WO2018131412A1 - 二相ステンレス鋼およびその製造方法 - Google Patents
二相ステンレス鋼およびその製造方法 Download PDFInfo
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- C—CHEMISTRY; METALLURGY
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
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- C21D6/00—Heat treatment of ferrous alloys
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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/10—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies
- C21D8/105—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies of ferrous alloys
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- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/08—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes
- C21D9/085—Cooling or quenching
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- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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- C22C38/004—Very low carbon steels, i.e. having a carbon content of less than 0,01%
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- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
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- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
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- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/001—Austenite
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- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/005—Ferrite
Definitions
- the present invention relates to a duplex stainless steel suitable for use in crude oil or natural gas oil wells, gas wells, etc. (hereinafter also simply referred to as oil wells) and a method for producing the same.
- the duplex stainless steel of the present invention has high strength and corrosion resistance, particularly carbon dioxide gas corrosion resistance under extremely severe corrosive environment including carbon dioxide (CO 2 ) and chlorine ions (Cl ⁇ ), and hydrogen sulfide ( Suitable for oil wells with excellent resistance to sulfide stress corrosion cracking at high temperatures (SCC resistance) and resistance to sulfide stress cracking at room temperature (SSC resistance) in environments containing H 2 S) Applicable to various stainless steel seamless steel pipes.
- Patent Document 1 discloses that the composition of steel is, by mass%, C ⁇ 0.03%, Si ⁇ 1.0%, Mn ⁇ 1.5%, P ⁇ 0.03%, S ⁇ 0.0015. %, Cr: 24.0 to 26.0%, Ni: 9.0 to 13.0%, Mo: 4.0 to 5.0%, N: 0.03 to 0.20%, Al: 0.0. 01 to 0.04%, O ⁇ 0.005%, Ca: 0.001 to 0.005%, limiting the addition amount of S, O, and Ca and greatly improving the phase balance that affects hot workability
- the amount of Cr, Ni, Mo, and N is optimized within that range.
- duplex stainless steel with improved H 2 S corrosion resistance is disclosed.
- Patent Document 1 has a problem that the yield strength can only be achieved at most about 80 ksi class, and can be applied only to some steel pipes for oil well pipes.
- Patent Document 2 in mass%, C: 0.03% or less, Si: 1% or less, Mn: 0.1 to 2%, Cr: 20 to 35%, Ni: 3 to 10%, Mo : Hot working of duplex stainless steel material containing 0-4%, W: 0-6%, Cu: 0-3%, N: 0.15-0.35%, the balance being Fe and impurities
- the processing degree Rd at the cross-section reduction rate in the final cold drawing is in the range of 5 to 35%.
- the duplex stainless steel containing Cu is heated to 1000 ° C. or higher and then hot-worked, and then rapidly cooled from a temperature of 800 ° C. or higher, followed by aging treatment to improve corrosion resistance.
- a method for producing a high-strength duplex stainless steel is disclosed.
- Patent Document 4 by weight, C: 0.03% or less, Si: 1% or less, Mn: 1.5% or less, P: 0.04% or less, S: 0.01% or less, Cr: 20-26%, Ni: 3-7%, Sol. Al: 0.03% or less, N: 0.25% or less, Cu: 1 to 4%, Mo: 2 to 6% and W: 4 to 10%, 1 or 2 types, Ca: 0 to 0 0.005%, Mg: 0 to 0.05%, B: 0 to 0.03%, Zr: 0 to 0.3%, Y, La and Ce as a total content of 0 to 0.03%
- the precipitation strengthened duplex stainless steel for seawater resistance satisfying the seawater resistance index PT value PT ⁇ 35 and the austenite fraction G value 70 ⁇ G ⁇ 30 is solution treated at 1000 ° C. or higher
- a method for producing a precipitation strengthened duplex stainless steel for seawater resistance obtained by aging heat treatment at 450 to 600 ° C. is disclosed.
- Patent Document 5 a solution treatment material of austenite-ferritic duplex stainless steel containing Cu is subjected to cold working with a cross-section reduction rate of 35% or more, and then once heated to 50 ° C./sec or more. It is heated to a temperature range of 800 to 1150 ° C. at a speed and then rapidly cooled, and then subjected to warm processing at 300 to 700 ° C. and then cold processing again, or after this cold processing, 450 to A method for producing a high-strength duplex stainless steel material that can be used as an oil well logging line for deep oil wells and gas wells by aging treatment at 700 ° C. is disclosed.
- Patent Document 6 C: 0.02 wt% or less, Si: 1.0 wt% or less, Mn: 1.5 wt% or less, Cr: 21 to 28 wt%, Ni: 3 to 8 wt%, Mo: 1 to 4 wt% N: 0.1 to 0.3 wt%, Cu: 2 wt% or less, W: 2 wt% or less, Al: 0.02 wt% or less, Ti, V, Nb, Ta: all 0.1 wt% or less, Zr, B: Steel containing 0.01 wt% or less, P: 0.02 wt% or less, S: 0.005 wt% or less after solution heat treatment at 1000 to 1150 ° C., then at 450 to 500 ° C. for 30 to 120 minutes A method for producing a duplex stainless steel for sour gas well pipes that undergoes aging heat treatment is disclosed.
- the corrosion resistance means excellent carbon dioxide gas corrosion resistance at a high temperature of 200 ° C. or higher and excellent low temperature of 80 ° C. or lower, particularly in a severe corrosive environment containing CO 2 , Cl ⁇ , and H 2 S. It means having both sulfide stress corrosion cracking resistance (SCC resistance) and excellent sulfide stress cracking resistance (SSC resistance) at room temperature of 20 to 30 ° C. There is also a tendency to improve economic efficiency (cost and efficiency).
- Patent Document 2 shows improvement in corrosion resistance and strength, but is still insufficient.
- the manufacturing method that performs cold drawing is expensive.
- Patent Document 3 a yield strength of about 78.9 kgf / mm 2 can be obtained without cold drawing.
- the resistance to sulfide stress corrosion cracking and the resistance to sulfide stress cracking at a low temperature of 80 ° C. or less are poor.
- Patent Documents 4 to 6 a high strength with a yield strength of 758 MPa or more can be obtained without cold drawing.
- the resistance to sulfide stress corrosion cracking and the resistance to sulfide stress cracking at a low temperature of 80 ° C. or less are poor.
- the present invention is suitable for crude oil or natural gas oil wells, gas wells, etc., and has high strength and corrosion resistance (especially in the above severe corrosive environment, carbon dioxide gas corrosion resistance, sulfide stress corrosion resistance) It is an object of the present invention to provide a duplex stainless steel excellent in crack resistance and sulfide stress crack resistance (corrosion resistance) and a method for producing the same.
- “high strength” refers to a material having a yield strength of 110 ksi or more, that is, a yield strength of 758 MPa or more in accordance with the API-5CT standard.
- “excellent carbon dioxide corrosion resistance” means that a test solution held in an autoclave: 20 mass% NaCl aqueous solution (liquid temperature: 200 ° C., 30 atmospheres CO 2 gas atmosphere) This refers to a case where the corrosion rate is 0.125 mm / y or less when the piece is immersed and the immersion period is 336 hours.
- excellent resistance to sulfide stress corrosion cracking refers to a test solution retained in an autoclave: 10 mass% NaCl aqueous solution (liquid temperature: 80 ° C., 2 MPa CO 2 gas, 35 kPa H 2 The test piece is immersed in the S atmosphere), the immersion period is set to 720 hours, 100% of the yield stress is added as an additional stress, and the test piece after the test is not cracked.
- excellent sulfide stress cracking resistance means a test solution held in a test cell: 20% mass NaCl aqueous solution (liquid temperature: 25 ° C., CO 2 gas of 0.07 MPa, 0.03 MPa)
- the test piece is immersed in an aqueous solution adjusted to pH: 3.5 by adding acetic acid + Na acetate to the H 2 S atmosphere), so that the immersion period is 720 hours and 90% of the yield stress is added as an additional stress. In this case, the test piece after the test is not cracked.
- the present inventors diligently examined various factors affecting the strength, carbon dioxide corrosion resistance, sulfide stress corrosion cracking resistance, and sulfide stress cracking resistance of duplex stainless steel. did. As a result, the following knowledge was obtained.
- the steel structure is a composite structure containing 20 to 70% austenite phase and the second phase consisting of ferrite phase. Accordingly, and at high temperatures up to 200 ° C. or higher, CO 2, Cl -, further H hot corrosion environment containing 2 S, and CO 2, Cl -, further and yield strength being corrosive atmosphere containing H 2 S In an environment where a nearby stress is applied, a duplex stainless steel having excellent carbon dioxide gas corrosion resistance and excellent sulfide stress corrosion cracking resistance at high temperatures can be obtained.
- sulfide stress corrosion cracking and sulfide stress cracking are mainly caused by active dissolution above 80 ° C., but (1) hydrogen embrittlement is mainly caused below 80 ° C., (2 ) It was newly found that nitride becomes a hydrogen trap site and increases hydrogen storage capacity, thereby deteriorating hydrogen embrittlement resistance.
- sulfide stress corrosion cracking and sulfide stress cracking at 80 ° C. or lower it is effective to reduce N to less than 0.07% in order to suppress the formation of nitrides when aging heat treatment is performed. I found out.
- the present invention has been completed based on the above findings, and the gist thereof is as follows.
- C 0.03% or less, Si: 1.0% or less, Mn: 0.10 to 1.5%, P: 0.030% or less, S: 0.005% or less, Cr: 20.0-30.0%, Ni: 5.0-10.0%, Mo: 2.0-5.0%, Cu: 2.0-6.0%, N: 0.07% 1 or more selected from Al: 0.05-1.0%, Ti: 0.02-1.0%, Nb: 0.02-1.0% And having a composition composed of the remaining Fe and inevitable impurities, and the structure has a volume ratio of 20 to 70% austenite phase and 30 to 80% ferrite phase, and yield strength YS is 758 MPa or more.
- Duplex stainless steel [2] The duplex stainless steel according to [1], further including one or more selected from the following groups A to E in addition to the composition.
- the yield strength is 110 ksi or more (758 MPa or more), and the carbon dioxide gas corrosion resistance and the sulfide stress resistance are excellent even in a severe corrosive environment containing hydrogen sulfide.
- a duplex stainless steel having excellent corrosion resistance and corrosion cracking resistance and excellent sulfide stress cracking resistance can be obtained.
- the duplex stainless steel manufactured by this invention can be manufactured cheaply by applying to the stainless steel seamless steel pipe for oil wells, and there is a remarkable industrial effect.
- C 0.03% or less C is an element having an effect of stabilizing the austenite phase and improving strength and low-temperature toughness.
- the C content is 0.02% or less. More preferably, the C content is 0.01% or less. If a large amount of C is contained, a large amount of carbide may be precipitated during heat treatment described later, and excessive penetration of diffusible hydrogen into the steel may not be prevented. Therefore, the C content is preferably 0.0020% or more. More preferably, the C content is 0.0050% or more. More preferably, the C content is 0.0065% or more.
- Si 1.0% or less
- Si is an element effective as a deoxidizing agent.
- Si is preferably contained in an amount of 0.05% or more. More preferably, the Si content is 0.10% or more. More preferably, the Si content is 0.40% or more.
- Si content shall be 1.0% or less.
- the Si content is 0.7% or less. More preferably, the Si content is 0.6% or less.
- Mn 0.10 to 1.5%
- Mn is an element that is effective as a deoxidizing agent, like Si described above.
- Mn fixes S inevitably contained in the steel as a sulfide to improve hot workability.
- the Mn content is set to 0.10 to 1.5%.
- the Mn content is 0.15% or more and 1.0% or less. More preferably, the Mn content is 0.20% or more and 0.5% or less.
- P 0.030% or less
- P is preferably reduced as much as possible in the present invention in order to reduce the corrosion resistance such as carbon dioxide corrosion resistance, pitting corrosion resistance and sulfide stress cracking resistance, but the P content is 0. 0.030% or less is acceptable.
- the P content is set to 0.030% or less.
- the P content is 0.020% or less. More preferably, the P content is 0.015% or less.
- the lower limit of the P amount is preferably 0.005% or more. More preferably, the P content is 0.007% or more.
- S 0.005% or less
- S is an element that significantly reduces hot workability and hinders stable operation of the pipe manufacturing process. Therefore, although it is preferable to reduce as much as possible, if the S content is 0.005% or less, pipe production in a normal process becomes possible. Therefore, the S content is set to 0.005% or less. Preferably, the S content is 0.002% or less. More preferably, the S content is 0.0015% or less. Excessive S reduction is industrially difficult, and is accompanied by an increase in desulfurization cost and a decrease in productivity in the steel making process. Therefore, the lower limit of the S content is preferably 0.0001%. More preferably, the S content is 0.0005% or more.
- Cr 20.0-30.0% Cr is a basic component effective for maintaining corrosion resistance and improving strength. In order to obtain these effects, the Cr content needs to be 20.0% or more. However, if the Cr content exceeds 30.0%, the ⁇ phase tends to precipitate, and both corrosion resistance and toughness deteriorate. Therefore, the Cr content is 20.0 to 30.0%. In order to obtain higher strength, the Cr content is preferably 21.0% or more, and more preferably the Cr content is 21.5% or more. Further, from the viewpoint of sulfide stress cracking resistance and toughness, the Cr content is preferably 28.0% or less, and more preferably the Cr content is 26.0% or less.
- Ni 5.0 to 10.0%
- Ni is an element contained for stabilizing the austenite phase and obtaining a two-phase structure.
- the Ni content is less than 5.0%, a ferrite phase is the main component and a two-phase structure cannot be obtained.
- the Ni content exceeds 10.0%, a two-phase structure cannot be obtained due to austenite.
- the Ni content is 5.0 to 10.0%.
- the Ni content is 6.0% or more.
- the Ni content is 8.5% or less.
- Mo 2.0-5.0% Mo is, Cl - and low pH increases the resistance to pitting is an element to enhance the sulfide stress cracking resistance and sulfide stress corrosion cracking resistance.
- Mo needs to contain 2.0% or more.
- the Mo content is set to 2.0 to 5.0%.
- the Mo content is 2.5% or more and 4.5% or less. More preferably, the Mo content is 2.6% or more and 3.5% or less.
- Cu 2.0 to 6.0%
- Cu precipitates fine ⁇ -Cu by aging heat treatment, and greatly increases the strength. Furthermore, the protective film is strengthened to suppress hydrogen intrusion into the steel, and the resistance to sulfide stress cracking and the resistance to sulfide stress corrosion cracking are enhanced. Therefore, it is a very important element in the present invention.
- Cu needs to contain 2.0% or more.
- the low temperature toughness value decreases.
- ⁇ -Cu may be excessively precipitated, and the resistance to sulfide stress corrosion cracking and the resistance to sulfide stress cracking may be reduced. For this reason, Cu content shall be 6.0% or less.
- the Cu content is 2.5% or more and 5.5% or less. More preferably, Cu content is 2.7% or more and 3.5% or less.
- N Less than 0.07% In normal duplex stainless steel, N is known as an element that improves pitting corrosion resistance and contributes to solid solution strengthening, and 0.10% or more is actively added. . However, the inventors, when performing an aging heat treatment, N rather forms various nitrides, which lowers sulfide stress corrosion cracking resistance and sulfide stress cracking resistance at a low temperature of 80 ° C. or lower. It was newly clarified that such an effect is remarkable when the N content is 0.07% or more. For this reason, the N content is less than 0.07%. Preferably, the N content is 0.05% or less, more preferably the N content is 0.03% or less, and still more preferably the N content is 0.015% or less. In order to obtain the target characteristics of the present invention, the N content is preferably 0.001% or more. More preferably, the N content is 0.005% or more.
- One or more selected from Al: 0.05-1.0%, Ti: 0.02-1.0%, Nb: 0.02-1.0% Al, Ti, Nb are It is an element that generates an intermetallic compound with Ni in an aging heat treatment, and significantly increases the strength without reducing the resistance to sulfide stress corrosion cracking and sulfide stress cracking at a low temperature of 80 ° C. or lower. Therefore, although it is an extremely important element in the present invention, the effect cannot be obtained when Al: less than 0.05%, Ti: less than 0.02%, and Nb: less than 0.02%.
- the contents are respectively Al: 0.05 to 1.0%, Ti: 0.02 to 1.0%, and Nb: 0.02 to 1.0%.
- the contents are respectively Al: 0.10% or more and 0.75% or less, Ti: 0.15% or more and 0.75% or less, Nb: 0.15% or more, and 0.0. 75% or less.
- the contents are Al: 0.40% or more and 0.60% or less, Ti: 0.40% or more and 0.60% or less, Nb: 0.40% or more, 0 .60% or less.
- Al, Ti, and Nb may be added alone.
- the strength can be further improved.
- Al, Ti, and Nb are preferably made 1.0% or less in total.
- the balance is Fe and inevitable impurities.
- O oxygen
- the above components are basic components, and the duplex stainless steel of the present invention can achieve the desired characteristics.
- the following selective elements can be contained as required.
- W 0.02 to 1.5% W is useful as an element for improving the resistance to sulfide stress corrosion cracking and the resistance to sulfide stress cracking.
- W is desirably contained in an amount of 0.02% or more.
- the toughness may be lowered.
- the resistance to sulfide stress cracking may be lowered. Therefore, when W is contained, the W content is 0.02 to 1.5%.
- the W content is 0.3 to 1.2%. More preferably, the W content is 0.4 to 1.0%.
- V 0.02 to 0.20%
- V is useful as an element for improving the strength of steel by precipitation strengthening. In order to acquire such an effect, it is desirable to contain V 0.02% or more. On the other hand, when V exceeds 0.20%, toughness may be reduced. Further, when V is contained in a large amount, the resistance to sulfide stress cracking may be lowered. For this reason, the V content is desirably 0.20% or less. Therefore, when V is contained, the V content is 0.02 to 0.20%. Preferably, the V content is 0.03 to 0.08%. More preferably, the V content is 0.04 to 0.07%.
- Zr 0.50% or less
- B One or two selected from 0.0030% or less Zr and B are both useful as elements contributing to strength increase, and are selected as necessary. Can be contained.
- Zr contributes to the above-described increase in strength and further contributes to the improvement of resistance to sulfide stress corrosion cracking. In order to obtain such an effect, it is desirable that Zr contains 0.02% or more. On the other hand, if Zr is contained in an amount exceeding 0.50%, the toughness may be lowered. Moreover, when Zr is contained in a large amount, the resistance to sulfide stress cracking may be lowered. For this reason, when Zr is contained, the Zr content is set to 0.50% or less. Preferably, the Zr content is 0.05 to 0.40%. More preferably, the Zr content is 0.10 to 0.30%.
- B is useful as an element that contributes to the above-described increase in strength and also contributes to an improvement in hot workability.
- B preferably contains 0.0005% or more.
- B when B exceeds 0.0030%, toughness and hot workability may be lowered. Further, when B is contained in a large amount, the resistance to sulfide stress cracking may be lowered. For this reason, when it contains B, B content shall be 0.0030% or less.
- the B content is 0.0008 to 0.0028%. More preferably, the B content is 0.0010 to 0.0027%.
- REM 0.005% or less, Ca: 0.005% or less, Sn: 0.20% or less, Mg: one or more selected from 0.0002 to 0.01% REM, Ca, Both Sn and Mg are useful as elements contributing to the improvement of resistance to sulfide stress corrosion cracking, and can be selected and contained as necessary.
- Ta 0.01 to 0.1%, Co: 0.01 to 1.0%, Sb: 0.01 to 1.0% Ta, Co, and Sb are Any of them is useful as an element that contributes to the improvement of the CO 2 corrosion resistance, sulfide stress cracking resistance and sulfide stress corrosion cracking resistance, and can be selected and contained as necessary.
- the content exceeds Ta: 0.1%, Co: 1.0%, Sb: 1.0%, the effect is saturated, and an effect commensurate with the content may not be expected.
- Ta 0.01 to 0.1%
- Co 0.01 to 1.0%
- Sb 0.01 to 1.0%
- Co increases the Ms point and contributes to an increase in strength. More preferably, Ta: 0.03-0.07%, Co: 0.03-0.3%, and Sb: 0.03-0.3%, respectively.
- volume ratio be a volume ratio with respect to the whole steel plate structure.
- the duplex stainless steel of the present invention has the above-described composition, and further has a composite structure containing 20 to 70% austenite phase and 30 to 80% ferrite phase by volume ratio.
- the austenite phase is set in the range of 20 to 70%.
- the austenite phase is 30-60%.
- the ferrite phase is in the range of 30 to 80%.
- the ferrite phase is 40-70%.
- the volume ratio of an austenite phase and a ferrite phase can be measured by the method as described in the Example mentioned later. In the present invention, in order to obtain a composite structure containing 20 to 70% of the austenite phase and 30 to 80% of the ferrite phase, it is controlled by performing a solution heat treatment described later.
- the volume fraction of the ferrite phase is determined by observing a surface perpendicular to the rolling direction and a surface at the center of the plate thickness with a scanning electron microscope.
- the above-mentioned specimen for observing the structure is corroded with Villera reagent, the structure is imaged with a scanning electron microscope (1000 times), and the average value of the area ratio of the ferrite phase is calculated using an image analyzer.
- the volume ratio (volume%) is used.
- the volume fraction of the austenite phase is measured using an X-ray diffraction method.
- a test specimen for measurement with the surface near the center of the plate thickness as the measurement surface was taken from the test piece material subjected to the above heat treatment (solution heat treatment and aging heat treatment), and the austenite phase ( ⁇ ) ( 220) and the diffraction X-ray integrated intensity of the (211) plane of the ferrite phase ( ⁇ ) are measured.
- I ⁇ ⁇ integrated intensity R ⁇ : ⁇ converted using crystallographically calculated value.
- precipitates such as intermetallic compounds, carbides, nitrides, and sulfides as phases other than the austenite phase and ferrite phase can be contained if the total is 1% or less. When these precipitates exceed 1% in total, the resistance to sulfide stress corrosion cracking and the resistance to sulfide stress cracking are significantly deteriorated.
- a steel piece having the above composition is used as a starting material.
- the method for producing the starting material is not particularly limited, and a generally known production method can be applied.
- the present invention can be applied not only to seamless steel pipes but also to thin plates, thick plates, UOE, ERW, spiral steel pipes, forged pipes, and the like.
- a thin plate, a thick plate, UOE, ERW, a spiral steel pipe, and a forged pipe they can be performed by a generally known production method.
- solution heat treatment is implemented after completion
- a molten steel having the above-described composition is melted by a conventional melting method such as a converter, and a steel pipe material such as a billet (starting method) by a generally known method such as a continuous casting method or an ingot-bundling rolling method (starting) Material).
- a steel pipe material such as a billet (starting method) by a generally known method such as a continuous casting method or an ingot-bundling rolling method (starting) Material).
- starting method a steel pipe material
- starting method a billet
- starting ingot-bundling rolling method
- the seamless steel pipe is preferably cooled to room temperature at an average cooling rate equal to or higher than air cooling.
- hardening and tempering processes can be performed as necessary.
- solution heat treatment is performed to cool to a temperature of 300 ° C. or lower at an average cooling rate of air cooling or higher, preferably 1 ° C./s or higher.
- the heating temperature of the solution heat treatment is less than 1000 ° C., the desired high toughness cannot be ensured.
- the heating temperature of solution heat treatment shall be 1150 degrees C or less from a viewpoint of preventing the coarsening of a structure
- the holding time at the heating temperature of the solution heat treatment is preferably 5 min or more from the viewpoint of making the temperature in the material uniform. The holding time at the heating temperature of the solution heat treatment is preferably 210 min or less.
- YS and TS increase because intermetallic compounds, carbides, nitrides, sulfides, and the like deposited before pipe formation cannot be dissolved.
- the average cooling rate of the solution heat treatment is less than 1 ° C./s, intermetallic compounds such as ⁇ phase and ⁇ phase may precipitate during the cooling, and the corrosion resistance may be significantly reduced. Therefore, the average cooling rate of the solution heat treatment is preferably 1 ° C./s or more. Note that the upper limit of the average cooling rate is not particularly limited. Here, the average cooling rate refers to the average cooling rate in the range from the heating temperature of the solution heat treatment to the cooling stop temperature.
- the cooling stop temperature of the solution heat treatment is set to 300 ° C. or lower.
- the cooling stop temperature of the solution heat treatment is 200 ° C. or lower.
- the seamless steel pipe that has undergone solution heat treatment is subjected to aging heat treatment that is heated to 350 to 600 ° C. and cooled.
- the added Cu is precipitated as ⁇ -Cu
- the added Al, Ti, and Nb form an intermetallic compound with Ni and contribute to the strength.
- it becomes a high strength duplex stainless steel seamless steel pipe having desired high strength and further excellent corrosion resistance.
- the heating temperature of the aging heat treatment exceeds 600 ° C. and becomes high, the intermetallic compound becomes coarse, and the desired high strength and further excellent corrosion resistance cannot be ensured.
- the heating temperature of the aging heat treatment is less than 350 ° C., the intermetallic compound is not sufficiently precipitated, and a desired high strength cannot be obtained.
- the heating temperature of the aging heat treatment is preferably in the range of 350 to 600 ° C. More preferably, the heating temperature of the aging heat treatment is in the range of 400 ° C to 550 ° C.
- the holding time in the aging heat treatment is preferably 5 min or more from the viewpoint of making the temperature in the material uniform.
- the holding time in the aging heat treatment is less than 5 minutes, the desired structure cannot be made uniform. More preferably, the holding time in the aging heat treatment is 20 min or more. The holding time in the aging heat treatment is preferably 210 min or less. More preferably, the holding time in the aging heat treatment is 100 min or less.
- the cooling in the aging heat treatment refers to cooling from a temperature range of 350 to 600 ° C. to room temperature at an average cooling rate equal to or higher than air cooling. Preferably, the average cooling rate in cooling by aging heat treatment is 1 ° C./s or more.
- molten steel having the composition shown in Table 1 is melted in a converter, cast into a billet (steel pipe material) by a continuous casting method, the steel pipe material is heated at 1150 to 1250 ° C., and then a heating model seamless rolling mill To make a seamless steel pipe having an outer diameter of 83.8 mm and a wall thickness of 12.7 mm.
- the seamless steel pipe was air-cooled after pipe making.
- the obtained seamless steel pipe was heated under the conditions shown in Table 2 and then subjected to solution heat treatment for cooling. Further, an aging heat treatment was performed by heating and air cooling under the conditions shown in Table 2.
- the specimens for structure observation are collected from the finally obtained seamless steel pipe after heat treatment in this way, and quantitative evaluation of the structural structure, tensile test, corrosion test, sulfide stress corrosion cracking resistance test (SCC resistance test) Test) and a sulfide stress cracking test (SSC test).
- the test method was as follows.
- volume ratio (volume%) of each phase in the entire structure of the steel plate The volume fraction of the ferrite phase was determined by observing the surface perpendicular to the rolling direction and the surface at the center of the plate thickness with a scanning electron microscope.
- the above-mentioned specimen for observing the structure is corroded with Villera reagent, the structure is imaged with a scanning electron microscope (1000 times), and the average value of the area ratio of the ferrite phase is calculated using an image analyzer.
- the volume ratio (% by volume) was used.
- the volume fraction of the austenite phase was measured using an X-ray diffraction method.
- a test specimen for measurement with the surface near the center of the plate thickness as the measurement surface was taken from the test piece material subjected to the above heat treatment (solution heat treatment and aging heat treatment), and the austenite phase ( ⁇ ) ( 220) and the diffraction X-ray integrated intensity of the (211) plane of the ferrite phase ( ⁇ ) were measured.
- I ⁇ ⁇ integrated strength
- R ⁇ ⁇ calculated crystallographic theoretical value
- I ⁇ ⁇ integrated strength
- R ⁇ converted using crystallographic theoretical calculated value of ⁇ : ⁇ .
- Corrosion test (CO2 corrosion resistance test) A corrosion test piece having a thickness of 3 mm, a width of 30 mm, and a length of 40 mm was produced by machining from the test piece material subjected to the above heat treatment, and a corrosion test was performed.
- the corrosion test was performed by immersing the test piece in a test solution: 20 mass% NaCl aqueous solution (liquid temperature: 200 ° C., CO 2 gas atmosphere of 30 atm) held in the autoclave and setting the immersion period to 336 hours. About the test piece after a test, the weight was measured and the corrosion rate calculated from the weight loss before and behind a corrosion test was calculated
- SSC resistance test Sulfide stress cracking resistance test
- NACE TM0177 Method A a round bar-shaped test piece (diameter: 6.4 mm ⁇ ) was produced by machining from the test piece material subjected to the above-described heat treatment, and an SSC resistance test was performed.
- the SSC resistance test was performed by adding acetic acid + Na acetate to a test solution: 20 mass% NaCl aqueous solution (liquid temperature: 25 ° C., atmosphere of H 2 S: 0.03 MPa, CO 2 : 0.07 MPa) to pH: 3.5.
- the test piece was immersed in the adjusted aqueous solution, the immersion period was set to 720 hours, and 90% of the yield stress was added as an additional stress.
- the test piece after the test was observed for cracks.
- produce in the test piece after a test was evaluated as the pass.
- Table 3 the case where no crack occurs is indicated by symbol ⁇ , and the case where crack occurs is indicated by symbol x.
- SCC test Sulfide stress corrosion cracking test
- the SCC resistance test was performed by immersing the test piece in a test solution held in an autoclave: 10 mass% NaCl aqueous solution (liquid temperature: 80 ° C., H 2 S: 35 kPa, CO 2 : 2 MPa), and setting the immersion period to 720 hours. 100% of the yield stress was added as an additional stress.
- the presence or absence of a crack was observed.
- produce in the test piece after a test was evaluated as the pass.
- Table 3 the case where no crack occurs is indicated by symbol ⁇ , and the case where crack occurs is indicated by symbol x.
- All of the examples of the present invention have a high yield strength: 758 MPa or more.
- it has excellent corrosion resistance (CO2 corrosion resistance) in a high-temperature corrosive environment of 200 ° C. or more containing CO 2 and Cl ⁇ , and there is no occurrence of cracks (SSC, SCC) in an environment containing H 2 S.
- SSC, SCC cracks
- Compare examples out of the scope of the present invention are high strength (yield strength: 758 MPa or more), carbon dioxide corrosion resistance, sulfide stress cracking resistance (SSC resistance), and sulfidation resistance. Any one or more of the physical stress corrosion cracking resistance (SCC resistance) was not satisfied.
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Abstract
Description
また、本発明において、「優れた耐炭酸ガス腐食性」とは、オートクレーブ中に保持された試験液:20mass%NaCl水溶液(液温:200℃、30気圧のCO2ガス雰囲気)中に、試験片を浸漬し、浸漬期間を336時間として実施した場合の腐食速度が0.125mm/y以下の場合をいう。また、本発明において、「優れた耐硫化物応力腐食割れ性」とは、オートクレーブ中に保持された試験液:10mass%NaCl水溶液(液温:80℃、2MPaのCO2ガス、35kPaのH2S雰囲気)に、試験片を浸漬し、浸漬期間を720時間として、降伏応力の100%を付加応力として付加し、試験後の試験片に割れが発生しない場合をいう。また、本発明において、「優れた耐硫化物応力割れ性」とは、試験セルに保持された試験液:20%massNaCl水溶液(液温:25℃、0.07MPaのCO2ガス、0.03MPaのH2S雰囲気)に酢酸+酢酸Naを加えて、pH:3.5に調節した水溶液中に、試験片を浸漬し、浸漬期間を720時間として、降伏応力の90%を付加応力として付加し、試験後の試験片に割れが発生しない場合をいう。
[1] 質量%で、C:0.03%以下、Si:1.0%以下、Mn:0.10~1.5%、P:0.030%以下、S:0.005%以下、Cr:20.0~30.0%、Ni:5.0~10.0%、Mo:2.0~5.0%、Cu:2.0~6.0%、N:0.07%未満を含有し、Al:0.05~1.0%、Ti:0.02~1.0%、Nb:0.02~1.0%のうちから選ばれた1種または2種以上を含有し、残部Feおよび不可避的不純物からなる組成を有し、組織は、体積率で、20~70%のオーステナイト相、30~80%のフェライト相を有する、降伏強さYSが758MPa以上である二相ステンレス鋼。
[2] 前記組成に加えてさらに、以下のA群~E群のうちから選ばれる1つまたは2つ以上を含有する[1]に記載の二相ステンレス鋼。
A群:質量%で、W:0.02~1.5%、
B群:質量%で、V:0.02~0.20%、
C群:質量%で、Zr:0.50%以下、B:0.0030%以下のうちから選ばれた1種または2種、
D群:質量%で、REM:0.005%以下、Ca:0.005%以下、Sn:0.20%以下、Mg:0.0002~0.01%のうちから選ばれた1種または2種以上、
E群:質量%で、Ta:0.01~0.1%、Co:0.01~1.0%、Sb:0.01~1.0%のうちから選ばれた1種または2種以上。
[3] [1]または[2]に記載の二相ステンレス鋼の製造方法であり、ステンレス鋼を、1000℃以上の加熱温度に加熱したのち、空冷以上の平均冷却速度で300℃以下の温度まで冷却する溶体化熱処理と、350~600℃の温度に加熱し冷却する時効熱処理とを施す、降伏強さYSが758MPa以上である二相ステンレス鋼の製造方法。
Cは、オーステナイト相を安定させて強度・低温靭性を向上させる効果を有する元素である。しかし、C含有量が0.03%を超えると、熱処理により炭化物の析出が過剰となり、鋼の耐食性を劣化させる。そのため、C含有量の上限は0.03%とする。好ましくは、C含有量は0.02%以下である。より好ましくは、C含有量は0.01%以下である。なお、Cを多量に含有すると、後述の熱処理時に、炭化物を多量に析出させ、拡散性水素の鋼中への過剰な侵入を阻止できない恐れがある。そのため、C含有量は0.0020%以上とすることが好ましい。より好ましくは、C含有量は0.0050%以上である。さらに好ましくは、C含有量は0.0065%以上である。
Siは、脱酸剤として有効な元素であり、この効果を得るためには、Siは0.05%以上の含有量が好ましい。より好ましくは、Si含有量は0.10%以上とする。さらに好ましくは、Si含有量は0.40%以上である。しかしながら、Si含有量が1.0%を超えると熱処理により金属間化合物の析出が過剰となり、鋼の耐食性を劣化させる。このため、Si含有量は1.0%以下とする。好ましくは、Si含有量は0.7%以下である。より好ましくは、Si含有量は0.6%以下である。
Mnは、上述のSiと同様に、脱酸剤として有効な元素である。これとともに、Mnは、鋼中に不可避的に含有されるSを硫化物として固定し熱間加工性を改善する。これらの効果はMnが0.10%以上の含有量で得られる。しかし、Mn含有量が1.5%を超えると熱間加工性が低下するだけでなく、耐食性に悪影響を及ぼす。このため、Mn含有量は0.10~1.5%とする。好ましくは、Mn含有量は0.15%以上であり、1.0%以下である。より好ましくは、Mn含有量は0.20%以上であり、0.5%以下である。
Pは、耐炭酸ガス腐食性、耐孔食性および耐硫化物応力割れ性等の耐食性を低下させるため、本発明ではできるだけ低減することが好ましいが、P含有量は0.030%以下であれば許容できる。このようなことから、P含有量は0.030%以下とする。好ましくは、P含有量は0.020%以下である。より好ましくは、P含有量は0.015%以下である。なお、過度のP低減は精錬コストを高騰させ、経済的に不利となる。よって、P量の下限は0.005%以上が好ましい。より好ましくは、P含有量は0.007%以上とする。
Sは、熱間加工性を著しく低下させ、パイプ製造工程の安定操業を阻害する元素である。そのため、できるだけ低減することが好ましいが、S含有量は0.005%以下であれば通常工程のパイプ製造が可能となる。このようなことから、S含有量は0.005%以下とする。好ましくは、S含有量は0.002%以下である。より好ましくは、S含有量は0.0015%以下である。なお、過度のS低減は工業的に困難であり、製鋼工程における脱硫コストの増加、生産性の低下を伴う。よって、S含有量の下限は0.0001%が好ましい。より好ましくは、S含有量は0.0005%以上とする。
Crは、耐食性を維持し、強度を向上するために有効な基本成分である。これらの効果を得るためには、Crの含有量を20.0%以上とする必要がある。しかし、Crの含有量が30.0%を超えると、σ相が析出し易くなり耐食性と靭性がともに劣化する。従って、Crの含有量は20.0~30.0%とする。より高強度を得るためには、好ましくはCr含有量は21.0%以上、より好ましくはCr含有量は21.5%以上である。また、耐硫化物応力割れ性と靱性の観点からは、好ましくはCr含有量は28.0%以下、より好ましくはCr含有量は26.0%以下である。
Niは、オーステナイト相を安定させ、二相組織を得るために含有される元素である。Ni含有量が5.0%未満の場合、フェライト相が主体となって二相組織が得られない。一方、Ni含有量が10.0%を超えると、オーステナイト主体となり二相組織が得られない。また、Niが高価な元素であるために経済性も損なわれる。従って、Ni含有量は5.0~10.0%とする。好ましくは、Ni含有量は6.0%以上である。好ましくは、Ni含有量は8.5%以下である。
Moは、Cl-や低pHによる孔食に対する抵抗性を増加させ、耐硫化物応力割れ性および耐硫化物応力腐食割れ性を高める元素である。本発明では、Moは2.0%以上の含有を必要とする。一方、Moが5.0%を超える多量の含有は、σ相を析出させ、靭性、耐食性を低下させる。従って、Mo含有量は2.0~5.0%とする。好ましくは、Mo含有量は2.5%以上であり、4.5%以下である。より好ましくは、Mo含有量は2.6%以上であり、3.5%以下である。
Cuは、時効熱処理にて微細なε-Cuを析出し、強度を大幅に上昇させる。さらに、保護皮膜を強固にして鋼中への水素侵入を抑制し、耐硫化物応力割れ性および耐硫化物応力腐食割れ性を高める。そのため、本発明において非常に重要な元素である。これらの効果を得るためには、Cuは2.0%以上の含有を必要とする。一方、Cuは6.0%を超える含有は、低温靭性値を低下させる。また、ε-Cuが過剰に析出し、耐硫化物応力腐食割れ性および耐硫化物応力割れ性が低下する恐れがある。このため、Cu含有量は6.0%以下とする。好ましくは、Cu含有量は2.5%以上であり、5.5%以下である。より好ましくは、Cu含有量は2.7%以上であり、3.5%以下である。
Nは、通常の二相ステンレス鋼においては、耐孔食性を向上させ、また固溶強化に寄与する元素として知られ、0.10%以上が積極的に添加される。しかしながら、発明者らは、時効熱処理を行う場合には、Nはむしろ種々の窒化物を形成し、80℃以下の低温での耐硫化物応力腐食割れ性および耐硫化物応力割れ性を低下させる元素であり、このような作用はN含有量が0.07%以上で顕著であることを新たに明らかにした。このことから、N含有量は0.07%未満とする。好ましくはN含有量は0.05%以下、より好ましくはN含有量は0.03%以下、さらに好ましくはN含有量は0.015%以下である。なお、本発明の目的とする特性を得るためには、N含有量を0.001%以上とすることが好ましい。より好ましくは、N含有量は0.005%以上とする。
Al、Ti、Nbは、時効熱処理においてNiと金属間化合物を生成し、80℃以下の低温での耐硫化物応力腐食割れ性および耐硫化物応力割れ性を低下させることなく強度を大幅に上昇させる元素である。そのため、本発明において極めて重要な元素であるが、Al:0.05%未満、Ti:0.02%未満、Nb:0.02%未満ではその効果を得ることができない。一方、Al:1.0%超え、Ti:1.0%超え、Nb:1.0%超えて含有すると、金属間化合物が過剰に析出し、逆に80℃以下の低温での耐硫化物応力腐食割れ性および耐硫化物応力割れ性が低下する。そのため、含有量はそれぞれ、Al:0.05~1.0%、Ti:0.02~1.0%、Nb:0.02~1.0%とする。好ましくは、含有量はそれぞれ、Al:0.10%以上、0.75%以下であり、Ti:0.15%以上、0.75%以下であり、Nb:0.15%以上、0.75%以下である。より好ましくは、含有量はそれぞれ、Al:0.40%以上、0.60%以下であり、Ti:0.40%以上、0.60%以下であり、Nb:0.40%以上、0.60%以下である。なお、Al、Ti、Nbはそれぞれ単独で添加してもよい。
本発明において、Al、Ti、Nbのうちから選ばれた2種以上の元素を複合して添加する場合は、より強度を向上できる。Al、Ti、Nbのうちから選ばれた2種以上の元素を複合して添加する場合には、Al、TiおよびNbを合計で1.0%以下とすることが好ましい。
Wは、耐硫化物応力腐食割れ性、耐硫化物応力割れ性を向上させる元素として有用である。このような効果を得るためには、Wは0.02%以上含有することが望ましい。一方、Wは1.5%を超えて多量に含有すると、靭性を低下させる場合がある。また、Wは多量に含有すると、耐硫化物応力割れ性が低下する場合がある。従って、Wを含有する場合には、W含有量は0.02~1.5%とする。好ましくは、W含有量は0.3~1.2%である。より好ましくは、W含有量は0.4~1.0%である。
Vは、析出強化により鋼の強度を向上させる元素として有用である。このような効果を得るためには、Vは0.02%以上含有することが望ましい。一方、Vは0.20%を超えて含有すると、靭性を低下させる場合がある。また、Vは多量に含有すると、耐硫化物応力割れ性が低下する場合がある。このため、V含有量は0.20%以下が望ましい。従って、Vを含有する場合には、V含有量は0.02~0.20%とする。好ましくは、V含有量は0.03~0.08%である。より好ましくは、V含有量は0.04~0.07%である。
Zr、Bは、いずれも、強度増加に寄与する元素として有用であり、必要に応じて選択して含有できる。
REM、Ca、Sn、Mgはいずれも、耐硫化物応力腐食割れ性の改善に寄与する元素として有用であり、必要に応じて選択して含有できる。このような効果を確保するためには、それぞれREM:0.001%以上、Ca:0.001%以上、Sn:0.05%以上、Mg:0.0002%以上を含有することが望ましい。より好ましくは、それぞれREM:0.0015%以上、Ca:0.0015%以上、Sn:0.09%以上、Mg:0.0005%以上とする。一方、REM:0.005%、Ca:0.005%、Sn:0.20%、Mg:0.01%をそれぞれ超えて含有しても、効果が飽和し、含有量に見合う効果が期待できなくなり、経済的に不利となる場合がある。このため、含有する場合には、それぞれREM:0.005%以下、Ca:0.005%以下、Sn:0.20%以下、Mg:0.01%以下とする。より好ましくは、それぞれREM:0.004%以下、Ca:0.004%以下、Sn:0.15%以下、Mg:0.005%以下とする。
Ta、Co、Sbはいずれも耐CO2腐食性、耐硫化物応力割れ性および耐硫化物応力腐食割れ性の改善に寄与する元素として有用であり、必要に応じて選択して含有できる。このような効果を確保するためには、それぞれTa:0.01%以上、Co:0.01%以上、Sb:0.01%以上含有することが望ましい。一方、Ta:0.1%、Co:1.0%、Sb:1.0%を超えて含有しても効果が飽和し、含有量に見合う効果が期待できなくなる場合がある。このため、含有する場合には、それぞれTa:0.01~0.1%、Co:0.01~1.0%、Sb:0.01~1.0%とする。なお、Coは、上述の効果に加えて、Ms点を高め、強度増加にも寄与する。より好ましくはそれぞれTa:0.03~0.07%、Co:0.03~0.3%、Sb:0.03~0.3%とする。
本発明では、上記したオーステナイト相を20~70%含有し、フェライト相を30~80%含有する複合組織を得るため、後述の溶体化熱処理を行うことで制御する。
また、オーステナイト相の体積率は、X線回折法を用いて測定する。上述の熱処理(溶体化熱処理および時効熱処理)を施された試験片素材から板厚中央位置付近の面を測定面とする測定用試験片を採取し、X線回折によりオーステナイト相(γ)の(220)面、フェライト相(α)の(211)面、の回折X線積分強度を測定する。そして、オーステナイト相の体積率は、次式
γ(体積率)=100/(1+(IαRγ/IγRα))
ここで、Iα:αの積分強度
Rα:αの結晶学的理論計算値
Iγ:γの積分強度
Rγ:γの結晶学的理論計算値
を用いて換算する。
なお、本発明は、継目無鋼管のみならず、薄板、厚板、UOE、ERW、スパイラル鋼管、鍛接管等にも適用できる。薄板、厚板、UOE、ERW、スパイラル鋼管、鍛接管に適用する場合、それぞれ通常公知の製造方法で行うことができる。なお、溶体化熱処理は、いずれの製造方法においても熱間圧延終了後に実施する。
フェライト相の体積率は、圧延方向に垂直な面かつ板厚中央位置の面を走査型電子顕微鏡で観察することにより求めた。上述の組織観察用の試験片をビレラ試薬で腐食して走査型電子顕微鏡(1000倍)で組織を撮像し、画像解析装置を用いて、フェライト相の面積率の平均値を算出し、これを体積率(体積%)とした。
γ(体積率)=100/(1+(IαRγ/IγRα))
ここで、Iα:αの積分強度
Rα:αの結晶学的理論計算値
Iγ:γの積分強度
Rγ:γの結晶学的理論計算値
を用いて換算した。
上述の熱処理を施された試験片素材から、API-5CT規格に準拠して、引張方向が管軸方向となるようにAPI弧状引張試験片を採取し、引張試験を実施し、引張特性(降伏強さYS、引張強さTS)を求めた。本発明では、降伏強度は、758MPa以上を合格と評価した。
上述の熱処理を施された試験片素材から、厚さ3mm×幅30mm×長さ40mmの腐食試験片を機械加工によって作製し、腐食試験を実施した。
上述の熱処理を施された試験片素材から、NACE TM0177 Method Aに準拠して、丸棒状の試験片(直径:6.4mmφ)を機械加工によって作製し、耐SSC試験を実施した。
また、上述の熱処理された試験片素材から、機械加工により、厚さ3mm×幅15mm×長さ115mmの4点曲げ試験片を採取し、耐SCC試験を実施した。
Claims (3)
- 質量%で、
C:0.03%以下、
Si:1.0%以下、
Mn:0.10~1.5%、
P:0.030%以下、
S:0.005%以下、
Cr:20.0~30.0%、
Ni:5.0~10.0%、
Mo:2.0~5.0%、
Cu:2.0~6.0%、
N:0.07%未満
を含有し、
Al:0.05~1.0%、
Ti:0.02~1.0%、
Nb:0.02~1.0%
のうちから選ばれた1種または2種以上を含有し、残部Feおよび不可避的不純物からなる組成を有し、
組織は、体積率で、20~70%のオーステナイト相、30~80%のフェライト相を有する、
降伏強さYSが758MPa以上である二相ステンレス鋼。 - 前記組成に加えてさらに、以下のA群~E群のうちから選ばれる1つまたは2つ以上を含有する請求項1に記載の二相ステンレス鋼。
A群:質量%で、W:0.02~1.5%、
B群:質量%で、V:0.02~0.20%、
C群:質量%で、Zr:0.50%以下、B:0.0030%以下のうちから選ばれた1種または2種、
D群:質量%で、REM:0.005%以下、Ca:0.005%以下、Sn:0.20%以下、Mg:0.0002~0.01%のうちから選ばれた1種または2種以上、
E群:質量%で、Ta:0.01~0.1%、Co:0.01~1.0%、Sb:0.01~1.0%のうちから選ばれた1種または2種以上。 - 請求項1または2に記載の二相ステンレス鋼の製造方法であり、
ステンレス鋼を、
1000℃以上の加熱温度に加熱したのち、空冷以上の平均冷却速度で300℃以下の温度まで冷却する溶体化熱処理と、
350~600℃の温度に加熱し冷却する時効熱処理とを施す、
降伏強さYSが758MPa以上である二相ステンレス鋼の製造方法。
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