WO2017149572A1 - 油井用低合金高強度厚肉継目無鋼管 - Google Patents
油井用低合金高強度厚肉継目無鋼管 Download PDFInfo
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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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- C22C38/00—Ferrous alloys, e.g. steel alloys
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- C21D2211/00—Microstructure comprising significant phases
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- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/08—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes
Definitions
- the present invention relates to a high-strength, thick-walled seamless steel pipe excellent in sulfide stress corrosion cracking resistance (SSC resistance) for oil wells and gas wells, particularly in a sour environment containing hydrogen sulfide.
- SSC resistance stress corrosion cracking resistance
- “high strength” means that the yield strength is 758 MPa or more (110 ksi or more)
- “thick” means that the thickness of the steel pipe is 40 mm or more.
- C 0.2 to 0.35%
- Cr 0.2 to 0.7%
- Mo 0.1 to 0.5%
- weight% V Low well steel containing 0.1 to 0.3%, oil well steel with excellent resistance to sulfide stress corrosion cracking, which defines the total amount of precipitated carbide and the proportion of MC type carbide Is disclosed.
- Patent Document 2 by mass, C: 0.15 to 0.30%, Si: 0.05 to 1.0%, Mn: 0.10 to 1.0%, P: 0.025 %: S: 0.005% or less, Cr: 0.1-1.5%, Mo: 0.1-1.0%, Al: 0.003-0.08%, N: 0.008%
- B 0.0005 to 0.010%
- Nb 0.05% or less
- Zr 0.0.
- V For steel inclusions containing one or more selected from 0.30% or less, the maximum length of continuous non-metallic inclusions and the number of particles having a particle size of 20 ⁇ m or more An oil well steel material excellent in sulfide stress corrosion cracking resistance is disclosed.
- Patent Document 3 in mass%, C: 0.15 to 0.35%, Si: 0.1 to 1.5%, Mn: 0.1 to 2.5%, P: 0.025%
- An oil well steel having excellent resistance to sulfide stress corrosion cracking in which the hardness of the composite oxide and the steel is defined by HRC is disclosed.
- the resistance to sulfide stress corrosion cracking of steels of the techniques disclosed in these Patent Documents 1 to 3 refers to a round bar tensile test piece defined in NACE (abbreviation of National Association of Corrosion Engineering) TM0177 method A. This means the presence or absence of SSC when immersed for 720 hours in a test bath described in NACE TM0177 under constant stress.
- NACE abbreviation of National Association of Corrosion Engineering
- Patent Document 4 by mass, C: 0.2 to 0.35%, Si: 0.05 to 0.5%, Mn: 0.05 to 1.0%, P: 0.025 %: S: 0.01% or less, Al: 0.005 to 0.10%, Cr: 0.1 to 1.0%, Mo: 0.5 to 1.0%, Ti: 0.002 to [211] of steel containing 0.05%, V: 0.05 to 0.3%, B: 0.0001 to 0.005%, N: 0.01% or less, O: 0.01% or less
- a steel for low-alloy oil country tubular goods having a yield strength of 861 MPa or more, which is excellent in sulfide stress corrosion cracking resistance, is disclosed by prescribing a formula consisting of a half-value width and a hydrogen diffusion coefficient to a predetermined value.
- K ISSC values are also described.
- a member called a thick-walled coupling which is larger in diameter than the steel pipe size mainly used is required. Since the coupling is also exposed to a sour environment, it is required to have excellent resistance to sulfide stress corrosion cracking (SSC resistance) in the same manner as the main steel pipe.
- SSC resistance sulfide stress corrosion cracking
- the seamless steel pipe for coupling is thick, it is difficult to increase the strength, and in particular, it is difficult to realize a product with a yield strength of 758 MPa.
- an object of the present invention is to provide a low-alloy high-strength thick-walled seamless steel pipe for oil wells that exhibits a stable and high K ISSC value.
- the present inventors firstly used NACE from a seamless steel pipe having a yield strength of 758 MPa or more and a wall thickness of 44.5 to 56.1 mm having various chemical compositions and steel microstructures. Based on TM0177 method D, 3 or more DCB test pieces each having a thickness of 10 mm, a width of 25 mm, and a length of 100 mm were collected and subjected to a DCB test.
- the test bath for the DCB test was a 5 mass% NaCl + 0.5 mass% CH 3 COOH aqueous solution at 24 ° C. saturated with 1.0 atm (0.1 MPa) hydrogen sulfide gas.
- FIG. 1 is a schematic diagram of a DCB test piece.
- h is the height of each arm of the DCB test piece
- B is the thickness of the DCB test piece
- Bn is the web thickness of the DCB test piece.
- the target of the K ISSC value was set to 26.4 MPa ⁇ m or more (24 ksi ⁇ inch or more) based on the assumed maximum notch defect of the oil well pipe and the load weighting condition.
- FIG. 2 shows a graph in which the obtained K ISSC values are arranged by the average hardness (Rockwell C scale hardness) of the seamless steel pipe provided with the test piece.
- the K ISSC value obtained in the DCB test tended to decrease as the hardness of the seamless steel pipe increased, but it was found that the values varied greatly even at the same hardness.
- FIG. 3 shows an example of a stress-strain curve.
- the stress-strain curves (solid line A and broken line B) of the two steel pipes shown in FIG. 3 do not change the stress value of 0.5 to 0.7% strain corresponding to the yield stress, but one (broken line B) is continuous. Yield is occurring, and the other (solid line A) has an upper yield point. It was also found that the steel having a continuous yield type stress-strain curve (broken line B) has a larger variation in KISSC values.
- the inventors of the present invention conducted further research and found that the variation in the K ISSC value was determined by comparing the stress at the time of 0.7% strain with respect to the stress at the time of 0.4% strain ( ⁇ 0.4 ) of the stress-strain curve. (sigma 0.7) performs organized by the value ( ⁇ 0.7 / ⁇ 0.4) of the ratio of, as shown in FIG. 4, the ⁇ 0.7 / ⁇ 0.4 of seamless steel pipe 1.02 It has been found that by making the following (see the black circle in the figure), the variation of the K ISSC value can be reduced as compared with the case of exceeding 1.02 (see the white circle in the figure).
- the K ISSC has a low value of the ratio of the stress at the time of 0.7% strain ( ⁇ 0.7 ) to the stress at the time of 0.4% strain ( ⁇ 0.4 ) in the stress-strain curve of the seamless steel pipe.
- the following reasons can be considered as reasons why the variation in values can be reduced.
- the stress-strain curve should not be a continuous yield type.
- the precipitation Mo which precipitated before hardening is made into a primary precipitate, and it melts at the time of hardening, and Mo which precipitated after tempering is made into a secondary precipitate.
- the quenching temperature is lower.
- DQ is hot At the end of rolling, it indicates that quenching is performed immediately from a state where the steel pipe temperature is still high.
- FIG. 5 shows a Mo concentration distribution diagram in the measurement plane.
- the dark region is the Mo thickening part.
- the hardness of the steel increased up to 1.1 times at the maximum in such a Mo-concentrated portion.
- a KISSC value becomes low in the local hardening area accompanying Mo segregation.
- the Mo content is large in order to ensure high strength, and the occurrence of low K ISSC values due to such Mo segregation becomes significant.
- the segregation area was examined derive indicators of sufficient segregation to suppress the local generation of low K ISSC value.
- the present inventors statistically processed a value obtained by dividing the Mo concentration value (EPMA Mo value) at each measurement point by the EPMA quantitative surface analysis measurement by the average Mo concentration (EPMA Mo ave.) At all measurement points. Thereafter, a cumulative frequency rate graph as shown in FIG. 6 was prepared. And in this cumulative frequency rate graph, the present inventors used EPMA Mo value / EPMA Mo ave. If the cumulative frequency ratio of 1.5 or more (hereinafter also referred to as Mo segregation degree) is 1% or less (black circle in the figure), the generation of a low K ISSC value is suppressed as shown in FIG. Together with the black circles in the figure, it was found that the K ISSC value had a small variation and was stably at least 26.4 MPa ⁇ m.
- Mo atoms are diffused in the solid by holding the cast slab at a high temperature for a long time. Specifically, it is preferable to hold at 1100 ° C. or higher for at least 5 hours. About this long time holding at high temperature, it is once a rectangular cross section than when carrying out billet heating in hot rolling when seamlessly casting a continuous round pipe billet with a continuous casting facility etc.
- the bloom is kept at a high temperature for a long time, specifically, at 1200 ° C. or higher for 20 hours or longer.
- the billet heating at the time of hot rolling in seamless steel pipe forming need not be performed at a high temperature and for a long time, and the coarsening of the crystal grains is suppressed, so that the ⁇ 0.4 value is relatively increased and ⁇ 0 .7 / ⁇ 0.4 is stable because it can be stably reduced to 1.02 or less.
- a high K ISSC value can be stably obtained while increasing the strength of a thick seamless steel pipe used in a sour environment containing hydrogen sulfide.
- the present invention has been completed based on these findings and comprises the following gist.
- the value of the ratio of Ti content to N content (Ti / N) is 3.0 to 4.0, Having a composition consisting of the balance Fe and inevitable impurities,
- the cumulative frequency rate of the measurement points which is defined by the following formula (A) and has a Mo segregation degree of 1.5 or more in the tube longitudinal orthogonal cross section full
- the EPMA Mo value is the Mo concentration (% by mass) at each measurement point at the time of EPMA quantitative surface analysis.
- EPMA Mo ave. Is the average Mo concentration (% by mass) at all measurement points during EPMA quantitative surface analysis.
- W 0.1-0.2%
- Zr 0.005 to 0.03%
- high strength means that the yield strength is 758 MPa or more (110 ksi or more), and “thick” means that the thickness of the steel pipe is 40 mm or more.
- the upper limit of yield strength is not particularly limited, but is preferably 950 MPa.
- the upper limit value of the wall thickness is not particularly limited, but is preferably 60 mm.
- the low-alloy high-strength seamless steel pipe for oil wells of the present invention is excellent in sulfide stress corrosion cracking resistance (SSC resistance).
- SSC resistance sulfide stress corrosion cracking resistance
- the present invention has a high yield strength of 758 MPa or more, and further has excellent sulfide stress corrosion cracking resistance (SSC resistance) under a hydrogen sulfide gas saturated environment (sour environment), in particular, stably. It is possible to provide a low alloy high-strength thick-walled seamless steel pipe for oil wells that exhibits a high K ISSC value. This steel pipe can be used as a low alloy high-strength thick-walled seamless steel pipe for coupling.
- K is a diagram showing stress-strain curves of steel pipes with different variations in ISSC value. It is a figure which shows that the dispersion
- EPMA electron beam microanalyzer
- the steel pipe of the present invention is, in mass%, C: 0.25 to 0.31%, Si: 0.01 to 0.35%, Mn: 0.55 to 0.70%, P: 0.010% or less S: 0.001% or less, O: 0.0015% or less, Al: 0.015 to 0.040%, Cu: 0.02 to 0.09%, Cr: 0.8 to 1.5%, Mo: 0.9 to 1.6%, V: 0.04 to 0.10%, Nb: 0.005 to 0.05%, B: 0.0015 to 0.0030%, Ti: 0.005 to 0.020%, N: 0.005% or less, the ratio of Ti content to N content (Ti / N) is 3.0 to 4.0, the balance Fe and inevitable impurities
- the cumulative frequency rate of the measurement points at which the Mo segregation degree is 1.5 or more in the total thickness of the pipe longitudinal cross section defined by the following formula (A) is 1% or less, Power - the ratio of the values of 0.7% strain when the stress to 0.4% strain at the stress at strain curve ( ⁇ 0.7
- the EPMA Mo value is the Mo concentration (% by mass) at each measurement point at the time of EPMA quantitative surface analysis.
- EPMA Mo ave. Is the average Mo concentration (% by mass) at all measurement points during EPMA quantitative surface analysis.
- C 0.25 to 0.31%
- C has an effect of increasing the strength of steel and is an important element for ensuring a desired high strength.
- C is an element that improves hardenability, and particularly in a thick seamless steel pipe having a thickness of 40 mm or more, in order to achieve a high strength with a yield strength of 758 MPa or more, it is 0.25% or more.
- C content is required.
- the content of C exceeding 0.31% causes a significant increase in ⁇ 0.7 / ⁇ 0.4 and increases the variation of the K ISSC value. Therefore, C is set to 0.25 to 0.31%.
- C is 0.29% or less.
- Si 0.01 to 0.35%
- Si is an element that acts as a deoxidizer and has a function of increasing the strength of the steel by dissolving in steel and suppressing rapid softening during tempering. In order to obtain such an effect, it is necessary to contain 0.01% or more of Si. On the other hand, the inclusion of Si exceeding 0.35% forms coarse oxide inclusions and increases the variation of the K ISSC value. For this reason, Si is made 0.01 to 0.35%. Preferably, Si is 0.01 to 0.04%.
- Mn 0.55 to 0.70%
- Mn is an element that has the effect of increasing the strength of steel through the improvement of hardenability, and binding to S and fixing S as MnS, thereby preventing grain boundary embrittlement due to S.
- Mn is set to 0.55 to 0.70%.
- Mn is 0.55 to 0.65%.
- P 0.010% or less
- P has a tendency to segregate at grain boundaries in the solid solution state and cause grain boundary embrittlement cracks, etc., and is desirably reduced as much as possible in the present invention. acceptable. Therefore, P is set to 0.010% or less.
- S 0.001% or less S is mostly present as sulfide inclusions in steel, and lowers corrosion resistance such as ductility, toughness, and resistance to sulfide stress corrosion cracking.
- a part of S may exist in a solid solution state, but in this case, it segregates at the grain boundaries and tends to cause grain boundary embrittlement cracks. For this reason, it is desirable to reduce S as much as possible in the present invention. However, excessive reduction increases the refining cost. For this reason, in the present invention, S is set to 0.001% or less where the adverse effect is acceptable.
- O (oxygen) 0.0015% or less
- O (oxygen) is present as an inevitable impurity in the steel as an oxide such as Al or Si.
- O (oxygen) is made 0.0015% or less to which the adverse effect is allowable.
- O (oxygen) is 0.0010% or less.
- Al acts as a deoxidizer and combines with N to form AlN and contribute to the reduction of solid solution N. In order to acquire such an effect, Al needs to contain 0.015% or more. On the other hand, if the Al content exceeds 0.040%, oxide inclusions increase and the variation of the K ISSC value increases. For this reason, Al is made 0.015 to 0.040%. Preferably, Al is 0.020% or more. Preferably, Al is 0.030% or less.
- Cu 0.02 to 0.09%
- Cu is an element that has the effect of improving corrosion resistance. When added in a trace amount, a dense corrosion product is formed, and the formation and growth of pits starting from SSC is suppressed, and the resistance to sulfide stress corrosion cracking. In the present invention, it is necessary to contain 0.02% or more of Cu. On the other hand, when it contains Cu exceeding 0.09%, the hot workability at the time of the manufacturing process of a seamless steel pipe will fall. For this reason, Cu is made 0.02 to 0.09%.
- Cu is 0.03% or more.
- Cu is 0.05% or less.
- Cr 0.8 to 1.5% Cr is an element that contributes to an increase in the strength of steel through an increase in hardenability and improves the corrosion resistance. Also, Cr combines with C during tempering to form carbides such as M 3 C, M 7 C 3 and M 23 C 6 systems, and especially M 3 C carbides improve temper softening resistance. Reduces strength change due to tempering and contributes to improved yield strength. In order to achieve a yield strength of 758 MPa or more, it is necessary to contain 0.8% or more of Cr. On the other hand, even if Cr is contained exceeding 1.5%, the effect is saturated, which is economically disadvantageous. Therefore, Cr is set to 0.8 to 1.5%. Preferably, Cr is 0.9% or more. Preferably, Cr is 1.3% or less.
- Mo 0.9-1.6%
- Mo is an element that contributes to an increase in the strength of steel through an increase in hardenability and improves the corrosion resistance.
- Mo forms M 2 C-based carbides.
- M 2 C-based carbides that are secondarily precipitated after tempering improve temper softening resistance, reduce strength change due to tempering, and improve yield strength. This has the effect of changing the stress-strain curve of steel from a continuous yield type to a yield type shape.
- Mo is made 0.9 to 1.6%.
- Mo is 0.9 to 1.5%.
- V 0.04 to 0.10%
- V is an element that forms carbides or nitrides and contributes to the strengthening of steel.
- it is necessary to contain 0.04% or more of V.
- V is set in the range of 0.04 to 0.10%.
- V is 0.045% or more.
- V is 0.055% or less.
- Nb 0.005 to 0.05% Nb delays recrystallization in the austenite ( ⁇ ) temperature range, contributes to the refinement of ⁇ grains, and acts extremely effectively on the refinement of the substructure (eg, packets, blocks, lath) of steel immediately after quenching. Element. In order to acquire such an effect, 0.005% or more of content is required. On the other hand, the content of Nb exceeding 0.05% promotes the precipitation of coarse precipitates (NbN) and causes a decrease in resistance to sulfide stress corrosion cracking. For this reason, Nb is made 0.005 to 0.05%.
- a packet is defined as a region composed of a group of laths having the same crystal habit plane arranged in parallel, and a block is composed of a group of laths parallel and in the same orientation.
- Nb is 0.008% or more.
- Nb is 0.045% or less.
- B 0.0015 to 0.0030%
- B is an element that contributes to improving the hardenability when contained in a very small amount.
- B needs to contain 0.0015% or more of B.
- the effect is saturated or the formation of Fe boride (Fe-B) makes it impossible to expect the desired effect, which is economically disadvantageous. .
- B is made 0.0015 to 0.0030%.
- B is 0.0020 to 0.0030%.
- Ti 0.005 to 0.020%
- Ti is an element that forms nitrides and reduces the surplus N in the steel to make the effect of B described above effective, and contributes to the prevention of coarsening due to the pinning effect of austenite grains during steel quenching. .
- it is necessary to contain 0.005% or more of Ti.
- the Ti content exceeding 0.020% promotes the formation of coarse MC-type nitride (TiN) during casting, and causes coarsening of austenite grains during quenching. For this reason, Ti is made 0.005 to 0.020%.
- Ti is 0.009% or more.
- Ti is 0.016% or less.
- N 0.005% or less N is an unavoidable impurity in steel and forms MN-type precipitates by combining with nitride-forming elements such as Ti, Nb, and Al. Further, the remaining surplus N that forms these nitrides combines with B to form BN precipitates. At this time, since the effect of improving hardenability due to the addition of B is lost, it is preferable to reduce surplus N as much as possible, and N is set to 0.005% or less.
- Ti / N is specified.
- Ti / N is less than 3.0, surplus N is generated, and as a result of the formation of BN, the solid solution B at the time of quenching is insufficient, so that the microstructure at the end of quenching is martensite and bainite, or martensite and ferrite.
- the stress-strain curve after tempering such a composite structure becomes a continuous yield type, and the value of ⁇ 0.7 / ⁇ 0.4 increases.
- Ti / N exceeds 4.0, the austenite grain pinning effect is reduced by the coarsening of TiN, and the required fine grain structure cannot be obtained. Therefore, Ti / N is set to 3.0 to 4.0.
- the balance other than the above components is Fe and inevitable impurities.
- W 0.1-0.2%
- Zr 0.005-0
- One or two selected from 0.03% may be selected and contained.
- from Ca and Al containing 0.0005 to 0.0030% Ca, mass%, composition ratio (CaO) / (Al 2 O 3 ) ⁇ 4.0, and having a major axis of 5 ⁇ m or more.
- the number of non-metallic inclusions in the oxide-based steel may be 20 or less per 100 mm 2 .
- W 0.1-0.2% W, like Mo, forms carbides and contributes to an increase in strength by precipitation hardening, and also forms a solid solution, segregates at the prior austenite grain boundaries, and contributes to an improvement in resistance to sulfide stress corrosion cracking.
- Zr 0.005 to 0.03%
- Zr is effective in suppressing austenite grain growth during quenching by forming a nitride and pinning the same as Ti.
- Zr is set to 0.005 to 0.03%.
- Ca 0.0005 to 0.0030%
- Ca is effective in preventing nozzle clogging during continuous casting, and in order to obtain a necessary effect, it is desirable to contain 0.0005% or more of Ca.
- Ca forms oxide-based non-metallic inclusions complexed with Al.
- Ca exceeds 0.0030%, a large number of coarse substances exist, and resistance to sulfide stress corrosion cracking is present. Reduce.
- the major axis has a particularly adverse effect, so that the major axis is 5 ⁇ m or more.
- the number of inclusions satisfying the expression (1) is 20 or less per 100 mm 2 .
- the number of inclusions is obtained by taking a sample for a scanning electron microscope (SEM) having a cross section orthogonal to the longitudinal direction of the pipe from an arbitrary circumferential position on the end of the steel pipe. At least the outer surface of the pipe, the center of the wall, It can be calculated from the SEM observation of inclusions at three locations on the surface and the analysis result of the chemical composition with the characteristic X-ray analyzer attached to the SEM. Therefore, when Ca is contained, the Ca content is set to 0.0005 to 0.0030%.
- the number of non-metallic inclusions in the oxide-based steel composed of Ca and Al having a major axis of 5 ⁇ m or more satisfying the following formula (1) in mass% is 20 or less per 100 mm 2.
- Ca is 0.0010% or more.
- Ca is 0.0016% or less.
- the number of inclusions described above is to control the amount of Al input during Al deoxidation treatment after decarburization refining and to add an amount of Ca according to the analytical values of Al, O, and Ca in the molten steel before Ca addition. Can be controlled.
- the manufacturing method of the steel pipe material having the above composition is not particularly limited, but the molten steel having the above composition is melted by a generally known melting method such as a converter, an electric furnace or a vacuum melting furnace. And then cast into a rectangular slab with a continuous casting method or ingot-splitting rolling method, soak the bloom slab at 1250 ° C for 20 hours or more, and then hot-roll the steel pipe material. It is preferable to reduce Mo segregation by forming into a round cross-section billet.
- the steel pipe material is formed into a seamless steel pipe by hot forming.
- hot rolling is performed until appropriate diameter reduction rolling.
- DQ direct quenching
- the microstructure at the end of DQ becomes a composite structure such as martensite and bainite or martensite and ferrite, the crystal grain size of steel after the quenching and tempering heat treatment, 2
- the end of hot rolling is 950 ° C.
- the temperature of the steel pipe at the end of DQ is 200 ° C. or less.
- the steel pipe is quenched (Q) and tempered (T) in order to achieve a target yield strength of 758 MPa or more. From the viewpoint of refining the crystal grains of the steel, it is preferable to repeat the quenching and tempering heat treatment at least twice.
- the quenching temperature at this time is preferably 930 ° C. or less from the viewpoint of finer graining.
- the quenching temperature is preferably 860 to 930 ° C.
- the tempering temperature needs to be not more than Ac 1 temperature, but if it is less than 650 ° C., the secondary precipitation amount of Mo cannot be secured, and therefore it is preferably at least 650 ° C. or more.
- the cumulative frequency ratio of Mo segregation degree 1.5 or more of the total thickness of the longitudinal cross section of the tube is 1% or less.
- segregation of Mo affects the decrease of the K ISSC value.
- the inventors divided the Mo concentration (EPMA Mo value) at each measurement point obtained by EPMA surface analysis by the average value (EPMA Mo ave.) Of all measurement points.
- a value was defined as the Mo segregation degree, and a method for defining a Mo segregation state capable of suppressing a decrease in the K ISSC value was derived from a cumulative frequency rate graph obtained by statistically processing the Mo segregation degree.
- the Mo segregation degree is 1.5 or more and the local hardness of the segregation part is remarkably increased.
- the cumulative frequency ratio is 1% or less, the K ISSC value is hardly affected. Then, let the cumulative frequency rate of the measurement point whose Mo segregation degree is 1.5 or more be 1% or less.
- Mo segregation is not reduced by directly casting the steel pipe material into a round billet, but once it is made into a bloom slab, it is formed into a round billet by hot rolling after high-temperature and long-time soaking treatment of the bloom slab, Even in the case of a direct cast round billet, it can be achieved by a method of performing a normalizing process for a long time after hot rolling on a seamless steel pipe, before quenching and tempering.
- EPMA measurement uses the sample of the tube length orthogonal cross-section full thickness sample
- region is all the thickness directions.
- the measurement conditions for EPMA are an acceleration voltage of 20 kV, a beam current of 0.5 ⁇ A, and a beam diameter of 10 ⁇ m. The above rectangular region is measured, and the Mo concentration (% by mass) is calculated for each individual measurement point using a calibration curve prepared in advance from the characteristic X-ray intensity of Mo—K shell excitation.
- the value ( ⁇ 0.7 / ⁇ 0.4 ) of the ratio of the stress at the time of 0.7% strain ( ⁇ 0.7 ) to the stress at the time of 0.4% strain ( ⁇ 0.4 ) in the stress-strain curve is 1.02 or less
- the variation of the K ISSC value varies greatly depending on the shape of the stress-strain curve of the steel.
- the value ( ⁇ 0 ) of the ratio of the stress at the time of 0.7% strain ( ⁇ 0.7 ) to the stress at the time of 0.4% strain ( ⁇ 0.4 ). .7 / ⁇ 0.4 ) is 1.02 or less, it has been found that variation in K ISSC value is reduced. Therefore, ⁇ 0.7 / ⁇ 0.4 is set to 1.02 or less.
- the yield strength, the stress at 0.4% strain ( ⁇ 0.4 ), and the stress at 0.7% strain ( ⁇ 0.7 ) are measured by a tensile test based on JIS Z2241. be able to.
- microstructure of the present invention is not particularly limited, but the main phase is martensite, and the other remaining structures are one type or two types or more of ferrite, retained austenite, pearlite, bainite, etc. And if it is 5% or less, the objective of this invention can be achieved.
- the steel pipe is cooled to the room temperature (35 ° C. or less) by direct quenching (DQ) or air cooling (0.1 to 0.3 ° C./s), and then the steel pipe heat treatment conditions shown in Tables 2 to 4 (Q1 temperature: 1 Heat treatment was performed at the second quenching temperature, T1 temperature: first tempering temperature, Q2 temperature: second quenching temperature, T2 temperature: second tempering temperature).
- Q1 temperature 1 Heat treatment was performed at the second quenching temperature
- T1 temperature first tempering temperature
- Q2 temperature second quenching temperature
- T2 temperature second tempering temperature
- a normalization (N) process was performed in which the steel pipe was heated to 1100 ° C. or higher, held for at least 5 hours and then air-cooled.
- an EPMA measurement sample having a cross section perpendicular to the longitudinal direction of the pipe, and a tensile test piece and a DCB test piece were taken in parallel with the longitudinal direction of the pipe from one arbitrary position in the circumferential direction of the pipe end. Three or more DCB test pieces were collected from each steel pipe.
- EPMA quantitative surface analysis was performed on a predetermined rectangular area under the conditions of an acceleration voltage of 20 kV, a beam current of 0.5 ⁇ A, and a beam diameter of 10 ⁇ m (number of measurement points: 6750000), and characteristics of Mo-K shell excitation.
- the Mo concentration was calculated for each individual measurement point. This value was divided by the average value of all measurement points to obtain the Mo segregation degree, and after statistical processing, a cumulative frequency rate graph was created to read the cumulative frequency rate of the measurement points having a Mo segregation degree of 1.5 or more.
- the DCB test was implemented based on NACETM0177 methodD using the extract
- the test bath for the DCB test was a 5 mass% NaCl + 0.5 mass% CH 3 COOH aqueous solution at 24 ° C. saturated with 1.0 atm (0.1 MPa) hydrogen sulfide gas. After immersing the DCB test piece in which the wedge was introduced into this test bath under predetermined conditions for 336 hours, the length a of the crack generated in the DCB test piece during the immersion and the wedge opening stress P were measured, and the following equation (2 ) To calculate K ISSC (MPa ⁇ m).
- h is the height of each arm of the DCB test piece
- B is the thickness of the DCB test piece
- Bn is the web thickness of the DCB test piece.
- Comparative Examples 16 (steel No. J), 18 (steel No. L), 20 (steel No. N), 21 (steel) in which the chemical compositions C, Mn, Cr, and Mo were below the lower limit of the scope of the present invention. No. O) could not achieve the target yield strength of 758 MPa or more.
- Comparative Example 23 (steel No. Q) in which the chemical composition B was below the lower limit of the range of the present invention had a large K ISSC value as a result of ⁇ 0.7 / ⁇ 0.4 being out of the range of the present invention. As a result, it was not satisfied that 2 out of 3 DCB tests were over 26.4 MPa ⁇ m.
- Comparative Example 24 (steel No. R) in which the Ti / N ratio was lower than the lower limit of the inventive example, ⁇ 0.7 / ⁇ 0.4 was outside the scope of the present invention, and as a result, the K ISSC value varied greatly. Two of the three DCB tests did not satisfy the target of 26.4 MPa ⁇ m or more. Further, in Comparative Example 25 (steel No. S) in which the Ti / N ratio exceeded the upper limit of the inventive example, ⁇ 0.7 / ⁇ 0.4 was out of the scope of the present invention, and as a result, the K ISSC value was large. Variations did not satisfy the target of 26.4 MPa ⁇ m or more, one of the three DCB tests.
- the steel having the composition shown in Table 5 by the converter method After melting the steel having the composition shown in Table 5 by the converter method, it was made into a bloom slab by the continuous casting method.
- the bloom slab was formed into a billet with a round cross section by hot rolling. Further, using this billet as a raw material, after heating to the billet heating temperature shown in Table 6, the Mannesmann piercing-plug mill rolling-reducing rolling is carried out hot, and the rolling is finished at the rolling completion temperature shown in Table 6 to be seamless. Molded into a steel pipe.
- the steel pipe is directly quenched (DQ) or cooled to air cooling (0.2 to 0.5 ° C / s) chamber temperature (35 ° C or less), and then the steel pipe heat treatment conditions shown in Table 6 (Q1 temperature: first quenching) Temperature, T1 temperature: first tempering temperature, Q2 temperature: second quenching temperature, T2 temperature: second tempering temperature).
- Q1 temperature first quenching
- T1 temperature first tempering temperature
- Q2 temperature second quenching temperature
- T2 temperature second tempering temperature
- the DCB test was implemented based on NACETM0177 methodD using the extract
- the test bath for the DCB test was a 5 mass% NaCl + 0.5 mass% CH 3 COOH aqueous solution at 24 ° C. saturated with 1.0 atm (0.1 MPa) hydrogen sulfide gas. After immersing the DCB test piece into which the wedge was introduced into this test bath under predetermined conditions for 336 hours, the length a of the crack generated in the DCB test piece during the immersion and the wedge opening stress P were measured. K ISSC (MPa ⁇ m) was calculated.
- Steel pipes 2-1 to 2-5 in which the chemical composition and the number of inclusions, the cumulative frequency rate of EPMA measurement points having a Mo segregation degree of 1.5 or more, and ⁇ 0.7 / ⁇ 0.4 were within the scope of the present invention is a both yield strength 758MPa or more, K ISSC values obtained in DCB tests each triplet was satisfied all 26.4MPa ⁇ m to target without significantly vary either.
- Comparative Example 2-6 (steel No. Y) in which the upper limit of Ca exceeded the upper limit of the range of the present invention, the K ISSC value greatly varied, and one target in 3 DCB tests was 26.4 MPa. ⁇ m was not satisfied.
- Comparative Example 2-7 (steel No. Z), considering that the amount of Ca in the molten steel before addition of Ca is high due to impurities Ca contained in the alloy iron of other elements added during secondary refining. Ca was within the scope of the present invention because Ca was added, but the number of non-metallic inclusions in the oxide-based steel composed of Ca and Al satisfying the formula (1) was 5 mm or more. The upper limit of the range was exceeded, the K ISSC value varied greatly, and one of the three DCB tests did not satisfy the target of 26.4 MPa ⁇ m.
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Abstract
Description
[1]質量%で、
C:0.25~0.31%、
Si:0.01~0.35%、
Mn:0.55~0.70%、
P:0.010%以下、
S:0.001%以下、
O:0.0015%以下、
Al:0.015~0.040%、
Cu:0.02~0.09%、
Cr:0.8~1.5%、
Mo:0.9~1.6%、
V:0.04~0.10%、
Nb:0.005~0.05%、
B:0.0015~0.0030%、
Ti:0.005~0.020%、
N:0.005%以下、
を含有し、
N含有量に対するTi含有量の比の値(Ti/N)が3.0~4.0であり、
残部Feおよび不可避的不純物からなる組成を有し、
下記(A)式で定義される、管長手直交断面全厚でのMo偏析度1.5以上となる測定点の累積度数率が1%以下であり、
応力-歪曲線における0.4%歪時の応力に対する0.7%歪時の応力の比の値(σ0.7/σ0.4)が1.02以下である、肉厚40mm以上、かつ、降伏強度が758MPa以上である油井用低合金高強度厚肉継目無鋼管。
Mo偏析度=EPMA Mo値/EPMA Mo ave. ・・・(A)
(式(A)中、
EPMA Mo値は、EPMA定量面分析時の個々測定点のMo濃度(質量%)であり、
EPMA Mo ave.は、EPMA定量面分析時の全測定点の平均Mo濃度(質量%である。)
[2]前記組成に加えてさらに、質量%で、
W:0.1~0.2%、
Zr:0.005~0.03%
のうちから選ばれた1種または2種を含有する[1]に記載の油井用低合金高強度厚肉継目無鋼管。
[3]前記組成に加えてさらに、質量%で、
Ca:0.0005~0.0030%
を含有し、さらに、質量%で、組成比が下記(1)式を満足する長径5μm以上のCaとAlとからなる酸化物系の鋼中非金属介在物の個数が100mm2当り20個以下である[1]または[2]に記載の油井用低合金高強度厚肉継目無鋼管。
(CaO)/(Al2O3)≧4.0 (1)
Mo偏析度=EPMA Mo値/EPMA Mo ave. ・・・(A)
(式(A)中、
EPMA Mo値は、EPMA定量面分析時の個々測定点のMo濃度(質量%)であり、
EPMA Mo ave.は、EPMA定量面分析時の全測定点の平均Mo濃度(質量%である。)
まず、本発明鋼管の化学組成限定理由について説明する。以下、特に断わらないかぎり質量%は単に%で記す。
Cは、鋼の強度を増加させる作用を有し、所望の高強度を確保するために重要な元素である。また、Cは、焼入れ性を向上させる元素であり、特に肉厚40mm以上の厚肉の継目無鋼管において、降伏強度が758MPa以上の高強度化を実現するためには、0.25%以上のCの含有を必要とする。一方、0.31%を超えるCの含有は、σ0.7/σ0.4の著しい上昇を引き起こし、KISSC値のばらつきを大きくする。このため、Cは0.25~0.31%とする。好ましくは、Cは0.29%以下である。
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、焼戻時の急激な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、0.01%以上のSiの含有を必要とする。一方、0.35%を超えるSiの含有は、粗大な酸化物系介在物を形成し、KISSC値のばらつきを大きくする。このため、Siは0.01~0.35%とする。好ましくは、Siは0.01~0.04%である。
Mnは、焼入れ性の向上を介して、鋼の強度を増加させるとともに、Sと結合しMnSとしてSを固定して、Sによる粒界脆化を防止する作用を有する元素であり、特に肉厚40mm以上の厚肉の継目無鋼管において、降伏強度758MPa以上の高強度化をするためには、0.55%以上のMnの含有を必要とする。一方、0.70%を超えるMnの含有は、σ0.7/σ0.4の著しい上昇を引き起こし、KISSC値のばらつきを大きくする。このため、Mnは0.55~0.70%とする。好ましくは、Mnは0.55~0.65%である。
Pは、固溶状態では粒界等に偏析し、粒界脆化割れ等を引き起こす傾向を示し、本発明ではできるだけ低減することが望ましいが、0.010%までは許容できる。このようなことから、Pは0.010%以下とする。
Sは、鋼中ではほとんどが硫化物系介在物として存在し、延性、靭性や、耐硫化物応力腐食割れ性等の耐食性を低下させる。Sの一部は固溶状態で存在する場合があるが、その場合には粒界等に偏析し、粒界脆化割れ等を引き起こす傾向を示す。このため、Sは、本発明ではできるだけ低減することが望ましいが、過剰な低減は精錬コストを高騰させる。このようなことから、本発明では、Sは、その悪影響が許容できる0.001%以下とする。
O(酸素)は不可避的不純物として、AlやSi等の酸化物として鋼中に存在する。特に、その粗大な酸化物の数が多いと、KISSC値のばらつきを大きくする要因となる。このため、O(酸素)は、その悪影響が許容できる0.0015%以下とする。好ましくは、O(酸素)は0.0010%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AlNを形成して固溶Nの低減に寄与する。このような効果を得るために、Alは0.015%以上の含有を必要とする。一方、0.040%を超えてAlを含有すると、酸化物系介在物が増加しKISSC値のばらつきを大きくする。このため、Alは0.015~0.040%とする。好ましくは、Alは0.020%以上である。好ましくは、Alは0.030%以下である。
Cuは、耐食性を向上させる作用を有する元素であり、微量添加した場合、緻密な腐食生成物が形成され、SSCの起点となるピットの生成・成長が抑制されて、耐硫化物応力腐食割れ性が顕著に向上するため、本発明では、0.02%以上のCuの含有を必要とする。一方、0.09%を超えてCuを含有すると、継目無鋼管の製造プロセス時の熱間加工性が低下する。このため、Cuは0.02~0.09%とする。好ましくは、Cuは0.03%以上である。好ましくは、Cuは0.05%以下である。
Crは、焼入れ性の増加を介して、鋼の強度の増加に寄与するとともに、耐食性を向上させる元素である。また、Crは、焼戻時にCと結合し、M3C系、M7C3系、M23C6系等の炭化物を形成し、とくにM3C系炭化物は焼戻軟化抵抗を向上させ、焼戻しによる強度変化を少なくして、降伏強度の向上に寄与する。758MPa以上の降伏強度の達成には、0.8%以上のCrの含有を必要とする。一方、1.5%を超えてCrを含有しても、効果が飽和するため、経済的に不利となる。このため、Crは0.8~1.5%とする。好ましくは、Crは0.9%以上である。好ましくは、Crは1.3%以下である。
Moは、焼入れ性の増加を介して、鋼の強度の増加に寄与するとともに、耐食性を向上させる元素である。また、Moは、M2C系の炭化物を形成し、特に焼戻し後に2次析出するM2C系炭化物は焼戻軟化抵抗を向上させ、焼戻による強度変化を少なくして、降伏強度の向上に寄与し、鋼の応力-歪曲線を連続降伏型から降伏型の形状にさせる効果がある。特に、肉厚40mm以上の厚肉の継目無鋼管において、このような効果を得るためには、0.9%以上のMoの含有を必要とする。一方、1.6%を超えてMoを含有すると、Mo2C炭化物が粗大化し、硫化物応力腐食割れの起点となってむしろKISSC値が低下する原因となる。このため、Moは0.9~1.6%とする。好ましくは、Moは0.9~1.5%である。
Vは、炭化物あるいは窒化物を形成し、鋼の強化に寄与する元素である。特に肉厚40mm以上の厚肉の継目無鋼管において、このような効果を得るためには、0.04%以上のVの含有を必要とする。一方、0.10%を超えてVを含有すると、V系炭化物が粗大化して硫化物応力腐食割れの起点となり、むしろKISSC値が低下する。このため、Vは0.04~0.10%の範囲とする。好ましくは、Vは0.045%以上である。好ましくは、Vは0.055%以下である。
Nbは、オーステナイト(γ)温度域での再結晶を遅延させ、γ粒の微細化に寄与し、焼入直後の鋼の下部組織(例えばパケット、ブロック、ラス)の微細化に極めて有効に作用する元素である。このような効果を得るためには、0.005%以上の含有を必要とする。一方、0.05%を超えるNbの含有は、粗大な析出物(NbN)の析出を促進し、耐硫化物応力腐食割れ性の低下を招く。このため、Nbは0.005~0.05%とする。ここで、パケットとは、平行に並んだ同じ晶癖面を持つラスの集団から成る領域と定義され、ブロックは、平行でかつ同じ方位のラスの集団から成る。好ましくは、Nbは0.008%以上である。好ましくは、Nbは0.045%以下である。
Bは、微量の含有で焼入れ性向上に寄与する元素であり、本発明では0.0015%以上のBの含有を必要とする。一方、0.0030%を超えてBを含有しても、効果が飽和するかあるいはFe硼化物(Fe-B)の形成により、逆に所望の効果が期待できなくなり、経済的に不利となる。このため、Bは0.0015~0.0030%とする。好ましくは、Bは0.0020~0.0030%である。
Tiは、窒化物を形成し、鋼中の余剰Nを低減させて上述のBの効果を有効にするほか、鋼の焼入れ時にオーステナイト粒をピン止め効果によって粗大化の防止に寄与する元素である。このような効果を得るためには、0.005%以上のTiを含有することを必要とする。一方、0.020%を超えるTiの含有は、鋳造時に粗大なMC型窒化物(TiN)の形成が促進され、かえって焼入れ時のオーステナイト粒の粗大化を招く。このため、Tiは0.005~0.020%とする。好ましくは、Tiは0.009%以上である。好ましくは、Tiは0.016%以下である。
Nは、鋼中不可避的不純物であり、Ti、Nb、Al等の窒化物形成元素と結合しMN型の析出物を形成する。さらに、これらの窒化物を形成した残りの余剰Nは、Bと結合してBN析出物も形成する。この際、B添加による焼入れ性向上効果が失われるため、余剰Nはできるだけ低減することが好ましく、Nは0.005%以下とする。
Ti含有によるTiN窒化物形成でのオーステナイト粒ピン止め効果、および余剰N抑制によるBN形成防止を通じたB添加による焼入れ性向上効果を両立させるために、Ti/Nを規定する。Ti/Nが3.0を下回る場合、余剰Nが発生し、BN形成することで焼入れ時の固溶Bが不足する結果、焼入れ終了時のミクロ組織がマルテンサイトとベイナイト、あるいはマルテンサイトとフェライトの複合組織となり、このような複合組織を焼戻した後の応力-歪曲線が連続降伏型となって、σ0.7/σ0.4の値が上昇する。一方、Ti/Nが4.0を超える場合、TiNの粗大化によってオーステナイト粒ピン止め効果が低減し、必要とする細粒組織が得られない。このため、Ti/Nは3.0~4.0とする。
Wは、Moと同様に、炭化物を形成し析出硬化により強度の増加に寄与するとともに、固溶して、旧オーステナイト粒界に偏析して耐硫化物応力腐食割れ性の向上に寄与する。このような効果を得るためには、0.1%以上のWを含有することが望ましいが、0.2%を超えるWの含有は、耐硫化物応力腐食割れ性を低下させる。このため、Wを含有する場合、Wは0.1~0.2%とする。
ZrはTiと同様に、窒化物を形成しピン止め効果によって、焼入れ時のオーステナイト粒成長抑制に有効である。必要な効果を得るためには、0.005%以上のZrを含有することが望ましい。一方、0.03%を超えてZrを含有しても効果が飽和する。このため、Zrは0.005~0.03%とする。
Caは、連続鋳造時のノズル詰まり防止に有効で、必要な効果を得るためには0.0005%以上のCaを含有することが望ましい。一方、Caは、Alと複合した酸化物系非金属介在物を形成し、特に0.0030%を超えてCaを含有した場合、粗大なものが多数存在し、耐硫化物応力腐食割れ性を低下させる。具体的には、Ca酸化物(CaO)とAl酸化物(Al2O3)との組成比が、質量%で(1)式を満たす介在物が特に悪影響を及ぼすことから、長径が5μm以上かつ(1)式を満たす介在物の個数を100mm2当り20個以下とすることが望ましい。なお、この介在物の個数は、鋼管管端の周方向任意1箇所より管長手直交断面の走査型電子顕微鏡(SEM)用試料を採取し、該試料について、少なくとも管外面、肉厚中央、管内面の3か所について介在物のSEM観察、およびSEMに付随する特性X線分析装置での化学組成の分析結果によって算出することができる。このため、Caを含有する場合、Caは0.0005~0.0030%とする。また、この場合、質量%で、組成比が下記(1)式を満足する長径5μm以上のCaとAlとからなる酸化物系の鋼中非金属介在物の個数が100mm2当り20個以下であるようにする。好ましくは、Caは0.0010%以上である。好ましくは、Caは0.0016%以下である。
(CaO)/(Al2O3)≧4.0 (1)
上記の介在物の個数は、脱炭精錬終了後に行うAl脱酸処理時のAl投入量の管理、およびCa添加前の溶鋼中Al、O、Ca分析値に応じた量のCaを添加することにより制御することができる。
前述したように、Moの偏析がKISSC値の低下に影響する。このMoの偏析の定量化のために、本発明者らはEPMA面分析で得られる個々測定点のMo濃度(EPMA Mo値)を、全測定点の平均値(EPMA Mo ave.)で割った値をMo偏析度とし、このMo偏析度を統計処理して得られる累積度数率グラフから、KISSC値の低下を抑制しうるMo偏析状態を定義する手法を導出した。そして、Mo偏析度が1.5以上で、偏析部の局所硬さの上昇が著しいが、その累積度数率が1%以下であれば、KISSC値への影響がほとんどなくなることから、本発明では、Mo偏析度が1.5以上の測定点の累積度数率を1%以下とする。Moの偏析の軽減は、鋼管素材を直接丸ビレットに鋳造するのではなく、一旦ブルーム鋳片とし、ブルーム鋳片の高温・長時間の均熱処理後、熱間圧延で丸ビレットに成形するか、直接鋳造丸ビレットの場合でも、継目無鋼管に熱間圧延後、焼入れ、焼戻し前に長時間の焼きならし処理を行う等の方法で達成できる。なお、EPMA測定は、最終焼戻しが終了した段階で採取した管端サンプルの周方向の任意1箇所から、さらに採取した管長手直交断面全厚試料を使用し、その測定領域は肉厚方向全てと、その肉厚の約1/3に相当する周方向で定義される長方形領域とする。EPMAの測定条件は、加速電圧20kV、ビーム電流0.5μA、ビーム径10μmとする。上述の長方形領域の測定を行い、Mo-K殻励起の特性X線強度からあらかじめ作成しておいた検量線を使用して、個々測定点ごとにMo濃度(質量%)を算出する。
前述したように、KISSC値のばらつきは鋼の応力-歪曲線の形状によって大きく異なる。この点について、本発明者等が鋭意研究した結果、0.4%歪時の応力(σ0.4)に対する0.7%歪時の応力(σ0.7)の比の値(σ0.7/σ0.4)が1.02以下の場合に、KISSC値のばらつきが低減することを知見した。このため、σ0.7/σ0.4は1.02以下とする。
(CaO)/(Al2O3)≧4.0 (1)
また、採取したEPMA測定試料を用いて、加速電圧20kV、ビーム電流0.5μA、ビーム径10μmの条件でEPMA定量面分析を所定の長方形領域について行い(測定点数:6750000)、Mo―K殻励起の特性X線強度よりあらかじめ作成しておいた検量線を使用して、個々測定点ごとにMo濃度(質量%)を算出した。この値を全測定点平均値で割ってMo偏析度とし、統計処理の後、累積度数率グラフを作成して、Mo偏析度1.5以上である測定点の累積度数率を読み取った。
Claims (3)
- 質量%で、
C:0.25~0.31%、
Si:0.01~0.35%、
Mn:0.55~0.70%、
P:0.010%以下、
S:0.001%以下、
O:0.0015%以下、
Al:0.015~0.040%、
Cu:0.02~0.09%、
Cr:0.8~1.5%、
Mo:0.9~1.6%、
V:0.04~0.10%、
Nb:0.005~0.05%、
B:0.0015~0.0030%、
Ti:0.005~0.020%、
N:0.005%以下、
を含有し、
N含有量に対するTi含有量の比の値(Ti/N)が3.0~4.0であり、
残部Feおよび不可避的不純物からなる組成を有し、
下記(A)式で定義される、管長手直交断面全厚でのMo偏析度1.5以上となる測定点の累積度数率が1%以下であり、
応力-歪曲線における0.4%歪時の応力に対する0.7%歪時の応力の比の値(σ0.7/σ0.4)が1.02以下である、肉厚40mm以上、かつ、降伏強度が758MPa以上である油井用低合金高強度厚肉継目無鋼管。
Mo偏析度=EPMA Mo値/EPMA Mo ave. ・・・(A)
(式(A)中、
EPMA Mo値は、EPMA定量面分析時の個々測定点のMo濃度(質量%)であり、
EPMA Mo ave.は、EPMA定量面分析時の全測定点の平均Mo濃度(質量%である。) - 前記組成に加えてさらに、質量%で、
W:0.1~0.2%、
Zr:0.005~0.03%
のうちから選ばれた1種または2種を含有する請求項1に記載の油井用低合金高強度厚肉継目無鋼管。 - 前記組成に加えてさらに、質量%で、
Ca:0.0005~0.0030%
を含有し、さらに、質量%で、組成比が下記(1)式を満足する長径5μm以上のCaとAlとからなる酸化物系の鋼中非金属介在物の個数が100mm2当り20個以下である請求項1または2に記載の油井用低合金高強度厚肉継目無鋼管。
(CaO)/(Al2O3)≧4.0 (1)
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