than before. Therefore, it is possible to suppress a decrease in productivity due to a change in the setting of the tempering temperature during operation.

[0055] Furthermore, high toughness necessary for oil well steel can be obtained by satisfying the formula (2).

[0056] When martensitic stainless steel does not contain Ti and Cu as selective elements, "Ti" and "Cu" in formulas (1) and (2) are "0".

[0057] In the above, a force obtained by using martensitic stainless steel as a steel pipe may be martensitic stainless steel.

Example 1

[0058] Specimens having the chemical composition shown in Table 1 were manufactured, and the tempering temperature range in which the yield stress was 758 to 860 MPa was investigated in each specimen. In addition, the toughness and corrosion resistance of each specimen were investigated.

[table 1]

OAV

[0059] Steel having the chemical composition shown in Table 1 was melted. As shown in Table 1, the chemical compositions of the test materials 1 to 11 were within the range of the chemical composition of the present invention.

[0060] Here, F1 and F2 of each test material were obtained with Fl on the left side of Equation (1) and F2 on the left side of Equation (2). At this time, “Ti” in F1 is set to “0” for specimens that do not contain Ti, and “Cu” in F2 is set to “0” for specimens that do not contain Cu. It was.

[0061] Specimen 1 to Specimen 11 both had F1 and F2 within the scope of the present invention. Specifically, F1 was 600 or more and F2 was 0 or more.

[0062] On the other hand, in the test materials 12 and 13, although the chemical composition was within the range of the present invention, the F1 value was less than 600. In specimens 14 to 16, the C content exceeded the upper limit of the present invention. Further, the specimen 14 had an F1 value of less than 600, and the specimen 15 had an F2 value of less than 0.

[0063] Specimens 1 to 16 molten steel was forged into continuous forged materials. The manufactured continuous forging material was hot forged and hot rolled into a plurality of steel plates 15 mm thick, 120 mm wide and 1000 mm long. The steel sheet after hot forging and hot rolling was air-cooled to room temperature. The following tests were conducted using the obtained steel plate.

[0064] 1. Tempering temperature range

First, the obtained plurality of steel plates were quenched. At this time, the quenching temperature was set to 910 ° C. Subsequently, tempering was performed on the quenched steel sheet. At this time, the tempering temperature was changed within a temperature range of 450 to 650 ° C. Tensile tests were performed using steel plates that had been tempered at each tempering temperature. Specifically, a round bar test piece having a parallel part diameter of 6.35 mm and a parallel part length of 25.4 mm was prepared. A tensile test was performed at room temperature based on JIS Z2241 using the prepared round bar test piece, and the yield stress was determined. After the tensile test, the tempering temperature range ΔΤ where the yield stress was in the range of 758 to 860 MPa was determined for each specimen. The yield stress was 0.2% resistance.

[0065] Table 2 shows the tempering temperature range in which the yield stress of each specimen is 758 to 860 MPa.

[Table 2] 厶 T (.C)

* Issue

Book 1 1 10

2 80

3 100

Departure 4 1 10

5 45

6 80

Light 7 50

8 55

9 40

Steel 10 1 10

1 1 100

Ratio 12 10

Comparison 13 10

Steel 14 20

15 25

16 20

[0066] ΔΤ in Table 2 is a difference value between the highest temperature and the lowest temperature among the tempering temperatures at which the yield stress of each specimen becomes 758 to 860 MPa. The unit is ° C.

[0067] As shown in Table 2, Specimens 1 to L 1 all had a Δ repulsive force of 0 ° C or higher. On the other hand, specimens 12 and 13 had an F1 value of less than 600, so the Δ repulsive force was less than 0 ° C. Specimen 14 had a high C content and an F1 value of less than 600, so the Δ repulsive force was less than 0 ° C. Specimens 15 and 16 had a high C content, so the Δ repulsive force was less than 0 ° C.

[0068] FIG. 2 shows the relationship between the tempering temperature and the yield stress in Sample 1 and Sample 14. As shown in Fig. 2, the tempering curve C1 of Specimen 1 with an F1 value of 600 or more had a tempering temperature range ΔΤ1 of 110 ° C with a small gradient in the yield stress range of 758 to 860 MPa. On the other hand, the slope of the tempering curve C2 in the yield stress range of 758 to 860 MPa for specimen 14 with an F1 value of less than 600 was large, and the tempering temperature range ΔΤ2 was as low as 20 ° C.

[0069] 2. Toughness

Table 3 shows the results of determining the toughness value of each specimen.

[Table 3] No. F2 Yield stress (MPa) Absorption energy of -40 ° C '(J)

Book 1 0.20 804.0 185

2 0.25 782.0 182

3 0.14 812.0 176

Departure 4 0.24 805.0 185

5 0.03 813.0 188

6 0.26 838.0 187

7 0.18 854.0 192

3 0.30 808.0 90

9 0.06 841.0 188

Steel 10 0.06 807.0 190

1 1 0.04 773.0 194

12 0.63 848.0 160

Ratio 13 0.12 815.0 165

14 0.98 851.0 167 Steel 15 -1.32 844.0 81

16 0.34 841.0 171

[0070] The toughness test was performed as follows. The obtained steel sheet was quenched at 910 ° C. and subsequently tempered so that the yield stress was as shown in Table 3. From tempered steel sheet

A 10 mm wide V-notch specimen based on JISZ2202 was prepared.

[0071] A Charpy impact test was performed at -40 ° C based on JIS Z2242 using the manufactured V-notch test piece, and the absorbed energy was obtained.

[0072] The unit of absorbed energy in Table 3 is ί お. Specimens 1 ~: All L 1 had F2 value of 0 or more, so the absorbed energy exceeded 100J and showed high toughness. On the other hand, since the test material 15 had an F2 value less than SO, the absorbed energy was low.

[0073] 3. Corrosion resistance

Corrosion resistance in a wet carbon dioxide environment was evaluated by carrying out the following carbon dioxide corrosion test. A test piece having a width of 20 mm, a thickness of 3 mm, and a length of 50 mm was cut out for a steel plate that had been quenched and tempered under the same conditions as in the toughness evaluation. The surface of the cut specimen is 60

After polishing with No. 0 emery paper, it was degreased and dried.

[0074] The prepared test piece was saturated with 9.73 atmospheres of CO gas and 0.014 atmospheres of H 2 S 25%

twenty two

It was immersed in an aqueous NaCl solution for 720 hours. During the test, the temperature of the aqueous solution was maintained at 165 ° C.

[0075] After the test, the corrosion weight loss of the test piece was determined. Specifically, the value obtained by subtracting the weight of the test piece after the test from the weight of the test piece before the test was taken as the corrosion weight loss. Furthermore, the surface of the test piece is visually The presence or absence of local corrosion was confirmed. If the corrosion weight loss was less than 7.7 g and local corrosion did not occur, it was judged that the corrosion resistance in a wet carbon dioxide environment was high.

[0076] Furthermore, the SSC resistance in a wet sulfurous acid environment was evaluated by conducting the following SSC test. Tensile strength test pieces with a diameter of 6.3 mm in the parallel part and a length of 25.4 mm in the parallel part were prepared by quenching and tempering under the same conditions as for toughness evaluation. A proofing test was performed based on NACE TM0177-96 Method A using the prepared tensile specimen. At this time, 20% NaCl saturated with 0.03atm H 2 S (CO bal.)

twenty two

The test piece was immersed in the aqueous solution for 720 hours. The pH of the aqueous NaCl solution was 4.5, and the temperature of the aqueous solution was maintained at 25 ° C during the test. After the test, the presence or absence of cracks was confirmed visually.

[0077] Table 4 shows the results of the corrosion resistance test.

[Table 4]

[0078] “◯” in the carbon dioxide corrosion test in the table indicates that the corrosion weight loss is less than 7.7 g and no local corrosion has occurred. In addition, “◯” in the SSC corrosion test indicates that cracking has occurred. The specimens 1-1 were high in deviation and corrosion resistant.

Although the embodiments of the present invention have been described above, the above-described embodiments are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately modifying the above-described embodiment without departing from the spirit of the invention.

Industrial applicability The martensitic stainless steel according to the present invention can be used as a steel material used in a corrosive environment containing corrosive substances such as hydrogen sulfide, carbon dioxide gas, and chlorine ions. In particular, it can be used for steel materials for production facilities, steel materials for geothermal power generation facilities, steel materials for carbon dioxide gas removal equipment, and steel pipes used as oil well pipes in wet sulfur and hydrogen environments such as oil wells and gas wells. .

Claims

The scope of the claims

Mass ^{_{0/0, C: 0.001~0.01%}} , Si: 0.5% or less, Mn: 0.1~3.0%, P: 0.04% or less, S: 0.01% or less, Cr: 10~15%, Ni: 4~8 %, Mo: 2.8 to 5.0%, A1: 0.001 to 0.10%, N: 0.07% or less, Ti: 0 to 0.25%, V: 0 to 0.25%, Nb: 0 to 0.25%, Zr: 0 to Contains 0.25%, Cu: 0 to l.0%, Ca: 0 to 0.005%, Mg: 0 to 0.005%, La: 0 to 0.005%, Ce: 0 to 0.005%, the balance being Fe and impurities Satisfying the formula (1) and the formula (2),

Martensitic stainless steel with a yield stress of 758-860 MPa.

30C + 0.5Mn + Ni + 0.5Cu— 1.5Si— Cr— Mo + 7.9≥0 (2)

Here, the symbol in a formula is content (mass%) of each element.

The martensitic stainless steel according to claim 1,

A martensitic stainless steel characterized by containing at least one of Ti: 0.005-0.25%, V: 0.005-0.25%, Nb: 0.005-0.25%, Zr: 0.00 5-0.25%.

The martensitic stainless steel according to claim 1,

Martensitic stainless steel characterized by containing Cu: 0.05-1.0%. The martensitic stainless steel according to claim 2,

Martensitic stainless steel characterized by containing Cu: 0.05-1.0%. The martensitic stainless steel according to any one of claims 1 to 4, wherein

A martensite stainless steel maoka characterized by containing at least one of Ca: 0.0002 to 0.005%, Mg: 0.0002 to 0.005%, La: 0.0002 to 0.005%, Ce: 0.0002-0.005%.