WO2008023702A1

WO2008023702A1 - Martensitic stainless steel

Info

Publication number: WO2008023702A1
Application number: PCT/JP2007/066194
Authority: WO
Inventors: Hideki Takabe; Tomoki Mori; Masakatsu Ueda
Original assignee: Sumitomo Metal Industries, Ltd.
Priority date: 2006-08-22
Filing date: 2007-08-21
Publication date: 2008-02-28
Also published as: EP2060644A1; BRPI0719904A2; CN101506400A; NO20090712L; RU2416670C2; BRPI0719904B1; US20090162239A1; MX2009001836A; JP5124857B2; EP2060644A4; JPWO2008023702A1; RU2009110199A

Abstract

A martensitic stainless steel which contains, in terms of mass%, 0.010-0.030% C, 0.30-0.60% Mn, up to 0.040% P, up to 0.0100% S, 10.00-15.00% Cr, 2.50-8.00% Ni, 1.00-5.00% Mo, 0.050-0.250% Ti, up to 0.25% V, and up to 0.07% N, and contains at least either of up to 0.50% Si and up to 0.10% Al, the remainder being iron and incidental impurities. The martensitic stainless steel satisfies the relationship (1) and has a yield strength of 758-862 MPa. Due to this constitution, this martensitic stainless steel has a strength of the 110-ksi class (yield strength of 758-862 MPa), and the value obtained by subtracting the yield strength from the tensile strength is 20.7 MPa or more. 6.0≤Ti/C≤10.1 (1)

Description

Specification

Martensitic stainless steel

[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] In recent years, deepening of oil and gas fields has been progressing. Since these oil wells and gas wells (hereinafter collectively referred to as oil wells) are deep, high yield strength is required for steel materials used as oil well pipes in these wells. Recently, steel with a yield strength of l lOksi class (0.6% total elongation yield of 758 MPa to 862 MPa) has been used as an oil well pipe.

[0003] Furthermore, the oil well contains hydrogen sulfide, carbon dioxide, and chlorine ions. Therefore, steel materials for oil well pipes are required to have excellent SSC (Sulfide stress corrosion cracking) resistance and carbon dioxide gas corrosion resistance.

[0004] Generally, steel containing a lot of alloy components is used for oil wells. For example, SUS420 martensitic stainless steel with carbon dioxide corrosion resistance is used for oil wells containing carbon dioxide. However, SUS420 martensitic stainless steel is not suitable for oil wells containing hydrogen sulfide. This is because the SSC resistance to hydrogen sulfide is low.

[0005] Therefore, martensitic stainless steels that have not only carbon dioxide corrosion resistance but also SSC resistance have been developed. Japanese Laid-Open Patent Publication No. 5-287455 (hereinafter referred to as Patent Document 1) discloses martensitic stainless steel for oil wells having high SSC resistance and carbon dioxide gas corrosion resistance in oil wells containing hydrogen sulfide, carbon dioxide gas, and the like. ing. In order to improve SSC resistance, it is effective to reduce the tensile strength. Therefore, in Patent Document 1 described above, high S / S resistance is obtained by reducing the bow I tension strength of martensitic stainless steel. Furthermore, by reducing the tensile strength, the variation in tensile strength after tempering is reduced.

[0006] By the way, recently, in steel materials for oil well pipes, the above-mentioned high strength, SSC resistance and carbon dioxide gas resistance have been proposed. In addition to properties such as corrosivity, there is a need for properties that do not break immediately even when plastic deformation occurs in steel due to external forces. More specifically, the value obtained by subtracting the yield strength (0.6% total elongation resistance) from the tensile strength is required to be 20.7 MPa (= 3 ksi) or more.

[0007] The martensitic stainless steel for oil wells disclosed in Patent Document 1 is designed to have a low tensile strength. Therefore, when the yield strength of steel is llOksi class (758MPa ~ 832MPa), there is a problem that the value obtained by subtracting the yield strength from the tensile strength is less than 20.7MPa.

[0008] Further, as described above, the steel material for oil country tubular goods is also required to have SSC resistance. If the hardness of the same steel material is large, the SSC resistance decreases. Therefore, it is necessary to suppress the hardness variation in the steel material for oil well pipes.

Disclosure of the invention

[0009] An object of the present invention is a martensite of llOksi class (yield strength is 758MPa to 862MPa) having a value obtained by subtracting yield strength from tensile strength of 20.7MPa or more and capable of suppressing variation in hardness. Is to provide stainless steel.

[0010] The present inventors calculated the yield strength (Yield Stress) from the ratio of Ti content to C content in steel (hereinafter also referred to as Ti / C) and tensile strength (hereinafter also referred to as TS). : We have newly found that there is a correlation with the value (hereinafter also referred to as TS-YS) minus (hereinafter also referred to as YS). Hereinafter, this knowledge will be described.

[0011] The present inventors, in mass%, C: 0.010-0.030%, Mn: 0.30-0.60%, P: 0.0 40% or less, S: 0.0100% or less, Cr: 10.00-15.00%, Ni: 2.50 —8.00%, Mo: l.00—5.00%, Ti: 0.050—0.250%, V: 0.25% or less, N: 0.07% or less, Si: 0.50% or less, A1: 0.10% or less A plurality of martensitic stainless steels with the balance of Fe and impurities and Ti / C of 7-4-10.7 were manufactured. Quenching and tempering were performed during production, and the tempering temperature was adjusted to set the yield strength of each martensitic stainless steel to the llOksi class (758 MPa to 862 MPa). Each manufactured martensitic stainless steel was subjected to a tensile test at room temperature to determine the tensile strength and yield strength. In addition, 0.6% total elongation resistance based on ASTM standards was considered as yield strength. [0012] Figure 1 shows the survey results. The horizontal axis in Fig. 1 is Ti / C, and the vertical axis is TS—YS (ksi). Referring to Fig. 1, Ti / C and TS—YS showed a negative correlation. Specifically, TS-YS became larger as Ti / C became smaller. Based on this new finding, the present inventors have found that TS-YS≥20.7 MPa (3 ksi) can be satisfied by satisfying the equation (A).

Ti / C≤ 10.1 (A)

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

[0013] Furthermore, the present inventors have also newly found that if Ti / C is too small, the variation in hardness becomes large. In other words, it was found that by setting Ti / C within an appropriate range, the TS-YS force is ¾0.7 MPa or more, and the force S can be suppressed to suppress variation in hardness.

[0014] Based on the above technical idea, the present inventors have completed the following invention.

[0015] The martensitic stainless steel according to the present invention is, in mass%, C: 0.010—0.030%, Mn: 0.30—0.60%, P: 0.040% or less, S: 0.0100% or less, Cr: 10.00—15.0% , Ni: 2.50—8.00%, Μο: 1.00—5.00%, Ti: 0.050—0.250%, V: 0.25% or less, N: 0.07% or less, Si: 0.50% or less, A1: 0.10% or less One or more of them, and the balance consists of Fe and impurities. The martensitic stainless steel of the present invention further satisfies the formula (1) and has a yield strength of 758 to 862 MPa. Yield strength here refers to 0.6% total elongation resistance based on ASTM standards.

6.0≤Ti / C≤10.1 (1)

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

[0016] Preferably, the martensitic stainless steel further includes Nb: 0.25 instead of a part of Fe.

% Or less, Zr: contain one or more of 0.25% or less.

[0017] Preferably, in the martensitic stainless steel, Cu: 1.00

Contains% or less.

[0018] Preferably, in the martensitic stainless steel, in place of a part of Fe, Ca: 0.005% or less, Mg: 0.005% or less, La: 0.005% or less, Ce: 0.005% or less Contains more than seeds.

Brief Description of Drawings [0019] FIG. 1 is a graph showing the relationship between Ti / C and the value obtained by subtracting the yield strength from the tensile strength.

FIG. 2 is a cross-sectional view of a steel pipe for explaining hardness measurement points.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[0021] 1. Chemical composition

The martensitic stainless steel according to the embodiment of the present invention has the following composition. Hereinafter,% related to elements means mass%.

[0022] C: 0. 010—0. 030%

If carbon (C) is contained excessively, the hardness after tempering becomes too high, and the susceptibility to sulfide stress corrosion cracking becomes high. If the C content is too small, TS—YS ≥ 20.7 MPa cannot be satisfied when the yield strength of steel is at least lOksi class (758 MPa to 862 MPa). Therefore, the C content shall be 0.0010-0.030%. A preferable C content is 0.012 to 0.018%.

[0023] Mn: 0.30—0.60%

Manganese (Μη) improves hot workability. However, if Μη is contained excessively, the effect is saturated. Therefore, the Μη content is 0.30-0.60%.

[0024] Ρ: 0. 040% or less

Phosphorus (Ρ) is an impurity. Ρ reduces SSC resistance. Therefore, the soot content is not more than 0.040%.

[0025] S: 0.0100% or less

Sulfur (S) is an impurity. S decreases hot workability. Therefore, the lower the S content, the better. The S content is not more than 0.0100%.

[0026] Cr: 10. 00—15. 00%

Chromium (Cr) improves carbon dioxide corrosion resistance. However, excessive Cr content prevents the tempered structure from becoming a martensite phase. Therefore, the Cr content is set to 10.00-15.00%.

[0027] Ni: 2. 50—8.00%

Nickel (Ni) is effective to make the structure after tempering mainly martensite. . If the Ni content is too low, many ferrite phases will precipitate in the tempered structure. On the other hand, if the Ni content is too large, the structure after tempering mainly becomes an austenite phase. Was once, Ni content (or 2. 50-8. To 00 ^_0/0. Is preferably Rere Ni content (or 4. 00-7. 00%.

[0028] Mo: l. 00—5.00%

Molybdenum (Mo) improves the SSC resistance of high-strength steel in environments containing hydrogen sulfide. However, if Mo is contained excessively, the effect is saturated. Therefore, the Mo content should be 1.00-5.00%.

[0029] Ti: 0. 050—0. 250%

Titanium (Ti) improves toughness by suppressing the coarsening of the structure. However, excessive Ti content prevents the structure after tempering from becoming a martensite phase, and as a result, toughness and corrosion resistance (SSC resistance and carbon dioxide corrosion resistance) decrease. Therefore, the Ti content is set to 0.05-0.250%. The preferred Ti content is between 0.050 and 0.150%

Yes

[0030] N: 0.07% or less

Nitrogen (N) is an impurity. If N is contained excessively, a large amount of nitride inclusions are precipitated in the steel, resulting in a decrease in corrosion resistance. Therefore, the N content is 0.07% or less. Preferably, the N content is 0.03% or less, more preferably the N content is 0.02% or less. More preferably, the N content is 0.01% or less.

[0031] V: 0.25% or less

Vanadium (V) fixes carbides in steel by forming carbides, raises the tempering temperature, and improves SSC resistance. However, excessive addition of V has the effect of preventing the martensitic phase. Therefore, the V content is 0.25% or less. A preferable lower limit of the V content is 0.01%.

[0032] The martensitic stainless steel according to the present embodiment further contains at least one of Si and A1.

[0033] Si: 0.50% or less

A1: 0.10% or less Both silicon (Si) and aluminum (Al) are effective as deoxidizers. However, if Si is contained excessively, toughness and hot workability are reduced. In addition, if A1 is contained excessively, many inclusions are formed in the steel, which lowers the corrosion resistance. Therefore, the Si content is 0.50% or less, and the A1 content is 0.10% or less. The lower limit of the preferred Si content is 0.10%, and the lower limit of the preferred A1 content is 0.001%. Even if Si and / or A1 below the lower limit is contained, the above effect can be obtained to some extent.

[0034] The balance of the martensitic stainless steel according to the present embodiment is made of Fe. Note that impurities other than the above-described impurities may be included due to various factors.

[0035] Further, the Ti content and the C content in the chemical composition satisfy the formula (1).

[0036] 6. 0≤Ti / C≤10. 1 (1)

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

[0037] As shown in FIG. 1, TS-YS increases as Ti / C decreases. If Ti / C exceeds 10 · 1, TS -YS≥ 20. 7MPa cannot be satisfied! / ,.

[0038] On the other hand, if Ti / C is too small, the variation in hardness increases. Specifically, the following formula

Hardness variation (HRC) specified in (2) is 2.5 or more.

[0039] Hardness variation (HRC) = Hmax— Hmin (2)

Here, Hmax and Hmin are measured by the following method. In the cross-section corresponding to the center of the steel pipe as shown in Fig. 2, the Rockwell hardness C scale (hereinafter simply referred to as Rockwell hardness) of the thickness center P1 to P4 at intervals of 90 ° in the circumferential direction. The unit is HRC). Of the four measured Rockwell hardnesses, the maximum value is Hmax and the minimum value is Hmin.

[0040] If the hardness variation is 2.5 or more, the SSC resistance tends to decrease. If Ti / C is 6.0 or more, the hardness variation is less than 2.5, and the strength S can be suppressed. The reason for this is not clear, but the following reason is presumed. If Ti / C is too small, the Ti content in the steel is low. Therefore, multiple VCs precipitate during tempering. The size of each deposited VC is non-uniform depending on the deposition location in the steel pipe. As a result, the hardness variation increases. On the other hand, if Ti / C is large, the Ti content in the steel is large. Therefore, TiC precipitates during tempering and VC precipitation is suppressed. As a result, the hardness variation is reduced. [0041] The martensitic stainless steel according to the present invention satisfies the formula (1), so that TS-YS is 20.7 MPa or more and the hardness variation is less than 2.5.

A preferable upper limit value of Ti / C is 9.6, and a more preferable upper limit value of Ti / C is 9.0.

[0042] The martensitic stainless steel according to the present embodiment further contains at least one of Nb and Zr instead of a part of Fe, if necessary.

[0043] Nb: 0.25% or less

Zr: 0.25% or less

Niobium (Nb) and zirconium (Zr) are both selective elements. Together, these elements reduce the strength variation after tempering by forming carbides and fixing C in the steel. However, excessive inclusion of these elements prevents the structure after tempering from becoming mainly martensitic. Therefore, the Nb content and Zr content are 0.25% or less, respectively. The lower limit of the preferred Nb content and the lower limit of the Zr content are each 0.05%. Note that the above effect can be obtained to some extent even if it contains less than 0.005% Nb and Zr.

[0044] The martensitic stainless steel according to the present embodiment further contains Cu instead of a part of Fe, if necessary.

[0045] Cu: l. 00% or less

Copper (Cu) is a selective element. Cu, like Ni, is effective in making the structure after tempering mainly martensite. However, if Cu is contained excessively, hot workability is reduced. Therefore, Cu content should be 1.00% or less. The preferred lower limit of Cu content is 0.05%. Note that the above effect can be obtained to some extent even if it contains less than 0.05% of Cu.

[0046] The martensitic stainless steel according to the present embodiment further contains at least one of Ca, Mg, La, and Ce instead of part of Fe, if necessary.

[0047] Ca: 0.005% or less

Mg: 0.005% or less

La: 0.005% or less Ce: 0.005% or less

Calcium (Ca), magnesium (Mg), lanthanum (La) and cerium (Ce) are all selective elements. All of these elements improve hot workability. However, if these elements are contained excessively, coarse oxides are formed, and as a result, the corrosion resistance decreases. Therefore, the content of these elements should be 0.005% or less. The preferable lower limit of the content of these elements is 0.0002%. Even if Ca, Mg, La, Ce is contained in less than 0.0002%, the above effect can be obtained to some extent. Preferably, among these elements, Ca and / or La is contained.

[0048] 2. Manufacturing method

A method for manufacturing martensitic stainless steel according to the present embodiment will be described. The molten steel having the chemical composition described in 1 above is formed into a slab or billet by a continuous forging method or the like. Alternatively, molten steel is made into an ingot by the ingot-making method. Slabs and ingots are hot-worked by ingot rolling, etc. to form billets.

[0049] The manufactured billet is heated in a heating furnace, and the steel piece or steel piece extracted from the heating furnace is drilled in the axial direction by a punching machine. After that, it is processed into a seamless steel pipe of a predetermined size by a mandrel mill and reducer. After processing, heat treatment (quenching and tempering) is performed. At this time, the quenching temperature and the tempering temperature are adjusted so that the 0.6% total elongation yield strength of the martensitic stainless steel after tempering falls within the range of 758 to 862 MPa (110 ksi class).

[0050] It should be noted that the above-described manufacturing method is a method for manufacturing a martensitic stainless steel seamless steel pipe! / The force described in the above is a method for manufacturing a martensitic stainless steel welded steel pipe by a known manufacturing method. Also good.

Example

[0051] Seamless steel pipes with various chemical compositions were manufactured, and TS-Y of the manufactured seamless steel pipes

S and hardness variations were investigated.

[0052] [Investigation method]

Billets were manufactured by melting steel having the chemical composition shown in Table 1 for each test number in Table 1. Each billet produced was hot forged and hot rolled to produce a seamless steel pipe.

[table 1]

Subsequently, quenching and tempering were carried out so that the 0.6% total elongation yield strength of each manufactured seamless steel pipe was within the range of 758 MPa 862 MPa. Specifically, the quenching temperature was 910 ° C, and the tempering temperature was adjusted within the range of 560 ° C and 630 ° C.

After quenching and tempering, each steel pipe has a 0.6% total elongation resistance (YS) and tensile strength ( TS) was measured. Along the axial direction of each seamless steel pipe, a parallel bar length of 25.4 mm and a parallel section cross section diameter of 6. 35111111 round bar specimens (according to A3 ^^ [A370]) were collected. Tensile tests were performed on the collected round bar specimens at room temperature, and 0.6% total elongation resistance YS (MPa) and tensile strength TS (MPa) based on ASTM standards were measured. After the measurement, TS-YS was determined for each test number.

[0054] Further, the hardness variation of each seamless steel pipe was determined. Specifically, each seamless steel pipe was cut at the center in the transverse direction. In the cross section of the cut seamless steel pipe as shown in Fig. 2, the Rockwell hardness C scale (HRC) of the central thickness P1-P4 was measured every 90 ° in the circumferential direction. Of the four measured Rockwell hardnesses, the maximum value was Hmax and the minimum value was Hmin. Using the obtained Hmax and Hmin, hardness variation (HRC) was determined from equation (2).

[0055] [Survey results]

Table 1 shows the survey results. “Ti / C” in the table is the ratio of the Ti content (mass%) to the C content (mass%) of each test number. “TS” in the table indicates the tensile strength (MPa) of each test number, and “YS” indicates the 0.6% total elongation resistance (MPa) of each test number. “TS—YS” in the table indicates the value (MPa) obtained by subtracting 0.6% total elongation resistance from tensile strength. “Hardness variation” in the table indicates the hardness variation (HRC) obtained by equation (2). It should be noted that numerical values with an underlined bow in the table are outside the scope of the present invention.

[0056] Referring to Table 1, the 0.6% total elongation resistance (YS) of all the test numbers was in the range of 758 to 862 MPa.

[0057] The seamless steel pipes having test numbers 1 to 49 had a chemical composition within the scope of the present invention, and Ti / C satisfied the formula (1). Therefore, TS-YS of all seamless steel pipes was 20 · 7MPa or more. Furthermore, the hardness variation (HRC) of all seamless steel pipes was less than 2.5.

[0058] In contrast, the seamless steel pipes with test numbers 50 and 51 had a chemical composition within the scope of the present invention, but Ti / C did not satisfy formula (1), and Ti / C was 10 · 1. Exceeded. Therefore, TS —YS was less than 20.7 MPa.

[0059] The seamless steel pipes of test numbers 52 to 69 all have a C content lower limit of the C content of the present invention. Was less than. Therefore, TS-YS was less than 20.7 MPa.

[0060] The seamless steel pipes having test numbers 70 to 73 all had a chemical composition within the scope of the present invention, but had a Ti / C of less than 6.0. Therefore, the hardness variation was 2.5 or more.

[0061] In addition, SSC tests were conducted on seamless steel pipes with test numbers 1 to 49 and 70 to 73 in Table 1 to evaluate SSC resistance. Specifically, tensile test pieces having a parallel part diameter of 6.3 mm and a parallel part length of 25.4 mm were produced from each seamless steel pipe. A proof ring test was performed based on NACE TM0177-96 Method A using the prepared tensile test piece. At this time, the test piece was placed in a 20% NaCl aqueous solution saturated with 0.03 atm of H 2 S (CO bal.).

twenty two

Soaked 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 visually confirmed.

[0062] As a result of the test, no crack occurred in any of the tensile test pieces of test numbers 1 to 49. On the other hand, cracks were confirmed in the tensile test pieces of test numbers 70 to 73.

[0063] 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

[0064] The martensitic stainless steel according to the present invention is widely applied to steel materials used in corrosive environments containing corrosive substances such as hydrogen sulfide, carbon dioxide, and chlorine ions. Specifically, it is suitable for steel materials used in oil and natural gas production facilities, carbon dioxide removal equipment, and geothermal power generation facilities. Especially suitable for oil well pipes used in oil wells and gas wells.

Claims

The scope of the claims

[1] At mass ⁰ / o, C: 0.010—0.030%, Mn: 0.30—0.60%, P: 0.040% or less, S:

0.0100% or less, Cr: 10.00—15.00%, Ni: 2.50—8.00%, Mo: l.00—5.00%, Ti: 0.050—0.250%, V: 0.25% or less, N: 0.07% or less, Si: 0.50% or less and A1: 0. 10% or less, the balance being Fe and impurities, satisfying formula (1), and having a yield strength of 758 to 862 MPa Martensitic stainless steel maoka.

6.0≤Ti / C≤10.1 (1)

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

[2] The martensitic stainless steel according to claim 1, further comprising:

A martensitic stainless steel characterized by containing at least one of Nb: 0.25% or less and Zr: 0.25% or less in place of a part of Fe.

[3] The martensitic stainless steel according to claim 1,

A martensitic stainless steel maoka characterized by containing Cu: 100% or less in place of a part of the Fe.

[4] The martensitic stainless steel according to claim 2,

[5] The force according to any one of claims 1 to 4, and the martensitic stainless steel according to claim 1,

Martensite stainless steel characterized by containing at least one of Ca: 0.005% or less, Mg: 0.005% or less, La: 0.005% or less, Ce: 0.005% or less instead of part of Fe Maoka.