WO2018074109A1 - 油井用高強度継目無鋼管およびその製造方法 - Google Patents
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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/10—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies
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- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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- C21D2211/00—Microstructure comprising significant phases
- C21D2211/004—Dispersions; Precipitations
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- C21D2211/00—Microstructure comprising significant phases
- C21D2211/008—Martensite
Definitions
- the present invention relates to a high-strength seamless steel pipe suitable for oil country pipes (oil country pipes) and line pipes, and is particularly resistant to sulfide stress corrosion cracking (resistant to wet hydrogen sulfide environments (sour environments)). It is related to the improvement of SSC resistance and SSCC resistance (sulfide stress corrosion cracking resistance).
- Patent Document 1 In response to such a demand, for example, in Patent Document 1, C: 0.2 to 0.35%, Cr: 0.2 to 0.7%, Mo: 0.1 to 0.5%, V: 0.1 to 0.3%, and C, There has been proposed a method for producing oil-well steel in which a low-alloy alloy steel adjusted with Cr, Mo, and V is quenched at the Ac 3 transformation point or higher and then tempered at 650 ° C. or higher and the Ac 1 transformation point or lower. According to the technique described in Patent Document 1, the total amount of precipitated carbide can be adjusted to 2 to 5% by weight, and the proportion of MC type carbide in the total amount of carbide can be adjusted to 8 to 40% by weight. It is said that oil well steel having excellent resistance to sulfide stress corrosion cracking can be obtained.
- Patent Document 2 discloses a low alloy containing, 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%.
- the hot working is finished at 1000 ° C or higher and subsequently quenched from 900 ° C or higher, then tempered at 550 ° C or higher and below the Ac 1 transformation point, and further 850-1000 ° C.
- the total amount of precipitated carbide is 1.5 to 4% by mass, and the proportion of MC type carbide in the total amount of carbide is 5 to 45% by mass.
- M 23 C 6 type carbide This ratio can be adjusted to 200 / t (t: wall thickness (mm)) mass% or less, and it is said that the oil well steel is excellent in toughness and resistance to sulfide stress corrosion cracking.
- Patent Document 3 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 to 0.08%, N: 0.008% or less, B: 0.0005 to 0.010%, Ca + O: 0.008% or less, Ti: 0.005 to 0.05%, Nb: 0.05% or less , Zr: 0.05% or less, V: One or more of 0.30% or less, the maximum length of continuous non-metallic inclusions by cross-sectional microscope observation is 80 ⁇ m or less, non-metallic inclusions by cross-sectional microscope observation There have been proposed oil well steels in which the number of particles having a particle diameter of 20 ⁇ m or more is 10/100 mm 2 or less. Thereby, it is said that a low alloy steel material for oil wells having high strength required for oil wells and excellent SSC resistance commensur
- Patent Document 4 in 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, O: 0.01% or less and sulfide stress corrosion resistance satisfying 12V + 1-Mo ⁇ 0 Low alloy oil well pipe steels with excellent cracking properties have been proposed.
- Cr 0.6% or less may be contained so as to satisfy Mo ⁇ (Cr + Mn) ⁇ 0, and Nb: 0.1% or less, Ti : 0.1% or less, Zr: One or more of 0.1% or less may be contained, and Ca: 0.01% or less may be contained.
- Patent Document 5 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, Cr: 0.6% to 1.7%, Mo: 1.0% to 3.0%, V: 0.02 to 0.3%, Nb: 0.001 to 0.02%, B: 0.0003 to 0.0030%, O (oxygen) ): 0.0030% or less, Ti: 0.003 to 0.025%, Ti and N are adjusted so as to satisfy Ti / N: 2.0 to 5.0, and the balance is composed of Fe and inevitable impurities.
- the tempered martensite has a volume ratio of 95% or more, the prior austenite grains have a grain size number of 8.5 or more, and a nitride inclusion with a grain size of 4 ⁇ m or more is 100 mm 2 in a cross section perpendicular to the rolling direction.
- 100 per below the particle size is nitride inclusions of less than 4 ⁇ m is 100 mm 2 per 1000 or less, particle size oxide inclusions of more than 4 ⁇ m is 100 mm 2 per 40 or less, oxidation particle size of less than 4 ⁇ m
- Material inclusions A high-strength seamless steel pipe for oil wells with a yield stress of YS: 862 MPa or more having a structure of 400 or less per 100 mm 2 has been proposed.
- Japanese Unexamined Patent Publication No. 2000-178682 JP 2000-297344 A Japanese Patent Laid-Open No. 2001-172739 Japanese Unexamined Patent Publication No. 2007-16291 Patent No. 5930140 (International Publication No. 2016/079908)
- An object of the present invention is to solve the problems of the prior art and to provide a high-strength seamless steel pipe for oil wells excellent in sulfide stress corrosion cracking resistance and a method for producing the same.
- “high strength” as used herein refers to the case where the yield stress YS is 125 ksi (862 MPa) or more.
- “excellent in resistance to sulfide stress corrosion cracking” as used herein means 5.0 mass by which 10 kPa of hydrogen sulfide is saturated and the pH is adjusted to 3.5 in accordance with the test method specified in NACE TM0177 Method A. A constant load test was performed in an acetic acid-sodium acetate aqueous solution (liquid temperature: 24 ° C) containing a 1% sodium chloride aqueous solution, and cracking did not occur for more than 720h with a stress of 90% of the yield stress of the material under test. It shall be a case.
- Nitride inclusions with a particle size of 4 ⁇ m or more and oxide inclusions with a particle size of 4 ⁇ m or more are the starting points of sulfide stress corrosion cracking (SSC). The larger the size, the more likely SSC is generated. I found out.
- Nitride inclusions having a particle size of less than 4 ⁇ m do not become the starting point of SSC even if they are present alone, but if they become a large number, they will adversely affect SSC resistance, and less than 4 ⁇ m It has been found that a large number of oxide inclusions adversely affects SSC resistance.
- the inventors reduced the number of nitride inclusions and oxide inclusions to an appropriate number or less depending on the size. I came up with the need to adjust.
- the amount of N and O is increased during the manufacture of steel pipe materials, especially during the melting and casting of molten steel. It is important to control so that the value falls within the desired range. Furthermore, it is important to manage manufacturing conditions in the steel refining process and the continuous casting process.
- the steel pipe described in Patent Document 5 is a Ti-containing steel, and a large amount of Ti nitride is generated. Therefore, there is a limit to the suppression of nitride generation that is a factor affecting SSC resistance.
- the present inventors have found that there is a case where further improvement of the SSC resistance may be inhibited. Ti nitrides and carbides can cause not only SSC resistance but also a reduction in toughness when coarsened. Furthermore, it has also been found that the effect of grain refinement by the pinning effect of TiN described in Patent Document 5 is small under the heat treatment conditions. In contrast, as a result of intensive studies, the present inventors have found that the desired characteristics can be obtained by setting the Ti content to less than 0.003% in the case where the recent stricter acceptance criteria for SSC resistance are required. Derived.
- the gist of the present invention is as follows. (1) 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: 0.03 to 0.1%, N: 0.006 % Or less, Cr: 0.6% to 1.7%, Mo: 1.0% to 3.0%, V: 0.02 to 0.3%, Nb: 0.001 to 0.02%, B: 0.0005 to 0.0040%, O (oxygen): 0.0030% or less , Ti: containing less than 0.003%, having a composition consisting of the balance Fe and inevitable impurities, tempered martensite at a volume ratio of 90% or more, and having a grain size of 4 ⁇ m or more in a cross section perpendicular to the rolling direction nitride inclusions 100 mm 2 per 50 or less, the particle size is nitride inclusions of less than 4 ⁇
- a method for producing a seamless steel pipe for an oil well in which a steel pipe material is heated and subjected to hot working to obtain a seamless steel pipe having a predetermined shape, and the oil well according to any one of (1) to (4)
- a high-strength seamless steel pipe manufacturing method wherein the heating temperature is in the range of 1050 to 1350 ° C., and after the hot working, the seamless steel pipe has a surface temperature of 200 or more at a cooling rate equal to or higher than air cooling.
- a method for producing a high-strength seamless steel pipe for oil wells that is tempered by cooling to a temperature below °C and heating to a temperature in the range of 640-740 °C.
- the steel is reheated to a temperature in the range of Ac 3 transformation point to 1000 ° C. and rapidly cooled to a temperature of 200 ° C. or less at the surface temperature.
- the manufacturing method of the high strength seamless steel pipe for oil wells which performs the quenching process to perform once or more.
- a high-strength seamless steel pipe for oil wells having a high strength of yield stress YS: 125 ksi (862 MPa) or more and excellent resistance to sulfide stress corrosion cracking.
- YS yield stress YS: 125 ksi (862 MPa)
- desired high strength for oil wells and excellent SSC resistance can be obtained.
- the high-strength seamless steel pipe to hold can be manufactured stably.
- the high-strength seamless steel pipe for oil wells of the present invention (hereinafter also simply referred to as high-strength seamless steel pipe) is in 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: 0.03-0.1%, N: 0.006% or less, Cr: 0.6% to 1.7%, Mo: 1.0% to 3.0%, V: 0.02 to 0.3%, Nb: 0.001 to 0.02%, B: 0.0005 to 0.0040%, O (oxygen): 0.0030% or less, Ti: Less than 0.003%, with the balance Fe and inevitable impurities, tempered martensite In a cross section perpendicular to the rolling direction, the volume ratio is 90% or more, and 50 or less nitride inclusions with a particle size of 4 ⁇ m or more per 100 mm 2 and nitride inclusions with a particle size of less than 4 ⁇
- C 0.20 to 0.50% C dissolves and contributes to increasing the strength of the steel, improves the hardenability of the steel, and contributes to the formation of a structure whose main phase is the martensite phase during quenching. In order to acquire such an effect, C needs to contain 0.20% or more. On the other hand, if the content of C exceeds 0.50%, cracking occurs during quenching, and the productivity is significantly reduced. Therefore, the C content is in the range of 0.20 to 0.50%. Preferably, the C content is 0.20 to 0.35%. More preferably, the C content is 0.22 to 0.32%.
- Si 0.05 to 0.40%
- Si is an element that acts as a deoxidizer, has a function of increasing the strength of the steel by solid solution in the steel, and further suppressing softening during tempering. In order to acquire such an effect, it is necessary to contain Si 0.05% or more.
- the content of Si exceeding 0.40% promotes the formation of a ferrite phase which is a softening phase, inhibits the desired increase in strength, and further promotes the formation of coarse oxide inclusions. Reduces SSC resistance and toughness.
- Si is an element that segregates and locally hardens the steel, and the content of Si exceeding 0.40% has an adverse effect of forming a locally hardened region and lowering the SSC resistance. Therefore, in the present invention, the Si content is in the range of 0.05 to 0.40%.
- the Si content is 0.05 to 0.30%. More preferably, the Si content is 0.20 to 0.30%.
- Mn 0.3-0.9%
- Mn is an element that improves the hardenability of steel and contributes to an increase in steel strength. In order to acquire such an effect, Mn needs to contain 0.3% or more.
- Mn is an element that segregates and locally hardens the steel, and the content of Mn exceeding 0.9% forms a locally hardened region and has an adverse effect of lowering the SSC resistance. Therefore, in the present invention, the Mn content is in the range of 0.3 to 0.9%. Preferably, the Mn content is 0.4 to 0.8%. More preferably, the Mn content is 0.5 to 0.8%.
- P 0.015% or less
- P is an element that not only segregates at grain boundaries to cause grain boundary embrittlement but also segregates and locally hardens steel.
- P is an inevitable impurity as much as possible.
- the P content is acceptable up to 0.015%. Therefore, the P content is 0.015% or less.
- the P content is 0.012% or less.
- S 0.005% or less
- S is an unavoidable impurity, most of which is present as sulfide inclusions in steel, and lowers ductility, toughness and SSC resistance. Up to 0.005% is acceptable. For this reason, S content shall be 0.005% or less. Preferably, the S content is 0.003% or less. Furthermore, it is preferably 0.0015% or less.
- Al acts as a deoxidizer and combines with N to form AlN, contributing to the refinement of austenite grains during heating.
- Al fixes N, prevents solute B from binding to N, and suppresses reduction in the effect of improving the hardenability of B.
- Al is an element that is difficult to dissolve in cementite, coarse cementite is suppressed by suppressing the formation of cementite from austenite containing Al.
- Cementite is one of the carbides that are easily coarsened, and reducing the number of coarse cementite leads to a decrease in the number of coarse carbides. In order to acquire such an effect, Al needs to contain 0.03% or more.
- the Al content is in the range of 0.03 to 0.1%.
- the Al content is 0.04 to 0.09%. More preferably, the Al content is 0.05 to 0.08%. More preferably, the Al content is 0.05 to 0.08%.
- the carbide referred to in the present invention is a compound of carbon (C) and another metal element, and cementite is one of the carbides, which is a compound of iron (Fe) and carbon (C).
- N 0.006% or less N exists in steel as an unavoidable impurity, but forms AlN by combining with Al. If Ti is contained, TiN is formed to refine crystal grains and toughness. It has the effect
- Cr 0.6% to 1.7% or less
- Cr is an element that increases the strength of steel through the improvement of hardenability and improves the corrosion resistance.
- Cr is an element that combines with C during tempering treatment to form carbides such as M 3 C, M 7 C 3 and M 23 C 6 (M is a metal element) and improves temper softening resistance.
- M is a metal element
- M 3 C type carbide has a strong effect of improving the temper softening resistance.
- the Cr content needs to exceed 0.6%.
- the Cr content exceeds 1.7%, a large amount of carbides such as M 7 C 3 and M 23 C 6 are formed, acting as hydrogen trap sites, and reducing the SSC resistance.
- the Cr content is in the range of more than 0.6% and not more than 1.7%.
- the Cr content is 0.8 to 1.5%. More preferably, the Cr content is 0.8 to 1.3%.
- Mo More than 1.0% and less than 3.0% Mo is an element that forms carbides and contributes to strengthening steel by precipitation strengthening, and is effective in securing desired high strength after reducing dislocation density by tempering. Contribute to. SSC resistance is improved by reducing the dislocation density. Mo dissolves in the steel and segregates at the prior austenite grain boundaries, contributing to the improvement of SSC resistance. Furthermore, Mo has an action of densifying the corrosion product and further suppressing the generation and growth of pits that are the starting points of cracks. In order to obtain such an effect, the Mo content needs to exceed 1.0%.
- Mo content shall be 1.0% and 3.0% or less of range.
- the Mo content is more than 1.1% and not more than 3.0%. More preferably, the Mo content is more than 1.2% and not more than 2.8%. More preferably, the Mo content is 1.45 to 2.5%. Even more preferably, the Mo content is 1.45 to 1.80%.
- V 0.02 to 0.3%
- V is an element that forms carbides and carbonitrides and contributes to the strengthening of steel. In order to acquire such an effect, V needs to contain 0.02% or more. On the other hand, even if it contains V exceeding 0.3%, the effect is saturated and an effect commensurate with the content cannot be expected, which is economically disadvantageous. Therefore, the V content is in the range of 0.02 to 0.3%. Preferably, the V content is 0.03 to 0.20%. More preferably, the V content is 0.15% or less.
- Nb 0.001 to 0.02%
- Nb forms carbides and / or carbonitrides, contributes to increasing the strength of the steel by precipitation strengthening, and also contributes to refinement of austenite grains. In order to acquire such an effect, Nb needs to contain 0.001% or more.
- Nb precipitates are likely to be the propagation path of SSC (sulfide stress corrosion cracking), and the presence of a large amount of Nb precipitates based on a large amount of Nb content exceeding 0.02% is particularly high strength steel with yield stress of 125 ksi or more. In this case, the SSC resistance is significantly reduced. Therefore, from the viewpoint of achieving both desired high strength and excellent SSC resistance, the Nb content is set to 0.001 to 0.02% in the present invention. Preferably, the Nb content is 0.001% or more and less than 0.01%.
- B 0.0005-0.0040% B segregates at the austenite grain boundaries and suppresses the ferrite transformation from the grain boundaries, thereby having the effect of enhancing the hardenability of the steel even when contained in a small amount.
- B needs to contain 0.0005% or more.
- the B content is in the range of 0.0005 to 0.0040%.
- the B content is 0.0010 to 0.0030%.
- Ti Less than 0.003% Ti has a large binding force with N, and even if it is a trace amount, Ti is generated as inclusions (nitride inclusions) in steel, and the SSC resistance is lowered. Further, as the amount of Ti added increases, the amount of nitride (the amount of nitride inclusions) increases and tends to become coarser, and the SSC resistance decreases. Therefore, Ti is not added, and even when Ti is mixed, the Ti content is less than 0.003%. Preferably, the Ti content is 0.002% or less.
- O (oxygen) 0.0030% or less
- O (oxygen) exists as an oxide inclusion in steel as an inevitable impurity. Since these inclusions become the starting point of SSC (sulfide stress corrosion cracking) and reduce the SSC resistance, in the present invention, it is preferable to reduce O (oxygen) as much as possible. However, excessive reduction of O (oxygen) leads to an increase in refining costs, so it is acceptable up to 0.0030%. For this reason, the O (oxygen) content is 0.0030% or less. Preferably, the O content is 0.0020% or less.
- the balance other than the above components is composed of Fe and inevitable impurities.
- unavoidable impurities Mg: 0.0008% or less, Co: 0.05% or less are acceptable.
- the above-mentioned components are basic components.
- one or two elements selected from Cu: 1.0% or less, Ni: 1.0% or less, and W: 3.0% or less are further selected as the selective elements. More than species, and / or Ca: 0.0005-0.005%.
- Cu is an element that contributes to increasing the strength of steel and has the effect of improving toughness and corrosion resistance. In particular, it is an extremely effective element for improving SSC resistance in severe corrosive environments.
- a dense corrosion product is formed and the corrosion resistance is improved, and further, the generation and growth of pits as the starting point of cracking are suppressed.
- the Cu content is preferably limited to 1.0% or less.
- Ni is an element that contributes to increasing the strength of steel and further improves toughness and corrosion resistance. In order to acquire such an effect, it is desirable to contain Ni 0.03% or more. On the other hand, even if Ni is contained in an amount exceeding 1.0%, the effect is saturated, and an effect commensurate with the content cannot be expected, which is disadvantageous for economic efficiency. For this reason, when it contains Ni, it is preferable to limit Ni content to 1.0% or less.
- W is an element that forms carbides and contributes to increasing the strength of the steel by precipitation strengthening, and also dissolves and segregates at the prior austenite grain boundaries to contribute to the improvement of SSC resistance.
- W is preferably contained in an amount of 0.03% or more.
- W content it is preferable to limit W content to 3.0% or less.
- Ca 0.0005 to 0.005%
- Ca is an element that combines with S to form CaS and effectively acts to control the morphology of sulfide inclusions, and improves toughness and SSC resistance through the morphology control of sulfide inclusions. Contribute to. In order to acquire such an effect, Ca needs to contain 0.0005% or more. On the other hand, even if Ca is contained in excess of 0.005%, the effect is saturated, and an effect commensurate with the Ca content cannot be expected, which is disadvantageous for economy. For this reason, when Ca is contained, the Ca content is preferably limited to a range of 0.0005 to 0.005%.
- the high-strength seamless steel pipe of the present invention has the above-described composition, and further has a tempered martensite as a main phase and a volume ratio of 90% or more, and a nitriding with a grain size of 4 ⁇ m or more in a cross section perpendicular to the rolling direction.
- -based inclusions 100 mm 2 per 50 or less
- the particle size is nitride inclusions of less than 4 ⁇ m is 100 mm 2 per 500 or less
- particle size: oxide inclusions of more than 4 ⁇ m is 100 mm 2 per 40 or less
- Particle size It has a structure in which oxide inclusions less than 4 ⁇ m are 400 or less per 100 mm 2 .
- Main phase Tempered martensite phase
- the structure is mainly composed of martensite phase.
- a tempered martensite phase obtained by tempering the martensite phase is used as a main phase.
- the term “main phase” as used herein refers to a case where the phase is a single phase having a volume ratio of 100%, or the phase includes 90% or less of the second phase that has a volume ratio that does not affect the characteristics. The case where it is more than%.
- examples of the second phase include a bainite phase, a retained austenite phase, pearlite, or a mixed phase thereof.
- the above structure of the high-strength seamless steel pipe of the present invention can be adjusted by appropriately selecting the heating temperature in the quenching process according to the steel components and the cooling rate during cooling.
- the number of nitride inclusions and oxide inclusions is within an appropriate range depending on the size (particle size) in order to improve SSC resistance. adjust.
- the nitride inclusions and oxide inclusions are identified by automatic detection using a scanning electron microscope.
- the nitride inclusions are mainly composed of Al, and the oxide inclusions are Al.
- Ca, Mg are the main components.
- the number of inclusions is a value measured in a cross section perpendicular to the rolling direction of the steel pipe (cross section perpendicular to the pipe axis direction: C cross section).
- the particle size of each inclusion is used as the size of the inclusion.
- the particle size of the inclusion is obtained by obtaining the area of the inclusion particle and calculating the equivalent circle diameter to obtain the particle size of the inclusion particle.
- Nitride inclusions with a grain size of 4 ⁇ m or more are 50 or less per 100 mm 2
- Nitride inclusions are the starting point of SSC (sulfide stress corrosion cracking) in high strength steel pipes with yield stress of 125 ksi class or higher.
- SSC semiconductor stress corrosion cracking
- the adverse effect increases.
- the number of nitride inclusions having a particle size of 4 ⁇ m or more is limited to 50 or less per 100 mm 2 .
- the number is preferably 40 or less.
- nitride inclusions with a particle size of less than 4 ⁇ m per 100 mm 2 Fine nitride inclusions with a particle size of less than 4 ⁇ m are the origin of SSC (sulfide stress corrosion cracking) even if they exist alone
- SSC sulfuride stress corrosion cracking
- the number of high-strength steel pipes with yield stress YS: 125 ksi class or higher increases, and if it exceeds 500 per 100 mm 2 , the adverse effect on SSC resistance becomes unacceptable.
- the number of nitride inclusions having a particle size of less than 4 ⁇ m is limited to 500 or less per 100 mm 2 .
- the number is preferably 450 or less.
- Oxide inclusions are the starting point of SSC (sulfide stress corrosion cracking) in high strength steel pipes with yield stress YS: 125 ksi class or higher
- SSC sulfuride stress corrosion cracking
- the adverse effect increases. Therefore, it is desirable to reduce the number of oxide inclusions having a particle size of 4 ⁇ m or more as much as possible, but if the number is 40 or less per 100 mm 2 , an adverse effect on SSC resistance can be tolerated. For this reason, the number of oxide inclusions having a particle size of 4 ⁇ m or more is limited to 40 or less per 100 mm 2 .
- the number is preferably 35 or less.
- the ladle from the ladle to the tundish is reduced so that the number of nitride inclusions and oxide inclusions is less than the number per unit area described above.
- sealing with an inert gas is performed, and electromagnetic stirring is performed in the mold to achieve floating separation of inclusions.
- One of the carbides that tends to coarsen is cementite.
- the number of carbides having an equivalent circle diameter of 175 nm or more is preferably 100 or less per 100 ⁇ m 2 . More preferably, the number of carbides having an equivalent circle diameter of 175 nm or more is 80 or less per 100 ⁇ m 2 . More preferably, the number of carbides having an equivalent circle diameter of 175 nm or more is 60 or less per 100 ⁇ m 2 .
- the number of carbides is a value measured in a cross section perpendicular to the rolling direction including the thickness center of the steel pipe (cross section perpendicular to the pipe axis direction: C cross section).
- the equivalent circle diameter of each carbide is used.
- requires the area of a carbide particle, calculates a circle equivalent diameter, and makes it the circle equivalent diameter of the said carbide
- the steel pipe material having the above composition is heated and hot-worked to obtain a seamless steel pipe having a predetermined shape.
- the steel pipe material used in the present invention is prepared by melting molten steel having the above composition by a conventional melting method such as a converter, and by a conventional casting method such as a continuous casting method. It is preferable to do.
- the slab may be further hot-rolled to obtain a round steel piece having a predetermined shape, or a round steel piece that has undergone ingot-bundling rolling.
- the number of nitride inclusions and oxide inclusions is reduced to be equal to or less than the number per unit area described above. . For this reason, it is necessary to reduce the steel pipe material (slab or steel slab) as much as possible within the ranges of N (nitrogen): 0.006% or less and O (oxygen): 0.0030% or less.
- the heat stirring and refining treatment has a treatment time of 30 min or more
- the RH vacuum degassing treatment has a treatment time of 20 min or more
- the RH reflux rate is 85 ton / min or more. If the RH reflux rate is less than 85 ton / min, the desired number of inclusions cannot be reduced.
- the ladle from the ladle to the tundish is reduced so that the number of nitride inclusions and oxide inclusions is less than the number per unit area described above.
- the slab (steel pipe material) having the above composition is heated to a heating temperature of 1050 to 1350 ° C. and hot-worked to obtain a seamless steel pipe having a predetermined size.
- Heating temperature 1050-1350 ° C
- the heating temperature is less than 1050 ° C.
- the dissolution of carbides in the steel pipe material becomes insufficient.
- crystal grains become coarse, precipitates such as TiN precipitated during solidification become coarse, and cementite becomes coarse, so that the steel pipe toughness decreases.
- heating to a high temperature exceeding 1350 ° C. is not preferable from the viewpoint of energy saving because a thick scale layer is formed on the surface of the steel pipe material, causing surface flaws and the like during rolling and increasing energy loss.
- the heating temperature is limited to a temperature in the range of 1050 to 1350 ° C.
- it is 1100-1300 ° C.
- the heated steel pipe material is then subjected to hot working (pipemaking) using a Mannesmann-plug mill type or Mannesmann-mandrel type hot rolling mill to obtain a seamless steel pipe having a predetermined dimension.
- a Mannesmann-plug mill type or Mannesmann-mandrel type hot rolling mill to obtain a seamless steel pipe having a predetermined dimension.
- it is good also as a seamless steel pipe by the hot extrusion by a press system.
- the obtained seamless steel pipe is subjected to a cooling process of cooling at a cooling rate of air cooling or higher until the surface temperature becomes 200 ° C. or lower after the hot working is finished.
- Cooling after completion of hot working Cooling rate: Air cooling or higher, Cooling stop temperature: 200 ° C. or lower
- Cooling rate Air cooling or higher
- Cooling stop temperature 200 ° C. or lower
- the “cooling rate over air cooling” refers to 0.1 ° C./s or more, and water cooling is also possible.
- the cooling rate depends on the thickness of the steel pipe and the water cooling method.
- the cooling rate is less than 0.1 ° C./s, the metal structure after cooling becomes non-uniform, and the metal structure after the subsequent heat treatment becomes non-uniform.
- ⁇ Temperature treatment is performed after cooling at a cooling rate higher than air cooling.
- the tempering process is a process of heating to a temperature in the range of 640 to 740 ° C.
- Tempering temperature 640-740 ° C
- the tempering treatment is performed for the purpose of reducing dislocation density and improving toughness and SSC resistance.
- the tempering temperature is less than 640 ° C.
- the reduction of dislocations is insufficient, so that excellent SSC resistance cannot be ensured.
- the temperature exceeds 740 ° C.
- the tissue is remarkably softened and the desired high strength cannot be ensured.
- the tempering temperature was limited to a temperature in the range of 640 to 740 ° C.
- the temperature is preferably 660 to 710 ° C.
- Reheating temperature for quenching treatment Ac 3 transformation point or more and 1000 ° C or less If the reheating temperature is less than the Ac 3 transformation point, the austenite single phase region is not heated, so a structure with the martensite phase as the main phase is obtained. Absent. On the other hand, when the temperature exceeds 1000 ° C., in addition to coarsening of crystal grains and lowering toughness, there are adverse effects such as thickening of the surface oxide scale, easy peeling and causing wrinkling on the surface of the steel sheet. Furthermore, the load on the heat treatment furnace becomes excessive, which causes a problem from the viewpoint of energy saving. For these reasons and from the viewpoint of energy saving, the reheating temperature for quenching is limited to the Ac 3 transformation point or higher and 1000 ° C. or lower. Preferably it is 950 degrees C or less.
- quenching treatment is performed.
- the quenching cooling is preferably performed by water cooling at an average cooling rate of 2 ° C./s or more to a temperature of 400 ° C. or less at the center position of the plate thickness, and the surface temperature is It is preferable to cool to 200 ° C. or lower, preferably to a temperature of 100 ° C. or lower.
- the quenching process may be repeated twice or more.
- the hot metal discharged from the blast furnace was desulfurized and dephosphorized in the hot metal pretreatment, decarburized and dephosphorized in the converter, and as shown in Table 2, the heat treatment and refining treatment (LF ) And RH vacuum degassing treatment with a reflux rate of 120 ton / min and a treatment time of 10 to 40 min to obtain molten steel having the composition shown in Table 1, and a slab (round slab: 190 mm ⁇ ) by a continuous casting method. .
- a tundish Ar gas shield and electromagnetic stirring with a mold were performed.
- the obtained slab was placed in a heating furnace as a steel pipe material, heated to the heating temperature shown in Table 2, and held (holding time: 2 h).
- the heated steel pipe material was hot-worked using a Mannesmann-plug mill type hot rolling mill to obtain a seamless steel pipe (outer diameter 100 to 200 mm ⁇ ⁇ thickness 12 to 30 mm).
- it air-cooled and the quenching tempering process was performed on the conditions shown in Table 2. In some cases, after hot working, it was cooled with water, and then tempered or quenched and tempered.
- Specimens were collected from the obtained seamless steel pipe and subjected to a structure observation, a tensile test, and a sulfide stress corrosion cracking test.
- the test method was as follows.
- main component refers to a case where the elements are 65% by mass or more in total.
- the number of particles identified as inclusions was obtained, the area of each particle was obtained, and the equivalent circle diameter was calculated to obtain the particle size of the inclusions. Then, the number density (inclusions / 100 mm 2 ) of inclusions having a particle size of 4 ⁇ m or more and inclusions having a particle size of less than 4 ⁇ m was calculated. Inclusions whose long sides were less than 2 ⁇ m were not analyzed. Further, the number of carbides was obtained by collecting a structure observation specimen from a position including the center of the thickness of the obtained seamless steel pipe, and a cross section perpendicular to the rolling direction (cross section perpendicular to the longitudinal direction of the pipe (C cross section)).
- the polished observation surface is corroded with nital to reveal the structure, and the structure is observed using a scanning electron microscope (magnification: 13000 times).
- the area of 550 ⁇ m 2 was used as the observation surface.
- the equivalent circle diameter of the carbide was determined by using image processing software.
- a sulfide stress corrosion cracking test was performed in accordance with the test method specified in NACE TM TM0177 Method A.
- the tensile test piece described above was prepared using the test solution: (acetic acid-sodium acetate aqueous solution containing 5.0 mass% saline solution saturated with 10 kPa hydrogen sulfide and adjusted to pH 3.5 (liquid temperature: 24 ° C))) and is a constant load test that holds 90% of the yield stress YS obtained in the tensile test. Sulfide stress corrosion resistance when no breakage occurs before 720 hours It was evaluated that the crackability was good.
- the sulfide stress corrosion cracking test was not performed.
- the sulfide stress corrosion cracking test is performed under a severe test condition with a load stress larger than the load stress described in each patent document of the background art. Therefore, a general load stress as shown in these patent documents, specifically, a state in which a stress of 85% of the yield stress YS obtained in the tensile test is applied is held, and the rest is the same as described above. A sulfide stress corrosion cracking test was conducted under the test conditions.
- All examples of the present invention are seamless steel pipes having both high strength of yield stress YS: 862 MPa and excellent SSC resistance.
- the yield stress YS is reduced and the desired high strength cannot be secured, or the SSC resistance is reduced.
- Steel pipe No. 13 (steel No. I) in which C deviated from the scope of the present invention has not secured the desired high strength.
- Steel pipe No. 14 (steel No. J) in which C deviated from the scope of the present invention has a low SSC resistance at the tempering temperature within the scope of the present invention.
- Steel pipe No. 15 (steel No. K), in which Mo deviates from the scope of the present invention, has secured the desired high strength, but has reduced SSC resistance.
- Steel pipe No. 16 (steel No. L), in which Cr is out of the scope of the present invention and the number of inclusions is out of the scope of the present invention, can secure the desired high strength, but is resistant to SSC. Has fallen.
- Steel tube No. 17 (steel No. 17 (steel No.
- steel pipe No. 24 (steel No. H) in which the number of inclusions (nitride inclusions and oxide inclusions) is outside the scope of the present invention is resistant. SSC property is reduced.
- Steel pipes No. 25 (steel No. R) and No. 26 (steel No. S) in which Al deviated from the scope of the invention had a larger number of coarse carbides having an equivalent circle diameter of 175 nm or more than the scope of the invention. In many cases, SSC resistance is reduced.
Abstract
Description
(1)質量%で、C:0.20~0.50%、Si:0.05~0.40%、Mn:0.3~0.9%、P :0.015%以下、S:0.005%以下、Al:0.03~0.1%、N :0.006%以下、Cr:0.6%超え1.7%以下、Mo:1.0%超え3.0%以下、V:0.02~0.3%、Nb:0.001~0.02%、B:0.0005~0.0040%、O(酸素):0.0030%以下、Ti:0.003%未満を含有し、残部Feおよび不可避的不純物からなる組成を有し、焼戻マルテンサイトを体積率で90%以上とし、圧延方向に垂直な断面において、粒径が4μm以上の窒化物系介在物が100mm2あたり50個以下、粒径が4μm未満の窒化物系介在物が100mm2あたり500個以下、粒径が4μm以上の酸化物系介在物が100mm2あたり40個以下、粒径が4μm未満の酸化物系介在物が100mm2あたり400個以下である組織を有する、降伏応力YS:862MPa以上である油井用高強度継目無鋼管。
(2)前記(1)において、前記組織は、さらに、圧延方向に垂直な断面において、円相当径が175nm以上の炭化物が100μm2あたり100個以下を有する油井用高強度継目無鋼管。
(3)前記(1)または(2)において、前記組成に加えてさらに、質量%で、Cu:1.0%以下、Ni:1.0%以下、W:3.0%以下のうちから選ばれた1種または2種以上を含有する油井用高強度継目無鋼管。
(4)前記(1)ないし(3)のいずれかにおいて、前記組成に加えてさらに、質量%で、Ca:0.0005~0.0050%を含有する油井用高強度継目無鋼管。
(5)鋼管素材を加熱し、熱間加工を施して所定形状の継目無鋼管とする油井用継目無鋼管の製造方法であって、前記(1)ないし(4)のいずれかに記載の油井用高強度継目無鋼管の製造方法であり、前記加熱の加熱温度を、1050~1350℃の範囲の温度とし、前記熱間加工後に、前記継目無鋼管に空冷以上の冷却速度で表面温度が200℃以下となる温度まで冷却を施し、640~740℃の範囲の温度に加熱する焼戻処理を施す油井用高強度継目無鋼管の製造方法。
(6)前記(5)において、前記冷却後で、前記焼戻処理の前に、Ac3変態点以上1000℃以下の範囲の温度に再加熱し、表面温度で200℃以下となる温度まで急冷する焼入れ処理を1回以上施す油井用高強度継目無鋼管の製造方法。
Cは、固溶して鋼の強度増加に寄与するとともに、鋼の焼入性を向上させ、焼入れ時にマルテンサイト相を主相とする組織の形成に寄与する。このような効果を得るためには、Cは0.20%以上の含有を必要とする。一方、Cの0.50%を超える含有は、焼入れ時に割れを発生させ、製造性を著しく低下させる。このため、C含有量は0.20~0.50%の範囲とする。好ましくは、C含有量は0.20~0.35%である。より好ましくは、C含有量は0.22~0.32%である。
Siは、脱酸剤として作用するとともに、鋼中に固溶して鋼の強度を増加させ、さらに焼戻時の軟化を抑制する作用を有する元素である。このような効果を得るためには、Siは0.05%以上含有する必要がある。一方、Siの0.40%を超える含有は、軟化相であるフェライト相の生成を促進し、所望の高強度化を阻害したり、さらに粗大な酸化物系介在物の形成を促進したりして、耐SSC性や靭性を低下させる。また、Siは偏析して局部的に鋼を硬化させる元素であり、0.40%を超えるSiの含有は、局部的硬化領域を形成し、耐SSC性を低下させるという悪影響をおよぼす。このようなことから、本発明では、Si含有量は0.05~0.40%の範囲とする。好ましくは、Si含有量は0.05~0.30%である。より好ましくは、Si含有量は0.20~0.30%である。
Mnは、Cと同様に、鋼の焼入性を向上させ、鋼の強度増加に寄与する元素である。このような効果を得るためには、Mnは0.3%以上の含有を必要とする。一方、Mnは、偏析して局部的に鋼を硬化させる元素であり、0.9%を超えるMnの含有は、局部的硬化領域を形成し、耐SSC性を低下させるという悪影響をおよぼす。このため、本発明では、Mn含有量は0.3~0.9%の範囲とする。好ましくは、Mn含有量は0.4~0.8%である。より好ましくは、Mn含有量は0.5~0.8%である。
Pは、粒界に偏析して粒界脆化を引き起こすだけでなく、偏析して局部的に鋼を硬化させる元素であり、本発明では、Pは不可避的不純物として、できるだけ低減することが好ましいが、P含有量は0.015%までは許容できる。このため、P含有量は0.015%以下とする。好ましくは、P含有量は0.012%以下である。
Sは、不可避的不純物として、鋼中ではそのほとんどが硫化物系介在物として存在し、延性、靭性、さらには耐SSC性を低下させるため、できるだけ低減することが好ましいが、0.005%までは許容できる。このため、S含有量は0.005%以下とする。好ましくは、S含有量は0.003%以下である。さらに、好ましくは、0.0015%以下である。
Alは、脱酸剤として作用するとともに、Nと結合してAlNを形成して、加熱時のオーステナイト粒の微細化に寄与する。また、Alは、Nを固定し、固溶BがNと結合することを防止して、Bの焼入性向上効果の低減を抑制する。さらに、Alはセメンタイトに固溶しにくい元素であるため、Alを含有するオーステナイトからセメンタイトが生成するのを抑制することにより、粗大なセメンタイトが抑制される。セメンタイトは粗大化しやすい炭化物の1つであり、粗大なセメンタイト個数を低減することは、粗大な炭化物の数の減少につながる。このような効果を得るために、Alは0.03%以上の含有を必要とする。特に、Ti含有量を0.003%未満に規定する本発明の鋼管では、上記の効果を得るために、Al含有量を0.03%以上とすることは重要である。一方、0.1%を超えるAlの含有は、酸化物系介在物の増加をもたらし、鋼の清浄度を低下させて、延性、靭性、さらには耐SSC性の低下を招く。このため、Al含有量は0.03~0.1%の範囲とする。好ましくは、Al含有量は0.04~0.09%である。より好ましくは、Al含有量は0.05~0.08%である。より好ましくは、Al含有量は0.05~0.08%である。ここで、本発明で言う炭化物とは、炭素(C)と他の金属元素との化合物であり、セメンタイトとは炭化物の一つであり、鉄(Fe)と炭素(C)の化合物を言う。
Nは、不可避的不純物として鋼中に存在するが、Alと結合してAlNを形成し、また、Tiを含有する場合はTiNを形成して、結晶粒を微細化し、靭性を向上させる作用を有する。しかし、0.006%を超えるNの含有は、形成される窒化物が粗大化し、耐SSC性や靭性を著しく低下させる。このため、N含有量は0.006%以下とする。
Crは、焼入性の向上を介して鋼の強度を増加させるとともに、耐食性を向上させる元素である。また、Crは、焼戻処理時にCと結合し、M3C、M7C3および、M23C6(Mは金属元素)などの炭化物を形成し、焼戻軟化抵抗を向上させる元素であり、とくに鋼管の高強度化に際しては必要な元素である。特にM3C型炭化物は、焼戻軟化抵抗を向上させる作用が強い。このような効果を得るためには、Crは0.6%超えの含有を必要とする。一方、1.7%を超えてCrを含有すると、多量のM7C3および、M23C6などの炭化物を形成し、水素のトラップサイトとして作用して耐SSC性を低下させる。このようなことから、Cr含有量は、0.6%超え1.7%以下の範囲とする。好ましくは、Cr含有量は0.8~1.5%である。より好ましくは、Cr含有量は0.8~1.3%である。
Moは、炭化物を形成し、析出強化により鋼の強化に寄与する元素であり、焼戻により転位密度を低減させたうえで所望の高強度を確保するのに有効に寄与する。転位密度の低減により耐SSC性が向上する。また、Moは、鋼中に固溶して、旧オーステナイト粒界に偏析して、耐SSC性の向上に寄与する。さらに、Moは、腐食生成物を緻密化し、さらに割れの起点となるピットの生成および成長を抑制する作用を有する。このような効果を得るためには、Moは1.0%超えの含有を必要とする。一方、3.0%を超えるMoの含有は、針状のM2C析出物(炭化物)や、場合によってはLaves相(Fe2Mo)の形成を促進して、耐SSC性を低下させる。このため、Mo含有量は1.0%超え3.0%以下の範囲とする。好ましくは、Mo含有量は、1.1%超え3.0%以下である。より好ましくは、Mo含有量は1.2%超え2.8%以下である。さらに好ましくは、Mo含有量は1.45~2.5%である。さらにより好ましくは、Mo含有量は1.45~1.80%である。
Vは、炭化物や炭窒化物を形成し、鋼の強化に寄与する元素である。このような効果を得るためには、Vは0.02%以上の含有を必要とする。一方、0.3%を超えてVを含有しても、効果が飽和し、含有量に見合う効果を期待できなくなり、経済的に不利となる。このため、V含有量は0.02~0.3%の範囲とする。好ましくは、V含有量は0.03~0.20%である。より好ましくは、V含有量は0.15%以下である。
Nbは、炭化物やあるいはさらに炭窒化物を形成し、析出強化により鋼の強度増加に寄与するとともに、オーステナイト粒の微細化にも寄与する。このような効果を得るためには、Nbは0.001%以上の含有を必要とする。一方、Nb析出物は、SSC(硫化物応力腐食割れ)の伝播経路と成りやすく、0.02%を超える多量のNb含有に基づく多量のNb析出物の存在は、とくに降伏応力125ksi以上の高強度鋼材において、耐SSC性の顕著な低下に繋がる。このため、所望の高強度と優れた耐SSC性との両立の観点から、本発明では、Nb含有量は0.001~0.02%とする。好ましくは、Nb含有量は0.001%以上、0.01%未満である。
Bは、オーステナイト粒界に偏析し、粒界からのフェライト変態を抑制することにより、微量の含有でも、鋼の焼入性を高める作用を有する。このような効果を得るためには、Bは0.0005%以上の含有を必要とする。一方、0.0040%超えてBを含有すると、炭窒化物等として析出し、焼入性が低下し、したがって靭性が低下する。このため、B含有量は0.0005~0.0040%の範囲とする。好ましくは、B含有量は0.0010~0.0030%である。
Tiは、Nとの結合力が大きく、微量であっても鋼中の介在物(窒化物系介在物)として生成して、耐SSC性が低下する。また、Tiの添加量が多いほど窒化物量(窒化物系介在物量)が増加し、粗大化する傾向があり、耐SSC性は低下する。従って、Tiは無添加とし、Tiが混入する場合でもTi含有量は0.003%未満とする。好ましくは、Ti含有量は0.002%以下とする。
O(酸素)は、不可避的不純物として、鋼中では酸化物系介在物として存在している。これら介在物は、SSC(硫化物応力腐食割れ)の発生起点となり、耐SSC性を低下させるため、本発明ではO(酸素)は、できるだけ低減することが好ましい。しかし、O(酸素)の過剰な低減は精錬コストの高騰を招くため、0.0030%までは許容できる。このため、O(酸素)含有量は0.0030%以下とする。好ましくは、O含有量は0.0020%以下である。
Cu、Ni、Wはいずれも、鋼の強度増加に寄与する元素であり、必要に応じて1種または2種以上を選択して含有できる。
Caは、Sと結合しCaSを形成して、硫化物系介在物の形態制御に有効に作用する元素であり、硫化物系介在物の形態制御を介して、靭性および、耐SSC性の向上に寄与する。このような効果を得るためには、Caは0.0005%以上の含有を必要とする。一方、Caを0.005%を超えて含有しても、その効果が飽和し、Caの含有量に見合う効果が期待できなくなり、経済性に不利となる。このため、Caを含有する場合には、Ca含有量は0.0005~0.005%の範囲に限定することが好ましい。
本発明の高強度継目無鋼管では、YS:125ksi(862MPa)級以上の高強度を確保するために、組織をマルテンサイト相主体の組織とするが、構造物として必要な延性や靭性を保持するために、マルテンサイト相を焼戻した焼戻マルテンサイト相を主相とする。ここでいう「主相」とは、当該相が体積率で100%である単相である場合、あるいは第二相を特性に影響しない程度である体積率で10%以下含む、当該相が90%以上である場合をいう。なお、本発明では第二相は、ベイナイト相、残留オーステナイト相、パーライトあるいはそれらの混合相が例示できる。
窒化物系介在物は、降伏応力125ksi級以上の高強度鋼管ではSSC(硫化物応力腐食割れ)の発生起点となり、その大きさ(粒径)が4μm以上と大きくなるほど、その悪影響が大きくなる。そのため、粒径が4μm以上の窒化物系介在物はできるだけ、少なくすることが望ましいが、100mm2あたり50個以下であれば、耐SSC性への悪影響は許容できる。このため、粒径が4μm以上の窒化物系介在物は100mm2あたり50個以下に限定する。なお、好ましくは40個以下である。
粒径が4μm未満の微細な窒化物系介在物は、単独で存在してもSSC(硫化物応力腐食割れ)の発生起点にはならないが、降伏応力YS:125ksi級以上の高強度鋼管では、その数が多くなり、100mm2あたり500個を超えると、耐SSC性への悪影響が許容できなくなる。このため、粒径が4μm未満の窒化物系介在物は100mm2あたり500個以下に限定する。なお、好ましくは450個以下である。
酸化物系介在物は、降伏応力YS:125ksi級以上の高強度鋼管では、SSC(硫化物応力腐食割れ)の発生起点となり、その大きさ(粒径)が4μm以上と大きくなるほど、その悪影響が大きくなる。そこで、粒径が4μm以上の酸化物系介在物はできるだけ、少なくすることが望ましいが、100mm2あたり40個以下であれば、耐SSC性への悪影響は許容できる。このため、粒径が4μm以上の酸化物系介在物は100mm2あたり40個以下に限定する。なお、好ましくは35個以下である。
酸化物系介在物は、降伏応力125ksi級以上の高強度鋼では、粒径が4μm未満と小さいものでもSSCの発生起点となり、その数が多くなるほど耐SSC性への悪影響が大きくなる。そのため、粒径が4μm未満の酸化物系介在物でもできるだけ少なくすることが望ましいが、100mm2あたり400個以下であれば、許容できる。このようなことから、粒径が4μm未満の酸化物系介在物は100mm2あたり400個以下に限定する。なお、好ましくは365個以下である。
円相当径が175nm以上の炭化物:100μm2あたり100個以下
粗大化しやすい炭化物の1つがセメンタイトである。粗大な炭化物はSSCの割れの伝播経路となるため、粗大なセメンタイト個数を低減することにより円相当径で175nm以上の粗大な炭化物の数の減少につながり、耐SSC性が向上する。このようなことから、円相当径が175nm以上の炭化物は100μm2あたり100個以下が好ましい。なお、より好ましいくは、円相当径が175nm以上の炭化物は、100μm2あたり80個以下である。さらに好ましくは、円相当径が175nm以上の炭化物は100μm2あたり60個以下である。
炭化物の個数は、鋼管の肉厚中央を含む圧延方向に垂直な断面(管軸方向に垂直な断面:C断面)において測定した値とする。炭化物の大きさは、各炭化物の円相当径を用いるものとする。なお、炭化物の円相当径は、炭化物粒子の面積を求め、円相当直径を計算し、当該炭化物の円相当径とする。
加熱温度が1050℃未満では、鋼管素材中の炭化物の溶解が不十分となる。一方、1350℃を超えて加熱されると、結晶粒が粗大化するとともに、凝固時に析出したTiNなどの析出物が粗大化し、また、セメンタイトが粗大化するため、鋼管靭性が低下する。また、1350℃を超える高温に加熱すると、鋼管素材表面にスケール層が厚く生成し、圧延時に表面疵等の発生原因になるとともに、エネルギーロスが増大し省エネルギーの観点から好ましくない。このようなことから、加熱温度は1050~1350℃の範囲の温度に限定する。好ましくは1100~1300℃である。
冷以上の冷却速度で冷却する冷却処理を施す。
本発明の組成範囲では、熱間加工後に空冷以上の冷却速度で冷却すれば、マルテンサイト相を主相とする組織を得ることができる。表面温度が200℃超えで空冷(冷却)を停止すると、変態が完全に完了していない場合がある。そのため、熱間加工後の冷却処理は、表面温度が200℃以下となるまで、空冷以上の冷却速度で冷却することとした。また、本発明において、「空冷以上の冷却速度」とは、0.1℃/s以上のことを指し、水冷も可能である。水冷の場合、冷却速度は鋼管の肉厚や水冷方法に依存する。0.1℃/s未満の冷却速度であると、冷却後の金属組織が不均一になり、その後の熱処理後の金属組織が不均一となる。
焼戻処理は、転位密度を減少させ、靭性および耐SSC性を向上させる目的で行なう。焼戻温度が640℃未満では、転位の減少が不十分であるため、優れた耐SSC性を確保できない。一方、740℃を超える温度では、組織の軟化が著しく、所望の高強度を確保できない。このため、焼戻温度は640~740℃の範囲の温度に限定した。なお、好ましくは660~710℃である。
再加熱温度が、Ac3変態点未満では、オーステナイト単相域に加熱されないため、マルテンサイト相を主相とする組織が得られない。一方、1000℃を超えると、結晶粒が粗大化し靭性が低下することに加え、表面の酸化スケールが厚くなり、剥離しやすくなり鋼板表面の疵発生の原因となる、などの悪影響がある。さらに、熱処理炉への負荷が過大となり、省エネルギーの観点からも問題となる。このようなことから、また、省エネルギーの観点から、焼入れ処理のための再加熱温度は、Ac3変態点以上1000℃以下に限定する。好ましくは950℃以下である。
(ここで、C、Si、Mn、Cu、Cr、Ni、Mo、V、Ti、Al、B:各元素の含有量(質量%))
Ac3変態点の計算にあたっては、上記した式に記載された元素を含有しない場合には、当該元素の含有量を零%として算出するものとする。
得られた継目無鋼管の、内面側1/4t位置から組織観察用試験片を採取し、管長手方向に直交する断面(C断面)を研磨し、腐食(ナイタール(nital(硝酸-エタノール混合液))腐食)して組織を現出させ、光学顕微鏡(倍率:1000倍)および走査型電子顕微鏡(倍率:2000~3000倍)を用いて、組織を観察し、視野:4箇所以上で撮像した。得られた組織写真に基づき、画像解析により、構成する相の同定、およびそれら相の組織分率を、それぞれ算出した。
また、介在物として識別した粒子の個数を求め、さらに各粒子の面積を求め、円相当直径を計算し当該介在物の粒径とした。そして、粒径:4μm以上の介在物と粒径:4μm未満の介在物の個数密度(個/100mm2)を算出した。なお、長辺が2μmに満たない介在物は分析しなかった。
さらに、炭化物の個数は、得られた継目無鋼管の、肉厚中央を含む位置から組織観察用試験片を採取し、圧延方向に垂直な断面(管長手方向に直交する断面(C断面))を研磨し、研磨した観察面をナイタールで腐食して組織を現出させ、走査型電子顕微鏡(倍率:13000倍)を用いて、組織を観察し、視野:任意の10箇所で撮影し、合わせて550μm2の面積を観察面とした。得られた組織写真に基づき、画像処理ソフトを用いることにより炭化物の円相当径を求めた。
得られた継目無鋼管の内面側1/4t位置から、JIS Z 2241の規定に準拠して、引張方向が管軸方向となるように、JIS 10号引張試験片(棒状試験片:平行部径12.5mmφ、平行部長さ:60mm、GL:50mm)を採取し、引張試験を実施し、引張特性(降伏応力YS(0.5%耐力))、引張応力TS)を求めた。
得られた継目無鋼管の内面側1/4t位置(t:管厚(mm))を中心として、管軸方向が引張方向となるように引張試験片(平行部径:6.35mmφ×平行部長さ25.4mm)を採取した。
また、Crが本発明の範囲を低く外れ、介在物の個数が本発明の範囲を外れた鋼管No.16(鋼No.L)は、所望の高強度を確保できているが、耐SSC性が低下している。また、Nbが本発明の範囲を高く外れ、介在物の個数が本発明の範囲を外れた鋼管No.17(鋼No.M)は、所望の高強度を確保できているが、耐SSC性が低下している。その他に、Nが本発明の範囲を高く外れ、介在物(窒化物系介在物)の個数が本発明の範囲を外れた鋼管No.18(鋼No.N)も所望の高強度を確保できているが、耐SSC性が低下している。
また、Oが本発明の範囲を高く外れ、介在物(酸化物系介在物)の個数が本発明の範囲を外れた鋼管No.19(鋼No.O)も所望の高強度を確保できているが、耐SSC性が低下している。また、Tiが本発明の範囲を高く外れ、介在物(窒化物系介在物)の個数が本発明の範囲を外れた鋼管No.20(鋼No.P)、21(鋼No.Q)も所望の高強度を確保できているが、耐SSC性が低下している。
また、成分は本発明の範囲内であるが、焼戻温度が本発明の範囲を高く外れた鋼管No.22(鋼No.F)は、強度が低下している。また、焼入れ処理の冷却停止温度が本発明の範囲を高く外れた鋼管No.23(鋼No.F)は、マルテンサイト相を主相とする所望の組織が得られず、強度が低下している。また、成分は本発明の範囲内であるが、介在物(窒化物系介在物および酸化物系介在物)の個数が本発明の範囲を外れた鋼管No.24(鋼No.H)は耐SSC性が低下している。
また、Alが発明の範囲を低く外れた鋼管No.25(鋼No.R)とNo.26(鋼No.S)は、円相当径が175nm以上の粗大な炭化物の数が発明の範囲より多く、耐SSC性が低下している。
Claims (6)
- 質量%で、
C :0.20~0.50%、 Si:0.05~0.40%、
Mn:0.3~0.9%、 P :0.015%以下、
S :0.005%以下、 Al:0.03~0.1%、
N :0.006%以下、 Cr:0.6%超え1.7%以下、
Mo:1.0%超え3.0%以下、 V :0.02~0.3%、
Nb:0.001~0.02%、 B :0.0005~0.0040%、
O(酸素):0.0030%以下、 Ti:0.003%未満
を含有し、残部Feおよび不可避的不純物からなる組成を有し、
焼戻マルテンサイトを体積率で90%以上とし、
圧延方向に垂直な断面において、
粒径が4μm以上の窒化物系介在物が100mm2あたり50個以下、
粒径が4μm未満の窒化物系介在物が100mm2あたり500個以下、
粒径が4μm以上の酸化物系介在物が100mm2あたり40個以下、
粒径が4μm未満の酸化物系介在物が100mm2あたり400個以下である組織を有する、降伏応力YS:862MPa以上である油井用高強度継目無鋼管。 - 前記組織は、さらに、圧延方向に垂直な断面において、円相当径が175nm以上の炭化物が100μm2あたり100個以下を有する請求項1に記載の油井用高強度継目無鋼管。
- 前記組成に加えてさらに、質量%で、
Cu:1.0%以下、
Ni:1.0%以下、
W:3.0%以下
のうちから選ばれた1種または2種以上を含有する請求項1または2に記載の油井用高強度継目無鋼管。 - 前記組成に加えてさらに、質量%で、
Ca:0.0005~0.005%を含有する請求項1ないし3のいずれかに記載の油井用高強度継目無鋼管。 - 鋼管素材を加熱し、熱間加工を施して所定形状の継目無鋼管とする油井用継目無鋼管の製造方法であって、請求項1ないし4のいずれかに記載の油井用高強度継目無鋼管の製造方法であり、
前記加熱の加熱温度を、1050~1350℃の範囲の温度とし、
前記熱間加工後に、前記継目無鋼管に空冷以上の冷却速度で表面温度が200℃以下となる温度まで冷却を施し、
640~740℃の範囲の温度に加熱する焼戻処理を施す油井用高強度継目無鋼管の製造方法。 - 前記冷却後で、前記焼戻処理の前に、Ac3変態点以上1000℃以下の範囲の温度に再加熱し、表面温度で200℃以下となる温度まで急冷する焼入れ処理を1回以上施す請求項5に記載の油井用高強度継目無鋼管の製造方法。
Priority Applications (5)
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JP2017567269A JP6451874B2 (ja) | 2016-10-17 | 2017-09-13 | 油井用高強度継目無鋼管およびその製造方法 |
US16/333,029 US11313007B2 (en) | 2016-10-17 | 2017-09-13 | High-strength seamless steel pipe for oil country tubular goods, and method for producing the same |
EP17863132.1A EP3527684B1 (en) | 2016-10-17 | 2017-09-13 | High-strength seamless steel pipe for oil country tubular goods, and method for producing the same |
BR112019004836-7A BR112019004836B1 (pt) | 2016-10-17 | 2017-09-13 | Tubo de aço contínuo de alta resistibilidade para poço de petróleo, e método para produção do mesmo |
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WO2019167945A1 (ja) * | 2018-02-28 | 2019-09-06 | 日本製鉄株式会社 | サワー環境での使用に適した鋼材 |
WO2020071217A1 (ja) * | 2018-10-04 | 2020-04-09 | 日本製鉄株式会社 | サワー環境での使用に適した鋼材 |
WO2023157897A1 (ja) * | 2022-02-17 | 2023-08-24 | 日本製鉄株式会社 | サワー環境での使用に適した鋼材 |
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JP6996641B2 (ja) * | 2018-10-01 | 2022-02-04 | 日本製鉄株式会社 | サワー環境での使用に適した継目無鋼管 |
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WO2023157897A1 (ja) * | 2022-02-17 | 2023-08-24 | 日本製鉄株式会社 | サワー環境での使用に適した鋼材 |
JP7406177B1 (ja) | 2022-02-17 | 2023-12-27 | 日本製鉄株式会社 | サワー環境での使用に適した鋼材 |
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BR112019004836B1 (pt) | 2022-10-11 |
EP3527684A1 (en) | 2019-08-21 |
JP6451874B2 (ja) | 2019-01-16 |
JPWO2018074109A1 (ja) | 2018-10-18 |
EP3527684A4 (en) | 2019-08-21 |
US20190226039A1 (en) | 2019-07-25 |
US11313007B2 (en) | 2022-04-26 |
MX2019003100A (es) | 2019-06-10 |
AR109945A1 (es) | 2019-02-06 |
EP3527684B1 (en) | 2020-12-16 |
BR112019004836A2 (pt) | 2019-06-04 |
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