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    • 专利摘要:
    it is a high strength seamless steel pipe for oil wells that has excellent resistance to cracking from sulfide stress corrosion. seamless steel tube includes, in % by mass, 0.20% to 0.50% of c, 0.05% to 0.40% of si, 0.3% to 0.9% of mn, 0.015 % or less of powder, 0.005% or less of s, 0.005% to 0.1% of al, 0.006% or less of n, more than 1.0%, but 3.0% or less of mo, 0, 01% or more, but less than 0.05% of v, 0.001% or more, but less than 0.01% of nb, 0.0003% to 0.0030% of b, 0.0030% or less of o, and 0.003% to 0.025% of ti, where ti/n is 2.0 to 5.0. seamless steel tube has a hardened martensite volume fraction of 95% or more; has a previous austenite grain size number of 8.5 or more; and within a cross-section that is orthogonal to the rolling direction has, by 100 mm2, 100 or less nitride inclusions that have a grain size of 4 µm or more, 1000 or less nitride inclusions that have a grain size less than 4 ¿m, 40 or less oxide inclusions that have a grain size of 4 ¿m or more, and 400 or less oxide inclusions that have a grain size less than 4 ¿m.
    公开号:BR112017012766B1
    申请号:R112017012766-0
    申请日:2015-09-10
    公开日:2021-06-01
    发明作者:Masao Yuga；Yasuhide Ishiguro；Seiji Nabeshima；Mitsuhiro Okatsu；Hiroki Ota
    申请人:Jfe Steel Corporation；
    IPC主号:
    专利说明:

    FIELD OF TECHNIQUE
    [001] The present invention relates to a high strength seamless steel tube suitable for tubular products of the oil industry and particularly relates to an improvement in resistance to sulfide stress cracking (hereinafter referred to as "SSC resistance") in a wet hydrogen sulfide environment (acidic environment). BACKGROUND TECHNIQUE
    [002] In recent years, from the point of view of stable protection of energy resources, oil wells and natural gas wells at a high depth in a severe corrosive environment have been developed. Therefore, for petroleum industry tubular products, resistance to SSC in an acidic environment containing hydrogen sulfide (H2S) is strongly required to be higher while maintaining a high YS flow limit of 862 MPa (125 ksi) or more.
    [003] In order to meet the requirements, for example, PTL 1 discloses a method of producing steel for tubular products of the petroleum industry, the method including: preparing low alloy steel which contains, in % by weight, C: 0, 2% to 0.35%, Cr: 0.2% to 0.7%; Mo: 0.1% to 0.5%, and V: 0.1% to 0.3%; harden the low alloy steel at a transformation point Ac3 or more; and tempering the low alloy steel over a temperature range of 650°C to an Ac1 transformation point. According to the technique disclosed in PTL 1, the low alloy steel can be adjusted so that a total amount of precipitated carbides is 2% by weight to 5% by weight, and a ratio of one carbide MC to the total amount of precipitated carbides is 8% by weight to 40% by weight. Therefore, steel for tubular products from the petroleum industry which has superior sulfide stress crack resistance can be obtained.
    [004] Additionally, PTL 2 discloses a steel production method for tubular products of the petroleum industry that has superior tenacity and resistance to sulfide stress cracking, the method including: preparing low alloy steel that contains, in % by mass, C : 0.15% to 0.3%, Cr: 0.2% to 1.5%; Mo: 0.1% to 1%, V: 0.05% to 0.3%, and Nb: 0.003% to 0.1%; heat the low alloy steel to 1150 °C or more; finish work hot at 1000 °C or more; and performing a quench-temper treatment on the low-alloy steel at least once the low-alloy steel is quenched at a temperature of 900°C or more, is tempered in a range of 550°C to a turning point Ac1 is quenched by reheating it over a range of 850 °C to 1000 °C, and is tempered over a range of 600 °C to the Ac1 transformation point. According to the technique revealed in PTL 2, the low alloy steel can be adjusted so that a total amount of precipitated carbides is 1.5% by mass and 4% by mass, a ratio of one MC carbide to the amount. total precipitated carbides is 5% by mass to 45% by mass, and a ratio of an M23C6 carbide to the total amount of precipitated carbides is 200/t (t: wall thickness (mm)) or less. Therefore, steel for tubular products from the petroleum industry which has superior toughness and resistance to sulfide stress cracking can be obtained.
    [005] Additionally, PTL 3 discloses steel for tubular products of the petroleum industry that contains, in % by mass, C: 0.15% to 0.30%, Si: 0.05% to 1.0%, Mn: 0, 10% to 1.0%, P: 0.025% or less, S: 0.005% or less, Cr: 0.1% to 1.5%, Mo: 0.1% to 1.0%, Al: 0.003% at 0.08%, N: 0.008% or less, B: 0.0005% to 0.010%, and Ca+O (oxygen): 0.008% or less and which additionally contains one element or two or more Ti elements: 0.005 % to 0.05%, Nb: 0.05% or less, Zr: 0.05% or less, and V: 0.30% or less, where a maximum continuous length of non-metallic inclusions in cross-sectional observation is 80 µm or shorter, and the number of non-metallic inclusions that have a grain size of 20 µm or more in cross-sectional observation is 10 inclusions/100 mm2 or less. As a result, low alloy steel for petroleum industry tubular products which has high strength required for petroleum industry tubular products and has superior SSC strength corresponding to strength can be obtained.
    [006] Additionally, PTL 4 reveals low alloy steel for tubular products of the petroleum industry that has superior sulfide stress cracking resistance, the steel contains, in % by mass, C: 0.20% to 0.35 %, Si: 0.05% to 0.5%, Mn: 0.05% to 0.6%, P: 0.025% or less, S: 0.01% or less, Al: 0.005% to 0.100%, Mo: 0.8% to 3.0%, V: 0.05% to 0.25%, B: 0.0001% to 0.005%, N: 0.01% or less, and O: 0.01% or less, where 12V+1-Mo>0 is satisfied. According to the technique disclosed in PTL 4, in addition to the composition described above, the steel may additionally contain, in % by mass, Cr: 0.6% or less so that Mo-(Cr+Mn)>0 is satisfied, it may additionally contain one or more elements of Nb: 0.1% or less, Ti: 0.1% or less, and Zr: 0.1% or less, or it may additionally contain Ca: 0.01% or less. OF QUOTES PATENT LITERATURE
    [007] [PTL 1] JP-A-2000-178682
    [008] [PTL 2] JP-A-2000-297344
    [009] [PTL 3] JP-A-2001-172739
    [0010] [PTL 4] JP-A-2007-16291 SUMMARY OF THE INVENTION PROBLEM OF THE TECHNIQUE
    [0011] However, there are several factors that affect the resistance to cracking by sulfide stress (resistance to SSC). Therefore, it cannot be said that applying only the techniques disclosed in PTLS 1 to 4 is sufficient to improve the SSC strength of a high strength seamless steel tube that has a yield point (YS) of 862 MPa (125 ksi) or more to a degree that is sufficient for petroleum industry tubular products in a severe corrosive environment. Furthermore, there are problems related to the difficulty of stably adjusting the types and amounts of carbides disclosed in PTLS 1 and 2 and the shapes and numbers of the non-metallic inclusions disclosed in PTL 3 being within the desired ranges.
    [0012] The present invention was produced in order to solve the problems of the technique in question, and an objective of the same is to provide a high strength seamless steel tube for tubular products of the petroleum industry that have superior sulfide stress cracking resistance ; and a method of producing it.
    [0013] The "high strength" described herein refers to a yield point (YS) that is 862 MPa (125 ksi) or more. Additionally, "superior sulfide stress cracking resistance" described herein refers to a case where no cracking occurs with an applied stress of 85% of a sample's yield point for more than 720 hours (time ) when a constant load test is performed on an acetic acid-sodium acetate solution (liquid temperature: 24 °C) saturated with hydrogen sulfide at 10 kPa, which has a pH adjusted to 3.5, and which contains 5.0% by mass of sodium chloride solution according to a test method defined in NACE TMO177 Method A. SOLUTION TO THE PROBLEM
    [0014] In order to achieve the objectives described above, it is necessary to simultaneously realize the desired high strength and higher SSC strength. Therefore, the present inventors have thoroughly investigated several factors that affect SSC strength and strength. As a result, it has been found that, in a high strength steel tube that has a YS yield strength of 862 MPa (125 ksi) or more, nitride-based inclusions and oxide-based inclusions have a significant effect on strength. SSC although the effect varies depending on their sizes. Nitride-based inclusions that have a grain size of 4 µm or more and oxide-based inclusions that have a grain size of 4 µm or more have been found to cause sulfide stress cracking (SSC), and SSC is likely occur as their sizes increase. It has been found that the presence of a single nitride-based inclusion that has a grain size less than 4 µm does not cause SSC; however, nitride-based inclusions that have a grain size of less than 4 µm adversely affect the SSC strength when their number is high. Additionally, it has also been found that oxide-based inclusion having a grain size of less than 4 µm adversely affects the SSC strength when the number of the same is high.
    [0015] Therefore, the present inventors have found that, in order to further improve the SSC strength, it is necessary to adjust the numbers of nitride-based inclusions and oxide-based inclusions to be appropriate numbers or less size-dependent. In order to adjust the numbers of nitride-based inclusions and oxide-based inclusions to be appropriate numbers or less, it is important to control the N content and the O (oxygen) content to be in the desired ranges during preparation of a steel pipe raw material, particularly during the melting and casting of molten steel. Furthermore, control in a molten steel refining process is important. Furthermore, the control of production conditions in a refining process and a continuous molten steel casting process is important.
    [0016] The present inventors performed further investigation based on the above findings and completed the present invention. That is, the summary of the present invention is as follows.
    [0017] (1) A high strength seamless steel tube for tubular products of the petroleum industry which has a yield strength (YS) of 862 MPa or more, the steel tube including, as a composition, % by mass,
    [0018] C: 0.20% to 0.50%,
    [0019] Si: 0.05% to 0.40%,
    [0020] Mn: 0.3% to 0.9%,
    [0021] P: 0.015% or less,
    [0022] S: 0.005% or less,
    [0023] Al: 0.005% to 0.1%,
    [0024] N: 0.006% or less,
    [0025] Mo: more than 1.0% and 3.0% or less,
    [0026] V: 0.01% or more and less than 0.05%,
    [0027] Nb: 0.001% or more and less than 0.01%,
    [0028] B: 0.0003% to 0.0030%,
    [0029] O (oxygen): 0.0030% or less,
    [0030] Ti: 0.003% to 0.025%, and
    [0031] a remainder that includes Fe and unavoidable impurities, in which
    [0032] Ti and N contents are adapted to satisfy Ti/N: 2.0 to 5.0,
    [0033] tempered martensite has a volume fraction of 95% or more,
    [0034] grains of prior austenite have a grain size number of 8.5 or more, and
    [0035] in a cross-section perpendicular to a delamination direction, the number of nitride-based inclusions that have a grain size of 4 µm or more is 100 or less per 100 mm2, the number of nitride-based inclusions that have a grain size of less than 4 μm is 1000 or less per 100 mm2, the number of oxide-based inclusions that have a grain size of 4 μm or more is 40 or less per 100 mm2, and the number of oxide-based inclusions oxide base that has a grain size less than 4 µm is 400 or less per 100 mm2.
    [0036] (2) The high strength seamless steel tube for tubular products of the oil industry, according to (1), which additionally includes,
    [0037] an element or more elements selected from, in % by mass,
    [0038] Cr: 0.6% or less,
    [0039] Cu: 1.0% or less,
    [0040] Ni: 1.0% or less, and
    [0041] W: 3.0% or less.
    [0042] (3) The high strength seamless steel tube for tubular products of the petroleum industry according to (1) or (2), which additionally comprises, in % by mass,
    [0043] Ca: 0.0005% to 0.0050%.
    [0044] (4) A method of producing a high strength seamless steel tube for tubular products of the oil industry,
    [0045] seamless steel tube is high strength seamless steel tube for oil industry tubular products according to any one of (1) to (3), and
    [0046] the method includes:
    [0047] heat the steel tube raw material to a heating temperature within a range of 1050°C to 1350°C;
    [0048] perform hot work on the heated steel tube raw material to form a seamless steel tube having a predetermined shape;
    [0049] cool the seamless steel tube at a cooling rate equal to or greater than that of air cooling after hot work until a seamless steel tube surface temperature reaches 200 °C or less; and
    [0050] perform a tempering treatment in which the seamless steel tube is heated to a temperature in a range of 600 °C to 740 °C.
    [0051] (5) The method of producing a high strength seamless steel tube for tubular products of the oil industry according to (4),
    [0052] perform a quench treatment on the seamless steel tube at least once after cooling and before the tempering treatment in which the seamless steel tube is reheated to a temperature in a range from a transformation point Ac3 to 1000 °C or less and is rapidly cooled until the surface temperature of the seamless steel tube reaches 200 °C or less. ADVANTAGEOUS EFFECTS OF THE INVENTION
    [0053] According to the present invention, a high strength seamless steel tube for tubular products of the petroleum industry that has a high YS flow limit of 862 MPa (125 ksi) or higher sulfide stress cracking resistance and superior can be easily produced at a low cost, and industrially significant advantages are exhibited. According to the present invention, suitable alloying elements are contained in appropriate amounts, and the production of nitride-based inclusions and oxide-based inclusions is suppressed. As a result, a high strength seamless steel tube that has a high strength desired for tubular products from the oil industry and superior SSC strength can be stably produced. DESCRIPTION OF MODALITIES
    [0054] First, the reason for limiting the composition of a high strength seamless steel tube according to the present invention will be described. Hereinafter, "% by mass" in the composition will be indicated simply as "%".
    [0055] C: 0.20% to 0.50%
    [0056] C contributes to an increase in the strength of steel by being solubilized in solid in it and also contributes to the formation of a microstructure that contains martensite as a main phase during quenching improving the hardenability of steel. In order to obtain the effects described above, the C content is necessarily 0.20% or more. On the other hand, when the C content is greater than 0.50%, cracking occurs during quenching, and productivity significantly decreases. Therefore, the C content is limited to a range of 0.20% to 0.50%. Preferably, the C content is 0.20% to 0.35%. More preferably, the C content is 0.24% to 0.32%.
    [0057] Si: 0.05% to 0.40%
    [0058] Si is an element that functions as a deoxidizing agent and has an effect of increasing the strength of steel by being solubilized in solid in it and a suppression effect of softening during tempering. In order to obtain the effects described above, the Si content is necessarily 0.05% or more. On the other hand, when the Si content is greater than 0.40%, the formation of ferrite as a soft phase is promoted, the desired high strength strengthening is inhibited, the formation of coarse oxide-based inclusions is promoted, and SSC strength and toughness deteriorate. Additionally, Si is an element that locally hardens steel by being segregated. Therefore, the addition of a large amount of Si, greater than 0.40%, has an adverse effect that a locally hard region is formed to deteriorate the SSC strength. Therefore, in the present invention, the Si content is limited to a range of 0.05% to 0.40%. Preferably, the Si content is 0.05% to 0.30%. More preferably, the Si content is 0.24% to 0.30%.
    [0059] Mn: 0.3% to 0.9%
    [0060] Like C, Mn is an element that improves the hardenability of steel and contributes to an increase in steel strength. In order to obtain the effects described above, the Mn content is necessarily 0.3% or more. On the other hand, Mn is an element that locally hardens steel by being segregated. Therefore, the addition of a large amount of Mn has an adverse effect that a locally hard region is formed to deteriorate the SSC strength. Therefore, in the present invention, the Mn content is limited to a range of 0.3% to 0.9%. Preferably, the Mn content is 0.4% to 0.8%.
    [0061] P: 0.015% or less
    [0062] P is an element that causes grain boundary embrittlement by being segregated into grain boundaries and locally hardens the steel by being segregated therein. In the present invention, P is an unavoidable impurity. Therefore, it is preferred that the P content is reduced as much as possible. However, a P content of 0.015% or less is permissible. Therefore, the P content is limited to be 0.015% or less. Preferably, the P content is 0.012% or less.
    [0063] S: 0.005% or less
    [0064] S is an unavoidable impurity, is present in steel as a sulphide-based inclusion in many cases and deteriorates ductility, toughness and SSC strength. Therefore, it is preferable that the S content is reduced as much as possible. However, an S content of 0.005% or less is permissible. Therefore, the S content is limited to be 0.005% or less. Preferably, the S content is 0.003% or less.
    [0065] Al: 0.005% to 0.1%.
    [0066] Al works as a deoxidizing agent and contributes to the refinement of austenite grains during heating by being bound to N to form AlN. Additionally, Al fixes N, prevents the binding of solid solution B with N, and suppresses a decrease in the effect of B which improves hardenability. In order to obtain the effects described above, the Al content is necessarily 0.005% or more. On the other hand, the addition of more than 0.1% Al causes an increase in the number of oxide-based inclusions, deteriorates steel cleanliness, and causes a deterioration in ductility, toughness and SSC strength. Therefore, the Al content is limited to a range of 0.005% to 0.1%. Preferably, the Al content is 0.01% to 0.08%. More preferably, the Al content is 0.02% to 0.05%.
    [0067] N: 0.006% or less
    [0068] N is present in steel as an unavoidable impurity. However, N has an effect of refining crystal grains and enhancing toughness when it is bonded to Al to form AlN or, in a case where Ti is contained, when it is bonded to Ti to form TiN. However, the addition of more than 0.006% N thickens the nitrides to be formed and significantly deteriorates SSC strength and toughness. Therefore, the N content is limited to be 0.006% or less.
    [0069] Mo: more than 1.0% and 3.0% or less
    [0070] Mo is a carbide-forming element and contributes to the strength strengthening of steel through precipitation strength. Mo effectively contributes to protecting the desired high strength after reduction in displacement density by tempering. Due to the reduction in displacement density, the SSC resistance is improved. Additionally, Mo contributes to SSC strength enhancement by being solubilized in solid in steel and segregated into pre-austenite grain boundaries. Furthermore, Mo has an effect of densifying a corrosion product and suppressing the formation and growth of a crack that causes cracking. In order to obtain the effects described above, the Mo content is necessarily greater than 1.0%. On the other hand, the addition of more than 3.0% Mo promotes the formation of a needle-like M2C precipitate or, in some cases, a Laves phase (Fe2Mo) and deteriorates the SSC strength. Therefore, the Mo content is limited to a range of more than 1.0% and 3.0% or less. The Mo content is preferably 1.45% to 2.5%.
    [0071] V: 0.01% or more and less than 0.05%
    [0072] V is an element that forms a carbide or a carbon nitride and contributes to the strength strengthening of steel. In order to obtain the effects described above, the V content is necessarily 0.01% or more. On the other hand, when the V content is greater than 0.05%, the effect is saturated, and an effect corresponding to the content cannot be expected, which is economically disadvantageous. Therefore, the V content is limited to a range of 0.01% or more and less than 0.05%.
    [0073] Nb: 0.001% or more and less than 0.01%
    [0074] Nb forms a carbide or a carbon nitride, contributes to an increase in steel strength through precipitation strength strengthening, and also contributes to the refinement of austenite grains. In order to obtain the effects described above, the Nb content is necessarily 0.001% or more. On the other hand, an Nb precipitate is likely to function as an SSC propagation path (sulfide stress cracking), and the presence of a large amount of Nb precipitate based on the addition of a large amount of 0.01 % Nb or more leads to significant deterioration in SSC strength, particularly in high strength steel that has a yield point of 862 MPa (125 ksi) or more. Therefore, in the present invention, the Nb content is limited to a range of 0.001% or more and less than 0.01% from the standpoint of simultaneously realizing the desired high strength and higher SSC strength.
    [0075] B: 0.0003% to 0.0030%
    [0076] B is segregated at austenite grain boundaries and suppresses ferrite transformation at grain boundaries. As a result, even with a small amount of B addition, a steel hardenability enhancing effect can be obtained. In order to obtain the effects described above, the B content is necessarily 0.0003% or more. On the other hand, when the B content is greater than 0.0030%, B is precipitated as a carbon nitride or similar, which deteriorates hardenability and toughness. Therefore, the B content is limited to a range of 0.0003% to 0.0030%. Preferably, the B content is 0.0007% to 0.0025%.
    [0077] O (oxygen): 0.0030% or less
    [0078] O (oxygen) is an unavoidable impurity and is present in steel as an oxide-based inclusion. This inclusion causes SSC and deteriorates SSC resistance. Therefore, in the present invention, it is preferable that the O (oxygen) content is reduced as much as possible. However, excessive reduction of O (oxygen) causes an increase in the refinement cost and therefore an O content of 0.0030% or less is permissible. Therefore, the O (oxygen) content is limited to be 0.0030% or less. Preferably, the O (oxygen) content is 0.0020% or less.
    [0079] Ti: 0.003% to 0.025%
    [0080] Ti is precipitated as fine TiN when bonded to N during the solidification of molten steel and, due to its deposition effect, contributes to the refinement of austenite grains. In order to obtain the effects described above, the Ti content is necessarily 0.003% or more. When the Ti content is less than 0.003%, the effect is low. On the other hand, when the Ti content is greater than 0.025%, TiN is thickened, the depositing effect described above cannot be exhibited and the toughness deteriorates. Additionally, thick TiN causes a deterioration in SSC strength. Therefore, the Ti content is limited to a range of 0.003% to 0.025%.
    [0081] Ti/N: 2.0 to 5.0
    [0082] When Ti/N is less than 2.0, the fixation of N is insufficient, BN is formed, and the effect of B that improves hardenability decreases. On the other hand, when Ti/N is greater than 5.0, TiN is more likely to be thickened, and toughness and SSC strength deteriorate. Therefore, Ti/N is limited to a range of 2.0% to 5.0%. Preferably, Ti/N is 2.5% to 4.5%.
    [0083] The elements described above are basic elements. In addition to the basic composition, the high strength seamless steel tube according to the present invention may additionally contain one element or more elements of Cr: 0.6% or less, Cu: 1.0% or less, Ni: 1 0.0% or less, and W: 3.0% or less and/or Ca: 0.0005% to 0.0050% as optional elements.
    [0084] One Element or More Elements of Cr: 0.6% or less, Cu: 1.0% or less, Ni: 1.0% or less, and W: 3.0% or less.
    [0085] Cr, Cu, Ni and W are elements that contribute to an increase in the strength of steel, and an element or more elements selected from these elements can be optionally contained.
    [0086] Cr is an element that increases the strength of steel with the improvement of hardenability and improves corrosion resistance. Additionally, Cr is an element that is bonded to C to form a carbide, such as M3C, M7C3 or M23C6 (M represents a metallic element) during a temper treatment and improves softening resistance in tempering and is a required element. In order to obtain the effects described above, the Cr content is necessarily greater than 0.10% or more. On the other hand, when the Cr content is greater than 0.6%, a large amount of M7C3 or M23C6 is formed and functions as a trap site for hydrogen to deteriorate SSC strength. Therefore, in the case of containing Cr, the Cr content is limited to a range of 0.6% or less.
    [0087] Cu is an element that contributes to an increase in the strength of steel and has an effect of improving toughness and corrosion resistance. In particular, Cu is extremely effective in improving SSC resistance in a severe corrosive environment. When Cu is contained, corrosion resistance is enhanced by a dense corrosion product that is formed, and the formation and growth of a crack that causes cracking is suppressed. In order to obtain the effects described above, the Cu content is preferably 0.03% or more. On the other hand, when the Cu content is greater than 1.0%, the effect is saturated, and an effect corresponding to the content cannot be expected, which is economically disadvantageous. Therefore, when Cu is contained, it is preferable that the Cu content is limited to be 1.0% or less.
    [0088] Ni is an element that contributes to an increase in steel strength and improves toughness and corrosion resistance. In order to obtain the effects described above, the Ni content is preferably 0.03% or more. On the other hand, when the Ni content is greater than 1.0%, the effect is saturated, and an effect corresponding to the content cannot be expected, which is economically disadvantageous. Therefore, when Ni is contained, it is preferable that the Ni content be limited to be 1.0% or less.
    [0089] W is a carbide-forming element, contributes to an increase in steel strength through precipitation strength strengthening, and also contributes to SSC strength enhancement by being solubilized into solid and segregated into previous austenite grain boundaries. In order to obtain the effects described above, the W content is preferably 0.03% or more. On the other hand, when the W content is greater than 3.0%, the effect is saturated, and an effect corresponding to the content cannot be expected, which is economically disadvantageous. Therefore, when W is contained, it is preferable that the W content be limited to be 3.0% or less.
    [0090] Ca: 0.0005% to 0.0050%
    [0091] Ca is an element that is S-linked to form CaS and efficiently serves to control the form of sulfide-based inclusions, and contributes to SSC toughness and strength enhancement by controlling the form of sulfide-based inclusions . In order to obtain the effects described above, the Ca content is 0.0005% or more. On the other hand, when the Ca content is greater than 0.0050%, the effect is saturated, and an effect corresponding to the content cannot be expected, which is economically disadvantageous. Therefore, when Ca is contained, it is preferable that the Ca content is limited to a range of 0.0005% to 0.0050%.
    [0092] A different remainder of the components described above include Fe and unavoidable impurities. As unavoidable impurities, Mg: 0.0008% or less and Co: 0.05% or less are permissible.
    [0093] The high strength seamless steel tube according to the present invention contains the composition described above, wherein quenched martensite is a main phase and has a volume fraction of 95% or more, prior austenite grains have a number of grain size of 8.5 or more, and in a cross-section perpendicular to a rolling direction, the number of nitride-based inclusions that have a grain size of 4 µm or more is 100 or less per 100 mm2. the number of nitride-based inclusions that have a grain size of less than 4 µm is 1000 or less per 100 mm2, the number of oxide-based inclusions that have a grain size of 4 µm or more is 40 or less per 100 mm2, and the number of oxide-based inclusions that have a grain size less than 4 µm is 400 or less per 100 mm2.
    [0094] Tempered martensitic phase: 95% or more
    [0095] In the high strength seamless steel tube according to the present invention, to acquire a high strength of class 862 MPa (125 ksi) or more than YS safely and to maintain the ductility and toughness necessary for the steel pipe as a construction, a quenched martensitic phase formed by tempering the martensitic phase is established as a main phase. The "main phase" described in this document represents a case where this phase is a single phase that has a volume fraction of 100% or a case where this phase is contained in the microstructure at a volume fraction of 95% or more and a second phase is contained in the microstructure in a volume fraction of 5% or less in which the volume fraction does not affect microstructure characteristics. In the present invention, examples of the second phase include bainite, remaining austenite, perlite and a mixed phase thereof.
    [0096] In the high strength seamless steel tube according to the present invention, the composition described above can be adjusted by appropriately selecting a heating temperature during a quench treatment and a cooling rate during cooling according to the components of steel.
    [0097] Previous Austenite Grain Size Number: 8.5 or More
    [0098] When the grain size number of prior austenite grains is less than 8.5, an inferior martensite microstructure to be formed is thickened, the SSC strength deteriorates. Therefore, the grain size number of prior austenite grains is limited to be 8.5 or more. As the grain size number, a measured value according to JIS G 0551 is used.
    [0099] In the present invention, the grain size number of pre-austenite grains can be adjusted by changing a heating rate, a heating temperature and a holding temperature during a quench treatment and changing the number of times of the tempering treatment.
    [00100] Additionally, in the high strength seamless steel tube according to the present invention, in order to improve the SSC strength, the numbers of nitride-based inclusions and oxide-based inclusions are adjusted to be in appropriate ranges depending on sizes. Nitride-based inclusions and oxide-based inclusions are identified by automatic detection using a scanning electron microscope. Nitride-based inclusions contain Ti and Nb as major components, and oxide-based inclusions contain Al, Ca, Mg as major components. Inclusion numbers are values measured in a perpendicular cross section in a rolling direction of the steel tube (cross section perpendicular to a direction of tube axis: C cross section). Like the sizes of the inclusions, the grain sizes of the respective inclusions are used. In relation to the inclusion grain sizes, the inclusion grain areas are obtained, and their equivalent circular diameters are calculated to obtain the inclusion grain grain sizes.
    [00101] Number of Nitride-Based Inclusions That Have a Grain Size of 4 µm or More: 100 or Less per 100 mm2
    [00102] Nitride-based inclusions cause SSC in high strength steel pipe to have a yield point of 862 MPa (125 ksi) or more, and as their size increases to be 4 µm or more, a their adverse effect increases. Therefore, it is preferable that the number of nitride-based inclusions that have a grain size of 4 µm or more be decreased as much as possible. However, when the number of nitride-based inclusions that have a grain size of 4 µm or more is 100 or less per 100 mm2, an adverse effect on the SSC strength is permissible. Therefore, the number of nitride-based inclusions that have a grain size of 4 µm or more is limited to be 100 or less per 100 mm2. Preferably, the number of nitride-based inclusions that have a grain size of 4 µm or more is 84 or less.
    [00103] Number of Nitride-Based Inclusions That Have Grain Size Less Than 4 µm: 1000 or Less per 100 mm2
    [00104] The presence of a single fine nitride-based inclusion that has a grain size less than 4 µm does not cause SSC. However, in high strength steel tube that has a YS yield point of 862 MPa (125 ksi) or more, when the number of nitride-based inclusions that have a grain size of less than 4 µm is greater than 1000 per 100 mm2, an adverse effect of these on the SSC strength is not permissible. Therefore, the number of nitride-based inclusions that have a grain size less than 4 µm is limited to be 1000 or less per 100 mm2. Preferably, the number of nitride-based inclusions that have a grain size less than 4 µm is 900 or less.
    [00105] Number of Oxide-Based Inclusions That Have a Grain Size of 4 µm or More: 40 or Less per 100 mm2
    [00106] Oxide-based inclusions cause SSC in high strength steel pipe to have a yield point of 862 MPa (125 ksi) or more, and as their size increases to be 4 µm or more, a their adverse effect increases. Therefore, it is preferable that the number of oxide-based inclusions that have a grain size of 4 µm or more be decreased as much as possible. However, when the number of oxide-based inclusions that have a grain size of 4 µm or more is 40 or less per 100 mm2, an adverse effect of them on the SSC strength is permissible. Therefore, the number of oxide-based inclusions that have a grain size of 4 µm or more is limited to be 40 or less per 100 mm2. Preferably, the number of oxide-based inclusions that have a grain size of 4 µm or more is 35 or less.
    [00107] Number of Oxide-Based Inclusions That Have Grain Size Less Than 4 µm: 400 or Less per 100 mm2
    [00108] Even a small oxide-based inclusion that has a grain size less than 4 µm causes SSC in high strength steel pipe that has a yield point of 862 MPa (125 ksi) or greater, and bespoke as its number increases, an adverse effect of it on the SSC resistance increases. Therefore, it is preferable that the number of oxide-based inclusions that have a grain size less than 4 µm be decreased as much as possible. However, when the number of oxide-based inclusions that have a grain size less than 4 µm is 400 or less per 100 mm2, an adverse effect of them on the SSC strength is permissible. Therefore, the number of oxide-based inclusions that have a grain size less than 4 µm is limited to be 400 or less per 100 mm2. Preferably, the number of oxide-based inclusions that have a grain size less than 4 µm is 365 or less.
    [00109] In the present invention, in order to adjust the numbers of nitride-based inclusions and oxide-based inclusions, in particular, control in a molten steel refining process is important. Desulfurization and dephosphorization are carried out in a molten iron preparation treat- ment, decarbonization and dephosphorization are carried out in a steelmaking converter and then a heat-agitation-refinement (LF) and an RH vacuum degassing treatment are carried out in a melting pot. The treatment time of the heating-stirring-refinement (LF) treatment is sufficiently ensured. Additionally, the treatment time of the RH vacuum degassing treatment is ensured. Additionally, in order to prepare a molten plate (raw material of steel tube) using a continuous casting method, the molten steel is melted from the melting pot into a funnel ("tundish") so that the numbers of nitride-based inclusions and oxide-based inclusions per unit area are the values described above or less, and the molten steel is sealed using inert gas. Additionally, the molten steel is electromagnetically stirred in a mold to separate inclusions by flotation.
    [00110] Next, a preferable method for producing a high strength seamless steel tube according to the present invention will be described.
    [00111] In the present invention, the steel tube raw material having the composition described above is heated, and hot work is performed on the heated steel tube raw material to form a seamless steel tube having a predetermined format.
    [00112] It is preferable that the steel pipe raw material used in the present invention is prepared by preparing molten steel having the composition described above with a commonly used melting method using a steelmaking converter or the like and obtaining it. if a cast plate (round cast plate) using a commonly used casting method such as a continuous casting method. Furthermore, the cast slab can be hot rolled into a round steel slab which has a predetermined shape or can be subjected to ingot production and grinding to obtain a round steel slab.
    [00113] In the high strength seamless steel tube according to the present invention, in order to further improve the SSC strength, the numbers of nitride-based inclusions and oxide-based inclusions per unit area are reduced to be the values described above or less. Therefore, in steel pipe raw material (cast plate or steel plate), it is necessary to reduce the N content and O content as much as possible in order to satisfy the N (nitrogen) ranges: 0.006% or less and O (oxygen): 0.0030% or less.
    [00114] In order to adjust the numbers of nitride-based inclusions and oxide-based inclusions per unit area to be the values described above or less, control in the molten steel refining process is important. In the present invention, it is preferable to carry out desulfurization and dephosphorization in a molten iron preparation treatment, to carry out decarbonization and dephosphorization in a steelmaking converter, and then to carry out a heating-agitation-refinement (LF) treatment and treatment. RH vacuum degassing in a melting pot. As the LF time increases, the CaO concentration or the CaS concentration in the inclusions decreases, MgO-Al2O3 inclusions are formed, and SSC resistance is improved. Furthermore, as the RH time increases, the oxygen concentration in the molten steel decreases, the size of oxide-based inclusions decreases, and their number decreases. Therefore, it is preferable that the treatment time of the heat-agitation-refinement (LF) treatment is 30 minutes or longer, the treatment time of the RH vacuum degassing treatment is 20 minutes or longer.
    [00115] Additionally, in order to prepare a molten slab (raw material of steel pipe) using a continuous casting method, it is preferable that the molten steel is melted from the casting ladle in a funnel ("tundish"). so that the numbers of nitride-based inclusions and oxide-based inclusions per unit area are the values described above or less, and the molten steel is sealed using inert gas. Furthermore, it is preferable that the molten steel is stirred electromagnetically in a mold to separate inclusions by flotation. As a result, the amounts and sizes of nitride-based inclusions and oxygen-based inclusions can be adjusted.
    [00116] Then, the cast plate is heated to a heating temperature of 1050°C to 1350°C, and hot work is carried out on the cast plate (raw material for steel tube) which have the composition described above to form a seamless steel tube that has a predetermined dimension.
    [00117] Heating Temperature: 1050 °C to 1350 °C
    [00118] When the heating temperature is less than 1050 °C, the melting of carbides in the steel tube raw material is insufficient. On the other hand, when the molten plate is heated to more than 1350°C, crystal grains are thickened, precipitates such as TiN precipitated during solidification are thickened, and cementite is thickened. As a result, the toughness of the steel tube deteriorates. Additionally, the cast plate is heated to a high temperature greater than 1350 °C, a thick scale layer is formed on the surface of the steel tube raw material, which causes surface defects to be generated during lapping. mination. Furthermore, energy loss increases, which is not preferable from an energy saving point of view. Therefore, the heating temperature is limited to be in a range of 1050°C to 1350°C. Preferably, the heating temperature is in a range of 1100 °C to 1300 °C.
    [00119] Then, hot working (pipe production) is carried out on the heated steel tube raw material using a hot rolling mill from the Mannesmann plug mill process ("Mannesmann-plug mill process") or the Mannesmann-mandrel mill process to form a seamless steel tube having a predetermined dimension. Seamless steel tube can be obtained by hot extrusion using a pressing process.
    [00120] After completion of the hot work, a cooling treatment is performed on the seamless steel tube obtained in which the seamless steel tube is cooled to a cooling rate equal to or greater than that of air cooling to a its surface temperature reaches 200 °C or less.
    [00121] Cooling Treatment After Completion of Hot Work: Cooling Rate: Air Cooling Rate or More, Cooling Stop Temperature: 200°C or Less
    [00122] When the seamless steel tube in the composition range according to the present invention is cooled to a cooling rate equal to or greater than that of air cooling after hot work, a microstructure that contains martensite as a phase main can be obtained. When air cooling (cooling) is stopped at a surface temperature greater than 200 °C, the transformation may not be completely completed. Therefore, after hot work, the seamless steel tube is cooled at a cooling rate equal to or greater than that of air cooling until its surface temperature reaches 200 °C or less. Additionally, in the present invention, "a cooling rate equal to or greater than that of air cooling" represents 0.1°C/second or more. When the cooling rate is less than 0.1 °C/second, a metallographic microstructure after cooling is non-uniform, and a metallographic microstructure after a heat treatment subsequent to cooling is non-uniform.
    [00123] After the cooling treatment of cooling the seamless steel tube to a cooling rate equal to or greater than that of air cooling, a tempering treatment is performed. In the tempering treatment, the seamless steel tube is heated to a temperature in a range of 600 °C to 740 °C.
    [00124] Tempering Temperature: 600 °C to 740 °C
    [00125] Tempering treatment is performed in order to decrease displacement density to improve toughness and SSC strength. When the tempering temperature is less than 600 °C, a decrease in displacement is insufficient and therefore superior SSC resistance cannot be assured. On the other hand, when the tempering temperature is above 740 °C, the softening of the microstructure becomes severe, and the desired high strength cannot be ensured. Therefore, the tempering temperature is limited to a temperature in a range of 600 °C to 740 °C. Preferably, the tempering temperature is in a range of 670 °C to 710 °C.
    [00126] In order to stably ensure desired characteristics, after the hot work and cooling treatment to cool the seamless steel tube to a cooling rate equal to or greater than that of air cooling, an air-cooling treatment Tempering is carried out in which the seamless steel tube is reheated and quickly cooled by cooling water or the like. Then, the tempering treatment described above is carried out.
    [00127] Reheat Temperature During Quench Treatment: from Transformation Point Ac3 to 1000 °C
    [00128] When the reheat temperature is less than an Ac3 transformation point, the seamless steel tube is not heated to a single-phase region of austenite. Therefore, a microstructure that contains martensite as a main phase cannot be obtained. On the other hand, when the reheat temperature is above 1000 °C, there are several adverse effects. For example, crystal grains are thickened, toughness deteriorates, the thickness of the oxide inlay on the surface increases, and flaking is likely to occur, which leads to defects that are generated on the surface of the steel tube. Additionally, an excessive amount of load is applied to a heat treatment furnace, which leads to a problem from an energy saving point of view. Therefore, from an energy saving point of view, the reheat temperature during the annealing treatment is limited to a range from an Ac3 transformation point to 1000 °C. Preferably, the reheat temperature during the quench treatment is 950°C or less.
    [00129] Additionally, in tempering treatment, it is preferable that the cooling after reheating is performed by cooling water at an average cooling rate of 2 °C/second until the temperature at a central position of wall thickness reaches 400 ° C or less and then is carried out until the surface temperature reaches 200 °C or less and preferably 100 °C or less. Tempering treatment can be repeated twice or more.
    [00130] As the transformation point Ac3, a value calculated from the following equation can be used.
    [00131] Transformation point Ac3 (°C) = 937-476.5C+56Si-19.7Mn-16.3Cu-4.9Cr-26.6Ni+38.1Mo+124.8V+136.3Ti+198Al+ 3315B
    [00132] (where C, Si, Mn, Cu, Cr, Ni, Mo, V, Ti, Al, B: content (% by mass) of each element)
    [00133] In calculating the Ac3 transformation point, when an element shown in the equation described above is not contained, the element content is calculated as 0%.
    [00134] After the hardening treatment and the tempering treatment, optionally, a correction treatment to correct shape defects of the steel tube can be carried out in a warm or cold environment. MODALITY
    [00135] Hereinafter, the present invention will be described in more detail based on the Modality.
    [00136] In relation to molten iron extracted from a blast furnace, desulfurization and dephosphorization were carried out in a molten iron preparation treatment, decarbonization and dephosphorization were carried out in a steel production converter, a heating treatment - agitation-refinement (LF) was carried out under conditions of a treatment time of 60 minutes, as shown in Table 2, and an RH vacuum degassing treatment was carried out under conditions of a reflux amount of 120 tons/minute and one treatment time from 10 to 40 minutes. As a result, molten steel having a composition shown in Table 1 was obtained, and a cast plate (round cast plate: 190 mmΦ) was obtained using a continuous casting method. In the continuous casting method, an Ar gas shielding in a funnel ("tundish") was performed except for Steel No P and No S and electromagnetic stirring in a mold was performed except for Steel No N and No S.
    [00137] The cast plate obtained was loaded into a heating furnace as a steel tube raw material, was heated to a heating temperature shown in Table 2, and was held at that temperature (retention time: 2 hours). Hot work was carried out on the heated steel pipe feedstock using a Mannesmann plug mill process hot rolling mill to form a seamless steel pipe (outer diameter 100 mmΦ to 230 mmΦxwall thickness 12 mm to 30 mm). After the hot work, air cooling was carried out, and the quenching and tempering treatments were carried out under conditions shown in Table 2. In relation to a part of the seamless steel tubes, after the hot work, the cooling of water was performed, and then a tempering treatment or tempering and tempering treatments were performed.
    [00138] A sample was collected from each of the obtained seamless steel tubes, and the microstructure observation, a tensile test, and a sulfide stress cracking test were performed. The test methods were as follows.(1) MICROSTRUCTURE OBSERVATION
    [00139] A sample for microstructure observation was collected from a 1/4t position of the inner surface side (t: wall thickness) of each of the seamless steel tubes obtained. A cross section (cross section C) perpendicular to a longitudinal direction of the tube was polished and eroded (Nital corrosion (mixed solution of nitric acid-ethanol) to expose a microstructure. The exposed microstructure was observed and images were formed using a optical microscope (magnification: 1000 times) and a scanning electron microscope (magnification: 2000 times to 3000 times) in four or more fields of view. By analyzing the obtained microstructure images, the phases that constitute the microstructure were identified, and a ratio of phases in the microstructure was calculated.
    [00140] Furthermore, using the sample for microstructure observation, the grain sizes of prior austenite grains (y) were measured. The cross section (cross section C) of the sample for observation of microstructure perpendicular to the longitudinal direction of the tube was polished and eroded (with Picral solution (mixed picric acid-ethanol solution)) to expose the previous y grain boundaries. The exposed previous y grain boundaries were observed and images were formed using an optical microscope (magnification: 1000 times) in three or more fields of view. From the microstructure images obtained, the grain size number of previous y grains was obtained using a cutting method according to JIS G 0551.
    [00141] Furthermore, in relation to the sample for microstructure observation, the microstructure in a region which has a size of 400 mm2 was observed using a scanning electron microscope (magnification: 2000 times to 3000 times). Inclusions were automatically detected based on the light and shadow of the images. Simultaneously, quantitative analysis of the inclusions was automatically performed using an EDX (energy dispersion X-ray analysis) provided in the scanning electron microscope to measure the types, sizes and numbers of the inclusions. Inclusion types were determined based on quantitative analysis using the EDX. Inclusions have been classified into nitride-based inclusions which contain Ti and Nb as major components and oxide-based inclusions which contain Al, Ca and Mg as major components. The "main component" described in this document represents a case where the element content is 65% or more in total.
    [00142] Furthermore, the numbers of grains identified as inclusions were obtained. Additionally, the areas of the respective grains were obtained, and their equivalent circular diameters were calculated to obtain the grain sizes of the inclusions. The numerical densities (grains/100 mm2) of inclusions that have a grain size of 4 µm or more and inclusions that have a grain size less than 4 µm were calculated. Inclusions that have a long side length less than 2 µm have not been analyzed.(2) TENSION TEST
    [00143] JIS sample No. 10 for a tensile test (bar sample: parallel portion diameter: 12.5 mmΦ, parallel portion length: 60 mm, GL (Gage Length)): 50 mm) was collected from a 1/4t position of the inner surface side (t: wall thickness) of each of the seamless steel tubes obtained according to JIS Z 2241 so that one direction of pull was one direction of tube axis. Using this sample, the tensile test was performed to obtain tensile characteristics (YS yield strength (0.5% yield strength), TS tensile strength).(3) SULFIDE STRESS CRACKING TEST
    [00144] A sample for a tensile test (parallel portion diameter: 6.35 mmΦxparallel portion length: 25.4 mm) was collected by centering a 1/4t position of the inner surface side (t: wall thickness) of each of the seamless steel tubes obtained so that a direction of the tube axis was a direction of pull.
    [00145] Using the obtained sample for a tensile test, a sulfide stress crack test was performed according to a test method defined in NACE TMO177 Method A. The sulfide stress crack test was a load test constant in which the sample described above for a tensile test was dipped in a test solution (an acetic acid-sodium acetate solution (liquid temperature: 24 °C) saturated with hydrogen sulfide at 10 kPa, having a pH adjusted to 3.5 and containing 5.0 mass% sodium chloride solution) and was retained with an applied load of 85% yield point YS. The rating "O: good" (satisfactory) was given to cases where the sample did not break before 720 hours, and the rating "x : bad" (unsatisfactory) was given to other cases where the sample broke before 720 hours hours. When a target flow limit was not assured, the sulfide stress cracking test was not performed.
    [00146] The results obtained are shown in Table 3.







    [00147] In all the Examples seamless steel tubes according to the present invention, a high yield limit YS of 862 MPa or greater and superior SSC strength were obtained. On the other hand, in the seamless steel tubes of the Comparative Examples which were outside the ranges of the present invention, a desired high strength was not able to be ensured due to the low yield point YS, or deteriorated SSC strength.
    [00148] In Steel Pipe No. 9 in which the quench temperature was higher than the range of the present invention, previous austenite grains were thickened, and SSC strength deteriorated. Furthermore, in Steel Pipe No. 12 where the tempering temperature was greater than the range of the present invention, the strength decreased. Furthermore, in Steel Pipe No. 13 where the quenching treatment cooling stop temperature was greater than the range of the present invention, the desired microstructure containing martensite as a main phase was not able to be obtained, and the strength decreased. Furthermore, in the No. 16 Steel Pipe where the C content was less than the range of the present invention, the desired high strength was not able to be assured. Furthermore, in Steel Pipe No. 17 where the C content was greater than the range of the present invention, the strength increased, and the SSC strength deteriorated at the tempering temperature in the range of the present invention. Additionally, in No 18, No 19 and No 26 Steel Pipe where the Mo content, the Cr content and the V content were lower than the ranges of the present invention, and the SSC strength deteriorated. Additionally, in No. 20 Steel Pipe where the Nb content was greater than the ranges of the present invention, where the inclusion numbers were outside the ranges of the present invention, and the SSC strength deteriorated. Additionally, in No. 21 and No. 22 Steel Tubes where Ti/N were outside the ranges of the present invention, the inclusion numbers were outside the ranges of the present invention, and the SSC strength deteriorated. Additionally, in Steel Pipe No. 23 in which the RH treatment time of 10 minutes was less than the preferred ranges and the O (oxygen) content was greater than the range of the present invention, in Steel Pipe No. 24, the Ti content was higher than the range of the present invention, and in Steel Tube No 25, both the N content and the O (oxygen) content were higher than the range of the present invention, for these tubes, in that inclusion numbers were outside the ranges of the present invention, and SSC resistance deteriorated. Furthermore, in No 28 Steel Pipe where the components were within the ranges of the present invention but the inclusion numbers were outside the ranges of the present invention, the SSC strength deteriorated.
    权利要求:
    Claims (5)
    [0001]
    1. High strength seamless steel tube for tubular products in the oil industry, characterized by the fact that it has a yield point (YS) of 862 MPa or more, the steel tube comprises, in % by mass, C : 0.20% to 0.50%, Si: 0.05% to 0.40%, Mn: 0.3% to 0.9%, P: 0.015% or less, S: 0.005% or less, Al : 0.005% to 0.1%, N: 0.006% or less, Mo: more than 1.0% and 3.0% or less, V: 0.01% or more and less than 0.05%, Nb: 0.001% or more and less than 0.01%, B: 0.0003% to 0.0030%, O (oxygen): 0.0030% or less, Ti: 0.003% to 0.025%, and a remainder that includes Fe and unavoidable impurities, where Ti and N contents are adapted to satisfy Ti/N: 2.0 to 5.0, tempered martensite has a volume fraction of 95% or more, prior austenite grains have a grain size number of 8.5 or more, and in a cross section perpendicular to a rolling direction, the number of nitride-based inclusions that have a grain size of 4 µm or more is 100 or less per 100 mm2, the number of inclusions Nitride-based inclusions that have a grain size of less than 4 μm is 1000 or less per 100 mm2, the number of oxide-based inclusions that have a grain size of 4 μm or more is 40 or less per 100 mm2 , and the number of oxide-based inclusions that have a grain size less than 4 µm is 400 or less per 100 mm2.
    [0002]
    2. High strength seamless steel tube for tubular products of the oil industry, according to claim 1, characterized in that it further comprises an element or more elements selected from, in % by mass, Cr: 0.6 % or less, Cu: 1.0% or less, Ni: 1.0% or less, eW: 3.0% or less.
    [0003]
    3. High strength seamless steel tube for tubular products of the oil industry, according to claim 1 or 2, characterized in that it further comprises, in % by mass, Ca: 0.0005% to 0.0050% .
    [0004]
    4. Production method of high strength seamless steel tube for oil industry tubular products, seamless steel tube is high strength seamless steel tube for oil industry tubular products as defined in any of claims 1 to 3, and the method characterized in that it comprises: heating the steel tube raw material to a heating temperature within a range of 1050°C to 1350°C; performing hot work on the raw material of heated steel tube to form a seamless steel tube that has a predetermined shape; cool the seamless steel tube at a cooling rate equal to or greater than that of air cooling after hot work until a seamless steel tube surface temperature reaching 200 °C or less; and perform a temper treatment in which the seamless steel tube is heated to a temperature in a range of 600 °C to 740 °C.
    [0005]
    5. Method of producing a high strength seamless steel tube for tubular products of the oil industry, according to claim 4, characterized by carrying out a quenching treatment on the seamless steel tube at least once after cooling and prior to the tempering treatment in which the seamless steel tube is reheated to a temperature in the range of an Ac3 transformation point up to 1000 °C and is rapidly cooled until the surface temperature of the seamless steel tube reaches 200 ° C or less.
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    法律状态:
    2019-08-27| B06U| Preliminary requirement: requests with searches performed by other patent offices: procedure suspended [chapter 6.21 patent gazette]|
    2021-03-09| B09A| Decision: intention to grant [chapter 9.1 patent gazette]|
    2021-06-01| B16A| Patent or certificate of addition of invention granted|Free format text: PRAZO DE VALIDADE: 20 (VINTE) ANOS CONTADOS A PARTIR DE 10/09/2015, OBSERVADAS AS CONDICOES LEGAIS. |
    优先权:
    申请号 | 申请日 | 专利标题
    JP2014-260217|2014-12-24|
    JP2014260217|2014-12-24|
    PCT/JP2015/004621|WO2016103537A1|2014-12-24|2015-09-10|High-strength seamless steel pipe for oil wells, and production method for high-strength seamless steel pipe for oil wells|
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