WO2014030392A1 - Highly strong, highly tough and highly corrosion-resistant martensitic stainless steel - Google Patents

Highly strong, highly tough and highly corrosion-resistant martensitic stainless steel Download PDF

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Publication number
WO2014030392A1
WO2014030392A1 PCT/JP2013/062004 JP2013062004W WO2014030392A1 WO 2014030392 A1 WO2014030392 A1 WO 2014030392A1 JP 2013062004 W JP2013062004 W JP 2013062004W WO 2014030392 A1 WO2014030392 A1 WO 2014030392A1
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stainless steel
less
martensitic stainless
steel
strength
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PCT/JP2013/062004
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French (fr)
Japanese (ja)
Inventor
修司 橋爪
友美 谷口
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エヌケーケーシームレス鋼管株式会社
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Priority to EP13830674.1A priority Critical patent/EP2889390B1/en
Priority to US14/421,927 priority patent/US20150191809A1/en
Priority to BR112015003178-1A priority patent/BR112015003178B1/en
Publication of WO2014030392A1 publication Critical patent/WO2014030392A1/en
Priority to US15/704,461 priority patent/US20180002791A1/en

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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
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
    • C—CHEMISTRY; METALLURGY
    • 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
    • C—CHEMISTRY; METALLURGY
    • 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/008—Heat treatment of ferrous alloys containing Si
    • C—CHEMISTRY; METALLURGY
    • 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
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/005—Modifying the physical properties by deformation combined with, or followed by, heat treatment of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • 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
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
    • 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
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • 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
    • C22C38/004—Very low carbon steels, i.e. having a carbon content of less than 0,01%
    • 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
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • 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
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • 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
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • 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
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
    • 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
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • 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
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
    • 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
    • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum

Definitions

  • the present invention relates to martensitic stainless steel having high strength, high toughness and corrosion resistance, particularly high strength, high toughness and corrosion resistance martensite suitable for use in an environment containing high-temperature wet carbon dioxide in drilling and transportation of oil and natural gas.
  • the present invention relates to a stainless steel and a manufacturing method thereof.
  • Oil and natural gas produced in recent years have increased in cases where they contain a large amount of wet carbon dioxide, and as a material for steel pipes and the like used for excavation and transportation, 13% Cr is used instead of conventional carbon steel. Martensitic stainless steel such as stainless steel has been used.
  • the conventional martensitic stainless steel has excellent corrosion resistance against wet carbon dioxide gas up to 180 ° C. (hereinafter referred to as “corrosion resistance”), the corrosion resistance becomes insufficient at a temperature higher than that. Furthermore, since steel pipes and the like used in oil and natural gas drilling and transportation are placed in a high-temperature and high-pressure environment, it is desirable that they have high strength and excellent toughness.
  • Patent Documents 1 to 7 Stainless steels that exhibit corrosion resistance at about 180 ° C. or higher are disclosed in Patent Documents 1 to 7.
  • the basic idea of the stainless steel disclosed in these documents is to increase the Cr content and maintain the corrosion resistance in a high temperature humid carbon dioxide environment.
  • Patent Documents 1 and 2 disclose high-strength stainless steel pipes for oil wells having specific compositions of C, Si, Mn, Cr, Ni, Mo, W, Cu, V, and N, while Patent Document 1 discloses strength. Manufactures stainless steel with a yield stress of up to 792 MPa and examines toughness and the like. Patent Document 2 has no description of strength, but only examines toughness. Neither Patent Documents 1 and 2 have studied about further high-strength stainless steel.
  • Patent Documents 3 to 7 have examples in which high-strength steels equivalent to or higher than Patent Documents 1 and 2 were examined, but there is no study on toughness. Generally, the toughness of stainless steel pipes deteriorates due to the increase in strength. Therefore, it is considered that the toughness of stainless steel disclosed in these documents is not completely resistant to oil and natural gas drilling and transportation.
  • an object of the present invention is to provide a high-performance stainless steel that exhibits corrosion resistance even in a very severe corrosive environment at 180 ° C. or higher, for example, 220 ° C. while maintaining strength and toughness.
  • the target performance is as follows in view of the performance required for oil pipes including carbon dioxide, natural gas drilling, and steel pipes for transportation.
  • a steel pipe although an oil well pipe is a main object of the present invention, a steel pipe for a transportation line pipe or the like that requires the same performance can also be an object.
  • Increasing the ferrite-forming elements Cr and Mo is effective in improving the corrosion resistance of martensitic stainless steels.
  • the increase in these elements produces a ⁇ -ferrite phase, and the hot workability and strength of the steel. ⁇ Toughness may be deteriorated.
  • the increase of Ni as an austenite forming element is effective in controlling the formation of the ⁇ -ferrite phase, the tempering temperature during production may be limited. Therefore, in the present invention, Cr, Mo, and Ni are set to the optimum ranges described later, thereby suppressing the formation of ⁇ -ferrite.
  • the tempering temperature is high and contains a large amount of austenite-forming elements, a small amount of austenite may precipitate, which hinders the strengthening of stainless steel, so the austenite phase generation must also be controlled.
  • austenite generation there is a strong tendency for austenite generation to be accelerated by an increase in Ni, and the amount of Ni is limited.
  • the inventors of the present invention have included a martensitic stainless steel containing an appropriate amount of Ta as an essential component after adding an appropriate amount of V to the stainless steel, when V or Ta is added individually, or V and Nb.
  • the synergistic effect was found that it is easier to disperse fine carbide precipitates in the stainless steel matrix after the heat treatment, and it is easier to increase the strength while maintaining the toughness, compared with the case of adding at the same time.
  • the present inventors have also found that the effect becomes more remarkable by adding Nb to V and Ta.
  • the present inventors have made steel contain a certain amount of V and Ta or V and Ta and Nb, and have a strength of 758 MPa or more, while taking into account the above-mentioned metal structure constraints.
  • heat treatment conditions are adjusted within a certain range, and carbides are uniformly dispersed and precipitated in the matrix, with high toughness and high strength that could not be realized with conventional martensitic stainless steel, A novel martensitic stainless steel excellent in corrosion resistance in an environment of 180 ° C. or higher and a manufacturing method thereof were completed.
  • the present invention limits the alloy composition and production conditions to a certain range, improves the corrosion resistance of conventional high-strength martensitic stainless steel, maintains toughness, and maintains a wet carbon dioxide environment at 180 ° C. or higher.
  • the present invention provides a martensitic stainless steel having a high strength of 758 MPa or more that can be used in the above.
  • the present invention is as follows. (1) By mass%, C: 0.005 to 0.05%, Si: 1.0% or less, Mn: 2.0% or less, Cr: 16 to 18%, Ni: 2.5 to 6.5 %, Mo: 1.5-3.5%, W: 3.5% or less, Cu: 3.5% or less, V: 0.01-0.08%, Sol.Al: 0.005-0.
  • a martensitic stainless steel comprising 10%, N: 0.05% or less, Ta: 0.01 to 0.06%, the balance being Fe and inevitable impurities.
  • the stainless steel is austenitized at a temperature of 800 ° C. or higher and 980 ° C. or lower, and subsequently 100 ° C. or lower.
  • a method for producing martensitic stainless steel comprising quenching and cooling to a temperature of 500 ° C. and then tempering at a temperature of 500 ° C. to 700 ° C.
  • the alloy composition and production conditions by specifying the alloy composition and production conditions, a high-strength, high-toughness, high-corrosion-resistant martensitic stainless steel having excellent toughness and good corrosion resistance against carbon dioxide corrosion in an environment of 180 ° C. or higher is obtained. It is possible to obtain.
  • Component composition range ⁇ C: 0.005 to 0.05%> C is a strong austenite-forming element, and is also an indispensable element for obtaining high strength in stainless steel. However, it combines with Cr during tempering in steel production and precipitates as carbide, which degrades the corrosion resistance and toughness of the steel. If the C content is less than 0.005%, sufficient strength cannot be obtained. If the C content exceeds 0.05%, the deterioration becomes significant, so the content is made 0.005 to 0.05%.
  • Si is an element necessary as a deoxidizer, but is also a strong ferrite-forming element. In the martensitic stainless steel of the present invention, it is necessary to control ferrite in order to achieve high strength and high toughness. If Si is contained in stainless steel in an amount exceeding 1.0%, it becomes difficult to control ferrite, so the upper limit of the content is 1.0%, preferably 0.5%, more preferably 0.3%. And
  • Mn is effective as a deoxidation and desulfurization agent.
  • the upper limit of its content is 2.0%, preferably 0.5%, more preferably 0.3%.
  • Cr is a basic element constituting martensitic stainless steel, and is an important element that develops corrosion resistance. Considering the corrosion resistance in a harsh environment of 180 ° C or higher, if the content is less than 16%, sufficient corrosion resistance cannot be obtained, and if it exceeds 18%, the effect is saturated and a problem arises in terms of economy. The amount is 16-18%.
  • Ni improves the corrosion resistance of stainless steel and is an extremely effective element for stabilizing austenite. However, if the content is less than 2.5%, the effect is not sufficiently exhibited. Since the point (Ac1 point) is lowered, restrictions are imposed on the tempering temperature during the production of the martensitic stainless steel of the present invention. Further, when the Ni content increases, austenite may precipitate, which hinders the strengthening of stainless steel. Therefore, the Ni content is set to 2.5 to 6.5%.
  • Mo is an element that is particularly effective for improving the corrosion resistance of stainless steel. However, if the content is less than 1.5%, the effect does not appear, and if it exceeds 3.5%, it becomes difficult to control ferrite. The content is 1.5 to 3.5%.
  • V is a strong carbide-forming element, and fine carbides are uniformly precipitated in the matrix grains of stainless steel, and are not preferentially precipitated at the grain boundaries, thereby making the carbide crystal grains finer and contributing to improving the strength of the steel. Since carbides are precipitated finely and uniformly in this way, the strength can be improved without reducing the toughness of the steel. If the content is less than 0.01%, the effect of improving the strength does not appear. If the content exceeds 0.08%, the effect is saturated, so the content is 0.01 to 0.08%, preferably 0.02 to 0.04%.
  • ⁇ Sol.Al 0.005 to 0.10%> Acid-soluble Al (Sol. Al) is added for deoxidation of stainless steel in the refining process and for improving the toughness of the steel by refinement of ⁇ grains by AlN precipitation. If the content is less than 0.005%, the effect of improving toughness cannot be obtained, and if it exceeds 0.10%, the toughness may be lowered, so the content is made 0.005 to 0.10%.
  • N is an element harmful to the corrosion resistance improvement of stainless steel, but is also an austenite generating element. If it exceeds 0.05%, it will precipitate as a nitride during tempering during the production of the martensitic stainless steel of the present invention, and the corrosion resistance and toughness of the steel will deteriorate. 05%, preferably 0.02%.
  • Ta is a strong carbide-forming element, and precipitates fine carbides uniformly in the matrix grains of stainless steel, contributing to the improvement of steel strength. Since fine carbides are uniformly deposited, the strength can be improved without reducing the toughness of the steel. In addition, by adding a certain amount together with the V, it is possible to achieve strength and toughness superior to the case where one is added. If the content is less than 0.01%, the effect of improving the strength does not appear, and if it exceeds 0.06%, the effect is almost saturated and is not preferable in terms of cost. 06%.
  • Nb 0.1% or less>
  • Nb may be contained in addition to the above basic components.
  • Nb is a strong carbide-forming element, and precipitates fine carbides to refine crystal grains and improve the strength of stainless steel. When the content exceeds 0.1%, the effect is saturated, so the content is set to 0.1% or less, preferably 0.05% or less.
  • the balance other than the components described above consists of Fe and inevitable impurities.
  • inevitable impurities represented by P, S, and O if P is 0.04% or less and S is 0.01% or less, a seamless steel pipe or a hot-rolled steel sheet in which the present invention can be considered is used. There is no problem in the production of ERW steel pipes. However, these impurities are all elements that degrade the hot workability of steel, and the smaller the content, the better. Further, it is preferable that the content of O and other inevitable impurities is as low as possible.
  • the 0.2% proof stress of the martensitic stainless steel of the present invention is 758 MPa or more and 965 MPa or less, and the absorbed energy in the Charpy full size test at ⁇ 20 ° C. is 100 J or more, preferably 200 J or more.
  • Corrosion rate in an environment of 20% NaCl aqueous solution, 220 ° C., 10 atm CO 2 is 0.5 mm / year or less, preferably 0.3 mm / year or less. If the 0.2% proof stress exceeds 965 MPa, when a small amount of hydrogen enters the stainless steel due to corrosion or the like, the steel tends to crack.
  • the martensitic stainless steel of the present invention can be manufactured by the following manufacturing method.
  • ⁇ ⁇ Martensitic stainless steel adjusted to the above component composition range is melted in a converter or electric furnace and made into a steel slab by ordinary ingot-making-slab rolling or continuous casting.
  • This steel slab is hot-worked and given a predetermined shape such as a seamless steel pipe or steel plate as necessary (in this state, the intended strength, toughness, etc. of the present invention are achieved.
  • the hot-worked steel is heated to a temperature of 800 ° C. or higher and 980 ° C. or lower to austenite, then quenched and cooled to a temperature of 100 ° C. or lower, and then tempered at a temperature of 500 ° C. or higher and 700 ° C. or lower.
  • the heating temperature in the quenching and tempering will be described.
  • Heating temperature 800 ° C. or higher and 980 ° C. or lower> If the heating temperature is less than 800 ° C., the stainless steel does not become austenite and the effect of quenching cannot be obtained, so the lower limit is set to 800 ° C. On the other hand, if the heating temperature exceeds 980 ° C., the crystal grains become coarse and the toughness deteriorates, so the upper limit is made 980 ° C.
  • Tempering temperature 500 ° C to 700 ° C>
  • the tempering process is a process generally performed to impart toughness to steel.
  • fine carbides of V and Ta are uniformly dispersed and precipitated, thereby increasing the strength of stainless steel. This is an essential process for increasing the strength.
  • the tempering temperature exceeds 700 ° C, a 0.2% proof stress of 758 MPa or more cannot be obtained, so the upper limit is 700 ° C.
  • the temperature is lower than 500 ° C., the precipitation of carbide is not sufficient, and the target 0.2% proof stress and toughness cannot be obtained, so the lower limit is 500 ° C.
  • the present inventors melted the inventive steels N1 to N8 and comparative steels C1 to C5 having chemical compositions shown in Table 1 below as test steels (martensitic stainless steels), and made steel plates with a thickness of 12 mm by hot rolling. Thereafter, heat treatment (quenching and tempering treatment) shown in Table 2 below was performed to produce martensitic stainless steel. In addition, after quenching heat processing, it cooled with water and cooled to room temperature, and then tempered heat processing.
  • the comparative steels C1 and C5 are steels that do not contain Ta. Further, the comparative steel C2 is a steel whose V content is less than the lower limit of the present invention, the comparative steel C3 is a steel whose Cr and Mo contents are less than the lower limit of the present invention, and the comparative steel C4 has a Ta content. Steel exceeding the upper limit of the present invention.
  • the steels of the present invention “N1, N2A, N2B, N3, N4, N5, N6A, N6B, N7, N8” all had the 0.2% proof stress and the absorbed energy in the Charpy full size test. Corrosion resistance also cleared the target value.
  • C1 and C5 are steels not containing Ta
  • C2 is a steel whose V content is less than the lower limit of the present invention
  • C3 is a Cr and Mo content that is the lower limit of the present invention
  • C4 is a steel whose Ta content exceeds the upper limit of the present invention.
  • the test results also do not achieve the target of 0.2% proof stress, toughness or corrosion resistance.
  • the strength of 758 MPa could not be obtained, and the toughness and corrosion resistance could not achieve the target values.
  • the composite addition of V + Ta improved the characteristics that could not be achieved by each single addition, and a synergistic effect by the composite addition was recognized.
  • the martensitic stainless steel pipe of the present invention has corrosion resistance, maintains toughness despite high strength of 758 MPa or higher, can be used in a wet carbon dioxide environment of 180 ° C. or higher, and contains petroleum and natural gas containing carbon dioxide. It can be suitably used as a steel pipe for excavation and transportation (an oil well, a gas well or a martensitic stainless steel pipe for a line pipe).

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Abstract

The purpose of the present invention is to provide high performance stainless steel which exhibits good corrosion resistance even in an extremely harsh corrosive environment at 180°C or higher, for example, at 220°C by improving the corrosion resistance of conventional high-strength martensitic stainless steel, while maintaining the strength and toughness thereof. The present invention is martensitic stainless steel which is characterized by containing, in mass%, 0.005-0.05% of C, 1.0% or less of Si, 2.0% or less of Mn, 16-18% of Cr, 2.5-6.5% of Ni, 1.5-3.5% of Mo, 3.5% or less of W, 3.5% or less of Cu, 0.01-0.08% of V, 0.005-0.10% of Sol. Al, 0.05% or less of N and 0.01-0.06% of Ta, with the balance made up of Fe and unavoidable impurities.

Description

高強度高靭性高耐食マルテンサイト系ステンレス鋼High strength, high toughness, high corrosion resistance martensitic stainless steel
　本発明は、高強度高靭性高耐食性を有するマルテンサイト系ステンレス鋼、特に石油、天然ガスの掘削、輸送における高温の湿潤炭酸ガスを含む環境での使用に適した高強度高靭性高耐食性マルテンサイト系ステンレス鋼及びその製造方法に関する。 The present invention relates to martensitic stainless steel having high strength, high toughness and corrosion resistance, particularly high strength, high toughness and corrosion resistance martensite suitable for use in an environment containing high-temperature wet carbon dioxide in drilling and transportation of oil and natural gas. The present invention relates to a stainless steel and a manufacturing method thereof.
　近年生産される石油、天然ガスは、湿潤炭酸ガスを多量に含む場合が増加しており、その掘削、輸送において使用される鋼管等の材料としては、従来の炭素鋼に替わって１３％Ｃｒ系ステンレス鋼などのマルテンサイト系ステンレス鋼が用いられてきている。 Oil and natural gas produced in recent years have increased in cases where they contain a large amount of wet carbon dioxide, and as a material for steel pipes and the like used for excavation and transportation, 13% Cr is used instead of conventional carbon steel. Martensitic stainless steel such as stainless steel has been used.
　しかし、従来のマルテンサイト系ステンレス鋼は、１８０℃までの湿潤炭酸ガスに対する耐食性（以下、「耐食性」という。）は優れているものの、それ以上の温度になると耐食性は十分ではなくなる。さらに、石油、天然ガスの掘削や輸送において使用される鋼管等は高温高圧の環境下におかれるので、高い強度と同時に優れた靭性を備えていることも望まれる。 However, although the conventional martensitic stainless steel has excellent corrosion resistance against wet carbon dioxide gas up to 180 ° C. (hereinafter referred to as “corrosion resistance”), the corrosion resistance becomes insufficient at a temperature higher than that. Furthermore, since steel pipes and the like used in oil and natural gas drilling and transportation are placed in a high-temperature and high-pressure environment, it is desirable that they have high strength and excellent toughness.
　従って、強度及び靭性を維持しつつ１８０℃以上、例えば２２０℃で耐食性を発揮するステンレス鋼が望まれている。 Therefore, a stainless steel that exhibits corrosion resistance at 180 ° C. or higher, for example, 220 ° C. while maintaining strength and toughness is desired.
　そのような１８０℃程度以上で耐食性を発揮するステンレス鋼が、特許文献１～７に開示されている。これらの文献に開示されたステンレス鋼の基本的思想は、Cr量を増加させて高温湿潤炭酸ガス環境での耐食性を維持させるというものである。 Stainless steels that exhibit corrosion resistance at about 180 ° C. or higher are disclosed in Patent Documents 1 to 7. The basic idea of the stainless steel disclosed in these documents is to increase the Cr content and maintain the corrosion resistance in a high temperature humid carbon dioxide environment.
　特許文献１及び２は、Ｃ、Ｓｉ、Ｍｎ、Ｃｒ、Ｎｉ、Ｍｏ、Ｗ、Ｃｕ、Ｖ及びＮを特定の組成で有する油井用高強度ステンレス鋼管を開示しているが、特許文献１は強度が降伏応力で７９２ＭＰａまでのステンレス鋼を製造し、靭性等を検討しており、特許文献２は強度については記載がなく、靭性を検討しているにとどまる。特許文献１及び２のいずれも、更なる高強度のステンレス鋼に関する検討は行っていない。 Patent Documents 1 and 2 disclose high-strength stainless steel pipes for oil wells having specific compositions of C, Si, Mn, Cr, Ni, Mo, W, Cu, V, and N, while Patent Document 1 discloses strength. Manufactures stainless steel with a yield stress of up to 792 MPa and examines toughness and the like. Patent Document 2 has no description of strength, but only examines toughness. Neither Patent Documents 1 and 2 have studied about further high-strength stainless steel.
　特許文献３～７には特許文献１、２と同等またはそれ以上の高強度鋼を検討した実施例があるが、靭性に関する検討はない。一般的にステンレス鋼管は高強度化によりその靭性が劣化するので、これらの文献に開示されたステンレス鋼の靭性は、石油、天然ガスの掘削や輸送に完全に耐えうるものではないと考えられる。 Patent Documents 3 to 7 have examples in which high-strength steels equivalent to or higher than Patent Documents 1 and 2 were examined, but there is no study on toughness. Generally, the toughness of stainless steel pipes deteriorates due to the increase in strength. Therefore, it is considered that the toughness of stainless steel disclosed in these documents is not completely resistant to oil and natural gas drilling and transportation.
特開２００８－８１７９３号公報JP 2008-81793 A 特開２０１０－２０９４０２号公報JP 2010-209402 A 特許第２８１４５２８号公報Japanese Patent No. 2814528 特許第４５７７４５７号公報Japanese Patent No. 4577457 特許第４７６１００８号公報Japanese Patent No. 4761008 特許第４９１１２６６号公報Japanese Patent No. 4911266 ＷＯ２００９／１１９０４８号公報WO2009 / 112048
　さらに、鋼管、特に油井管が遭遇する環境はますます苛酷化し、７５８ＭＰａ以上の高強度で１８０℃以上の湿潤炭酸ガスに対して良好な耐食性を示し、しかも靭性に優れた鋼管の必要性がでてきた。そこで、本発明は、強度、靭性を維持しつつ１８０℃以上、例えば２２０℃での非常に苛酷な腐食環境においても耐食性を発揮する高性能なステンレス鋼を提供することを目的とする。 Furthermore, the environment encountered by steel pipes, especially oil well pipes, is becoming increasingly severe, and there is a need for steel pipes that exhibit good corrosion resistance against wet carbon dioxide gas at a high strength of 758 MPa or higher and 180 ° C. or higher and that have excellent toughness. I came. Accordingly, an object of the present invention is to provide a high-performance stainless steel that exhibits corrosion resistance even in a very severe corrosive environment at 180 ° C. or higher, for example, 220 ° C. while maintaining strength and toughness.
　ここで、目標とする性能は、炭酸ガスを含む石油、天然ガスの掘削、輸送用鋼管に要求される性能に鑑み、以下の通りとした。また鋼管としては、油井管が本発明の主たる対象ではあるものの、同様の性能が要求される輸送用のラインパイプ用鋼管等も対象となり得る。 Here, the target performance is as follows in view of the performance required for oil pipes including carbon dioxide, natural gas drilling, and steel pipes for transportation. In addition, as a steel pipe, although an oil well pipe is a main object of the present invention, a steel pipe for a transportation line pipe or the like that requires the same performance can also be an object.
　強度：０．２％耐力で７５８ＭＰａ以上９６５ＭＰａ以下。
　靭性：－２０℃でのシャルピー・フルサイズ試験における吸収エネルギーが１００Ｊ以上。
　耐食性：２０％ＮａＣｌ水溶液、２２０℃、１０気圧ＣＯ２の環境下で、腐食速度が０．５ｍｍ／ｙｅａｒ以下。 Strength: 758 MPa to 965 MPa with 0.2% yield strength.
Toughness: Absorbed energy in Charpy full size test at -20 ° C is 100J or more.
Corrosion resistance: Corrosion rate of 0.5 mm / year or less in an environment of 20% NaCl aqueous solution, 220 ° C., 10 atm CO 2 .
　本発明者らは、上記課題を解決すべく鋭意研究を重ねた結果、以下の知見を得るに至った。 As a result of intensive studies to solve the above problems, the present inventors have obtained the following knowledge.
　マルテンサイト系ステンレス鋼の耐食性向上にはフェライト生成元素であるＣｒとＭｏの増加が有効であるが、これらの元素の増加は一方でδ－フェライト相を生成させ、鋼の熱間加工性と強度・靭性を劣化させることがある。δ―フェライト相の生成を制御するにはオーステナイト生成元素であるＮｉの増加が効果的であるものの、製造時の焼戻し温度を制限することがある。そこで本発明では、Ｃｒ，Ｍｏ，Ｎｉを後述する最適範囲とし、それによりδ-フェライトの生成を抑制することとした。 Increasing the ferrite-forming elements Cr and Mo is effective in improving the corrosion resistance of martensitic stainless steels. However, the increase in these elements, on the other hand, produces a δ-ferrite phase, and the hot workability and strength of the steel.・ Toughness may be deteriorated. Although the increase of Ni as an austenite forming element is effective in controlling the formation of the δ-ferrite phase, the tempering temperature during production may be limited. Therefore, in the present invention, Cr, Mo, and Ni are set to the optimum ranges described later, thereby suppressing the formation of δ-ferrite.
　また、オーステナイト生成元素を多く含み焼き戻し温度が高いと、少量のオーステナイトが析出する場合があり、ステンレス鋼の高強度化を阻害するので、オーステナイト相の生成も制御する必要がある。特にＮｉの増加によりオーステナイト生成が促進される傾向が強く、Ｎｉ量は制限される。 In addition, if the tempering temperature is high and contains a large amount of austenite-forming elements, a small amount of austenite may precipitate, which hinders the strengthening of stainless steel, so the austenite phase generation must also be controlled. In particular, there is a strong tendency for austenite generation to be accelerated by an increase in Ni, and the amount of Ni is limited.
　一般に、鋼を高強度化させると靭性が劣化する。本発明者らは、ステンレス鋼にＶを適量含有させた上で、さらに適量のＴａを必須成分として同時に含有させたマルテンサイト系ステンレス鋼は、Ｖ又はＴａを個々に添加した場合あるいはＶとＮｂを同時に添加した場合よりも、熱処理後にステンレス鋼のマトリックスに微細な炭化物の析出物を分散させやすく、靭性を維持したままでの高強度化が容易であるという相乗効果を見出した。また、ＶとＴａにさらにＮｂを加えることによりその効果がより顕著となることも本発明者らは知見した。 Generally, toughness deteriorates when steel is strengthened. The inventors of the present invention have included a martensitic stainless steel containing an appropriate amount of Ta as an essential component after adding an appropriate amount of V to the stainless steel, when V or Ta is added individually, or V and Nb. The synergistic effect was found that it is easier to disperse fine carbide precipitates in the stainless steel matrix after the heat treatment, and it is easier to increase the strength while maintaining the toughness, compared with the case of adding at the same time. The present inventors have also found that the effect becomes more remarkable by adding Nb to V and Ta.
　以上の知見に基づき、本発明者らは、上記のような金属組織の制約を考慮しつつ、鋼にＶとＴａ、またはＶとＴａとＮｂとを一定量含有させ、かつ７５８ＭＰａ以上の強度を安定して得るために熱処理条件を一定範囲内に調整し、炭化物をマトリックス内に均一に分散析出させるようにして、従来のマルテンサイト系ステンレス鋼では実現しえなかった高靭性、高強度で、１８０℃以上の環境での耐食性に優れた新規なマルテンサイト系ステンレス鋼及びその製造方法を完成させた。 Based on the above findings, the present inventors have made steel contain a certain amount of V and Ta or V and Ta and Nb, and have a strength of 758 MPa or more, while taking into account the above-mentioned metal structure constraints. In order to obtain stably, heat treatment conditions are adjusted within a certain range, and carbides are uniformly dispersed and precipitated in the matrix, with high toughness and high strength that could not be realized with conventional martensitic stainless steel, A novel martensitic stainless steel excellent in corrosion resistance in an environment of 180 ° C. or higher and a manufacturing method thereof were completed.
　すなわち本発明は、合金組成及び製造条件を一定の範囲に限定することで、従来の高強度マルテンサイト系ステンレス鋼の耐食性を改善して、靭性を維持しつつ、１８０℃以上の湿潤炭酸ガス環境で使用できる７５８ＭＰａ以上という高強度のマルテンサイト系ステンレス鋼を提供するものである。 That is, the present invention limits the alloy composition and production conditions to a certain range, improves the corrosion resistance of conventional high-strength martensitic stainless steel, maintains toughness, and maintains a wet carbon dioxide environment at 180 ° C. or higher. The present invention provides a martensitic stainless steel having a high strength of 758 MPa or more that can be used in the above.
　すなわち本発明は、以下の通りである。
　（１）質量％で、Ｃ：０．００５～０．０５％、Ｓｉ：１．０％以下、Ｍｎ：２．０％以下、Ｃｒ：１６～１８％、Ｎｉ：２．５～６．５％、Ｍｏ：１．５～３．５％、Ｗ：３．５％以下、Ｃｕ：３．５％以下、Ｖ：０．０１～０．０８％、Ｓｏｌ.Ａｌ：０．００５～０．１０％、Ｎ：０．０５％以下、Ｔａ：０．０１～０．０６％を含み、残部がＦｅ及び不可避的不純物からなることを特徴とするマルテンサイト系ステンレス鋼。 That is, the present invention is as follows.
(1) By mass%, C: 0.005 to 0.05%, Si: 1.0% or less, Mn: 2.0% or less, Cr: 16 to 18%, Ni: 2.5 to 6.5 %, Mo: 1.5-3.5%, W: 3.5% or less, Cu: 3.5% or less, V: 0.01-0.08%, Sol.Al: 0.005-0. A martensitic stainless steel comprising 10%, N: 0.05% or less, Ta: 0.01 to 0.06%, the balance being Fe and inevitable impurities.
　（２）質量％で、Ｎｂ：０．１％以下をさらに含む、上記（１）に記載のマルテンサイト系ステンレス鋼。 (2) The martensitic stainless steel according to the above (1), further including Nb: 0.1% or less by mass%.
　（３）０．２％耐力が７５８ＭＰａ以上９６５ＭＰａ以下であり、－２０℃でのシャルピー・フルサイズ試験における吸収エネルギーが１００Ｊ以上であり、２０％ＮａＣｌ水溶液、２２０℃、１０気圧ＣＯ２の環境下での腐食速度が０．５ｍｍ／ｙｅａｒ以下である、上記（１）又は（２）に記載のマルテンサイト系ステンレス鋼。 (3) 0.2% proof stress is 758 MPa or more and 965 MPa or less, absorbed energy in Charpy full size test at −20 ° C. is 100 J or more, in an environment of 20% NaCl aqueous solution, 220 ° C., 10 atm CO 2 . The martensitic stainless steel according to (1) or (2) above, which has a corrosion rate of 0.5 mm / year or less.
　（４）上記（１）～（３）のいずれかに記載のマルテンサイト系ステンレス鋼からなる油井、ガス井又はラインパイプ用マルテンサイト系ステンレス鋼管。 (4) A martensitic stainless steel pipe for an oil well, gas well or line pipe made of the martensitic stainless steel according to any one of (1) to (3) above.
　（５）上記（１）又は（２）に記載の組成を有するマルテンサイト系ステンレス鋼を熱間加工した後、該ステンレス鋼を８００℃以上９８０℃以下の温度でオーステナイト化し、続いて１００℃以下の温度に焼入れ冷却し、次いで５００℃以上７００℃以下の温度で焼戻しを行う工程を有する、マルテンサイト系ステンレス鋼の製造方法。 (5) After hot working the martensitic stainless steel having the composition described in (1) or (2) above, the stainless steel is austenitized at a temperature of 800 ° C. or higher and 980 ° C. or lower, and subsequently 100 ° C. or lower. A method for producing martensitic stainless steel, comprising quenching and cooling to a temperature of 500 ° C. and then tempering at a temperature of 500 ° C. to 700 ° C.
　本発明によれば、合金組成及び製造条件を特定することにより、靭性に優れ、また１８０℃以上の環境での炭酸ガス腐食に対する耐食性が良好である高強度高靭性高耐食マルテンサイト系ステンレス鋼を得ることが可能である。 According to the present invention, by specifying the alloy composition and production conditions, a high-strength, high-toughness, high-corrosion-resistant martensitic stainless steel having excellent toughness and good corrosion resistance against carbon dioxide corrosion in an environment of 180 ° C. or higher is obtained. It is possible to obtain.
　以下に、本発明のマルテンサイト系ステンレス鋼における合金元素の添加理由、その量の限定理由及び製造条件の限定理由について、説明する。なお、鋼中の各合金元素の含有量は、特に断りのない限り鋼全体を１００％とする質量基準の値である。 Hereinafter, the reason for adding the alloy element in the martensitic stainless steel of the present invention, the reason for limiting the amount thereof, and the reason for limiting the manufacturing conditions will be described. In addition, content of each alloy element in steel is a value of the mass reference | standard which makes the whole steel 100% unless there is particular notice.
　（１）成分組成範囲
　＜Ｃ：０．００５～０．０５％＞
　Ｃは強力なオーステナイト生成元素であり、また、ステンレス鋼において高強度を得るためにも欠かせない元素である。しかし、鋼製造における焼戻し時にＣｒと結合して炭化物として析出し、これが鋼の耐食性及び靭性を劣化させる。Ｃの含有量が０．００５％未満では十分な強度が得られず、０．０５％を超えると前記劣化が顕著になるため、その含有量を０．００５～０．０５％とする。 (1) Component composition range <C: 0.005 to 0.05%>
C is a strong austenite-forming element, and is also an indispensable element for obtaining high strength in stainless steel. However, it combines with Cr during tempering in steel production and precipitates as carbide, which degrades the corrosion resistance and toughness of the steel. If the C content is less than 0.005%, sufficient strength cannot be obtained. If the C content exceeds 0.05%, the deterioration becomes significant, so the content is made 0.005 to 0.05%.
　＜Ｓｉ：１．０％以下＞
　Ｓｉは脱酸剤として必要な元素であるが、強力なフェライト生成元素でもある。本発明のマルテンサイト系ステンレス鋼においては、高強度及び高靭性を達成するためにはフェライトを制御する必要がある。Ｓｉをステンレス鋼に１．０％を超えて含有させるとフェライトを制御するのが難しくなるため、その含有量の上限を１．０％、好ましくは０．５％、より好ましくは０．３％とする。 <Si: 1.0% or less>
Si is an element necessary as a deoxidizer, but is also a strong ferrite-forming element. In the martensitic stainless steel of the present invention, it is necessary to control ferrite in order to achieve high strength and high toughness. If Si is contained in stainless steel in an amount exceeding 1.0%, it becomes difficult to control ferrite, so the upper limit of the content is 1.0%, preferably 0.5%, more preferably 0.3%. And
　＜Ｍｎ：２．０％以下＞
　Ｍｎは脱酸、脱硫剤として有効である。しかし、Ｍｎはステンレス鋼の耐食性を低下させるので、その含有量の上限を２．０％、好ましくは０．５％、より好ましくは０．３％とする。 <Mn: 2.0% or less>
Mn is effective as a deoxidation and desulfurization agent. However, since Mn lowers the corrosion resistance of stainless steel, the upper limit of its content is 2.0%, preferably 0.5%, more preferably 0.3%.
　＜Ｃｒ：１６～１８％＞
　Ｃｒはマルテンサイト系ステンレス鋼を構成する基本的な元素で、しかも耐食性を発現する重要な元素である。１８０℃以上の苛酷な環境における耐食性を考慮した場合、含有量が１６％未満では十分な耐食性が得られず、１８％を超えると効果は飽和し経済性の点で問題が生じるため、その含有量を１６～１８％とする。 <Cr: 16-18%>
Cr is a basic element constituting martensitic stainless steel, and is an important element that develops corrosion resistance. Considering the corrosion resistance in a harsh environment of 180 ° C or higher, if the content is less than 16%, sufficient corrosion resistance cannot be obtained, and if it exceeds 18%, the effect is saturated and a problem arises in terms of economy. The amount is 16-18%.
　＜Ｎｉ：２．５～６．５％＞
　Ｎｉはステンレス鋼の耐食性を向上させるとともに、オーステナイトの安定化に極めて有効な元素であるが、２．５％未満ではその効果が十分に発揮されず、一方、含有量が増加するとマルテンサイトの変態点（Ａｃ１点）を下げるので、本発明のマルテンサイト系ステンレス鋼の製造時における焼戻し温度に制約を与える。さらにＮｉの含有量が高まるとオーステナイトが析出する場合があり、ステンレス鋼の高強度化を阻害する。そこで、Ｎｉの含有量を２．５～６．５％とする。 <Ni: 2.5-6.5%>
Ni improves the corrosion resistance of stainless steel and is an extremely effective element for stabilizing austenite. However, if the content is less than 2.5%, the effect is not sufficiently exhibited. Since the point (Ac1 point) is lowered, restrictions are imposed on the tempering temperature during the production of the martensitic stainless steel of the present invention. Further, when the Ni content increases, austenite may precipitate, which hinders the strengthening of stainless steel. Therefore, the Ni content is set to 2.5 to 6.5%.
　＜Ｍｏ：１．５～３．５％＞
　Ｍｏは特にステンレス鋼の耐食性の向上に有効な元素であるが、１．５％未満の含有量ではその効果が現れず、また３．５％を超えるとフェライトを制御することが難しくなるため、その含有量を１．５～３．５％とする。 <Mo: 1.5-3.5%>
Mo is an element that is particularly effective for improving the corrosion resistance of stainless steel. However, if the content is less than 1.5%, the effect does not appear, and if it exceeds 3.5%, it becomes difficult to control ferrite. The content is 1.5 to 3.5%.
　＜Ｗ、Ｃｕ：３．５％以下＞
　これらはいずれもステンレス鋼の強度及び耐食性の向上に有効な元素であり、添加する場合は、その量が３．５％を超えると鋼の熱間加工性が劣化するので、上限を３．５％とする。 <W, Cu: 3.5% or less>
These are all effective elements for improving the strength and corrosion resistance of stainless steel, and when added, if the amount exceeds 3.5%, the hot workability of steel deteriorates, so the upper limit is 3.5. %.
　＜Ｖ：０．０１～０．０８％＞
　Ｖは強力な炭化物生成元素であり、微細な炭化物をステンレス鋼のマトリックス粒内に均一に析出させ、粒界に優先析出させないことにより炭化物の結晶粒を微細化し、鋼の強度向上に寄与する。このように微細に均一に炭化物を析出させるので、鋼の靭性を低下させることなく、強度の向上を図ることができる。その含有量が０．０１％未満では強度の向上効果が現れず、０．０８％を超えるとその効果は飽和するため、含有量を０．０１～０．０８％、好ましくは０．０２～０．０４％とする。 <V: 0.01 to 0.08%>
V is a strong carbide-forming element, and fine carbides are uniformly precipitated in the matrix grains of stainless steel, and are not preferentially precipitated at the grain boundaries, thereby making the carbide crystal grains finer and contributing to improving the strength of the steel. Since carbides are precipitated finely and uniformly in this way, the strength can be improved without reducing the toughness of the steel. If the content is less than 0.01%, the effect of improving the strength does not appear. If the content exceeds 0.08%, the effect is saturated, so the content is 0.01 to 0.08%, preferably 0.02 to 0.04%.
　＜Ｓｏｌ.Ａｌ：０．００５～０．１０％＞
　酸可溶Ａｌ（Ｓｏｌ.Ａｌ）は精錬工程におけるステンレス鋼の脱酸のため、及びＡｌＮ析出によるγ粒微細化により鋼の靭性を向上させるために添加される。その含有量が０．００５％未満では靭性向上効果が得られず、０．１０％を超えると反対に靭性が低下することがあるため、含有量を０．００５～０．１０％とする。 <Sol.Al: 0.005 to 0.10%>
Acid-soluble Al (Sol. Al) is added for deoxidation of stainless steel in the refining process and for improving the toughness of the steel by refinement of γ grains by AlN precipitation. If the content is less than 0.005%, the effect of improving toughness cannot be obtained, and if it exceeds 0.10%, the toughness may be lowered, so the content is made 0.005 to 0.10%.
　＜Ｎ：０．０５％以下＞
　Ｎはステンレス鋼の耐食性向上に有害な元素であるが、オーステナイト生成元素でもある。０．０５％を超えて含有させると本発明のマルテンサイト系ステンレス鋼の製造時における焼戻し時に窒化物となって析出し、鋼の耐食性及び靭性が劣化するため、その含有量の上限を０．０５％、好ましくは０．０２％とする。 <N: 0.05% or less>
N is an element harmful to the corrosion resistance improvement of stainless steel, but is also an austenite generating element. If it exceeds 0.05%, it will precipitate as a nitride during tempering during the production of the martensitic stainless steel of the present invention, and the corrosion resistance and toughness of the steel will deteriorate. 05%, preferably 0.02%.
　＜Ｔａ：０．０１～０．０６％＞
　Ｔａは強力な炭化物生成元素であり、微細な炭化物をステンレス鋼のマトリックス粒内に均一に析出させ、鋼の強度向上に寄与する。微細な炭化物を均一に析出させることから、鋼の靭性を低下させることなく強度の向上を図ることができる。しかも、上記Ｖとともに一定量添加することによって、一方を添加した場合よりも優れた強度・靭性を達成することができる。その含有量が０．０１％未満では強度の向上効果が現れず、０．０６％を超えるとその効果はおおよそ飽和し、またコストの点でも好ましくないため、含有量を０．０１～０．０６％とする。 <Ta: 0.01 to 0.06%>
Ta is a strong carbide-forming element, and precipitates fine carbides uniformly in the matrix grains of stainless steel, contributing to the improvement of steel strength. Since fine carbides are uniformly deposited, the strength can be improved without reducing the toughness of the steel. In addition, by adding a certain amount together with the V, it is possible to achieve strength and toughness superior to the case where one is added. If the content is less than 0.01%, the effect of improving the strength does not appear, and if it exceeds 0.06%, the effect is almost saturated and is not preferable in terms of cost. 06%.
　＜Ｎｂ：０．１％以下＞
　本発明では、上記の基本成分以外にＮｂを含有してもよい。Ｎｂは強力な炭化物生成元素であり、微細な炭化物を析出させることにより結晶粒を微細化し、ステンレス鋼の強度を向上させる。その含有量が０．１％を超えるとその効果は飽和するため、含有量を０．１％以下、好ましくは０．０５％以下とする。 <Nb: 0.1% or less>
In the present invention, Nb may be contained in addition to the above basic components. Nb is a strong carbide-forming element, and precipitates fine carbides to refine crystal grains and improve the strength of stainless steel. When the content exceeds 0.1%, the effect is saturated, so the content is set to 0.1% or less, preferably 0.05% or less.
　＜残部＞
　また、本発明のマルテンサイト系ステンレス鋼において、以上説明した成分以外の残部はＦｅおよび不可避的不純物からなる。Ｐ、Ｓ、Ｏに代表される不可避的不純物のうち、Ｐは０．０４％以下、Ｓは０．０１％以下であれば、本発明の実施が考えられる継目無鋼管あるいは熱間圧延鋼板を素材とする電縫鋼管の製造に支障がない。しかし、これらの不純物はいずれも鋼の熱間加工性を劣化させる元素であり、その含有量は少ないほど好ましい。またＯ及びその余の不可避的不純物についても、含有量が極力低い方が好ましい。 <Remainder>
In the martensitic stainless steel of the present invention, the balance other than the components described above consists of Fe and inevitable impurities. Among inevitable impurities represented by P, S, and O, if P is 0.04% or less and S is 0.01% or less, a seamless steel pipe or a hot-rolled steel sheet in which the present invention can be considered is used. There is no problem in the production of ERW steel pipes. However, these impurities are all elements that degrade the hot workability of steel, and the smaller the content, the better. Further, it is preferable that the content of O and other inevitable impurities is as low as possible.
　＜マルテンサイト系ステンレス鋼の特性＞
　本発明に使用するステンレス鋼を上記の組成成分範囲に調整することにより、従来の高強度マルテンサイト系ステンレス鋼の耐食性を改善して、靭性を維持しつつ、１８０℃以上の湿潤炭酸ガス環境で使用できる７５８ＭＰａ以上のマルテンサイト系ステンレス鋼を得ることが可能である。 <Characteristics of martensitic stainless steel>
By adjusting the stainless steel used in the present invention to the above-mentioned composition component range, the corrosion resistance of the conventional high-strength martensitic stainless steel is improved, while maintaining toughness, in a wet carbon dioxide environment of 180 ° C. or higher. It is possible to obtain martensitic stainless steel of 758 MPa or more that can be used.
　具体的には、本発明のマルテンサイト系ステンレス鋼の０．２％耐力は７５８ＭＰａ以上９６５ＭＰａ以下であり、－２０℃でのシャルピー・フルサイズ試験における吸収エネルギーが１００Ｊ以上、好ましくは２００Ｊ以上であり、２０％ＮａＣｌ水溶液、２２０℃、１０気圧ＣＯ２の環境下での腐食速度が０．５ｍｍ／ｙｅａｒ以下、好ましくは０．３ｍｍ／ｙｅａｒ以下である。なお、０．２％耐力が９６５ＭＰａを超えると、腐食等によりステンレス鋼に少量の水素が入った場合に、鋼に割れが生じやすくなる。 Specifically, the 0.2% proof stress of the martensitic stainless steel of the present invention is 758 MPa or more and 965 MPa or less, and the absorbed energy in the Charpy full size test at −20 ° C. is 100 J or more, preferably 200 J or more. Corrosion rate in an environment of 20% NaCl aqueous solution, 220 ° C., 10 atm CO 2 is 0.5 mm / year or less, preferably 0.3 mm / year or less. If the 0.2% proof stress exceeds 965 MPa, when a small amount of hydrogen enters the stainless steel due to corrosion or the like, the steel tends to crack.
　本発明においては、ステンレス鋼の組成を上記の金属組成として所定の製造条件を実施することにより、苛酷な環境における使用に耐えうる適切な強度、高靭性及び高耐食性を達成している。 In the present invention, appropriate strength, high toughness, and high corrosion resistance that can withstand use in a harsh environment are achieved by carrying out predetermined manufacturing conditions with the stainless steel composition as the above metal composition.
　（２）マルテンサイト系ステンレス鋼の製造工程
　本発明のマルテンサイト系ステンレス鋼は、以下の製造方法により製造することができる。 (2) Manufacturing process of martensitic stainless steel The martensitic stainless steel of the present invention can be manufactured by the following manufacturing method.
　上記の成分組成範囲に調整したマルテンサイト系ステンレス鋼を転炉あるいは電気炉にて溶製し、普通造塊法－分塊圧延または連続鋳造法により鋼片にする。この鋼片を熱間加工し、必要に応じて継目無鋼管や鋼板など、所定の形状を付与する（この状態のマルテンサイト系ステンレス鋼では、本発明が目的とする強度や靭性等が達成されていない）。熱間加工した鋼を８００℃以上９８０℃以下の温度で加熱してオーステナイト化し、続いて１００℃以下の温度に焼入れ冷却し、次いで５００℃以上７００℃以下の温度で焼戻しを行う。以下、前記焼入れ及び焼戻しにおける加熱温度について説明する。 マ ル Martensitic stainless steel adjusted to the above component composition range is melted in a converter or electric furnace and made into a steel slab by ordinary ingot-making-slab rolling or continuous casting. This steel slab is hot-worked and given a predetermined shape such as a seamless steel pipe or steel plate as necessary (in this state, the intended strength, toughness, etc. of the present invention are achieved. Not) The hot-worked steel is heated to a temperature of 800 ° C. or higher and 980 ° C. or lower to austenite, then quenched and cooled to a temperature of 100 ° C. or lower, and then tempered at a temperature of 500 ° C. or higher and 700 ° C. or lower. Hereinafter, the heating temperature in the quenching and tempering will be described.
　＜ａ．加熱温度：８００℃以上９８０℃以下＞
　加熱温度が８００℃未満では、ステンレス鋼がオーステナイト化せず、焼入れの効果が得られないため、下限は８００℃とする。一方、加熱温度が９８０℃を超えると、結晶粒が粗大化し、靭性が劣化するため、上限は９８０℃とする。 <A. Heating temperature: 800 ° C. or higher and 980 ° C. or lower>
If the heating temperature is less than 800 ° C., the stainless steel does not become austenite and the effect of quenching cannot be obtained, so the lower limit is set to 800 ° C. On the other hand, if the heating temperature exceeds 980 ° C., the crystal grains become coarse and the toughness deteriorates, so the upper limit is made 980 ° C.
　＜ｂ．焼戻し温度：５００℃以上７００℃以下＞
　焼戻し処理は一般に鋼に靭性を付与するために行われる処理であるが、本発明においては、靭性の付与に加えて、Ｖ、Ｔａの微細な炭化物を均一に分散析出させて、ステンレス鋼の高強度化を図るために必須の工程である。ただし、焼戻し温度が７００℃を超えると、７５８ＭＰａ以上の０．２％耐力が得られないので、その上限は７００℃とする。また、５００℃より低いと炭化物の析出が十分ではなく、目標とする０．２％耐力及び靭性が得られないので、下限は５００℃とする。 <B. Tempering temperature: 500 ° C to 700 ° C>
The tempering process is a process generally performed to impart toughness to steel. In the present invention, in addition to imparting toughness, fine carbides of V and Ta are uniformly dispersed and precipitated, thereby increasing the strength of stainless steel. This is an essential process for increasing the strength. However, if the tempering temperature exceeds 700 ° C, a 0.2% proof stress of 758 MPa or more cannot be obtained, so the upper limit is 700 ° C. Further, if the temperature is lower than 500 ° C., the precipitation of carbide is not sufficient, and the target 0.2% proof stress and toughness cannot be obtained, so the lower limit is 500 ° C.
　以下、本発明の具体的な実施例について説明する。本発明者らは下記表１に示す化学組成の発明鋼Ｎ１～Ｎ８及び比較鋼Ｃ１～Ｃ５を試験鋼（マルテンサイト系ステンレス鋼）として溶製し、熱間圧延にて厚み１２ｍｍの鋼板とした後、下記表２に示す熱処理（焼入れおよび焼戻し処理）を行い、マルテンサイト系ステンレス鋼を製造した。なお、焼入れ熱処理後は水冷して室温まで冷却し、その後、焼戻し熱処理した。 Hereinafter, specific examples of the present invention will be described. The present inventors melted the inventive steels N1 to N8 and comparative steels C1 to C5 having chemical compositions shown in Table 1 below as test steels (martensitic stainless steels), and made steel plates with a thickness of 12 mm by hot rolling. Thereafter, heat treatment (quenching and tempering treatment) shown in Table 2 below was performed to produce martensitic stainless steel. In addition, after quenching heat processing, it cooled with water and cooled to room temperature, and then tempered heat processing.
　これらマルテンサイト系ステンレス鋼について、以下の条件で、機械的特性（強度と靭性）、及び耐食性の試験を行った。なお、比較鋼Ｃ１及びＣ５は、Ｔａを含有しない鋼である。また、比較鋼Ｃ２はＶ含有量が本発明の下限に満たない鋼であり、比較鋼Ｃ３はＣｒとＭｏ含有量が本発明の下限に満たない鋼であり、比較鋼Ｃ４はＴａ含有量が本発明の上限値を超える鋼である。 These martensitic stainless steels were tested for mechanical properties (strength and toughness) and corrosion resistance under the following conditions. The comparative steels C1 and C5 are steels that do not contain Ta. Further, the comparative steel C2 is a steel whose V content is less than the lower limit of the present invention, the comparative steel C3 is a steel whose Cr and Mo contents are less than the lower limit of the present invention, and the comparative steel C4 has a Ta content. Steel exceeding the upper limit of the present invention.
　強度：常温での０．２％耐力。
　靭性：－２０℃でのシャルピー・フルサイズ試験における吸収エネルギー（Ｊ）。
　耐食性：２０％ＮａＣｌ溶液、２２０℃、１０気圧ＣＯ２の環境下での２週間の腐食速度。 Strength: 0.2% proof stress at room temperature.
Toughness: Absorbed energy (J) in Charpy full size test at -20 ° C.
Corrosion resistance: Corrosion rate for 2 weeks in an environment of 20% NaCl solution, 220 ° C., 10 atm CO 2 .
　これらの機械的特性及び耐食性の試験結果を下記表３に示す。 These mechanical properties and corrosion resistance test results are shown in Table 3 below.
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000003
 ＊耐食性については、２週間の腐食速度から１年あたりの腐食速度を計算した。
Figure JPOXMLDOC01-appb-T000003
 * For corrosion resistance, the corrosion rate per year was calculated from the corrosion rate for two weeks.
　本発明鋼の「Ｎ１，Ｎ２Ａ，Ｎ２Ｂ，Ｎ３，Ｎ４，Ｎ５，Ｎ６Ａ，Ｎ６Ｂ，Ｎ７，Ｎ８」は、０．２％耐力、シャルピー・フルサイズ試験における吸収エネルギーはすべて目標範囲に入った。また、耐食性も目標値をクリアした。 The steels of the present invention “N1, N2A, N2B, N3, N4, N5, N6A, N6B, N7, N8” all had the 0.2% proof stress and the absorbed energy in the Charpy full size test. Corrosion resistance also cleared the target value.
　一方、比較鋼のうち、Ｃ１とＣ５は、Ｔａを含有しない鋼であり、Ｃ２はＶ含有量が本発明の下限に満たない鋼であり、Ｃ３はＣｒとＭｏ含有量が本発明の下限に満たない鋼であり、Ｃ４はＴａ含有量が本発明の上限値を超える鋼である。 On the other hand, among the comparative steels, C1 and C5 are steels not containing Ta, C2 is a steel whose V content is less than the lower limit of the present invention, and C3 is a Cr and Mo content that is the lower limit of the present invention. C4 is a steel whose Ta content exceeds the upper limit of the present invention.
　すなわち、いずれかの成分が本発明の範囲を外れているため、試験結果も０．２％耐力、靭性又は耐食性が目標を達成していない。特に、Ｃ２は、６００℃を下回る温度で焼き戻しを行っても、７５８ＭＰａ級の強度を得ることができず、靭性も耐食性も目標値を達成できなかった。以上から、Ｖ＋Ｔａの複合添加により、各々の単独添加では達成できなかった特性の向上が図られ、複合添加による相乗効果が認められた。 That is, since any component is out of the scope of the present invention, the test results also do not achieve the target of 0.2% proof stress, toughness or corrosion resistance. In particular, even when C2 was tempered at a temperature lower than 600 ° C., the strength of 758 MPa could not be obtained, and the toughness and corrosion resistance could not achieve the target values. From the above, the composite addition of V + Ta improved the characteristics that could not be achieved by each single addition, and a synergistic effect by the composite addition was recognized.
　本発明のマルテンサイト系ステンレス鋼管は、耐食性を有し、７５８ＭＰａ以上の高強度にもかかわらず靭性を維持し、１８０℃以上の湿潤炭酸ガス環境で使用でき、炭酸ガスを含む石油、天然ガスの掘削、輸送用鋼管（油井、ガス井又はラインパイプ用マルテンサイト系ステンレス鋼管）として好適に利用可能である。 The martensitic stainless steel pipe of the present invention has corrosion resistance, maintains toughness despite high strength of 758 MPa or higher, can be used in a wet carbon dioxide environment of 180 ° C. or higher, and contains petroleum and natural gas containing carbon dioxide. It can be suitably used as a steel pipe for excavation and transportation (an oil well, a gas well or a martensitic stainless steel pipe for a line pipe).

Claims (5)

  1. 　質量％で、Ｃ：０．００５～０．０５％、Ｓｉ：１．０％以下、Ｍｎ：２．０％以下、Ｃｒ：１６～１８％、Ｎｉ：２．５～６．５％、Ｍｏ：１．５～３．５％、Ｗ：３．５％以下、Ｃｕ：３．５％以下、Ｖ：０．０１～０．０８％、Ｓｏｌ.Ａｌ：０．００５～０．１０％、Ｎ：０．０５％以下、Ｔａ：０．０１～０．０６％を含み、残部がＦｅ及び不可避的不純物からなることを特徴とするマルテンサイト系ステンレス鋼。 By mass%, C: 0.005 to 0.05%, Si: 1.0% or less, Mn: 2.0% or less, Cr: 16 to 18%, Ni: 2.5 to 6.5%, Mo : 1.5 to 3.5%, W: 3.5% or less, Cu: 3.5% or less, V: 0.01 to 0.08%, Sol.Al: 0.005 to 0.10%, A martensitic stainless steel containing N: 0.05% or less, Ta: 0.01-0.06%, the balance being Fe and inevitable impurities.
  2. 　質量％で、Ｎｂ：０．１％以下をさらに含む、請求項１に記載のマルテンサイト系ステンレス鋼。 The martensitic stainless steel according to claim 1, further comprising Nb: 0.1% or less by mass%.
  3. 　０．２％耐力が７５８ＭＰａ以上９６５ＭＰａ以下であり、－２０℃でのシャルピー・フルサイズ試験における吸収エネルギーが１００Ｊ以上であり、２０％ＮａＣｌ水溶液、２２０℃、１０気圧ＣＯ２の環境下での腐食速度が０．５ｍｍ／ｙｅａｒ以下である、請求項１又は２に記載のマルテンサイト系ステンレス鋼。 0.2% proof stress is 758 MPa or more and 965 MPa or less, absorption energy in Charpy full size test at −20 ° C. is 100 J or more, corrosion in an environment of 20% NaCl aqueous solution, 220 ° C., 10 atm CO 2 The martensitic stainless steel according to claim 1 or 2, wherein the speed is 0.5 mm / year or less.
  4. 　請求項１～３のいずれかに記載のマルテンサイト系ステンレス鋼からなる油井、ガス井又はラインパイプ用マルテンサイト系ステンレス鋼管。 A martensitic stainless steel pipe for oil wells, gas wells or line pipes comprising the martensitic stainless steel according to any one of claims 1 to 3.
  5. 　請求項１又は２に記載の組成を有するマルテンサイト系ステンレス鋼を熱間加工した後、８００℃以上９８０℃以下の温度でオーステナイト化し、続いて１００℃以下の温度に焼入れ冷却し、次いで５００℃以上７００℃以下の温度で焼戻しを行う工程を有する、マルテンサイト系ステンレス鋼の製造方法。 After hot working the martensitic stainless steel having the composition according to claim 1 or 2, it is austenitized at a temperature of 800 ° C. or higher and 980 ° C. or lower, subsequently quenched and cooled to a temperature of 100 ° C. or lower, and then 500 ° C. The manufacturing method of the martensitic stainless steel which has the process of tempering at the temperature below 700 degreeC above.
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