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    • 专利摘要:
    The present invention relates to a high strength seamless steel tube for tubular products for the petroleum industry having excellent resistance to corrosion cracking under sulphide stress. The high strength seamless steel tube for tubular products for the petroleum industry has the composition containing, in % by mass, 0.20 to 0.50% of C, 0.05 to 0.40% of Si, 0 0.3 to 0.9% Mn, 0.015% or less P, 0.005% or less S, 0.005 to 0.1% Al, 0.008% or less N, 0.6 to 1.7% Cr 0.4 to 1.0% Mo, 0.01 to 0.30% V, 0.01 to 0.06% Nb, 0.0003 to 0.0030% B and 0.0030% or less than O (oxygen). The high strength seamless steel tube for tubular products for the petroleum industry has the microstructure in which a volume fraction of a quenched martensitic phase is 95% or more, and the earlier austenitic grains have a grain number of 8.5 or more, and a segregation degree index Ps which is defined by a formula Ps = 8.1 (XSi + XMn + XMo) + 1.2XP referring to XM which is a ratio of a segregated portion content to an average content is set to less than 65. (Here, XM: (segregated portion content (% by mass) of element M)(...).
    公开号:BR112017004534B1
    申请号:R112017004534-6
    申请日:2015-08-20
    公开日:2021-05-04
    发明作者:Masao Yuga；Yasuhide Ishiguro；Mitsuhiro Okatsu；Kazuki FUJIMURA
    申请人:Jfe Steel Corporation；
    IPC主号:
    专利说明:

    FIELD OF TECHNIQUE
    [001] The present invention relates to a seamless steel pipe, preferably used as tubular products for the oil industry, a line pipe or the like, and more particularly to a high strength seamless steel tube that exhibits excellent resistance to sulfide stress corrosion cracking (resistance to SSC) in a wet hydrogen sulfide environment (aggressive environment) and a production method of the same. BACKGROUND TECHNIQUE
    [002] Recently, from a point of view of obtaining a stable energy resource, the development of oil wells or natural gas fields having high depth in a severely corrosive environment is underway. To accomplish such development, tubular products for the oil industry and a line pipe for transport are strongly required for excellent resistance to SSC in an aggressive environment as well as a yield limit (YS) of 110 ksi or more.
    [003] To satisfy such demand, for example, in Patent Literature 1, a method of manufacturing steel for tubular products for the oil industry was proposed in which the low-alloy steel containing, in % by weight, 0.2 to 0.35% C, 0.2 to 0.7% Cr, 0.1 to 0.5% Mo, 0.1 to 0.3% V, and additionally containing C, Cr, Mo and V in an adjusted manner it is quenched to a transformation temperature of Ac3 or higher and thereafter it is tempered to a temperature of 650°C or higher and a transformation temperature of Ac1 or below. Using a technique described in Patent Literature 1, the steel composition for tubular products for the petroleum industry can be adjusted so that a total amount of precipitated carbide is 2 to 5% by weight, a carbide ratio. type MC between a total amount of carbide becomes 8 to 40% by weight, thereby producing a steel for tubular products for the petroleum industry having excellent resistance to cracking of corrosion under sulphide stress.
    [004] In Patent Literature 2, a method of manufacturing steel for tubular products for the petroleum industry was proposed having excellent toughness and resistance to corrosion cracking under sulphide stress in which the low alloy steel containing, % by mass , 0.15 to 0.3% C, 0.2 to 1.5% Cr, 0.1 to 1% Mo, 0.05 to 0.3% V and 0.003 to 0.1% Nb is processed by hot work being finished at 1000°C or higher after the low-alloy steel is heated to 1150°C or higher, is subsequently quenched from a temperature of 900°C or higher and then is quenched at 550°C or higher and a transformation temperature of Ac1 or lower, and additionally quenching and quenching treatment in which the low-alloy steel is reheated to a temperature of 850 to 1000°C, is quenched, and is tempered to 650°C or higher and a transformation temperature of Ac1 or lower is performed at least once. Using the technique described in Patent Literature 2, the steel composition for tubular products for the petroleum industry can be adjusted so that a total amount of precipitated carbide is 1.5 to 4% by mass, and a carbide ratio type MC out of a total amount of carbide is 5 to 45% by mass and a rate of type M23C6 carbide is 200/t (t: wall thickness (mm)) % by mass or below, thus making steel for tubular products for the oil industry having excellent toughness and excellent resistance to cracking from sulphide stress corrosion.
    [005] In Patent Literature 3, a steel material for tubular products for the oil industry was proposed containing, in % by mass, 0.15 to 0.30% of C, 0.05 to 1.0% of Si , 0.10 to 1.0% Mn, 0.1 to 1.5% Cr, 0.1 to 1.0% Mo, 0.003 to 0.08% Al, 0.008% or less N, 0.0005 to 0.010% B, 0.008% or less Ca+O, and additionally containing one type or two types or more of elements selected from a group consisting of 0.005 to 0.05% Ti, 0.05% or less than Nb, 0.05% or less than Zr, and 0.30% or less than V, where a maximum length of a continuous non-metal inclusion per cross-sectional observation is 80 µm or less, and the number of non-metal inclusions having a grain size of 20 µm or more by cross-sectional observation is 10 pieces/100 mm2 or less. With the use of such a steel material for tubular products for the oil industry, it is said that a low-alloy steel material for tubular products for the oil industry having high strength required for using tubular products for the oil industry and having excellent strength the SSC that corresponds to such resistance can be obtained.
    [006] In Patent Literature 4, a low alloy steel for tubular products for the petroleum industry was proposed having excellent resistance to corrosion cracking under sulphide stress containing, % by mass, 0.20 to 0.35% of C , 0.05 to 0.5% Si, 0.05 to 0.6% Mn, 0.025% or less P, 0.01% or less S, 0.005 to 0.100% Al, 0.8 to 3.0% Mo, 0.05 to 0.25% V, 0.0001 to 0.005% B, 0.01% or less N, and 0.01% or less O, where the ratio of 12V + 1 - Mo >0 is satisfied. In the technique described in Patent Literature 4, in addition to the composition mentioned above, low alloy steel for tubular products for the petroleum industry may additionally contain 0.6% or less of Cr as the ratio of Mo-(Cr+) Mn)>0 is satisfied, and the low-alloy steel for tubular products for the petroleum industry may additionally contain one type or more of elements selected from a group consisting of 0.1% or less of Nb, 0.1 % or less Ti, 0.1% or less Zr. Low alloy steel for tubular products for the petroleum industry may additionally contain 0.01% or less of Ca. QUOTE LIST Patent Literature
    [007] Patent Literature 1: Unexamined Japanese Patent Application Publication No. 2000 - 178682
    [008] Patent Literature 2: Unexamined Japanese Patent Application Publication No. 2000 - 297344
    [009] Patent Literature 3: Unexamined Japanese Patent Application Publication No. 2001 - 172739
    [0010] Patent Literature 4: Unexamined Japanese Patent Application Publication No. 2007 - 16291 SUMMARY OF THE INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION
    [0011] However, since factors influence the cracking resistance of corrosion under sulphide stress (resistance to SSC), several factors are considered. Consequently, the use only of the techniques described in Patent Literatures 1 to 4 is not considered sufficient since the technique to improve the SSC resistance of a high strength seamless steel tube having YS of 110 ksi class or higher at a level enough for oil well use is used under a severely corrosive environment. There is also a disadvantage that it is extremely difficult to adjust types and amounts of carbides described in Patent Literatures 1 and 2 and a shape and number of non-metal inclusions described in Patent Literature 3 within desired ranges in a stable manner.
    [0012] The present invention was made to overcome such disadvantages of the conventional technique, and it is an object of the present invention to provide a high strength seamless steel tube for tubular products for the oil industry having excellent resistance to corrosion cracking under sulfide stress (resistance to SSC) and a method of fabrication thereof.
    [0013] In this specification, "high strength" means a case where the steel has a YS yield point of class 110 ksi or more, ie a YS yield point of 758 MPa or more. In this specification, "excellent resistance to SSC" means a case where a constant load test is performed on a solution of 0.5% by mass acetic acid and 5.0% by mass sodium chloride where saturated with hydrogen sulfide (liquid temperature: 24°C) according to a test method stipulated in NACE Method A TM0177, and cracking does not occur even after durations of 720 hours with a constant stress that is 85% of a yield point of a material is applied. SOLUTION TO THE PROBLEM
    [0014] In view of the fact that it is necessary for a steel tube to acquire both the desired high strength and excellent SSC resistance to achieve the above-mentioned objective, the inventors of the present invention have extensively studied various factors influencing a strength and resistance to SSC of the steel tube. As a result, the inventors have found that it is important that a high strength seamless steel tube for tubular products for the petroleum industry strictly suppresses core segregation and microsegregation to obtain excellent resistance to SSC.
    [0015] The inventors of the present invention focused on the difference in influence exerted on SSC resistance when central segregation or microsegregation occurs with respect to the respective alloying elements, selected elements that exert a strong influence, and created a Ps value of segregation index that is defined by the following formula (1) having coefficients determined taking into account magnitudes of influences that the respective elements have sensitivity of the respective elements. Ps = 8.1 (Xsi + XMn + XMO) + 1.2XP ... (1)
    [0016] (Here, XM: (segregated portion content (% by mass))/(average content (% by mass)) of the element M). Along with the increase in the Ps value, the locally hardened regions are increased. These locally hardened regions accelerate crack propagation, thereby deteriorating SSC resistance. In view of the above, to increase resistance to SSC, it is important to suppress the generation of locally hardened regions. The inventors have found that when the Ps value is set to less than 65, the generation of locally hardened regions is suppressed and the resistance to SSC is considerably increased.
    [0017] Here, XM is (segregated portion content (% by mass))/(average content (% by mass)) of the element M. M indicates the respective elements Si, Mn, Mo and P.
    [0018] XM is a value obtained as follows.
    [0019] In a square region having sizes of 5 mm x 5 mm and having the center of it in a 1/4 t position (t: wall thickness) from an inner surface of a seamless steel tube, a Area analysis is performed in at least three fields of view with respect to an element M (Si, Mn, Mo, P) under a condition of 0.1 second by a point with a 20 μm pitch by a microanalyzer of electronic probe (EPMA) using a beam having a diameter of 20 µm. All acquired concentration values are arranged in descending order of concentration, and the grade corresponding to the cumulative frequency of occurrence of 0.0001 is obtained and the grade is adjusted as a segregated portion content of the element. To be more specific, the values measured in all fields of view are collected and are arranged in descending order of concentration, and measurement points x0.0001° of the value (when the value is not an integer, an integer value greater than this value and closest to the value) is adjusted as a segregated portion content. On the other hand, the content of each element is adjusted as an average element content based on the composition (representative value) of each seamless steel tube, and a ratio between the segregated portion concentration and the average concentration is obtained for each element, and the reason is set to XM. That is, XM = (segregated portion content of element M) / (average content of element M).
    [0020] The present invention was completed based on such finding as well as additional studies added to the finding. That is, the foundation of the present invention is as follows.
    [0021] (1) A high strength seamless steel tube for tubular products for the petroleum industry having the composition containing, in % by mass, 0.20 to 0.50% of C, 0.05 to 0. 40% Si, 0.3 to 0.9% Mn, 0.015% or less P, 0.005% or less S, 0.005 to 0.1% Al, 0.008% or less N, 0.6% to 1.7% Cr, 0.4 to 1.0% Mo, 0.01 to 0.30% V, 0.01 to 0.06% Nb, 0.0003 to 0.0030% B, 0.0030% or less of O (oxygen), and Fe and unavoidable impurities as balance, where the steel tube has the microstructure in which a volume fraction of a quenched martensitic phase is 95% or more, and grains Earlier austenitics have a grain size number of 8.5 or more, and a segregation degree index Ps which is defined by the following formula (1) using XM which is a ratio of a segregated portion content obtained by performing a area analysis of the respective elements by an electronic probe microanalyzer (EPMA) in a region having its center positioned at 1/4 t (t: thickness that of wall) from an inner surface of the steel tube and an average grade is set to less than 65, and a yield point YS is 758 MPa or more. Ps = 8.1 (Xsi + XMn + XMO) + 1.2Xp ... (1)
    [0022] (Here, XM: (segregated portion content (% by mass) of element M)/(average content (% by mass) of element M)
    [0023] (2) In the high strength seamless steel tube for tubular products for the oil industry having the constitution (1), the composition additionally contains, in % by mass, 0.005% to 0.030% Ti, so that Ti/N which is a ratio of the Ti content to the N content satisfies a value that is within a range of 2.0 to 5.0.
    [0024] (3) In the high strength seamless steel tube for an oil well having the constitution (1) or (2), the composition additionally contains one type or two types or more of elements selected from a group consisting of, in % by mass, 1.0% or less of Cu, 1.0% or less of Ni, and 2.0% or less of W.
    [0025] (4) In high strength seamless steel pipe for an oil well having any one of the constitutions (1) to (3), the composition additionally contains 0.0005 to 0.005% Ca.
    [0026] (5) A method of manufacturing a high strength seamless steel tube for tubular products for the oil industry which is a method of manufacturing a seamless steel tube having a predetermined shape by applying heating and hot working to a steel tube material, and which is a method of manufacturing high strength seamless steel tube for tubular products for the oil industry having any of the constitutions (1) to (4), wherein
    [0027] a heating temperature in heating is set to a temperature that is within a range of 1050 to 1350°C, cooling after hot work is carried out until a surface temperature becomes a temperature equal to or below 200 °C at a cooling rate equal to or greater than air cooling and, after cooling, a quench treatment in which the steel tube is reheated to a temperature that is within a transformation temperature range of Ac3 or above 1000°C or below and the steel tube is rapidly cooled to a temperature such that a surface temperature becomes 200°C or below is performed once or more, and after the quench treatment, the quench treatment which heats the steel pipe to a temperature that is within a range of 600 to 740°C is performed. ADVANTAGEOUS EFFECTS OF THE INVENTION
    [0028] According to the present invention, a high strength seamless steel tube for tubular products for the oil industry having a yield YS of 758 MPa or more and having excellent resistance to stress corrosion cracking. sulfide can be easily manufactured at a low cost and then the present invention can acquire the industrially remarkable advantageous effects. Additionally, in accordance with the present invention, by allowing the steel tube to contain adequate amounts of suitable alloying elements, it is possible to manufacture a high strength seamless steel tube having both the desired high strength and excellent SSC resistance required when used as a seamless steel tube for tubular products for the oil industry, WAY TO CARRY OUT THE INVENTION
    [0029] Firstly, the reasons for limiting the contents of respective constitutional elements of the high strength seamless steel tube according to the present invention are explained. Unless otherwise specified, the % by mass in the composition is simply indicated by "%" later in this document,
    [0030] C: 0.20 to 0.50%
    [0031] C contributes to the increase in strength of steel transforming into a state of solid solution in steel, increases the hardenability of steel and contributes to the formation of microstructure having a martensitic phase as a main phase at the time of sudden cooling. To allow the steel tube to achieve such an effect, the C content needs to be 0.20% or more. On the other hand, when the C content exceeds 0.50%, cracks occur at the moment of sudden cooling, thereby greatly deteriorating the manufacturing capacity. Accordingly, C is limited in a range of 0.20 to 0.50%, is preferably 0.20 to 0.35%, and is more preferably 0.22 to 0.32%,
    [0032] Si: 0.05 to 0.40%
    [0033] Si is an element that functions as a deoxidizing agent and has a function of increasing the strength of steel transforming it into a solid solution state in steel and suppressing the softening of steel at the time of quenching. To allow the steel tube to acquire such an effect, the Si content needs to be 0.05% or more. On the other hand, when the Si content is high and exceeds 0.40%, the generation of a ferrite phase which is a softening phase is accelerated, thereby preventing a desired high steel reinforcing effect, or accelerating the formation of coarse oxide-based inclusions, thereby deteriorating SSC resistance and toughness. Additionally, Si is an element that is segregated and locally hardens steel. Consequently, the high Si content gives rise to an adverse effect where a locally hardened region is formed so that the SSC resistance is deteriorated. Accordingly, in the present invention, Si is limited in a range of 0.05 to 0.40%, is preferably 0.05 to 0.30%, and is more preferably 0.20 to 0.30 %,
    [0034] Mn: 0.3 to 0.9%
    [0035] Like C, Mn is an element that increases the hardenability of steel and contributes to the increase in steel strength. To acquire such an effect, the Mn content needs to be 0.3% or more. On the other hand, Mn is an element that is segregated and locally hardens steel. Consequently, the high Mn content gives rise to an adverse effect where a locally hardened region is formed so that the SSC resistance is deteriorated. Accordingly, in the present invention, Mn is limited in a range of 0.3 to 0.9%, is preferably 0.4 to 0.8%, and is more preferably 0.5 to 0.8 %,
    [0036] P: 0.015% or less
    [0037] P is an element that not only induces the embrittlement of the grain boundary due to the segregation of the grain boundary but also locally hardens the steel due to its segregation. In the present invention, although it is preferable to reduce the P content as much as possible as an unavoidable impurity, the presence of P up to 0.015% is permissible. Consequently, P is limited to 0.015% or less, and is preferably 0.012% or less,
    [0038] S: 0.005% or less
    [0039] S is present as an unavoidable impurity, and most of S is present in steel as sulfide-based inclusions and deteriorates ductility, toughness and SSC resistance. Consequently, although it is preferable to reduce the S content as much as possible, the presence of S up to 0.005% is permissible. Consequently, S is limited to 0.005% or less, and is preferably 0.003% or less,
    [0040] Al: 0.005 to 0.1%
    [0041] Al functions as a deoxidizing agent and is added to deoxidize molten steel. Additionally, Al forms AlN by being bound to N, contributes to thinning the austenite grains at the time of heating and suppresses the deterioration of the hardenability enhancing effect of B preventing a solid solution B from being bound to N. To acquire such an effect, the Al content needs to be 0.005% or more. However, Al content exceeding 0.1% gives rise to an increase in oxide-based inclusions and reduces steel cleanliness, thereby inducing deterioration of ductility, toughness and SSC resistance. Accordingly, Al is limited in a range of 0.005 to 0.1%, is preferably 0.01 to 0.08%, and is more preferably 0.02 to 0.05%,
    [0042] N: 0.008% or less
    [0043] N is present in steel as an unavoidable impurity. N forms AlN because it is bonded to Al or forms TiN when Ti is contained and makes the crystal grains fine, thereby increasing toughness. However, when the N content exceeds 0.008%, the formed nitride becomes coarse in such a way that the SSC resistance and toughness are extremely deteriorated. Consequently, N is limited to 0.008% or less.
    [0044] Cr: 0.6 to 1.7%
    [0045] Cr is an element that increases the strength of steel by increasing a quenching property and increases corrosion resistance. Furthermore, Cr forms a carbide like M3C, M7C3, M23C6 (M: metal element) because it is bonded to C at the time of annealing treatment. Consequently, Cr is an element that increases the resistance to softening by quenching and, particularly, it is a necessary element to allow a steel tube to acquire greater strength. Particularly, an M3C type carbide exhibits a strong function to increase the softening resistance by quenching. To obtain such an effect, the Cr content has to be 0.6% or more. On the other hand, when the Cr content exceeds 1.7%, large amounts of M7C3 and M23C6 are formed, and these compounds act as a trap site for hydrogen, so the resistance to SSC is deteriorated. Accordingly, Cr is limited in a range of 0.6 to 1.7%, is preferably 0.8 to 1.5%, and is more preferably 0.8 to 1.3%.
    [0046] Mo: 0.4 to 1.0%
    [0047] Mo forms carbide and contributes to strengthening the steel by precipitation hardening. Mo effectively contributes to the determined acquisition of a desired high strength steel. Additionally, Mo transforms into a solid solution state in steel, is segregated into anterior austenite grain boundaries, and contributes to increased SSC resistance. Additionally, Mo has a function of making a corrosion product dense, thereby suppressing the generation and growth of pits that become crack initiation points. To obtain such effects, the Mo content must be 0.4% or more. On the other hand, when the Mo content exceeds 1.0%, acicular M2C precipitates or, in some cases, a Laves phase (Fe2Mo) is formed in such a way that the resistance to SSC is deteriorated. Accordingly, Mo is limited in a range of 0.4% to 1.0%, is preferably 0.6 to 1.0% and is more preferably 0.8 to 1.0%.
    [0048] V: 0.01 to 0.30%
    [0049] V is an element that forms carbide or carbonitride and contributes to steel reinforcement. To acquire such an effect, the V content needs to be 0.01% or more. On the other hand, even when the V content exceeds 0.30%, the effect is saturated in such a way that an additional effect corresponding to the further increase in V content cannot be expected and so is economically disadvantageous. Consequently, V is limited to 0.01 to 0.30%, and is preferably in a range of 0.03 to 0.25%.
    [0050] Nb: 0.01 to 0.06%
    [0051] Nb forms carbide or additionally forms carbonitride, contributes to steel reinforcement and also contributes to finer austenite grains. To acquire such an effect, the Nb content needs to be 0.001% or more. On the other hand, when the Nb content is high and exceeds 0.06%, coarse precipitates are formed, thereby preventing a high steel reinforcing effect and deterioration of SSC resistance. Consequently, Nb is limited in a range of 0.01 to 0.06% and Nb is preferably 0.02 to 0.05%.
    [0052] B: 0.0003 to 0.0030%
    [0053] B is segregated into austenite grain boundaries and has a function of increasing the hardenability of steel even when a trace amount of B is contained by suppressing the grain boundary ferrite transformation. To acquire such an effect, the B content needs to be 0.0003% or more. On the other hand, when the B content exceeds 0.0030%, B precipitates as carbonitride or the like, and a quench property is deteriorated such that toughness is deteriorated. Consequently, B is limited in a range of 0.0003 to 0.0030%, and is preferably in a range of 0.0005 to 0.0024%.
    [0054] O (oxygen): 0.0030% or less
    [0055] O (oxygen) is present as an unavoidable impurity and, in steel, is present in the form of oxide-based inclusions. These inclusions become SSC initiation points and deteriorate SSC resistance. Consequently, in the present invention, it is preferable to reduce the O (oxygen) content as much as possible. However, excessive oxygen reduction results in increased refining cost, so the presence of O up to 0.0030% is permissible. Consequently, O (oxygen) is limited to 0.0030% or less, and is preferably 0.0020% or less.
    [0056] The composition mentioned above is the basic composition. However, in addition to the basic composition, as selective components, 0.005 to 0.030% Ti and/or one type or two types or more of elements selected from a group consisting of 1.0% or less Cu, 1.0% or less Ni and 2.0% or less W and/or 0.0005 to 0.005% Ca may be contained.
    [0057] Ti: 0.005% to 0.030%
    [0058] Ti precipitates as fine TiN because it is bound to N at the time of coagulation of molten steel, and Ti contributes to making the austenite grains fine due to its pinning effect. To acquire such an effect, the Ti content needs to be 0.005% or more. When the Ti content is less than 0.005%, the effect is small. On the other hand, when the Ti content exceeds 0.030%, TiN becomes coarse and cannot exhibit the above mentioned depositing effect, so the toughness is deteriorated in reverse. Additionally, thick TiN deteriorates SSC resistance. Consequently, when Ti is contained, Ti is preferably limited in a range of 0.005 to 0.030%,
    [0059] Ti/N: 2.0 to 5.0
    [0060] When the steel tube contains Ti, Ti/N which is a ratio of the Ti content to the N content is adjusted to satisfy a value that is within a range of 2.0 to 5.0. When Ti/N is less than 2.0, the deposition of N becomes insufficient such that an effect of sharp cooling property enhancing by B is deteriorated. On the other hand, when Ti/N is large and exceeds 5.0, a tendency for TiN to become coarse appears considerably such that toughness and SSC resistance are deteriorated. Accordingly, Ti/N is preferably limited to a range of 2.0 to 5.0, and is more preferably 2.5 to 4.5.
    [0061] A type or two types or more of elements selected from a group consisting of 1.0% or less of Cu, 1.0% or less of Ni and 2.0% or less of W.
    [0062] All among Cu, Ni and W are elements that contribute to the increase in strength of steel and then one type or two types or more of elements from a group consisting of Cu, Ni, W may be contained when necessary.
    [0063] Cu is an element that contributes to the increase in the strength of steel and additionally has a function of increasing the toughness and corrosion resistance. In particular, Cu is an element that is extremely effective in increasing SSC resistance in a severely corrosive environment. When Cu is contained, dense corrosion products are formed so that corrosion resistance is increased and the generation and growth of pits that become crack initiation points is suppressed. To obtain such an effect, it is preferable to contain Cu of 0.03% or more. On the other hand, even when the Cu content exceeds 1.0%, the effect is saturated in such a way that an additional effect corresponding to the further increase in the Cu content cannot be expected and so is economically disadvantageous. Accordingly, when Cu is contained, Cu is preferably limited to 1.0% or less, and is more preferably 0.05 to 0.6%.
    [0064] Ni is an element that contributes to the increase in the strength of steel and additionally increases the toughness and corrosion resistance. To obtain such an effect, it is preferable to contain Ni of 0.03% or more. On the other hand, even when the Ni content exceeds 1.0%, the effect is saturated in such a way that an additional effect corresponding to the further increase in Ni content cannot be expected and so is economically disadvantageous. Accordingly, when Ni is contained, Ni is preferably limited to 1.0% or less, and is more preferably 0.05 to 0.6%.
    [0065] W is a carbide-forming element and contributes to the increase in strength of steel by precipitation hardening. W is also an element that becomes a solid solution state, is segregated into anterior austenite grain boundaries, and contributes to increased resistance to SSC. To obtain such an effect, it is preferable to contain W of 0.03% or more. On the other hand, even when the W content exceeds 2.0%, the effect is saturated in such a way that an additional effect corresponding to the further increase in W content cannot be expected and so is economically disadvantageous. Accordingly, when W is contained, W is preferably limited to 2.0% or less, and is more preferably 0.4 to 1.5%.
    [0066] Ca: 0.0005 to 0.005%
    [0067] Ca is an element that forms CaS because it is linked to S and works effectively for a sulfide-based inclusions configuration control. Ca contributes to increased toughness and SSC resistance through a sulfide-based inclusion setting control. To obtain such an effect, the Ca content needs to be at least 0.0005%. On the other hand, even when the Ca content exceeds 0.005%, the effect is saturated in such a way that an additional effect corresponding to the further increase in the Ca content cannot be expected and so is economically disadvantageous. Consequently, when Ca is contained, Ca is preferably limited to a range of 0.0005 to 0.005%.
    [0068] The balance except the above mentioned components is formed by Fe and unavoidable impurities. As unavoidable impurities, 0.0008% or less Mg and 0.05% or less Co are permissible.
    [0069] The high strength seamless steel tube according to the present invention has the above-mentioned composition and has the microstructure in which a quenched martensitic phase is a main phase and the grain size number of a previous austenite grain is 8.5 or more.
    [0070] Tempered martensitic phase: 95% or more
    [0071] In the high strength seamless steel pipe according to the present invention, to obtain a high strength of class 110 ksi or more YS with certainty and to maintain the ductility and toughness needed for the steel pipe as a construction , a quenched martensitic phase formed by quenching the martensitic phase is set as a main phase. The "main phase" described in this paragraph means that the phase is a single phase in which the composition contains 100% of the phase by a fraction of volume or the composition contains 95% or more of the phase and 5% or less of a second phase which does not influence the properties of the steel tube. In the present invention, a bainitic phase, a pearlite and retained austenitic phase or a mixed phase of these phases can be named as examples of the second phase.
    [0072] The microstructure mentioned above in the high strength seamless steel tube according to the present invention can be adjusted by properly selecting a heating temperature at the time of performing the quench cooling treatment and a cooling rate at the time of cooling corresponding to the steel component.
    [0073] Previous austenite grain size number: 8.5 or more
    [0074] When the grain size number of the previous austenite grain is less than 8.5, the generated martensitic phase substructure becomes coarse such that the resistance to SSC is deteriorated. Consequently, the grain size number of the anterior austenite grain is limited to 8.5 or more. Here, a measured value according to the stipulation of JIS G 0551 is used as the grain size number.
    [0075] In the present invention, the grain size number of the previous austenite grain can be adjusted by changing a heating rate, a heating temperature and a quench treatment retention time and the number of treatment times of sudden cooling.
    [0076] The high strength seamless steel tube of the present invention is a seamless steel tube in which a segregation degree index Ps which is defined by the following formula (1) using XM which is a ratio of a content of segregated portion obtained by performing an area analysis of the respective elements by an electronic probe microanalyzer (EPMA) in a region having its center positioned at 1/4 t (t: wall thickness) from an internal surface of the steel pipe and an average grade is set to less than 65. Ps = 8.1 (Xsi + XMn + XMO) + 1.2Xp... (1)
    [0077] (Here, XM: (segregated portion content (% by mass) of element M)/(average content (% by mass) of element M)
    [0078] The Ps mentioned above is a value obtained by selecting an element that greatly influences SSC resistance when segregation occurs, and is a value introduced to indicate a degree of deterioration of SSC resistance due to segregation. By increasing this value, a locally hardened region is increased and then the SSC resistance is deteriorated. When the Ps value is less than 65, the desired SSC resistance can be obtained. Consequently, in the present invention, the Ps value is limited to less than 65, and is preferably less than 60. The lower the Ps value, there is less bad influence caused by segregation and resistance to SSC shows a tendency to beneficence.
    [0079] Here, XM is a ratio between (segregated portion content) and (average content) in relation to the M element, that is, (segregated portion content)/(average content) in relation to the M element. is calculated as follows.
    [0080] In a region having sizes of 5 mm x 5 mm and having the center of it in a 1/4 t position (t: wall thickness) from an inner surface of a seamless steel tube, an analysis of area is performed in at least three fields of view with respect to an element M (Si, Mn, Mo, P in this mode) under a condition of 0.1 second by a point with a 20 µm pitch by a probe microanalyzer electronics (EPMA) using a beam having a diameter of 20 µm. Then, based on the result obtained from the area analysis, in relation to the M element, all the concentration values obtained in the measured region are arranged in descending order of concentration, the cumulative occurrence frequency distribution of the M element content is obtained, and the grade whose cumulative frequency of occurrence becomes 0.0001 is obtained, and the grade is set as a grade of segregated portion of the element M. On the other hand, the grade of each element (Si, Mn, Mo, P ) is adjusted as an average element content based on the composition (representative value) of each seamless steel tube.
    [0081] XM is a ratio between the segregation portion content mentioned above and the average content of the M element, that is, (segregation portion content)/(average content) of the M element.
    [0082] In the present invention, it is necessary to control Ps in a continuous casting step. To be more specific, Ps can be reduced by electromagnetic stirring in a mold and/or a shaft.
    [0083] In the following, a method of manufacturing a high strength seamless steel tube according to the present invention is explained.
    [0084] In the method of manufacturing a high strength seamless steel tube according to the present invention, the raw material of steel tube having the above-mentioned composition is subjected to heating and hot working and then is subjected to cooling so that a seamless steel tube having a predetermined shape is obtained. Then, the seamless steel tube is subjected to a quenching and quenching treatment.
    [0085] In the present invention, it is not particularly necessary to limit the method of manufacturing a steel tube raw material. However, it is desirable to manufacture a steel tube raw material such as a billet by making molten steel having the above mentioned composition by a melting furnace commonly used as a converter, an electric furnace or a vacuum melting furnace and forming it. if steel cast into a steel pipe raw material by a continuous casting method or similar.
    [0086] Firstly, a steel raw material having the above mentioned composition is heated to a heating temperature that is within a range of 1050 to 1350°C.
    [0087] Heating temperature: 1050 to 1350°C
    [0088] When the heating temperature is lower than 1050°C, a carbide in the steel pipe raw material is insufficiently dissolved. On the other hand, when the raw material of the steel tube is heated to a temperature exceeding 1350°C, the crystal grains become coarse and precipitates such as TiN precipitated at the time of coagulation become coarse and also cementite becomes coarse. it thickens and therefore the toughness of the steel tube is deteriorated. Furthermore, when the steel tube raw material is heated to a high temperature that exceeds 1350°C, a thick scale layer is generated on a surface of the steel tube raw material and the thick scale layer causes the generation of surface defects at the time of lamination. Consequently, also from an energy saving point of view, the heating temperature is limited in a range of 1050 to 1350°C.
    [0089] Then, hot work is applied to the steel tube raw material which is heated to the temperature mentioned above, and therefore a seamless steel tube having a predetermined size and a predetermined shape is formed.
    [0090] Any method of hot work using common seamless steel tube fabrication equipment is applicable to hot work in the present invention. As common seamless steel pipe fabrication equipment, equipment for fabrication of seamless steel pipe using a Mannesmann plug mill process ("Mannesmann-plug mill process") or a Mannesmann chuck fabrication process ( "Mannesmann-mandrel mill process") can be termed as an example. In addition, the press-type hot extrusion equipment can also be used to manufacture a seamless steel tube. Additionally, the hot working condition is not particularly limited as long as a seamless steel tube having a predetermined shape can be manufactured under such hot working condition. All commonly used hot working conditions can be used.
    [0091] Cooling after hot work: below a surface temperature of 200°C or below at an air cooling cooling rate or more.
    [0092] In the present invention, after the hot work mentioned above, the cooling process is applied to an acquired seamless steel tube until a surface temperature becomes a temperature of 200°C or below at a rate of cooling air cooling or more. In relation to the composition range of the present invention, as long as a cooling rate after hot work is air cooling or more, the seamless steel tube microstructure after cooling can be formed into a microstructure that has a martensitic phase as a main phase. In this case, the sudden cooling treatment carried out afterwards can be omitted. Consequently, to complete a martensitic transformation completely, it is necessary to cool the seamless steel tube below a surface temperature of 200°C or below the cooling rate mentioned above. When a cooling stop temperature exceeds a surface temperature of 200°C, there may be a case where a martensitic transformation is not completely completed. Consequently, by cooling the seamless steel tube after hot work, the seamless steel tube is cooled below a surface temperature of 200°C or below, at an air-cooling cooling rate or more. In the present invention, the "air cooling cooling rate or more" means 0.1°C/sec or more. When the cooling rate is less than 0.1°C/s, the metal structure after cooling becomes non-uniform, and the metal structure after subsequent heat treatment becomes non-uniform.
    [0093] In the present invention, as a next step, the quench treatment and the quench treatment are applied to the above-mentioned seamless steel tube to which the cooling after hot work is applied. There may be a case where the microstructure having a martensitic phase as a main phase cannot be obtained by the cooling mentioned above. Consequently, to stabilize the material quality, quench treatment and temper treatment are applied to the seamless steel tube.
    [0094] Reheat temperature for sudden cooling: transformation temperature from Ac3 to 1000°C
    [0095] In the quench treatment, the seamless steel tube is reheated to a temperature that is within a transformation temperature range of Ac3 or above and 1000°C or below, and then the blast chilling treatment is performed until a surface temperature becomes 200°C or below. When a reheat temperature for quenching is below a transformation temperature of Ac3, heating is not carried out to the extent that an austenitic single-phase region is formed and then the microstructure that has a martensitic phase as a main phase it cannot be obtained after sudden cooling. On the other hand, when a reheat temperature is a high temperature that exceeds 1000°C, the crystal grains become coarse and then the toughness of a steel tube is deteriorated. In addition, there may be a case where an oxide scale layer on a steel tube surface becomes thick and the oxide scale layer is peeled off, thereby causing faults on a steel tube surface. Furthermore, when the reheat temperature exceeds 1000°C, adverse effects such as the increase in a load of heat treatment furnaces are exerted and, at the same time, excessive energy is required for reheating, thus causing a problem from an energy saving point of view. Consequently, in the present invention, a reheat temperature for quenching is limited to a temperature that is within a transformation temperature range of Ac3 to 1000°C.
    [0096] Cooling after reheating for sudden cooling is performed by blast chilling. It is preferable that such cooling be performed by cooling with water so that a cooling rate is 2°C/sec or above on average at 700 to 400°C of core temperature obtained by the calculation, and a surface temperature is 200°C or below, preferably 100°C or below. The sudden cooling treatment can be carried out twice.
    [0097] A value obtained using the following formula is used as a transformation temperature of Ac3.
    [0098] Transformation temperature of Ac3 (°C) = 937 - 476.5C + 56Si - 19.7Mn - 16.3Cu - 4.9Cr - 26.6Ni + 38.1Mo + 124.8V + 136.3Ti + 198Al +3315B.
    [0099] (Here, C, Si, Mn, Cu, Cr, Ni, Mo, V, Ti, Al, B: The values calculated using the contents (% by mass) of respective elements are used). By calculating a transformation temperature of Ac3 using the formula mentioned above, in relation to the elements that are not contained in a steel tube between the elements described in the formula, the calculation is made by setting the element contents to "zero" .
    [00100] Quenching temperature from 600 to 740°C
    [00101] The quench treatment is carried out to increase the toughness and resistance to SSC by reducing the displacement density in the microstructure formed by treatment by sudden cooling (including cooling after hot work). In the present invention, in the quench treatment, a steel tube is heated to a temperature (temperature temperature) that is within a range of 600 to 740°C. It is preferable to carry out the air cooling treatment after such heating.
    [00102] When the quenching temperature is below 600°C, the displacement reduction is insufficient such that a steel tube cannot obtain excellent resistance to SSC. On the other hand, when the tempering temperature exceeds 740°C, the softening of the microstructure progresses considerably and therefore a steel tube cannot obtain a desired high strength.
    [00103] In the present invention, to correct a defective shape of a steel pipe as necessary, the shape correction treatment can be performed by hot work or cold work. EXAMPLE
    [00104] Later in this document, the present invention is further explained on the basis of an example.
    [00105] Cast steel having the composition shown in Table 1 was made by a converter, and was formed into slabs by a continuous casting method. Plates were used as raw materials for steel tube. Electromagnetic stirring was carried out in a mold or a shaft except for a Steel No. P steel. Electromagnetic stirring in a mold or a shaft was not carried out in relation to a Steel No. P steel. So these tube raw materials of steel were loaded into a heating furnace, and were heated to a heating temperature shown in Table 2 and were held at the heating temperature (retention time: 2 hours). Then, the heated steel tube raw materials were formed into tubes using a Mannesmann plug mill process ("Mannesmann-plug mill process"), thereby manufacturing seamless steel tubes having sizes described in Table 2 (diameter: 178.0 to 244.5 mm, wall thickness: 15 to 30 mm). After the hot work, cooling was carried out where the seamless steel tubes were air cooled to a surface temperature of 200°C or below shown below in Table 2.
    [00106] After the hot work, under conditions shown in Table 2, the quench treatment was additionally applied to the air-cooled seamless steel tubes. Alternatively, reheat, quench and quench treatments were additionally applied to the air-cooled seamless tubes. After the temper treatment, the seamless steel tubes were air cooled.
    [00107] The specimens were sampled from the obtained seamless steel tubes, and an observation of microstructure, a tensile test and a test in resistance to corrosion cracking under sulphide stress were performed on the specimens. The tests were carried out according to the following steps.
    [00108] (1) Microstructure observation
    [00109] Specimens for microstructure observation were sampled from seamless steel tubes obtained in such a way that a position that is 1/4 t (t: wall thickness) of an inner surface of the tube in an orthogonal cross section to a tube geometry axis direction (cross section C) is defined as an observation position. Specimens for microstructure observation were polished and eroded by nital (nitric acid-ethanol mixture), and the microstructures were observed and images were taken using an optical microscope (magnification: 1000 times) or a scanning electron microscope ( magnification: 2000 to 3000 times). The identification of the microstructure and the measurement of the microstructure fractions (% by volume) were performed by means of image analysis using obtained microstructure photographs.
    [00110] In addition, the specimens sampled for observation of the microstructure were polished, and were eroded by picral (picric acid-ethanol mixture) in order to expose the previous austenite contours. Microstructures were observed and images were taken in three or more fields of view or more using an optical microscope (magnification: 1000 times), and grain numbers were obtained using a cutting method according to JIS G 0551.
    [00111] In relation to specimens sampled for observation of microstructure, in a region having sizes of 5 mm x 5 mm and having the center of the same in a 1/4 t position (t: wall thickness) from an inner surface of a seamless steel tube, an area analysis was performed in at least three fields of view with respect to the respective elements Si, Mn, Mo, P under a condition of 0.1 second for a point with a 20 μm pitch by an electronic probe microanalyzer (EPMA) (beam diameter: 20 μm). Then, based on the result obtained from the area analysis, cumulative occurrence frequency distributions of the contents of the respective elements in the measured region were obtained with respect to the respective elements.
    [00112] Based on the acquired cumulative frequency of occurrence distributions, the content corresponding to the cumulative frequency of occurrence of 0.0001 was determined for each element and the content was defined as a segregated portion content of each element (( hereinafter also referred to as (segregated portion content)M). A compositional analysis result (representative value) of each seamless steel tube was referred to as an average content of each element of each seamless steel tube ( (later in this document also referred to as (average grade)M).
    [00113] In relation to the respective seamless steel tubes obtained, an XM ratio between a segregated portion content obtained from each element and an average content of each element (XM = (se-gregated portion content) M/(average content )M) was calculated and a Ps value of each seamless steel tube was calculated using the following formula (1). Ps = 8.1 (Xsi + XMn + XMO) + 1.2Xp ... (1)
    [00114] (2) Traction test
    [00115] JIS specimen No. 10 for a tensile test (bar specimen: parallel portion diameter: 12.5 mmΦ, parallel portion length: 60 mm, GL: 50 mm) was sampled from a side position of internal surface 1/4t (t: wall thickness) of each of the seamless steel tubes obtained according to JIS Z 2241 so that a direction of pull was a direction of geometric tube axis. Using this specimen, the tensile test was performed to obtain tensile characteristics (yield yield limit YS (test stress 0.5%), tensile strength TS).
    [00116] (3) Sulfide stress corrosion crack test
    [00117] Stick-shaped specimens (parallel portion diameter: 6.35 mmΦ, parallel portion length: 25.4 mm) were sampled from seamless steel tubes obtained from a region having the center of the same positioned at 1/4 t (t: wall thickness) from an inner surface of each steel tube so that the direction of the geometric axis of the tube coincides with the longitudinal direction of the specimen, and the crack test of Sulfide stress corrosion cracking was performed according to NACE Method A TM0177.
    [00118] As a test liquid, an aqueous solution of 0.5% by mass of acetic acid and 5.0% by mass of sodium chloride in which the hydrogen sulfide is saturated (liquid temperature: 24°C) it was used. In the test, the rod-shaped specimen was immersed in the test liquid and a constant load test in which a constant load (tension corresponding to 85% yield point) is applied to the specimen for 720 hours.
    [00119] The rating "O : good" (satisfactory) was given in cases where the specimen was not ruptured within 720 hours, and the rating "x : bad" (unsatisfactory) was given in other cases where the specimen was broken before 720 hours. The sulphide stress corrosion crack test was not performed on steel tubes that could not achieve a target yield limit (758 MPa) in the tensile test.
    [00120] The result obtained is shown in Table 3.




    [00121] In all examples of the present invention, a high strength of a yield point YS of 758 MPa or more was maintained and fractures did not occur even when a stress that is 85% of a yield point was applied to the specimen for 720 hours in 0.5% by mass of acetic acid in which hydrogen sulfide is saturated and 5.0% by mass of sodium chloride solution (liquid temperature: 24°C). Consequently, all examples of the present invention provide each high strength seamless steel tube having excellent resistance to cracking from sulphide stress corrosion. On the other hand, in comparative examples whose range is outside the range of the examples of the present invention, the desired high strength cannot be guaranteed or the SSC strength is deteriorated.
    [00122] In relation to the No. 7 Steel pipe, the sudden cooling temperature is a high temperature that exceeds 1000°C, such that the previous austenitic grains become coarse and, with it, the resistance to SSC is deteriorated. With respect to No. 10 Steel pipe, the temper temperature exceeds the upper limit in the range of the present invention, such that No. 10 Steel pipe cannot guarantee a desired high strength. With respect to No. 11 Steel pipe, the cooling cut-off temperature for sudden cooling exceeds the upper limit in the range of the present invention, such that No. 11 Steel pipe cannot acquire a desired microstructure in which a martensitic phase forms a main phase, so the No. 11 Steel tube cannot guarantee a desired high strength. With respect to No. 14 Steel pipe, the C content is lower than the lower limit in the range of the present invention, such that No. 14 Steel pipe cannot guarantee a desired high strength. With respect to No. 15 Steel pipe, the C content exceeds the upper limit in the range of the present invention and the Ps value of No. 15 Steel pipe also becomes 65 or more, such that the resistance to SSC is deteriorated. With respect to No. 16 Steel pipe, the Mo content is lower than the lower limit in the range of the present invention and the Ps value of No. 16 Steel pipe also becomes 65 or more, so that the resistance to SSC is deteriorated. With respect to No. 17 Steel pipe, the Cr content is lower than the lower limit in the range of the present invention, and the Ps value of No. 17 Steel pipe also becomes 65 or more, such that resistance to SSC is deteriorated. With respect to No. 18 Steel pipe, Ti/N exceeds the upper limit in the range of the present invention and the Ps value of No. 18 Steel pipe also becomes 65 or more, such that the resistance to SSC is deteriorated. With respect to No. 19 Steel pipe, Ti/N is lower than the lower limit in the range of the present invention and the Ps value of No. 19 Steel pipe also becomes 65 or more, so that the resistance to SSC is deteriorated. With respect to No. 20 Steel pipe, an amount of oxygen exceeds the upper limit in the range of the present invention, and the Ps value of No. 20 Steel pipe also becomes 65 or more, such that the resistance. the SSC is deteriorated. With respect to No. 23 Steel pipe, although the composition is within the range of the present invention, electromagnetic stirring is not carried out in a continuous casting process such that the Ps value of No. 23 Steel pipe becomes 65 or more, and with it, the resistance to SSC is deteriorated.
    权利要求:
    Claims (2)
    [0001]
    1. High strength seamless steel tube for tubular products for the oil industry, characterized by the fact that it has a composition containing, in % by mass, 0.20 to 0.50% of C, 0.05 to 0.40% Si, 0.3 to 0.9% Mn, 0.015% or less P, 0.005% or less S, 0.005 to 0.1% Al, 0.008% or less N, 0 0.6 to 1.7% Cr, 0.4 to 1.0% Mo, 0.01 to 0.30% V, 0.01% to 0.06% Nb, 0.0003 to 0. 0030% B, 0.0030% or less O (oxygen), optionally 0.005% to 0.030% Ti so that Ti/N which is a ratio of Ti content to N content is set to a value that is within a range of 2.5 to 4.5; optionally one type or two types or more of elements selected from a group consisting of, in % by mass, 1.1% or less of Cu, 1.2% or less of Ni, and 2.0% or less of W, optionally 0.0005 to 0.005% Ca in % by mass, and Fe and unavoidable impurities as a balance, in which the steel tube has the microstructure in which a volume fraction of a quenched martensitic phase is 95% or more, and Earlier austentitic grains have a grain size number of 8.5 or more, and a segregation degree index Ps which is defined by the following formula (1) using XM which is a ratio of a segregated portion content obtained by performing an area analysis of respective elements is performed by an electron probe microanalyzer (EPMA) in a region having its center positioned at 1/4 t (t: wall thickness) from an inner surface of the steel tube and an average grade is set to less than 65, and a YS runoff limit is 758 MPa or more. Note Ps = 8.1 (Xsi + XMn + XMO) + 1.2XP ... (1) (Here, XM: (segregated portion content (% by mass) of element M)/(average content (% by mass) ) of element M).
    [0002]
    2. Method of manufacturing a high strength seamless steel tube for tubular products for the oil industry, characterized in that it has a predetermined shape by applying heating and hot working to a steel tube material as defined in claim 1, wherein a heating temperature on heating is set to a temperature that is within a range of 1050 to 1350 °C, cooling after the hot work mentioned above is carried out until a surface temperature becomes an equal temperature or below 200oC at a cooling rate equal to or greater than air-cooling and, after cooling, a rough air-cooling treatment in which the steel tube is reheated to a temperature that is within a range of transformation temperature of Ac3 or above 1000oC or below and the steel tube is rapidly cooled to a cooling rate of 2°C/s or above a temperature so that a A surface temperature becomes 200oC or below is carried out once or more, and after the quench treatment, the quench treatment which heats the steel pipe to a temperature that is within a range of 600 to 740°C is carried out and in which the steel tube material has been prepared using the continuous casting process in which the molten steel is subjected to electromagnetic stirring in a mold and/or a shaft.
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    同族专利:
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    CN106687613A|2017-05-17|
    JP5971435B1|2016-08-17|
    WO2016038809A1|2016-03-17|
    AR101760A1|2017-01-11|
    US10472690B2|2019-11-12|
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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]|
    2021-02-17| B09A| Decision: intention to grant [chapter 9.1 patent gazette]|
    2021-05-04| B16A| Patent or certificate of addition of invention granted|Free format text: PRAZO DE VALIDADE: 20 (VINTE) ANOS CONTADOS A PARTIR DE 20/08/2015, OBSERVADAS AS CONDICOES LEGAIS. |
    优先权:
    申请号 | 申请日 | 专利标题
    JP2014-182043|2014-09-08|
    JP2014182043|2014-09-08|
    PCT/JP2015/004180|WO2016038809A1|2014-09-08|2015-08-20|High strength seamless steel pipe for use in oil wells and manufacturing method thereof|
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