WO2020241084A1 - 二相ステンレス鋼およびその製造方法、並びに二相ステンレス鋼管 - Google Patents
二相ステンレス鋼およびその製造方法、並びに二相ステンレス鋼管 Download PDFInfo
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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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Definitions
- the present invention relates to a duplex stainless steel having excellent corrosion resistance, high strength, and high toughness, which is suitable for use as an oil well pipe, and a method for producing the same.
- the duplex stainless steel used for an oil well pipe and its manufacturing method Regarding the manufacturing method.
- the present invention also relates to a duplex stainless steel pipe made of this duplex stainless steel.
- duplex stainless steel pipes are often used as oil well pipes used for mining in oil and gas fields in environments containing CO 2 , Cl ⁇ , and the like.
- Patent Document 1 Cu-containing austenite-ferritic two-phase stainless steel is heated to 1000 ° C. or higher for hot working, then rapidly cooled from a temperature of 800 ° C. or higher, and then aged.
- a method for producing a high-strength duplex stainless steel with improved corrosion resistance is disclosed.
- Patent Document 2 describes in terms of weight%, C: 0.03% or less, Si: 1% or less, Mn: 1.5% or less, P: 0.04% or less, and S: 0.01%.
- Cr 20 to 26%
- Ni 3 to 7%
- Sol-Al 0.03% or less
- N 0.25% or less
- Cu 1 to 4%
- Mo 2 to 6%
- W 4 to 10%
- Ca 0 to 0.005%
- Mg 0 to 0.05%
- B 0 to 0.03.
- the seawater resistance index PT value is PT ⁇ 35, and the austenite fraction G. It is assumed that a precipitation-strengthened two-phase stainless steel for seawater resistance having a value of 70 ⁇ G ⁇ 30 is obtained by solution-treating the stainless steel at 1000 ° C. or higher and then aging heat treatment at 450 to 600 ° C. A method for producing a precipitation reinforced two-phase stainless steel for seawater resistance is disclosed.
- Patent Document 3 a solution treated material of an austenite-ferritic two-phase stainless steel containing Cu is cold-worked with a cross-sectional reduction rate of 35% or more, and then once heated at 50 ° C./s or more. It is heated to a temperature range of 800 to 1150 ° C at a rate and then quenched, then warmed at 300 to 700 ° C and then cold again, or 450 to 450 after this cold working.
- a method for producing a high-strength duplex stainless steel material that can be used as an oil well inspection line for deep oil wells and gas wells by aging treatment at 700 ° C. is disclosed.
- Patent Document 4 describes 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 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 is solution heat treated at 1000 to 1150 ° C., and then subjected to solution heat treatment at 450 to 500 ° C. for 30 to 120 minutes.
- a method for producing duplex stainless steel for sour gas oil well pipes, which undergoes aging heat treatment, is disclosed.
- Patent Document 5 describes 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 to 30.0%, Ni: 5.0 to 10.0%, Mo: 2.0 to 5.0%, Cu: 2.0 to 6.0%, N: less than 0.07%
- Duplex heat treatment in which steel containing steel is heated to a heating temperature of 1000 ° C or higher and then cooled to a temperature of 300 ° C or lower at an average cooling rate of air cooling or higher, and aging to heat and cool to a temperature of 350 ° C to 600 ° C.
- a method for producing high-strength, high-toughness duplex stainless steel, which is subjected to heat treatment, is disclosed.
- JP-A-61-23713 Japanese Unexamined Patent Publication No. 10-60526 Japanese Unexamined Patent Publication No. 7-20737 Japanese Unexamined Patent Publication No. 61-157626 Special Republication 2018-43214
- excellent corrosion resistance means excellent carbon dioxide corrosion resistance at a high temperature of 200 ° C. or higher and a low temperature of 80 ° C. or lower, particularly in a severe corrosion environment containing CO 2 and Cl ⁇ and H 2 S. It means that it has both excellent sulfide stress corrosion cracking resistance (SCC resistance) and excellent sulfide stress cracking resistance (SSC resistance) at room temperature of 20 to 30 ° C. And there is a tendency that improvement of economic efficiency (cost and efficiency) is also required.
- Patent Documents 1 to 4 have a problem that the sulfide stress corrosion cracking resistance and the sulfide stress cracking resistance at a low temperature of 80 ° C. or lower are not taken into consideration.
- the steel described in Patent Document 5 is said to have good sulfide stress corrosion cracking resistance and sulfide stress cracking resistance at a low temperature of 80 ° C. or lower, but pitting corrosion at a low temperature of 80 ° C. or lower. There is no description about the occurrence of.
- an object of the present invention is to provide a duplex stainless steel having high strength, high toughness, and excellent corrosion resistance, and a method for producing the same.
- the excellent corrosion resistance means a corrosion resistance having excellent carbon dioxide gas corrosion resistance, excellent sulfide stress corrosion cracking resistance, and excellent sulfide stress cracking resistance even in the severe corrosion environment as described above. Point to.
- Such duplex stainless steel pipes are preferably used in harsh environments such as crude oil or natural gas oil wells and gas wells.
- “high strength” means that the yield strength is 95 ksi (655 MPa) or more.
- “high toughness” means low temperature toughness, that is, the absorbed energy vE- 10 of the Charpy impact test at ⁇ 10 ° C. has 40 J or more.
- “excellent carbon dioxide corrosion resistance” is defined in a test solution held in an autoclave: a 20% by mass NaCl aqueous solution (liquid temperature: 200 ° C., 3.0 MPa CO 2 gas atmosphere). It means that the corrosion rate is 0.125 mm / y or less and no pitting corrosion occurs when the test piece is immersed and the immersion period is 336 hours.
- excellent sulfide stress corrosion cracking resistance means a test solution held in an autoclave: a 10 mass% NaCl aqueous solution (liquid temperature: 80 ° C., 2 MPa CO 2 gas, 35 kPa H). When the test piece is immersed in (2S atmosphere), the immersion period is 720 hours, and 100% of the yield stress is applied as an additional stress, the test piece after the test does not crack and pitting corrosion occurs. It means that there is no. Further, in the present invention, “excellent sulfide stress cracking resistance” means a test solution held in a test cell: a 20 mass% NaCl aqueous solution (liquid temperature: 25 ° C., 0.07 MPa CO 2 gas, 0.
- the present inventors discuss various factors affecting the strength, toughness, carbon dioxide corrosion resistance, sulfide stress corrosion cracking resistance, and sulfide stress cracking resistance of duplex stainless steel. I examined it diligently. As a result, the following findings were obtained. 1) In duplex stainless steel containing 2.0% or more of Cu, Cu tends to be oversaturated in the ferrite phase during cooling after hot rolling, and as a result, coarse ⁇ -Cu is precipitated in the ferrite phase. thing. 2) Coarse ⁇ -Cu after hot rolling cannot be easily eliminated by ordinary solution treatment, and long-time heating is required to eliminate it.
- the coarse ⁇ -Cu remaining in the ferrite phase becomes the starting point of corrosion, and selective corrosion of the ferrite phase, which is the starting point of pitting corrosion, is likely to occur.
- a heat treatment is performed to precipitate a ⁇ phase that hardly dissolves Cu, thereby promoting the transfer of Cu from the ferrite phase to the austenite phase in a short heating time, and then a solution treatment.
- the amount of coarse ⁇ -Cu in the ferrite phase can be significantly reduced.
- the [% element symbol] in the above equation (1) represents the content (mass%) of the element in the steel
- the [% element symbol * F] is the content (mass%) of the element in the ferrite phase. Represents. If the element is not contained, it is set to zero.
- 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 0.040% or less
- S 0.01% or less
- Cr 20.0 to 28.0%
- Ni 2.0 to 10.0%
- Mo 2.0 to 5.0%
- Cu 2.0 to 6.0%
- Al 0.001 to It contains 0.05%
- N less than 0.070%
- the contents of Cr, Mo, Ni, N, Cu, and W satisfy the following formula (1), the yield strength YS is 655 MPa or more, and the test temperature: the absorbed energy vE- 10 of the Charpy impact test at -10 ° C is 40 J.
- duplex stainless steel 0.55 [% C] -0.056 [% Si] +0.018 [% Mn] -0.020 [% Cr] -0.087 [% Mo] +0.16 [% Ni] +0.28 [% N] -0.506 [% Cu] -0.035 [% W] + [% Cu * F] ⁇ 0.94 ...
- the [% element symbol] in the above equation (1) represents the content (mass%) of the element in the steel
- the [% element symbol * F] is the content (mass%) of the element in the ferrite phase. Represents. If the element is not contained, it is set to zero.
- Mg 0.01% or less, selected from one or more.
- Group E One or more selected from Ta: 0.1% or less
- Sb 1.0% or less.
- a steel material having the component composition according to [1] or [2] is heated to a temperature of 700 ° C. or higher and 950 ° C. or lower, and then cooled to a temperature of 300 ° C. or lower at an average cooling rate of air cooling or higher.
- ⁇ -phase precipitation treatment solution heat treatment that cools to a temperature of 300 ° C or lower at an average cooling rate of air cooling or higher after heating to a temperature of 1000 ° C or higher, and aging heat treatment that cools after heating to a temperature of 350 to 600 ° C.
- duplex stainless steel produced by the present invention By applying the duplex stainless steel produced by the present invention to stainless seamless steel pipes for oil wells, an industrially significant effect is achieved.
- 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 preferably 0.002% or more. .. More preferably, the C content is 0.005% or more.
- the C content is set to 0.03% or less.
- the C content is 0.02% or less. More preferably, the C content is 0.015% or less, and even more preferably, the C content is 0.012% or less.
- Si 1.0% or less
- Si is an element that functions as an antacid, and in order to obtain this effect, a content of 0.05% or more is preferable. More preferably, the Si content is 0.10% or more. However, if the Si content exceeds 1.0%, the intermetallic compound is excessively precipitated by the heat treatment, and the corrosion resistance of the steel is deteriorated. Therefore, the Si content is set to 1.0% or less. Preferably, the Si content is 0.8% or less, and more preferably, the Si content is 0.7% or less. More preferably, it is 0.6% or less.
- Mn 0.10 to 1.5%
- S which is inevitably contained in steel, is fixed as a sulfide to improve hot workability.
- the Mn content is set to 0.10% or more.
- the Mn content is 0.15% or more, more preferably 0.20% or more.
- the Mn content is set to 1.5% or less.
- the Mn content is 1.0% or less, more preferably 0.8% or less, still more preferably 0.5% or less.
- P 0.040% or less
- P is an element that lowers the corrosion resistance of duplex stainless steel, and if it exceeds 0.040%, the corrosion resistance is significantly lowered. Therefore, the P content is 0.040% or less. Preferably, the P content is 0.020% or less. However, in order to reduce P to less than 0.005%, it takes a long time to remove P in the process of melting molten steel, which leads to an increase in the manufacturing cost of duplex stainless steel. Therefore, P is preferably 0.005% or more.
- S 0.01% or less
- S is an element that reduces hot workability in the manufacturing process of duplex stainless steel, and if it exceeds 0.01%, it interferes with the production of duplex stainless steel. Therefore, S is set to 0.01% or less.
- the S content is 0.005% or less. From the viewpoint of preventing an increase in manufacturing cost, the S content is preferably 0.0005% or more.
- Cr 20.0 to 28.0% Cr is an effective basic component for maintaining corrosion resistance and improving strength.
- the Cr content is set to 20.0% or more.
- the Cr content is preferably 21.0% or more, and more preferably 23.0% or more.
- the Cr content is set to 28.0% or less. From the viewpoint of toughness, the Cr content is preferably 27.0% or less.
- Ni 2.0 to 10.0%
- Ni is an element contained to stabilize the austenite phase and obtain a two-phase structure. If Ni is less than 2.0%, the effect cannot be obtained. Therefore, the Ni content is set to 2.0% or more. Preferably, it is 3.0% or more. More preferably, it is 4.0% or more. On the other hand, when the Ni content exceeds 10.0%, the austenite phase becomes the main component, and the strength desired in the present invention cannot be obtained. Moreover, since Ni is an expensive element, economic efficiency is impaired. Therefore, the Ni content is set to 10.0% or less. It is preferably 8.0% or less.
- Mo 2.0-5.0%
- Mo is an element having an action of improving the corrosion resistance of duplex stainless steel, and particularly contributes to the prevention of pitting corrosion caused by Cl ⁇ . If Mo is less than 2.0%, the effect cannot be obtained. Therefore, the Mo content is set to 2.0% or more. Preferably, it is 2.5% or more. On the other hand, when the Mo content exceeds 5.0%, the ⁇ phase is precipitated and the toughness and corrosion resistance are lowered. Therefore, the Mo content is 5.0% or less. It is preferably 4.5% or less.
- Cu 2.0-6.0%
- fine ⁇ -Cu is precipitated by aging heat treatment, and the strength is significantly increased.
- Cu strengthens the protective film, suppresses hydrogen intrusion into steel, and enhances sulfide stress cracking resistance and sulfide stress corrosion cracking resistance. Therefore, it is a very important element in the present invention.
- the Cu content is set to 2.0% or more.
- the Cu content is 2.5% or more.
- the Cu content exceeds 6.0%, the low temperature toughness decreases. Therefore, the Cu content is set to 6.0% or less.
- the Cu content is 5.5% or less. More preferably, it is 5.0% or less.
- Al 0.001 to 0.05%
- Al is an element that functions as an antacid in the process of melting molten steel, which is a raw material for duplex stainless steel, and its effect cannot be obtained if it is less than 0.001%. Therefore, the Al content is set to 0.001% or more. It is preferably 0.005% or more.
- the Al content is set to 0.05% or less. It is preferably 0.04% or less.
- N Less than 0.070% N is known as an element that improves pitting corrosion resistance and contributes to solid solution strengthening in ordinary duplex stainless steel, and 0.10% or more is positively added. .. However, when aging heat treatment is performed, N is an element that rather forms various nitrides and lowers sulfide stress corrosion cracking resistance and sulfide stress cracking resistance at low temperatures of 80 ° C. or lower. When 0.070% or more is contained, the action becomes remarkable. Therefore, the N content is set to less than 0.070%. Preferably, the N content is 0.05% or less, more preferably 0.04% or less, still more preferably 0.03% or less, and even more preferably 0.015% or less. In order to obtain the desired properties of the present invention, the N content is preferably 0.001% or more. More preferably, the N content is 0.005% or more.
- the rest is Fe and unavoidable impurities.
- unavoidable impurities include O (oxygen), and O is acceptable if it is 0.01% or less.
- the above ingredients are the basic ingredients. Further, in addition to the above basic components, one group or two or more groups selected from the following groups A to E may be contained, if necessary.
- W 1.5% or less W is useful as an element for improving sulfide stress corrosion cracking resistance and sulfide stress cracking resistance.
- the W content is preferably 0.02% or more. More preferably, the W content is 0.3% or more, and even more preferably, the W content is 0.8% or more.
- the W content is set to 1.5% or less. More preferably, the W content is 1.2% or less.
- V 0.20% or less
- V is useful as an element that improves the strength of steel by precipitation strengthening.
- the V content is preferably 0.02% or more. More preferably, the V content is 0.04% or more.
- V is contained in an amount of more than 0.20%, the low temperature toughness may be lowered. Further, if it is contained in a large amount, the sulfide stress cracking resistance may decrease. Therefore, when V is contained, the V content is set to 0.20% or less. More preferably, the V content is 0.08% or less.
- Group C Zr: 0.50% or less
- B One or two selected from 0.0030% or less Zr and B are both useful as elements that contribute to the increase in strength, and if necessary. It may be selected and contained.
- Zr contributes to the above-mentioned increase in strength and also to the improvement of sulfide stress corrosion cracking resistance. In order to obtain such an effect, it is desirable that the Zr content is 0.02% or more. More preferably, the Zr content is 0.05% or more. On the other hand, if Zr is contained in an amount of more than 0.50%, the low temperature toughness may be lowered. Therefore, when Zr is contained, the content is 0.50% or less. More preferably, the Zr content is 0.20% or less.
- B is useful as an element that contributes to the above-mentioned increase in strength and also to the improvement of hot workability.
- the B content is 0.0005% or more. More preferably, the B content is 0.0010% or more.
- the B content is set to 0.0030% or less. More preferably, the B content is 0.0025% or less.
- Group D REM: 0.005% or less, Ca: 0.005% or less, Sn: 0.20% or less, Mg: 0.01% or less, one or more selected from REM, Ca, Both Sn and Mg are useful as elements that contribute to the improvement of sulfide stress corrosion cracking resistance, and may be selected and contained as necessary.
- REM 0.005% or less
- Ca 0.005% or less
- Sn 0.20% or less
- Mg 0.01% or less
- REM 0.004% or less
- Ca 0.004% or less
- Sn 0.15% or less
- Mg 0.005% or less
- Group E One or more selected from Ta: 0.1% or less, Co: 1.0% or less, Sb: 1.0% or less Ta, Co, and Sb are all carbon dioxide resistant gas. It is useful as an element that contributes to the improvement of corrosiveness, sulfide stress cracking resistance, and sulfide stress corrosion cracking resistance, and may be selected and contained as necessary. In order to ensure such an effect, when contained, Ta: 0.01% or more, Co: 0.01% or more, and Sb: 0.01% or more, respectively. More preferably, Ta: 0.02% or more, Co: 0.02% or more, and Sb: 0.02% or more, respectively.
- Ta: 0.1%, Co: 1.0%, and Sb: 1.0% are contained in excess, the effect may be saturated and an effect commensurate with the content may not be expected. Therefore, when they are contained, Ta: 0.1% or less, Co: 1.0% or less, and Sb: 1.0% or less, respectively. More preferably, Ta: 0.05% or less, Co: 0.5% or less, and Sb: 0.5% or less, respectively.
- [% element symbol] represents the content (mass%) of the element in steel
- [% element symbol * F] is the content (mass%) of the element in the ferrite phase. ) Means. If the element is not contained, it is set to zero. 0.55 [% C] -0.056 [% Si] +0.018 [% Mn] -0.020 [% Cr] -0.087 [% Mo] +0.16 [% Ni] +0.28 [% N] -0.506 [% Cu] -0.035 [% W] + [% Cu * F] ⁇ 0.94 ...
- the lvalue of Eq. (1) is an index of the amount of coarse ⁇ -Cu in the ferrite phase, and as the lvalue of Eq. (1) increases, the amount of coarse ⁇ -Cu increases and the pitting corrosion resistance deteriorates. To do. From the viewpoint of further improving pitting corrosion resistance, the lvalue of Eq. (1) is preferably 0.92 or less. The lower limit is not specified. From the viewpoint of ensuring stable strength, the lvalue of Eq. (1) is preferably 0.80 or more.
- the Cu content in the above-mentioned ferrite phase can be determined, for example, as follows.
- a test piece for microstructure observation is taken so that the cross section in the axial direction of the pipe is the observation surface, and the ferrite phase is identified by EBSP analysis (Electron Back Scattering Pattern). To do.
- EBSP analysis Electron Back Scattering Pattern
- any 20 points are measured with FE-EPMA (Field Emission Electron Probe Micro Analyzer) to obtain the Cu content.
- the value obtained by averaging the quantitative values of the obtained Cu content is taken as the Cu content in the ferrite phase of the steel.
- the two-phase stainless steel of the present invention has a structure containing an austenite phase and a ferrite phase.
- the volume fraction (%) of the austenite phase is preferably 20 to 70%.
- the volume fraction (%) of the ferrite phase is preferably 30 to 80%. If the austenite phase is less than 20%, the low temperature toughness, sulfide stress cracking resistance, and sulfide stress corrosion cracking resistance may be inferior. Further, when the austenite phase exceeds 70%, the strength may be inferior.
- the austenite phase is more preferably 25% or more, more preferably 65% or less. If the ferrite phase is less than 30%, the strength may be inferior.
- the ferrite phase exceeds 80%, the low temperature toughness, sulfide stress cracking resistance, and sulfide stress corrosion cracking resistance may be inferior.
- the ferrite phase is more preferably 35% or more, and more preferably 75% or less.
- the volume fraction of each phase can be measured by the method described in Examples described later.
- duplex stainless steel manufacturing method will be described as a duplex stainless steel manufacturing method of the present invention.
- a manufacturing method in the case where the duplex stainless steel of the present invention is a seamless steel pipe will be described.
- 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 steel material such as a billet having the above-mentioned composition is used as a starting material (hereinafter, may be referred to as a steel pipe material).
- the method for producing the starting material is not particularly limited, and a generally known production method can be applied.
- molten steel having the above-mentioned component composition is melted by a common melting method such as a converter, and a continuous casting method, an ingot-bulk rolling method, etc.
- a steel pipe material by a commonly known method.
- these steel pipe materials are heated and subjected to hot working such as an extrusion pipe manufacturing method such as the Eugene Sejurne method or a Mannesmann pipe manufacturing method, which is a commonly known pipe making method, to obtain the above-mentioned component composition of a desired size.
- an extrusion pipe manufacturing method such as the Eugene Sejurne method or a Mannesmann pipe manufacturing method, which is a commonly known pipe making method
- the heating temperature of the steel pipe material described above is preferably in the range of, for example, 1100 to 1300 ° C. If the temperature is lower than 1100 ° C., the workability of the material is lowered, and the outer surface of the steel pipe may be cracked during rolling. On the other hand, if the temperature exceeds 1300 ° C., the material temperature may exceed the melting point and melt due to heat generated by processing, making subsequent rolling difficult. Further, in the above-mentioned hot working, a large amount of dislocations and grain boundaries which are precipitation nuclei of Cu are introduced, and from the viewpoint of obtaining a high-strength material by the subsequent aging heat treatment, for example, the total reduction amount in the temperature range of 800 to 1300 ° C.
- the total reduction amount refers to the wall thickness reduction amount of the steel pipe rolled by an elongator, a plug mill, or the like, which is carried out after drilling with a piercer.
- the obtained seamless steel pipe is cooled.
- a duplex stainless steel pipe is produced by subjecting a seamless steel pipe after cooling to a ⁇ phase precipitation treatment, a solution heat treatment, and an aging heat treatment in this order.
- the ⁇ phase precipitation treatment which is important in the present invention, is performed.
- a seamless steel pipe having the above component composition is heated at a heating temperature of 700 ° C. or higher and 950 ° C. or lower, and then an average cooling rate of air cooling or higher, more specifically, 1 ° C./s or higher. Cool to a temperature of 300 ° C. or lower at an average cooling rate.
- the ⁇ phase is precipitated, and the Cu supersaturated state in the ferrite phase is eliminated.
- the degree of Cu supersaturation in the ferrite phase corresponds to Eq. (1).
- the heating temperature of the ⁇ phase precipitation treatment is preferably 900 ° C. or lower from the viewpoint of promoting the precipitation of the ⁇ phase. Further, preferably, the heating temperature of the ⁇ phase precipitation treatment is 750 ° C. or higher.
- the holding time of the ⁇ -phase precipitation treatment at the heating temperature is preferably 5 min or more from the viewpoint of making the temperature in the material uniform. More preferably, it is 10 min or more. Further, the holding time of the ⁇ phase precipitation treatment at the heating temperature is preferably 300 min or less. More preferably, it is 100 min or less.
- the average cooling rate of cooling in the ⁇ -phase precipitation treatment is preferably 2 ° C./s or more.
- the cooling method include air cooling and water cooling.
- the upper limit of the average cooling rate is not specified, but if the average cooling rate is large, the effect on the material properties is saturated, so that the average cooling rate is preferably 50 ° C./s or less.
- the average cooling rate means the average cooling rate in the range from the heating temperature to the cooling stop temperature.
- the cooling stop temperature of the ⁇ phase precipitation treatment is set to 300 ° C. or lower. More preferably, it is 250 ° C. or lower.
- the solution heat treatment is performed on the seamless steel pipe subjected to the ⁇ phase precipitation treatment.
- the seamless steel pipe subjected to the ⁇ phase precipitation treatment is further heated to a heating temperature of 1000 ° C. or higher, and then has an average cooling rate of air cooling or higher, more specifically, an average cooling rate of 1 ° C./s or higher. Cool to a temperature of 300 ° C or less.
- the intermetallic compounds, carbides, nitrides, sulfides, etc. precipitated before or during the ⁇ phase precipitation treatment are solid-solved to form a seamless steel pipe having a structure containing an appropriate amount of austenite phase and ferrite phase. can do.
- the heating temperature of the solution heat treatment is 1020 ° C. or higher.
- the heating temperature of the solution heat treatment is preferably 1150 ° C. or lower from the viewpoint of preventing coarsening of the structure. More preferably, the heating temperature of the solution heat treatment is 1130 ° C. or lower.
- the holding time of the solution heat treatment at the heating temperature is preferably 5 min or more from the viewpoint of making the temperature in the material uniform. More preferably, it is 10 min or more. Further, the holding time of the solution heat treatment at the heating temperature is preferably 210 min or less. More preferably, it is 100 min or less.
- the average cooling rate of the solution heat treatment is less than 1 ° C./s, intermetallic compounds such as ⁇ phase and ⁇ phase are precipitated during cooling, and the low temperature toughness and corrosion resistance are significantly lowered.
- the upper limit of the average cooling rate does not need to be particularly limited.
- the cooling rate of cooling in the solution heat treatment is preferably 2 ° C./s or more.
- the cooling stop temperature of the solution heat treatment exceeds 300 ° C., the added Cu precipitates as coarse ⁇ -Cu during cooling, and the desired high strength, high toughness and excellent corrosion resistance cannot be ensured. Therefore, the cooling stop temperature of the solution heat treatment is set to 300 ° C. or lower. More preferably, it is 250 ° C. or lower.
- the seamless steel pipe that has undergone solution heat treatment is subjected to aging heat treatment.
- the seamless steel pipe subjected to the solution heat treatment is heated to a temperature of 350 to 600 ° C. and then cooled.
- the added Cu is precipitated as fine ⁇ -Cu and contributes to the strength. Since fine ⁇ -Cu does not serve as a starting point for selective corrosion of the ferrite phase, it does not serve as a starting point for pitting corrosion.
- a high-strength duplex stainless steel pipe having desired high strength, high toughness, and excellent corrosion resistance can be obtained.
- the heating temperature of the aging heat treatment exceeds 600 ° C., ⁇ -Cu becomes coarse, and it becomes impossible to secure the desired high strength, high toughness, and excellent corrosion resistance.
- the heating temperature of the aging heat treatment is 550 ° C. or lower.
- the heating temperature of the aging heat treatment is less than 350 ° C., fine ⁇ -Cu is not sufficiently precipitated and the desired high strength cannot be obtained.
- the heating temperature of the aging heat treatment is 400 ° C. or higher.
- the holding time at the heating temperature of 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 min, the desired texture homogenization cannot be achieved. 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.
- cooling means 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. The average cooling rate of air cooling or higher is specifically 1 ° C./s or higher. The cooling rate of cooling in the aging heat treatment is preferably 2 ° C./s or more.
- Molten steel with the composition shown in Table 1 is melted in a converter, billets (steel pipe materials) are cast by a continuous casting method, the steel pipe materials are heated at 1150 to 1250 ° C, and then hot using a heating model seamless rolling mill.
- the pipe was formed by processing to obtain a seamless steel pipe having an outer diameter of 83.8 mm and a wall thickness of 12.7 mm.
- the obtained seamless steel pipe was air-cooled after pipe formation.
- the total reduction amount in the temperature range of 800 to 1300 ° C. was set to 20 to 60%.
- the obtained seamless steel pipe was subjected to a ⁇ -phase precipitation treatment in which it was heated under the conditions shown in Table 2 and cooled to a temperature of 300 ° C. or lower. Then, the seamless steel pipe subjected to the ⁇ phase precipitation treatment was heated under the conditions shown in Table 2 and then subjected to solution heat treatment for cooling to a temperature of 300 ° C. or lower. Then, the seamless steel pipe subjected to the solution heat treatment was further heated under the conditions shown in Table 2 and air-cooled at an average cooling rate of 1 ° C./s or more. In the ⁇ phase precipitation treatment and solution heat treatment, the average cooling rate when cooling is performed by air cooling is 1 ° C./s or more, and the average cooling rate when cooling is performed by water cooling is 10 ° C./s or more. is there.
- volume ratio (% by volume) of each phase in the entire structure of the steel pipe From the seamless steel pipe (duplex stainless steel pipe) obtained by the above heat treatment, the structure is observed in order to observe the cross section in the axial direction of the pipe. A test piece for observation was collected. The volume fractions of the ferrite phase and the austenite phase were determined by observing the observation surface with a scanning electron microscope. Specifically, the above-mentioned test piece for tissue observation is corroded with a virera reagent (a reagent in which picric acid, hydrochloric acid and ethanol are mixed at a ratio of 2 g, 10 ml and 100 ml, respectively) and scanned electron microscope (1000 times). The tissue was imaged with. From the obtained microstructure photograph, the average value of the area fractions of the ferrite phase and the austenite phase was calculated using an image analyzer, and this was taken as the volume fraction (volume%) of each.
- a virera reagent a reagent in which picric acid, hydro
- Corrosion test carbon dioxide resistance corrosion test
- seamless steel pipe duplex stainless steel pipe
- corrosion test pieces having a thickness of 3 mm, a width of 30 mm, and a length of 40 mm are produced by machining, and these test pieces are subjected to a corrosion test. Was carried out to evaluate the carbon dioxide corrosion resistance.
- the test piece was immersed in a test solution: 20% by mass NaCl aqueous solution (liquid temperature: 200 ° C., CO 2 : 3.0 MPa atmosphere) held in an autoclave, and the immersion period was 14 days (336 hours). ), The weight of the test piece after the test was measured, and the corrosion rate calculated from the weight loss before and after the corrosion test was calculated. In addition, the presence or absence of pitting corrosion on the surface of the test piece was observed using a magnifying glass with a magnification of 10 times for the test piece after the corrosion test.
- pitted corrosion refers to the presence of pitting corrosion having a diameter of 0.2 mm or more.
- the case where the corrosion rate is 0.125 mm / y or less and no pitting corrosion occurs is evaluated as acceptable.
- Table 3 the case where pitting corrosion did not occur was indicated by a symbol ⁇ , and the case where pitting corrosion occurred was indicated by a symbol ⁇ .
- SSC resistance test Sulfide stress crack resistance test
- test solution 20 wt% NaCl aqueous solution (liquid temperature: 25 °C, H 2 S: 0.03MPa, CO 2: atmosphere 0.07 MPa) pH by the addition of acetic acid + acetic acid Na in: 3.5
- the test piece was immersed in the aqueous solution adjusted to the above, the immersion period was 720 hours, and 90% of the yield stress was added as an additional stress.
- the presence or absence of cracks was visually observed on the test piece after the test.
- the presence or absence of pitting corrosion on the surface of the test piece was observed using a magnifying glass with a magnification of 10 times.
- the case where the test piece after the test is not cracked and no pitting corrosion is generated is evaluated as passing.
- Table 3 the case where cracks did not occur and no pitting corrosion occurred was indicated by a symbol ⁇ , and the case where cracks occurred and / or the case where pitting corrosion occurred was indicated by a symbol ⁇ . ..
- SCC resistance test Sulfide resistance stress corrosion cracking test
- All of the examples of the present invention have a yield strength of 655 MPa or more and a high toughness of absorption energy vE -10 ⁇ 40 J in the Charpy impact test, and further, it is said to be 200 ° C. or more containing CO 2 and Cl ⁇ . corrosion-resistant ( ⁇ acid gas corrosion resistance) in a high-temperature corrosive environment, no cracks in the environment (SSC and SCC) comprising H 2 S, excellent sulfide stress cracking resistance and sulfide stress corrosion It is a two-phase stainless steel pipe with crackability.
- the high strength which is the object of the present invention, cannot be achieved, the high toughness cannot be achieved, or the corrosion at a high temperature of 200 ° C. or higher containing CO 2 and Cl ⁇ . and pitting corrosion occurred in the environment, or corrosion rate becomes excessive, cracking in an environment containing H 2 S (SSC and / or SCC) is generated.
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Abstract
Description
1)Cuを2.0%以上含む二相ステンレス鋼では、熱間圧延後の冷却中にフェライト相でCuが過飽和状態となりやすく、その結果フェライト相内に粗大なε-Cuが析出していること。
2)熱間圧延後の粗大なε-Cuは、通常の溶体化処理では容易に解消せず、解消には長時間の加熱が必要となってしまうこと。
3)溶体化処理、時効処理した材料では、フェライト相内に残存した粗大なε-Cuが腐食起点となり、孔食の起点となるフェライト相の選択腐食が発生しやすいこと。
4)Cu過飽和状態を解消する手段として、Cuをほとんど固溶しないσ相を析出させる熱処理を行うことで、短い加熱時間でフェライト相からオーステナイト相へのCu移動を促進させ、その後の溶体化処理にてフェライト相中の粗大なε-Cuの量を格段に減らせること。
5)フェライト相中の粗大なε-Cuの有無は、Cuの過飽和度と相関しており、C、Si、Mn、Cr、Mo、Ni、N、Cu、Wの含有量が下記の(1)式を満足するように、それぞれの元素の含有量を下記の範囲内を満たせば、耐選択腐食性が向上すること。
0.55[%C]-0.056[%Si]+0.018[%Mn]-0.020[%Cr]-0.087[%Mo]+0.16[%Ni]+0.28[%N]-0.506[%Cu]-0.035[%W]+[%Cu*F] ≦0.94・・・(1)
ただし、上記(1)式における[%元素記号]は鋼中の当該元素の含有量(質量%)を表し、[%元素記号*F]はフェライト相中の当該元素の含有量(質量%)を表す。当該元素が含有されない場合はゼロとする。
[1] 質量%で、C:0.03%以下、Si:1.0%以下、Mn:0.10~1.5%、P:0.040%以下、S:0.01%以下、Cr:20.0~28.0%、Ni:2.0~10.0%、Mo:2.0~5.0%、Cu:2.0~6.0%、Al:0.001~0.05%、およびN:0.070%未満を含有し、残部がFeおよび不可避的不純物からなる成分組成を有し、オーステナイト相およびフェライト相を含む組織を有し、C、Si、Mn、Cr、Mo、Ni、N、Cu、Wの含有量が下記の(1)式を満たし、降伏強さYSが655MPa以上、試験温度:-10℃におけるシャルピー衝撃試験の吸収エネルギーvE-10が40J以上である、二相ステンレス鋼。
0.55[%C]-0.056[%Si]+0.018[%Mn]-0.020[%Cr]-0.087[%Mo]+0.16[%Ni]+0.28[%N]-0.506[%Cu]-0.035[%W]+[%Cu*F] ≦0.94・・・(1)
ただし、上記(1)式における[%元素記号]は鋼中の当該元素の含有量(質量%)を表し、[%元素記号*F]はフェライト相中の当該元素の含有量(質量%)を表す。当該元素が含有されない場合はゼロとする。
[2] 前記成分組成に加えてさらに、質量%で、以下のA群~E群のうちから選ばれた1群または2群以上を含有する、[1]に記載の二相ステンレス鋼。
A群:W:1.5%以下、
B群:V:0.20%以下、
C群: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:REM:0.005%以下、Ca:0.005%以下、Sn:0.20%以下、Mg:0.01%以下のうちから選ばれた1種または2種以上、
E群:Ta:0.1%以下、Co:1.0%以下、Sb:1.0%以下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を用いてなる、二相ステンレス鋼管。
[4] [1]または[2]に記載の成分組成を有する鋼素材に対し、700℃以上950℃以下の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σ相析出処理と、1000℃以上の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℃の温度に加熱したのち冷却する時効熱処理と、を施す、二相ステンレス鋼の製造方法。
以下に本発明の二相ステンレス鋼が有する成分組成の範囲の限定理由を説明する。なお、成分含有量に関する「%」は質量%である。
Cは、オーステナイト相を安定させて強度および低温靭性を向上させる効果を有する元素である。降伏強さが95ksi以上(655MPa以上)の高強度、シャルピー衝撃試験の吸収エネルギーvE-10が40J以上の低温靭性を実現するためには、C含有量は0.002%以上とすることが好ましい。さらに好ましくは、C含有量は0.005%以上である。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、耐食性に悪影響を及ぼす場合もある。そのため、C含有量は0.03%以下とする。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0.015%以下であり、さらに好ましくは、C含有量は0.012%以下である。
Siは、脱酸剤として機能する元素であり、この効果を得るためには、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%以上である。しかしながら、Si含有量は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0.8%以下であり、より好ましくは、Si含有量は0.7%以下である。さらに好ましくは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とともに、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含有量が0.10%以上で得られる。したがって、Mn含有量は0.10%以上とする。好ましくは、Mn含有量は0.15%以上であり、より好ましくは0.20%以上である。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1.5%以下とする。好ましくは、Mn含有量は1.0%以下であり、より好ましくは0.8%以下であり、さらに好ましくは0.5%以下である。
Pは、二相ステンレス鋼の耐食性を低下させる元素であり、0.040%を超えると、耐食性が著しく低下する。したがって、P含有量は0.040%以下とする。好ましくは、P含有量は0.020%以下である。ただしPを0.005%未満に低減するためには、溶鋼を溶製する過程で脱P処理に長時間を要し、二相ステンレス鋼の製造コストの上昇を招く。したがって、Pは0.005%以上が好ましい。
Sは、二相ステンレス鋼の製造過程における熱間加工性を低下させる元素であり、0.01%を超えると、二相ステンレス鋼の製造に支障を来す。したがって、Sは0.01%以下とする。好ましくは、S含有量は0.005%以下である。なお、製造コストの上昇を防止する観点より、好ましくは、S含有量は0.0005%以上である。
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、Cr含有量を20.0%以上とする。より高強度を得るためには、好ましくは、Cr含有量は21.0%以上であり、さらに好ましくは23.0%以上である。しかし、Crの含有量が28.0%を超えると、σ相が析出しやすくなり耐食性と靭性がともに劣化する。したがって、Cr含有量は28.0%以下とする。また、靱性の観点からは、好ましくは、Cr含有量は27.0%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が2.0%未満では、その効果が得られない。したがって、Ni含有量は2.0%以上とする。好ましくは、3.0%以上である。より好ましくは4.0%以上である。一方、Ni含有量が10.0%を超えると、オーステナイト相主体となり、本発明で目的とする強度が得られない。また、Niは高価な元素であるため経済性も損なわれる。したがって、Ni含有量は10.0%以下とする。好ましくは8.0%以下である。
Moは、二相ステンレス鋼の耐食性を向上する作用を有する元素であり、特にCl-に起因する孔食の防止に寄与する。Moが2.0%未満では、その効果が得られない。したがって、Mo含有量は2.0%以上とする。好ましくは、2.5%以上である。一方、Mo含有量が5.0%を超えると、σ相が析出し、靭性、耐食性が低下する。したがって、Mo含有量は5.0%以下とする。好ましくは4.5%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。また、Cuは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、Cu含有量は2.0%以上とする。好ましくは、Cu含有量は2.5%以上である。一方、Cuの含有量が6.0%を超えると、低温靭性が低下する。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5.5%以下である。より好ましくは、5.0%以下である。
Alは、二相ステンレス鋼の原材料の溶鋼を溶製する過程で脱酸剤として機能する元素であり、0.001%未満ではその効果が得られない。したがって、Al含有量は0.001%以上とする。好ましくは0.005%以上である。一方、Al含有量が0.05%を超えると、アルミナ系介在物が析出し易くなり、二相ステンレス鋼の製造過程における熱間加工性が低下し、靭性も劣化する。したがって、Al含有量は0.05%以下とする。好ましくは0.04%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、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、Nを0.070%以上含有するとその作用が顕著となる。したがって、N含有量は0.070%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0.04%以下、さらに好ましくは0.03%以下、さらに一層好ましくは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%以上である。
Wは、耐硫化物応力腐食割れ性および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含有量は0.02%以上であることが望ましい。より好ましくは、W含有量は0.3%以上であり、さらに好ましくは、W含有量は0.8%以上である。一方、Wは1.5%を超えて多量に含有すると、低温靭性を低下させる場合がある。したがって、Wを含有する場合には、W含有量は1.5%以下とする。より好ましくは、W含有量は1.2%以下である。
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含有量は0.02%以上であることが望ましい。より好ましくは、V含有量は0.04%以上である。一方、Vは0.20%を超えて含有すると、低温靭性を低下させる場合がある。また、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。したがって、Vを含有する場合には、V含有量は0.20%以下とする。より好ましくは、V含有量は0.08%以下である。
Zrおよび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させてもよい。
REM、Ca、Sn、および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させてもよい。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%以上である。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%以下である。
Ta、Co、およびSbはいずれも耐炭酸ガス腐食性、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させてもよい。このような効果を確保するためには、含有する場合には、それぞれTa:0.01%以上、Co:0.01%以上、Sb:0.01%以上とする。より好ましくは、それぞれTa:0.02%以上、Co:0.02%以上、Sb:0.02%以上である。一方、Ta:0.1%、Co:1.0%、Sb:1.0%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る場合がある。このため、含有する場合には、それぞれTa:0.1%以下、Co:1.0%以下、Sb:1.0%以下とする。より好ましくは、それぞれTa:0.05%以下、Co:0.5%以下、Sb:0.5%以下である。
0.55[%C]-0.056[%Si]+0.018[%Mn]-0.020[%Cr]-0.087[%Mo]+0.16[%Ni]+0.28[%N]-0.506[%Cu]-0.035[%W]+[%Cu*F] ≦0.94・・・(1)
C、Si、Mn、Cr、Mo、Ni、N、Cu、および必要に応じてWの含有量、ならびに、フェライト相中のCuの含有量が上記の(1)式を満足すれば、耐孔食性が向上する。(1)式の左辺のうち、各成分の含有量の1次式((1)式の左辺のうち[%Cu*F]を除く部分)の値に「-1」をかけた値は、フェライト相中のCuの含有量の平衡値に近似する。すなわち、(1)式の左辺値は、フェライト相中のCuの含有量の平衡値とフェライト相中のCuの含有量との差であり、Cuの過飽和度に対応している。(1)式の左辺値はフェライト相中の粗大なε-Cuの量の指標であり、(1)式の左辺値が大きくなるほど粗大なε-Cuの量が増加し、耐孔食性が劣化する。より耐孔食性を向上させる観点から、(1)式の左辺値は、0.92以下とすることが好ましい。下限は特に規定しない。安定した強度確保の観点から、(1)式の左辺値は、0.80以上とすることが好ましい。
本発明の二相ステンレス鋼は、オーステナイト相およびフェライト相を含む組織を有する。オーステナイト相の体積率(%)は、好ましくは20~70%である。フェライト相の体積率(%)は、好ましくは30~80%である。オーステナイト相が20%未満では、低温靱性、耐硫化物応力割れ性、および耐硫化物応力腐食割れ性に劣る場合がある。また、オーステナイト相が70%を超えると、強度に劣る場合がある。オーステナイト相は、より好ましくは25%以上であり、より好ましくは65%以下である。フェライト相が30%未満では、強度に劣る場合がある。また、フェライト相が80%を超えると、低温靱性、耐硫化物応力割れ性、および耐硫化物応力腐食割れ性に劣る場合がある。フェライト相は、より好ましくは35%以上であり、より好ましくは75%以下である。なお本発明では、各相の体積率は、後述する実施例に記載の方法で測定することができる。
本発明の二相ステンレス鋼の製造方法として、二相ステンレス鋼管の製造方法について説明する。以下では、本発明の二相ステンレス鋼が継目無鋼管である場合の製造方法について説明する。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。
なお、上記した鋼管素材の加熱温度は、例えば1100~1300℃の範囲とすることが好ましい。1100℃未満では材料の加工性が低下し、圧延時に鋼管外面に割れを生じる場合がある。一方、1300℃超えでは加工発熱によって材料温度が融点を超えて溶融し、その後の圧延が困難となる場合がある。
また、上記した熱間加工では、Cuの析出核となる転位および粒界を多く導入し、その後の時効熱処理で高強度の材料を得る観点から、例えば800~1300℃の温度域における合計圧下量を20~60%とすることが好ましい。800℃未満では、材料の加工性が低下し、圧延時に鋼管外面に割れを生じる場合がある。一方、1300℃超えでは、加工発熱によって材料温度が融点を超えて溶融し、その後の圧延が困難となる場合がある。上記温度域における合計圧下量が20%未満では、Cuの析出核となる転位および粒界の数が不足し、十分な高強度が得られない場合がある。一方、合計圧下量が60%超えでは、圧延時の加工発熱が過大となり、加工発熱によって材料温度が融点を超えて溶融し、その後の圧延が困難となる場合がある。ここで「合計圧下量」とは、ピアサーによる穿孔後に、実施されるエロンゲータ、プラグミル等によって圧延された鋼管の肉厚圧下量を指す。
造管後、得られた継目無鋼管は冷却される。上記した成分組成の場合、空冷以上の平均冷却速度で室温まで冷却することが好ましい。これにより、上記した組織とすることができる。
本発明では、冷却後の継目無鋼管に対して、σ相析出処理、溶体化熱処理、時効熱処理をこの順に施し、二相ステンレス鋼管を製造する。
次に、本発明で重要な、σ相析出処理を行う。本発明では具体的には、上記成分組成を有する継目無鋼管を700℃以上950℃以下の加熱温度にて加熱したのち、空冷以上の平均冷却速度、より具体的には1℃/s以上の平均冷却速度で300℃以下の温度まで冷却する。これにより、σ相が析出し、フェライト相中のCu過飽和状態が解消される。フェライト相中のCu過飽和度は、(1)式に対応する。σ相析出処理を行うことにより、上記の(1)式を満たす二相ステンレス鋼管とすることができる。なお、σ相析出処理の加熱温度は、σ相の析出を促進する観点から、900℃以下とすることが好ましい。また、好ましくは、σ相析出処理の加熱温度は750℃以上である。σ相析出処理の加熱温度における保持時間は、材料内の温度を均一にする観点から、5min以上が好ましい。より好ましくは10min以上である。また、σ相析出処理の加熱温度における保持時間は300min以下が好ましい。より好ましくは100min以下である。σ相析出処理における冷却の平均冷却速度は、好ましくは2℃/s以上である。冷却方法としては、例えば、空冷または水冷が挙げられる。特に平均冷却速度の上限は規定しないが、平均冷却速度が大きいと材料特性に及ぼす効果は飽和するため、50℃/s以下とすることが好ましい。本発明において、平均冷却速度とは、加熱温度から冷却停止温度までの範囲における冷却速度の平均をいう。σ相析出処理の冷却停止温度が300℃超えでは、添加したCuが冷却中に粗大なε-Cuとして析出し、後の溶体化処理で再びCuを固溶させるために非常に長い時間加熱することが必要となる。また、生産性を低下させる。後の溶体化熱処理でのCuの再固溶が十分でなかった場合、残存する粗大なε-Cuにより靭性が低下する。よって、σ相析出処理の冷却停止温度は300℃以下とする。より好ましくは250℃以下である。
σ相析出処理に引続き、本発明では、σ相析出処理を施した継目無鋼管に溶体化熱処理を施す。具体的には、σ相析出処理を施した継目無鋼管を、さらに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、より具体的には1℃/s以上の平均冷却速度で300℃以下の温度まで冷却する。これにより、σ相析出処理前またはσ相析出処理中に析出した金属間化合物や炭化物、窒化物、硫化物等を固溶し、適正量のオーステナイト相およびフェライト相を含む組織の継目無鋼管とすることができる。
溶体化熱処理の冷却停止温度が300℃超えでは、添加したCuが冷却中に粗大なε-Cuとして析出し、所望の高強度、さらには高靭性と優れた耐食性を確保できなくなる。よって、溶体化熱処理の冷却停止温度は300℃以下とする。より好ましくは250℃以下である。
次いで、溶体化熱処理を施した継目無鋼管に時効熱処理を施す。具体的には、溶体化熱処理を施した継目無鋼管を、350~600℃の温度に加熱したのち、冷却する。時効熱処理を施されることにより、添加したCuが微細なε-Cuとして析出し強度に寄与する。なお、微細なε-Cuはフェライト相の選択腐食の起点とならないため、孔食の起点ともならない。このような時効熱処理を継目無鋼管に施すことにより、所望の高強度と、高靭性さらには優れた耐食性を有する高強度二相ステンレス鋼管となる。
上述の熱処理を施して得られた継目無鋼管(二相ステンレス鋼管)から、管軸方向断面を観察するために、組織観察用の試験片を採取した。フェライト相およびオーステナイト相の体積率は、観察面を走査型電子顕微鏡で観察することにより求めた。具体的には、上述の組織観察用の試験片をビレラ試薬(ピクリン酸、塩酸およびエタノールをそれぞれ2g、10ml、および100mlの割合で混合した試薬)で腐食して走査型電子顕微鏡(1000倍)で組織を撮像した。得られた組織写真から、画像解析装置を用いて、フェライト相およびオーステナイト相の面積率の平均値を算出し、これをそれぞれの体積率(体積%)とした。
上述の組織観察を行ったものと同様の試験片に対して、EBSP解析にてフェライトを識別した。各試験片中のフェライトとして識別された相に対して、FE-EPMAにて、任意の20点を測定し、Cu含有量を得た。得られたCu含有量の定量値を平均した値を、その鋼のフェライト相中のCu含有量(質量%)とした。
上述の熱処理を施して得られた継目無鋼管(二相ステンレス鋼管)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た。採取された試験片に対し、API規格に準拠して引張試験を行なって、引張特性として降伏強さYS(MPa)および引張強さTS(MPa)を測定した。
上述の熱処理を施して得られた継目無鋼管(二相ステンレス鋼管)の肉厚中央部から、ISO-11960規格に準拠して、円周方向が試験片長さとなるようにVノッチ試験片(厚さ10mm)を採取した。採取した試験片に対し、試験温度を-10℃としてシャルピー衝撃試験を行なって、吸収エネルギーvE-10(J)を測定した。なお、試験片は、各鋼管からそれぞれ3本採取し、これらの試験片に対してシャルピー衝撃試験を行って得られた値の算術平均値を表3に示す。
上述の熱処理を施して得られた継目無鋼管(二相ステンレス鋼管)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、これらの試験片に対して腐食試験を実施して耐炭酸ガス腐食性を評価した。
上述の熱処理を施して得られた継目無鋼管(二相ステンレス鋼管)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、これらの試験片に対して耐SSC試験を実施した。
上述の熱処理を施して得られた継目無鋼管(二相ステンレス鋼管)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、これらの試験片に対して耐SCC試験を実施した。
Claims (4)
- 質量%で、
C:0.03%以下、
Si:1.0%以下、
Mn:0.10~1.5%、
P:0.040%以下、
S:0.01%以下、
Cr:20.0~28.0%、
Ni:2.0~10.0%、
Mo:2.0~5.0%、
Cu:2.0~6.0%、
Al:0.001~0.05%、および
N:0.070%未満
を含有し、残部がFeおよび不可避的不純物からなる成分組成を有し、
オーステナイト相およびフェライト相を含む組織を有し、
C、Si、Mn、Cr、Mo、Ni、N、Cu、Wの含有量が下記の(1)式を満たし、
降伏強さYSが655MPa以上、試験温度:-10℃におけるシャルピー衝撃試験の吸収エネルギーvE-10が40J以上である、二相ステンレス鋼。
0.55[%C]-0.056[%Si]+0.018[%Mn]-0.020[%Cr]-0.087[%Mo]+0.16[%Ni]+0.28[%N]-0.506[%Cu]-0.035[%W]+[%Cu*F] ≦0.94・・・(1)
ただし、上記(1)式における[%元素記号]は鋼中の当該元素の含有量(質量%)を表し、[%元素記号*F]はフェライト相中の当該元素の含有量(質量%)を表す。当該元素が含有されない場合はゼロとする。 - 前記成分組成に加えてさらに、質量%で、以下のA群~E群のうちから選ばれた1群または2群以上を含有する、請求項1に記載の二相ステンレス鋼。
A群:W:1.5%以下、
B群:V:0.20%以下、
C群: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:REM:0.005%以下、Ca:0.005%以下、Sn:0.20%以下、Mg:0.01%以下のうちから選ばれた1種または2種以上、
E群:Ta:0.1%以下、Co:1.0%以下、Sb:1.0%以下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を用いてなる、二相ステンレス鋼管。
- 請求項1または2に記載の成分組成を有する鋼素材に対し、
700℃以上950℃以下の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σ相析出処理と、
1000℃以上の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℃の温度に加熱したのち冷却する時効熱処理と、
を施す、二相ステンレス鋼の製造方法。
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