WO2017200033A1 - 継目無鋼管及びその製造方法 - Google Patents
継目無鋼管及びその製造方法 Download PDFInfo
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- WO2017200033A1 WO2017200033A1 PCT/JP2017/018618 JP2017018618W WO2017200033A1 WO 2017200033 A1 WO2017200033 A1 WO 2017200033A1 JP 2017018618 W JP2017018618 W JP 2017018618W WO 2017200033 A1 WO2017200033 A1 WO 2017200033A1
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- Prior art keywords
- pipe
- seamless steel
- steel pipe
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- temperature
- Prior art date
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- 229910000831 Steel Inorganic materials 0.000 title claims abstract description 275
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- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 2
- OKIZCWYLBDKLSU-UHFFFAOYSA-M N,N,N-Trimethylmethanaminium chloride Chemical compound [Cl-].C[N+](C)(C)C OKIZCWYLBDKLSU-UHFFFAOYSA-M 0.000 description 2
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- 229910052727 yttrium Inorganic materials 0.000 description 1
Images
Classifications
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/18—Hardening; Quenching with or without subsequent tempering
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21B—ROLLING OF METAL
- B21B19/00—Tube-rolling by rollers arranged outside the work and having their axes not perpendicular to the axis of the work
- B21B19/02—Tube-rolling by rollers arranged outside the work and having their axes not perpendicular to the axis of the work the axes of the rollers being arranged essentially diagonally to the axis of the work, e.g. "cross" tube-rolling ; Diescher mills, Stiefel disc piercers or Stiefel rotary piercers
- B21B19/04—Rolling basic material of solid, i.e. non-hollow, structure; Piercing, e.g. rotary piercing mills
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/10—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/10—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies
- C21D8/105—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies of ferrous alloys
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- 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
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F16—ENGINEERING ELEMENTS AND UNITS; GENERAL MEASURES FOR PRODUCING AND MAINTAINING EFFECTIVE FUNCTIONING OF MACHINES OR INSTALLATIONS; THERMAL INSULATION IN GENERAL
- F16L—PIPES; JOINTS OR FITTINGS FOR PIPES; SUPPORTS FOR PIPES, CABLES OR PROTECTIVE TUBING; MEANS FOR THERMAL INSULATION IN GENERAL
- F16L9/00—Rigid pipes
- F16L9/02—Rigid pipes of metal
Definitions
- the present invention relates to a seamless steel pipe and a manufacturing method thereof.
- the highly corrosive well is an environment containing a lot of corrosive substances, and the temperature of the highly corrosive well is from room temperature to about 200 ° C.
- the corrosive substance is, for example, a corrosive gas such as hydrogen sulfide.
- Hydrogen sulfide causes sulfide stress cracking (hereinafter referred to as “SSC”) in an oil well pipe made of a high-strength, low-alloy steel seamless steel pipe. Therefore, high SSC resistance is required for seamless steel pipes used in these highly corrosive wells.
- the oil well pipe used for the above-described highly corrosive well is required to have high strength.
- SSC resistance and strength are generally contradictory properties. Therefore, if the strength of the seamless steel pipe is increased, the SSC resistance of the seamless steel pipe is lowered.
- the main yield strength required for oil well pipes for the above-mentioned highly corrosive well applications is from 95 ksi (655 MPa) to less than 125 ksi (862 MPa). Therefore, there is a demand for a seamless steel pipe having excellent SSC resistance in a highly corrosive well even if it has a high yield strength of 655 MPa to less than 862 MPa.
- quenching and tempering are performed on the raw pipe after hot pipe making. There are two methods of quenching: offline quenching and in-line quenching.
- a seamless steel pipe is produced by hot pipe making (piercing, stretching rolling, and constant diameter rolling), and then cooled to room temperature. Thereafter, the raw tube is reheated and quenched (rapidly cooled).
- the quenching apparatus is disposed off-line apart from a pipe making line including a piercing mill, a drawing mill, a constant diameter mill, and a conveying line connecting these mills.
- offline quenching reverse transformation from ferrite to austenite occurs in the steel during heating before quenching. Thereby, the structure of steel is refined and the SSC resistance is increased.
- the raw tube after hot pipe forming is cooled to near room temperature, and then the raw tube is reheated with an off-line quenching apparatus to perform quenching. Therefore, productivity is low.
- in-line quenching in which quenching is performed on a pipe manufacturing line can increase productivity.
- a quenching device is arranged on the transfer line in the pipe making line. After manufacturing seamless steel pipes by pipe making processes (piercing, drawing rolling, constant diameter rolling, etc.), without cooling to room temperature, or after slightly heating in a reheating furnace, quenching is performed on the pipe making line. carry out.
- In-line quenching cannot use reverse transformation as in offline quenching, but can increase productivity. Therefore, there is a demand for a seamless steel pipe that can be manufactured not only by off-line quenching but also by in-line quenching and can achieve both high strength and excellent SSC resistance.
- Patent Document 1 Japanese Patent Application Laid-Open No. 2007-31756 (Patent Document 1) and International Publication No. 2008/123422 (Patent Document). 2).
- Patent Document 1 by mass, C: 0.15 to 0.20%, Si: 0.01% or more and less than 0.15%, Mn: 0.05 to 1.0%, Cr: 0.05 to 1.5%, Mo: 0.05 to 1.0%, Al: 0.10% or less, V: 0.01 to 0.2%, Ti: 0.002 to 0.03%, B: 0.00.
- a steel ingot containing 0003 to 0.005% and N: 0.002 to 0.01% with the balance being Fe and impurities is used. The steel ingot is heated to a temperature of 1000 to 1250 ° C., and the final rolling temperature is set to 900 to 1050 ° C. to finish the pipe-rolling.
- Patent Document 1 describes that a seamless steel pipe manufactured by this manufacturing method has 110 ksi class strength (758 to 861 MPa), high strength, excellent toughness, and SSC resistance. ing.
- the value of Ceq determined by / 5 is 0.65 or more, the balance is Fe and impurities, P in the impurities is 0.025% or less, S is 0.010% or less, and N is 0.007% or less And a steel slab in which B is less than 0.0003%.
- Patent Document 2 describes that a seamless steel pipe manufactured by this manufacturing method has high strength, excellent toughness, and SSC resistance.
- Patent Document 1 and Patent Document 2 may not provide excellent SSC resistance. Further, seamless steel pipes used as oil well pipes for highly corrosive well applications are required to suppress variations in strength in the circumferential direction and the axial direction for quality control. In particular, when in-line quenching is performed in the manufacturing process, stable yield strength may not be obtained.
- An object of the present invention is to provide a seamless steel pipe capable of achieving both high yield strength of 95 ksi (655 MPa) to less than 125 ksi (862 MPa) and excellent SSC resistance, and suppressing variation in strength in the circumferential and axial directions. It is to provide a manufacturing method.
- the seamless steel pipe according to this embodiment is a seamless steel pipe having a first pipe end and a second pipe end.
- the chemical composition of the seamless steel pipe was, by mass, C: 0.21 to 0.35%, Si: 0.10 to 0.50%, Mn: 0.05 to 1.00%, P: 0.00. 025% or less, S: 0.010% or less, Al: 0.005 to 0.100%, N: 0.010% or less, Cr: 0.05 to 1.50%, Mo: 0.10 to 1.
- a first pipe end region in a range from the first pipe end toward the second pipe end in the axial direction of the seamless steel pipe to a position of 500 mm, and from the second pipe end toward the first pipe end.
- the crystal grain size number of the former austenite crystal grain according to ASTM E112 is 7.0 or more, and the main body region
- the difference between the maximum value and the minimum value of the grain size number is 1.0 or less
- the yield strength in the main body region is less than 655 to 862 MPa
- the difference between the maximum value and the minimum value of the tensile strength in the main body region is 27. .6 MPa or less.
- the production method of the seamless steel pipe according to the present embodiment is, in mass%, C: 0.21 to 0.35%, Si: 0.10 to 0.50%, Mn: 0.05 to 1.00%, P : 0.025% or less, S: 0.010% or less, Al: 0.005 to 0.100%, N: 0.010% or less, Cr: 0.05 to 1.50%, Mo: 0.10 To 1.50%, Nb: 0.010 to 0.050%, B: 0.0003 to 0.0050%, Ti: 0.002 to 0.050%, V: 0 to 0.30%, Ca: A round billet containing 0 to 0.0050% and rare earth element: 0 to 0.0050%, with the balance being Fe and impurities, heated to 950 to 1100 ° C., and the round billet has an inclined roll Using a piercing machine, pierce with an inclined roll at 20 to 75 rpm, and then perform rolling.
- a pipe manufacturing process to produce a mother tube the final mother pipe temperature during rolling to 800 ⁇ 1000 ° C. and is manufactured by the tube process, a raw tube outer surface temperature A 3 transformation point ⁇ 1000 ° C. quenching
- the production method of the seamless steel pipe according to the present embodiment is, in mass%, C: 0.21 to 0.35%, Si: 0.10 to 0.50%, Mn: 0.05 to 1.00%, P : 0.025% or less, S: 0.010% or less, Al: 0.005 to 0.100%, N: 0.010% or less, Cr: 0.05 to 1.50%, Mo: 0.10 To 1.50%, Nb: 0.010 to 0.050%, B: 0.0003 to 0.0050%, Ti: 0.002 to 0.050%, V: 0 to 0.30%, Ca: A round billet containing 0 to 0.0050% and rare earth element: 0 to 0.0050%, with the balance being Fe and impurities, heated to 950 to 1100 ° C., and the round billet has an inclined roll Using a perforator, the peripheral speed of the inclined roll is perforated at 1450-5550 mm / second, and Conducted extended to produce a mother tube with a pipe-step the final mother pipe temperature during rolling to 800 ⁇ 1000
- a quenching process in which the average cooling rate from the outer surface temperature of the raw tube at the start of the rapid cooling to the outer surface temperature reaches 300 ° C is 15 ° C / second or more, and the outer surface of the raw tube quenched by the quenching process And a tempering step of producing a seamless steel pipe having a yield strength of less than 655 to 862 MPa by performing tempering at a temperature of 650 ° C. to Ac 1 transformation point.
- the seamless steel pipe according to the present embodiment can achieve both high yield strength of 95 ksi (655 MPa) to less than 125 ksi (862 MPa) and excellent SSC resistance, and can suppress variations in strength in the circumferential and axial directions.
- the manufacturing method according to this embodiment can manufacture the seamless steel pipe.
- FIG. 1 is a diagram showing a relationship between a steel material temperature during hot working and a grain size number of prior austenite crystal grains, assuming a raw pipe during pipe production.
- FIG. 2 is a perspective view of the seamless steel pipe of the present embodiment.
- FIG. 3 is a perspective view of a test piece used when measuring the prior austenite grain size.
- the present inventors have studied a method for obtaining excellent SSC resistance in a seamless steel pipe having a high yield strength of 95 ksi (655 MPa) to less than 125 ksi (862 MPa). As a result, the present inventors obtained the following knowledge.
- the seamless steel pipe of this embodiment contains 0.010 to 0.050% of Nb, thereby making the austenite crystal grains coarse in hot pipe making use of the pinning effect of Nb carbonitride and the like. Suppress.
- the grain size number in accordance with ASTM E112 is 7.0 or more,
- the difference between the maximum value and the minimum value of the crystal grain size number (hereinafter referred to as crystal grain size difference ⁇ GS) is set to 1.0 or less.
- the strength variation in the circumferential direction and the axial direction of the seamless steel pipe is that Nb dissolved in steel without being precipitated as Nb carbonitride or Nb nitride in the raw pipe after the quenching process and before the tempering process. This is due to the ratio (hereinafter referred to as Nb solid solution rate).
- Nb solid solution rate the ratio in the raw pipe after the quenching process and before the tempering process.
- the smaller the variation in the Nb solid solution ratio in the circumferential direction and the axial direction the more the intensity variation in the axial direction of the seamless steel pipe after the tempering process can be reduced.
- the Nb solid solution rate difference ⁇ SR the difference between the maximum value and the minimum value of the Nb solid solution rate in the circumferential direction and the axial direction in the main body region of the tube. Is 10% or less, the difference between the maximum value and the minimum value of the tensile strength in the circumferential direction and the axial direction in the main body region of the seamless steel pipe after the tempering process (hereinafter referred to as the tensile strength difference ⁇ TS) is 27. It becomes 6 MPa or less, and the strength variation can be sufficiently suppressed.
- yield strength is less than 655 to 862 MPa
- grain size number of old austenite grains conforming to ASTM E112 is 7.0 or more
- grain size difference ⁇ GS is 1.0 or less
- tensile strength difference ⁇ TS is 27.6 MPa or less.
- a seamless steel pipe having a can be manufactured by in-line quenching or offline quenching by satisfying the following production conditions.
- the present inventors investigated and examined a method for refining prior austenite crystal grains without using reverse transformation.
- the inventors set the rolling ratio (final finish length / billet length) during the normal pipe making process to 1.6 to 13.0 (the rolling ratio during the drilling process is 1.2 to 4.0).
- the relationship between the tube temperature and the prior austenite crystal grain size was investigated when the tube was manufactured by the following method.
- FIG. 1 is a diagram showing the relationship between the steel material temperature during hot working and the prior austenite grain size number in accordance with ASTM E112, assuming a raw tube during hot pipe production.
- FIG. 1 was obtained by the following method.
- a steel material (steel plate) satisfying the above chemical composition was produced.
- a round bar-shaped test piece having a diameter of 8 mm and a length of 12 mm was collected from the steel plate.
- a hot working test (cermec master test) was performed on the test piece.
- a cermek master Z (trade name) testing machine manufactured by Fuji Electric Koki was used.
- the environment of the compression test was a vacuum atmosphere. The test piece was heated to a predetermined temperature.
- the test piece After the test piece reached a predetermined temperature, the test piece was soaked for 5 minutes while applying a predetermined compressive strain (length change 50%) assuming a rolling ratio during normal pipe making. After soaking, the test piece was quenched with He gas. The prior austenite crystal grain size was measured at the center of the test piece after quenching, and the average value was defined as the prior austenite crystal grain size ( ⁇ m). The obtained prior austenite grain size was converted to the grain size number of the former austenite grain according to ASTM E112. FIG. 1 was prepared based on the obtained prior austenite grain size number.
- the prior austenite grain size number in the steel material is 4.0 or less. That is, the prior austenite crystal grains become coarse.
- the former austenite grain size number in the steel after hot working is substantially constant at 9.0 or more. That is, the grain size number of the prior austenite crystal grains changes discontinuously when the steel material temperature during hot pipe production is around 1100 ° C.
- the seamless steel pipe after pipe making can be made 7.0 or more in terms of the crystal grain size number based on ASTM E112.
- Nb in the steel combines with carbon and / or nitrogen to form fine Nb carbonitride and the like.
- Nb carbonitride, etc. exhibits a pinning effect, suppresses the austenite crystal grains from becoming coarse, and maintains the austenite crystal grains in a fine state.
- the raw tube temperature in hot pipe production exceeds 1100 ° C.
- the Nb carbonitride and the like once produced will be dissolved.
- austenite crystal grains become coarse.
- the old austenite crystal grains of the seamless steel pipe after pipe making are less than 7.0 in terms of the crystal grain size number according to ASTM E112.
- processing heat is generated when a material (round billet, raw pipe) is processed.
- the present inventors paid attention to this processing heat generation. Even if the heating temperature of the round billet before hot pipe production is set to 1100 ° C. or less, processing heat is generated unevenly in the axial direction and the circumferential direction of the raw pipe, If a portion exceeding 1100 ° C. occurs, the following phenomenon occurs. As described above, Nb carbonitride and the like are dissolved in the portion where the temperature exceeds 1100 ° C. due to processing heat generation. In the subsequent process, a part of the solid solution Nb is precipitated, but the amount of Nb carbonitride deposited is smaller than that in the part where there was no processing heat generation.
- the prior austenite crystal grains become coarse.
- Nb carbonitride exhibits a pinning effect, so that the prior austenite crystal grains become fine.
- the crystal grain size difference ⁇ GS of the prior austenite crystal grains exceeds 1.0, and the SSC resistance decreases.
- the pipe making process includes a drilling process and a rolling process.
- the rolling process includes, for example, a stretching rolling process and a constant diameter rolling process performed after stretching rolling.
- a round billet is pierced and rolled into a raw pipe using a piercing machine.
- the drawing and rolling process the raw tube is drawn and rolled using a drawing and rolling mill.
- the drawing mill is, for example, a plug mill or a mandrel mill.
- the constant diameter rolling step the raw pipe is constant diameter rolled using a constant diameter rolling mill.
- the constant diameter rolling mill is, for example, a sizer or a stretch reducer.
- the round billet which is a raw material is heated to 1100 ° C. or lower in a heating furnace in the pipe making process, processing heat is generated in the drilling process or the rolling process, and as a result, the raw material temperature is 1100 ° C. May exceed In this case, as described above, the prior austenite crystal grains become coarse or the crystal grain size difference ⁇ GS increases.
- the process having the highest rolling ratio among the drilling process and the rolling process is the drilling process.
- the round billet is pierced and rolled using a piercing machine having a pair of inclined rolls.
- the rotation speed (rpm) of the inclined roll is related to the processing heat generation amount. Specifically, if the peripheral speed of the inclined roll is high, the heat generated by the processing increases. If the peripheral speed is low, the heat generated by the work is suppressed.
- the heating temperature of the round billet is set to 1100 ° C. or less in the pipe making process, and the rotation speed of the inclined roll of the drilling machine in which the roll diameter of the gorge portion is 1390 to 1410 mm is set to 75 rpm or less in the punching process.
- the round billet (base pipe) temperature is unlikely to exceed 1100 ° C. due to heat generated during the pipe making process. Therefore, the Nb carbonitride generated in the drilling process does not dissolve again, and the pinning effect is exhibited during the pipe manufacturing process. Therefore, the prior austenite crystal grains can be refined without using reverse transformation, and the crystal grain size difference ⁇ GS can be reduced.
- the finishing temperature at the final rolling in the pipe making process (the outer surface temperature of the raw pipe at the rolling stand outlet side of the final rolling) is set to 1000 ° C. or less. In this case, even if processing heat is generated during the pipe making process, the raw pipe temperature is unlikely to exceed 1100 ° C. throughout the pipe making process. Therefore, the prior austenite crystal grains of the seamless steel pipe can be refined, and the crystal grain size difference ⁇ GS can be reduced.
- the heating temperature of the round billet which is the raw material
- the rotation speed of the inclined roll in the drilling process is set to 75 rpm or less (5550 mm / sec or less at the peripheral speed of the inclined roll). If the temperature is 1000 ° C. or less, the Nb solid solution difference ⁇ SR in the base tube after the quenching process and before the tempering process can be sufficiently reduced to 10% or less. Therefore, the tensile strength difference ⁇ TS in the circumferential direction and the axial direction of the seamless steel pipe can be sufficiently reduced to 27.6 MPa or less.
- the seamless steel pipe according to the present embodiment completed based on the above knowledge is a seamless steel pipe having a first pipe end and a second pipe end.
- the chemical composition of the seamless steel pipe was, by mass, C: 0.21 to 0.35%, Si: 0.10 to 0.50%, Mn: 0.05 to 1.00%, P: 0.00. 025% or less, S: 0.010% or less, Al: 0.005 to 0.100%, N: 0.010% or less, Cr: 0.05 to 1.50%, Mo: 0.10 to 1.
- a first pipe end region in a range from the first pipe end toward the second pipe end in the axial direction of the seamless steel pipe to a position of 500 mm, and from the second pipe end toward the first pipe end.
- the crystal grain size number of the former austenite crystal grain according to ASTM E112 is 7.0 or more, and the main body region
- the difference between the maximum value and the minimum value of the grain size number is 1.0 or less
- the yield strength in the main body region is less than 655 to 862 MPa
- the difference between the maximum value and the minimum value of the tensile strength in the main body region is 27. .6 MPa or less.
- the chemical composition of the above-described seamless steel pipe may contain V: 0.01 to 0.30%. Further, the chemical composition of the above-mentioned seamless steel pipe contains at least one selected from the group consisting of Ca: 0.0001 to 0.0050% and rare earth elements: 0.0001 to 0.0050%. Also good.
- the production method of the seamless steel pipe according to the present embodiment is, in mass%, C: 0.21 to 0.35%, Si: 0.10 to 0.50%, Mn: 0.05 to 1.00%, P : 0.025% or less, S: 0.010% or less, Al: 0.005 to 0.100%, N: 0.010% or less, Cr: 0.05 to 1.50%, Mo: 0.10 To 1.50%, Nb: 0.010 to 0.050%, B: 0.0003 to 0.0050%, Ti: 0.002 to 0.050%, V: 0 to 0.30%, Ca: A round billet containing 0 to 0.0050% and rare earth element: 0 to 0.0050%, with the balance being Fe and impurities, heated to 950 to 1100 ° C., and the round billet has an inclined roll Using a piercing machine, pierce with an inclined roll at 20 to 75 rpm, and then perform rolling.
- a pipe manufacturing process to produce a mother tube the final mother pipe temperature during rolling to 800 ⁇ 1000 ° C. and is manufactured by the tube process, a raw tube outer surface temperature A 3 transformation point ⁇ 1000 ° C. quenching
- the production method of the seamless steel pipe according to the present embodiment is, in mass%, C: 0.21 to 0.35%, Si: 0.10 to 0.50%, Mn: 0.05 to 1.00%, P : 0.025% or less, S: 0.010% or less, Al: 0.005 to 0.100%, N: 0.010% or less, Cr: 0.05 to 1.50%, Mo: 0.10 To 1.50%, Nb: 0.010 to 0.050%, B: 0.0003 to 0.0050%, Ti: 0.002 to 0.050%, V: 0 to 0.30%, Ca: A round billet containing 0 to 0.0050% and rare earth element: 0 to 0.0050%, with the balance being Fe and impurities, heated to 950 to 1100 ° C., and the round billet has an inclined roll Using a perforator, the peripheral speed of the inclined roll is perforated at 1450-5550 mm / second, and Conducted extended to produce a mother tube with a pipe-step the final mother pipe temperature during rolling to 800 ⁇ 1000
- a quenching process in which the average cooling rate from the outer surface temperature of the raw tube at the start of the rapid cooling to the outer surface temperature reaches 300 ° C is 15 ° C / second or more, and the outer surface of the raw tube quenched by the quenching process And a tempering step of producing a seamless steel pipe having a yield strength of less than 655 to 862 MPa by performing tempering at a temperature of 650 ° C. to Ac 1 transformation point.
- the above-described manufacturing method further heats the raw tube having an outer surface temperature of 400 ° C. to less than the Ar 3 transformation point before the quenching step and after the pipe making step, thereby setting the outer surface temperature of the raw tube to Ac.
- the hardening step the external surface temperature is heated by the auxiliary heat step to quench the mother pipe became A 3 transformation point ⁇ 1000 ° C..
- the manufacturing method described above further reheats the raw tube having an outer surface temperature of less than 400 ° C., which is manufactured in the pipe manufacturing step, before the quenching step and after the pipe making step, and sets the outer surface temperature of the raw tube to the Ac 3 transformation point to You may provide the reheating process which is 1000 degreeC.
- the quenching step is heated by the reheating step in the outer surface temperature to quench the mother pipe became A 3 transformation point ⁇ 1000 ° C..
- the round billet may contain V: 0.01 to 0.30%.
- the round billet may contain one or more selected from the group consisting of Ca: 0.0001 to 0.0050% and rare earth elements: 0.0001 to 0.0050%.
- FIG. 2 is a diagram illustrating an example of a seamless steel pipe according to the present embodiment.
- the seamless steel pipe 10 of this embodiment is provided with the 1st pipe end 1E and the 2nd pipe end 2E.
- the 2nd pipe end 2E is arrange
- the range from the first pipe end 1E toward the second pipe end 2E (toward the center in the axial direction of the seamless steel pipe 10) up to the 500 mm position in the axial direction of the seamless steel pipe 10 is It is defined as a tube end region 1A. Further, the range from the second pipe end 2E toward the first pipe end 1E (toward the center in the axial direction of the seamless steel pipe 10) up to the 500 mm position in the axial direction of the seamless steel pipe 10 is This is defined as area 2A. Furthermore, the area
- C 0.21 to 0.35% Carbon (C) increases the strength of the steel. If the C content is too low, this effect cannot be obtained. On the other hand, if the C content is too high, the sensitivity of the steel to fire cracking increases. In this case, special cooling means (quenching method) is required particularly in quenching of the steel pipe. If the C content is too high, the toughness of the steel may further decrease. Therefore, the C content is 0.21 to 0.35%.
- the minimum with preferable C content is 0.23%, More preferably, it is 0.25%.
- the upper limit with preferable C content is 0.30%, More preferably, it is 0.27%.
- Si 0.10 to 0.50% Silicon (Si) deoxidizes steel. If the Si content is too low, this effect cannot be obtained. On the other hand, if the Si content is too high, the SSC resistance and workability of the steel deteriorate. Therefore, the Si content is 0.10 to 0.50%.
- the minimum with preferable Si content is 0.15%, More preferably, it is 0.20%.
- the upper limit with preferable Si content is 0.40%, More preferably, it is 0.35%.
- Mn 0.05 to 1.00%
- Manganese (Mn) increases the hardenability of the steel and increases the strength of the steel. If the Mn content is too low, this effect cannot be obtained. On the other hand, if the Mn content is too high, Mn segregates at the grain boundaries and the SSC resistance of the steel decreases. Therefore, the Mn content is 0.05 to 1.00%.
- the minimum with preferable Mn content is 0.30%, More preferably, it is 0.40%.
- the upper limit with preferable Mn content is 0.95%, More preferably, it is 0.90%.
- Phosphorus (P) is an impurity and is unavoidably contained in steel. P segregates at the grain boundaries and decreases the SSC resistance of the steel. Therefore, the P content is 0.025% or less.
- the upper limit with preferable P content is 0.020%, More preferably, it is 0.015%.
- the P content is preferably as low as possible.
- S 0.010% or less Sulfur (S) is an impurity and is unavoidably contained in steel. S combines with Mn to form sulfide inclusions, thereby reducing the SSC resistance of the steel. Therefore, the S content is 0.010% or less.
- the upper limit with preferable S content is 0.006%, More preferably, it is 0.003%.
- the S content is preferably as low as possible.
- Al 0.005 to 0.100%
- Aluminum (Al) deoxidizes steel. If the Al content is too low, this effect cannot be obtained. On the other hand, if the Al content is too high, the effect is saturated. If the Al content is too high, a large number of coarse Al-based oxides are generated, and the SSC resistance of the steel is lowered. Therefore, the Al content is 0.005 to 0.100%.
- the minimum with preferable Al content is 0.010%, More preferably, it is 0.020%.
- the upper limit with preferable Al content is 0.070%, More preferably, it is 0.050%.
- the Al content means the content of so-called acid-soluble Al (sol. Al).
- N 0.010% or less Nitrogen (N) is inevitably contained in the steel. N forms a nitride. Since fine nitride prevents coarsening of crystal grains, N may be contained. On the other hand, coarse nitrides reduce the SSC resistance of steel. Therefore, the N content is 0.010% or less.
- the upper limit with preferable N content is 0.004%, More preferably, it is 0.003%.
- a preferable lower limit of the N content for obtaining a pinning effect due to the precipitation of fine nitride is 0.002%.
- Chromium (Cr) increases the hardenability of the steel and increases the strength of the steel. If the Cr content is too low, this effect cannot be obtained. On the other hand, if the Cr content is too high, the SSC resistance of the steel decreases. Therefore, the Cr content is 0.05 to 1.50%.
- the minimum with preferable Cr content is 0.20%, More preferably, it is 0.40%.
- the upper limit with preferable Cr content is 1.20%, More preferably, it is 1.15%.
- Mo 0.10 to 1.50% Molybdenum (Mo) increases the hardenability of the steel and increases the strength of the steel. Mo further increases the temper softening resistance of the steel and increases the SSC resistance due to high temperature tempering. If the Mo content is too low, this effect cannot be obtained. On the other hand, if the Mo content is too high, the effect is saturated and the manufacturing cost increases. Therefore, the Mo content is 0.10 to 1.50%.
- the minimum with preferable Mo content is 0.15%, More preferably, it is 0.20%.
- the upper limit with preferable Mo content is 0.80%, More preferably, it is 0.60%.
- Niobium combines with C and N to form fine Nb carbonitride, Nb carbide, and Nb nitride. Nb further forms a composite carbide with Ti and Al. These carbonitrides (Nb carbonitride, Nb carbide, Nb nitride, and composite carbide) increase the SSC resistance of the steel by refining crystal grains by the pinning effect. These carbonitrides further suppress variation in crystal grain size. If the Nb content is too low, this effect cannot be obtained. On the other hand, if the Nb content is too high, a large number of coarse Nb-based inclusions are generated, and the SSC resistance of the steel decreases.
- the Nb content is 0.010 to 0.050%.
- the minimum with preferable Nb content is 0.013%, More preferably, it is 0.015%, More preferably, it is 0.020%.
- the upper limit with preferable Nb content is 0.040%, More preferably, it is 0.035%.
- B 0.0003 to 0.0050% Boron (B) increases the hardenability of the steel and increases the strength of the steel. If the B content is too low, this effect cannot be obtained. On the other hand, if the B content is too high, carbonitrides precipitate at the grain boundaries, and the SSC resistance of the steel decreases. Therefore, the B content is 0.0003 to 0.0050%.
- the minimum with preferable B content is 0.0005%, More preferably, it is 0.0008%.
- the upper limit with preferable B content is 0.0030%, More preferably, it is 0.0020%.
- Ti 0.002 to 0.050% Titanium (Ti) combines with C and N to form fine Ti carbonitrides, and fixes N as an impurity. The generation of Ti nitride refines the crystal grains and further increases the strength of the steel. Further, when B is contained in the steel, since Ti suppresses the formation of B nitride, it promotes the improvement of hardenability by B. If the Ti content is too low, these effects cannot be obtained. On the other hand, if the Ti content is too high, Ti is dissolved in the Nb-based inclusions, and the Nb-based inclusions are coarsened. In this case, the SSC resistance of the steel decreases. Therefore, the Ti content is 0.002 to 0.050%. The minimum with preferable Ti content is 0.003%, More preferably, it is 0.004%. The upper limit with preferable Ti content is 0.035%, More preferably, it is 0.030%.
- the balance of the chemical composition of the seamless steel pipe according to the present embodiment is composed of Fe and impurities.
- impurities are mixed from ore, scrap, or production environment as raw materials when industrially manufacturing seamless steel pipes, and have an adverse effect on the seamless steel pipes of this embodiment. It means what is allowed in the range.
- the oxygen (O) content is 0.005% or less.
- the chemical composition of the seamless steel pipe described above may further contain V instead of a part of Fe.
- V 0 to 0.30%
- Vanadium (V) is an optional element and may not be contained. When contained, V produces fine carbides to increase the temper softening resistance and enable high temperature tempering. This increases the SSC resistance of the steel. However, if the V content is too high, carbides are generated excessively and the SSC resistance of the steel is rather lowered. Therefore, the V content is 0 to 0.30%.
- the minimum with preferable V content for acquiring the said effect more effectively is 0.01%, More preferably, it is 0.02%.
- the upper limit with preferable V content is 0.25%, More preferably, it is 0.20%.
- the chemical composition of the seamless steel pipe described above may further contain one or more selected from the group consisting of Ca and rare earth elements instead of a part of Fe.
- Ca 0 to 0.0050%
- Calcium (Ca) is an optional element and may not be contained. When contained, Ca spheroidizes sulfide inclusions in the steel. This increases the SSC resistance of the steel. If Ca is contained even a little, the above effect can be obtained. However, if the Ca content is too high, excessive inclusions are generated, and the SSC resistance of the steel decreases. Therefore, the Ca content is 0 to 0.0050%.
- the minimum with preferable Ca content is 0.0001%, More preferably, it is 0.0010%, More preferably, it is 0.0015%.
- the upper limit with preferable Ca content is 0.0040%, More preferably, it is 0.0030%.
- Rare earth element 0 to 0.0050%
- the rare earth element (REM) is an optional element and may not be contained. When contained, REM spheroidizes sulfide inclusions in the steel. This increases the SSC resistance of the steel. If REM is contained even a little, the above effect can be obtained. However, if the REM content is too high, excessive inclusions are generated, and the SSC resistance of the steel is lowered. Therefore, the REM content is 0 to 0.0050%.
- the minimum with preferable REM content is 0.0001%, More preferably, it is 0.0010%.
- the upper limit with preferable REM content is 0.0040%, More preferably, it is 0.0030%.
- REM in the present specification contains at least one of Sc, Y, and lanthanoid (La of atomic number 57 to Lu of 71), and the REM content means the total content of these elements To do.
- the microstructure of the seamless steel pipe of this embodiment is mainly composed of tempered martensite, and the balance is, for example, ferrite, bainite, pearlite, or a mixed phase thereof.
- “mainly” means that the total area ratio of tempered martensite in the microstructure is 90% or more.
- the area ratio of tempered martensite is proportional to the yield ratio YR of the seamless steel pipe of this embodiment. Therefore, the area ratio of tempered martensite is defined by the following method.
- each section is selected by dividing the main body area excluding the first pipe end area and the second pipe end area of the seamless steel pipe after tempering into 5 equal parts in the axial direction of the seamless steel pipe.
- arc-shaped tensile test specimens are collected from four positions at 90 ° pitch positions around the central axis of the seamless steel pipe.
- the cross section of the arc-shaped tensile test piece (cross section perpendicular to the axial direction of the seamless steel pipe) is arc-shaped, and the axial direction of the arc-shaped tensile test piece is parallel to the axial direction of the seamless steel pipe.
- a tensile test is performed at room temperature (25 ° C.) in accordance with API standard 5CT.
- the average of the yield strength (total of 20 locations) obtained with each arc-shaped specimen is defined as the yield strength YS (MPa) of the seamless steel pipe.
- the average of the tensile strength (total 20 locations) obtained with each arc-shaped test piece is defined as the tensile strength TS (MPa) of the seamless steel pipe.
- the yield strength YS is defined as follows. When the yield strength YS is 95 ksi grade (655 MPa to less than 758 MPa), the value of 0.5% total elongation is defined as the yield strength (MPa). When the yield strength YS is 110 ksi grade (758 MPa to less than 862 MPa), the value of 0.7% total elongation is defined as the yield strength (MPa).
- the definition of these yield strengths is based on the API standard 5CT.
- the yield ratio YR decreases.
- the yield strength YS obtained is 95 ksi class (less than 655 to 758 MPa)
- the yield ratio YR is 85.0% or more
- the area ratio of tempered martensite is 90% or more.
- the yield strength YS obtained is 110 ksi class (less than 758 to 862 MPa)
- the yield ratio is 90.0% or more
- the area ratio of tempered martensite is 90% or more.
- the crystal grain size number based on ASTM E112 of the prior austenite crystal grains is 7.0 or more. If the grain size number of the prior austenite crystal grains is less than 7.0, the prior austenite crystal grains are coarse. Therefore, the SSC resistance decreases. If the degree number of the prior austenite crystal grains is 7.0 or more, the crystal grains are sufficiently fine. Therefore, excellent SSC resistance can be obtained.
- the old austenite grain size number is realized by producing pipes at a lower temperature (1100 ° C. or lower) than in the conventional pipe making process and suppressing processing heat generated during drilling and rolling.
- the method for measuring the prior austenite grain size is as follows.
- the axial center position of each section is selected by dividing the main body region excluding the first pipe end region and the second pipe end region of the seamless steel pipe into five equal parts in the axial direction of the seamless steel pipe.
- In the cross-section perpendicular to the axial direction of the seamless steel pipe at each selected position it is parallel to the axial direction of the seamless steel pipe from the thickness central position of 8 positions at a 45 ° pitch position around the central axis of the seamless steel pipe.
- a test piece having a surface (observation surface) 100 is prepared. As shown in FIG.
- the observation surface 100 of the test piece is formed by scraping a region from the outer surface to a depth of 1.5 mm and a region from the inner surface to a depth of 1.5 mm in the thickness direction of the seamless steel pipe.
- the length in the thickness direction is the thickness T (mm) ⁇ (1.5 mm depth from the outer surface + 1.5 mm depth from the inner surface).
- the length of the observation surface 100 is set to 15 mm in the axial direction of the seamless steel pipe. That is, the observation surface 100 is a rectangle (thickness T-3.0 mm) ⁇ 15 mm.
- the observation surface of each test piece is mechanically polished.
- the observation surface after mechanical polishing is etched using a Picral corrosive solution to reveal prior austenite grain boundaries in the observation surface.
- the average value of the grain size numbers of the prior austenite crystal grains is determined in accordance with ASTM E112 with an arbitrary four visual fields (500 ⁇ m ⁇ 500 ⁇ m per visual field) using an optical microscope having a magnification of 200 times.
- required average value be a crystal grain size number of the prior austenite crystal grain in each measurement position.
- the grain size numbers of the former austenite crystal grains obtained at each measurement position total of 40 locations
- the smallest grain size number is defined as the crystal grain size number according to ASTM E112 of the former austenite crystal grains of the seamless steel pipe. .
- crystal grain size difference ⁇ GS In the microstructure of the seamless steel pipe of the present embodiment, in the main body region, the difference between the maximum value and the minimum value of the crystal grain size number measured at any of a plurality of portions in the circumferential direction and the axial direction of the seamless steel pipe (crystal The particle size difference ⁇ GS) is 1.0 or less.
- crystal grain size difference ⁇ GS exceeds 1.0, hydrogen entering the steel material causes embrittlement of the coarse grain portion in the sour environment, and as a result, the SSC resistance decreases.
- the crystal grain size difference ⁇ GS is 1.0 or less, excellent SSC resistance can be obtained.
- the crystal grains are refined by the pinning effect by Nb carbonitride and Nb nitride (hereinafter referred to as Nb carbonitride) generated during the pipe making process, and the crystal grain size difference ⁇ GS is It can be made 1.0 or less.
- Nb carbonitride Nb carbonitride
- the crystal grains are absolutely coarsened, and the crystal grain size difference ⁇ GS exceeds 1.0 due to the influence of temperature variations in the axial direction and circumferential direction of the pipe.
- the crystal grain size difference ⁇ GS is measured by the following method.
- the maximum value and the minimum value are selected from the grain size numbers at the 40 measurement positions at which the grain size numbers were obtained.
- the difference between the maximum value and the minimum value is defined as the crystal grain size difference ⁇ GS.
- the yield strength YS of the seamless steel pipe of this embodiment is 655 MPa (95 ksi) to less than 862 MPa (125 ksi). If the yield strength YS is 862 MPa or more, even if it has the above-mentioned microstructure, excellent SSC resistance cannot be obtained. On the other hand, if the yield strength is less than 655 MPa, the strength necessary for use as an oil well pipe for highly corrosive well applications cannot be obtained. Therefore, the yield strength YS of the seamless steel pipe of this embodiment is less than 655 to 862 MPa. Yield strength is defined as described above.
- yield strength when the yield strength is 95 ksi grade (655 MPa to 758 MPa), the value of 0.5% total elongation is defined as the yield strength (MPa). When the yield strength is 110 ksi grade (758 MPa to less than 876 MPa), the value of 0.7% total elongation is defined as the yield strength (MPa).
- MPa the yield strength
- the definition of these yield strengths is based on the API standard 5CT.
- the difference (tensile strength difference) ⁇ TS between the maximum value and the minimum value of the tensile strength TS is 27.6 MPa or less. Therefore, in the seamless steel pipe of the present embodiment, strength variations are suppressed in the circumferential direction and the axial direction.
- Yield strength YS and tensile strength TS are measured by the following methods.
- the axial center position of each section is selected by dividing the main body region excluding the first pipe end region and the second pipe end region of the seamless steel pipe into five equal parts in the axial direction of the seamless steel pipe.
- arc-shaped tensile test specimens are collected from four positions at 90 ° pitch positions around the central axis of the seamless steel pipe.
- the cross section of the arc-shaped tensile test piece (cross section perpendicular to the axial direction of the seamless steel pipe) is arc-shaped, and the axial direction of the arc-shaped tensile test piece is parallel to the axial direction of the seamless steel pipe.
- a tensile test is performed at room temperature (25 ° C.) in accordance with API standard 5CT.
- the average of the yield strength (total of 20 locations) obtained with each arc-shaped specimen is defined as the yield strength YS (MPa) of the seamless steel pipe.
- the difference between the maximum value and the minimum value of the tensile strength TS (20 locations) obtained with each arc-shaped test piece is defined as a tensile strength difference ⁇ TS (MPa).
- This manufacturing method is manufactured by heating a billet (heating step), manufacturing a blank pipe using the heated billet (tube manufacturing step), hot pipe manufacturing, and the outer surface temperature is A 3.
- the raw pipe after hot pipe making is reheated in-line with respect to the raw pipe having an outer surface temperature of less than Ac 3 points to 400 ° C.
- a supplementary heating step may be included.
- This manufacturing method also performs reheating off-line between the pipe making process and the quenching process, after the hot pipe making, at a temperature below 400 ° C to room temperature (25 ° C).
- a reheating step may be included.
- the reheating step and the quenching step may be sequentially performed after the quenching step, and then the tempering step may be performed.
- a reheating step, a quenching step, and a tempering step may be sequentially performed after the tempering step.
- the production method is as follows, for example.
- Case 1 Heating process-Pipe making process-Quenching process (direct quenching)-Tempering process
- Case 2 Heating process-Pipe making process-Supplementary heating process-Quenching process-Tempering process
- Case 3 Heating process-Pipe making process-Reheating Process-Quenching process-Tempering process
- Case 4 Heating process-Pipe making process-Supplementary heating process-Quenching process-Reheating process-Quenching process-Tempering process
- Case 5 Heating process-Pipe making process-Reheating process-Quenching process -Reheating process-Quenching process-Tempering process
- Case 6 Heating process-Pipe making process-(Supplementary heating process)-Quenching process-Tempering process-Reheating process-Quenching process-Tempering process
- Case 7 Heating process-Pipe making process-Reheating process-Quenching process-Tempering process-Reheating process-Qu
- Case 1 and Case 2 are so-called in-line quenching.
- Case 3 is so-called off-line quenching.
- a round billet having the above-described chemical composition is prepared.
- the manufacturing method of a round billet is not specifically limited.
- the round billet is manufactured by the following method.
- a molten steel having the above chemical composition is produced.
- a converter or the like is used for the production of molten steel.
- bloom is produced by continuous casting. You may manufacture an ingot by the ingot-making method using molten steel. The bloom and ingot are hot-rolled to produce a round billet having a circular cross section. You may manufacture a round billet by a continuous casting method using molten steel.
- a round billet is prepared by the above method.
- the heating temperature is 950 to 1100 ° C.
- the heating temperature here means the temperature in the furnace. If the furnace temperature is 950 to 1100 ° C., the outer surface temperature of the round billet is also 950 to 1100 ° C.
- the heating temperature of the round billet in the heating process (outer surface temperature of the round billet) is 1100 ° C. or less, the roll rotation speed (roll peripheral speed) and the finishing temperature in the drilling process described later are satisfied.
- the upper limit of the heating temperature of the round billet in the heating step is 1100 ° C. If the heating temperature of the round billet in the heating process is 1100 ° C. or lower, it is possible to further suppress variations in the Nb solid solution rate in the axial direction and the circumferential direction of the pipe, and in the pipe before the tempering process after the quenching process. Nb solid solution difference ⁇ SR can be suppressed to 10% or less.
- the tensile strength difference ⁇ TS in the main body region of the manufactured seamless steel pipe can be suppressed to 27.6 MPa or less.
- the heating temperature of the round billet in the heating step is too low, the deformation resistance of the round billet is increased. In this case, piercing and rolling becomes difficult. Therefore, the lower limit of the heating temperature of the round billet in the heating step is 950 ° C.
- the upper limit with the preferable heating temperature in a heating process is 1080 degreeC, and a preferable minimum is 1050 degreeC.
- a round billet heated by the heating process is punched and rolled to produce a raw pipe.
- the pipe making process includes a drilling process and a rolling process.
- the rolling process includes, for example, a drawing rolling process and a constant diameter rolling process.
- a round billet is pierced and rolled into a raw pipe using a piercing machine.
- the drawing and rolling process the raw tube is drawn and rolled using a drawing and rolling mill.
- the drawing mill is, for example, a plug mill or a mandrel mill.
- the constant diameter rolling step the raw pipe is constant diameter rolled using a constant diameter rolling mill.
- the constant diameter rolling mill is, for example, a sizer or a stretch reducer.
- the outer surface temperature of the round billet (element tube) in the drilling process and the drawing and rolling process during the pipe making process is 1100 ° C. or less. Furthermore, the outer surface temperature (finishing temperature) of the raw tube at the time of final rolling (rolling at the final reduction stand in the constant diameter rolling step) is 1000 ° C. or less.
- the heating temperature of the round billet in the heating process is 1100 ° C. or less
- the maximum value of the outer surface temperature (finishing temperature) in the main body region of the raw tube measured on the exit side of the constant diameter rolling mill is It is 1000 degrees C or less.
- a radiation thermometer is arranged on the exit side of the punching machine, the exit side of the final reduction stand of the drawing mill, and the exit side of the final reduction stand of the constant diameter rolling mill.
- the outer surface temperature of the portion corresponding to the main body region is the radiation temperature. Measure with a meter.
- the maximum temperature in the main body region in the measurement result from which noise has been removed is defined as the outer surface temperature of the above-described raw tube on the exit side of each facility (a piercing machine, a drawing mill, and a constant diameter rolling mill).
- the heating temperature of the round billet in the heating process is 950 to 1100 ° C. or lower
- the body area of the round billet (base pipe) during the pipe making process A portion where the outer surface temperature exceeds 1100 ° C. occurs.
- the fine Nb carbonitride, Nb carbide, and Nb nitride generated during the pipe making process are again dissolved.
- the pinning effect by Nb carbonitride, Nb carbide and Nb nitride in which Nb carbonitride or the like is dissolved is not obtained, and austenite grains are not refined.
- the prior austenite grain size in the main body region becomes less than 7.0, or the grain size difference ⁇ GS exceeds 1.0.
- the SSC resistance decreases.
- the Nb solid solution difference ⁇ SR in the base tube after the quenching process and before the tempering process exceeds 10%.
- the tensile strength difference ⁇ TS of the seamless steel pipe exceeds 27.6 MPa, and the strength variation becomes large.
- the rolling ratio is high particularly in the piercing process performed immediately after heating the round billet. Therefore, among the piercing process, the drawing and rolling process, and the constant diameter rolling process, heat generation is most likely to occur in the piercing process, and the outer surface temperature of the round billet (element tube) is likely to exceed 1100 ° C. in the piercing process. And if the outer surface temperature of a raw pipe exceeds 1100 degreeC in a piercing process, the outer surface temperature (finishing temperature) of the raw pipe in the fixed diameter rolling mill delivery side of a rolling process will exceed 1000 degreeC. In this case, the prior austenite grain size in the main body region becomes less than 7.0, the grain size difference ⁇ GS exceeds 1.0, or the tensile strength difference ⁇ TS exceeds 27.6 MPa.
- the roll rotation speed of the piercing machine is controlled to suppress processing heat generation in the piercing process, and the outer surface temperature of the round billet (element tube) during the pipe making process is suppressed to 1100 ° C. or lower.
- the perforating machine includes a plurality (for example, a pair) of inclined rolls and a plug disposed between the plurality of inclined rolls.
- the roll diameter of the inclined roll of the punching machine is 1390 to 1410 mm, and the rotation speed is 20 to 75 rpm.
- the peripheral speed of the inclined roll is 1450 to 5550 mm / sec.
- the roll diameter of an inclined roll means the roll diameter (mm) of the gorge part of an inclined roll.
- the rounding during the piercing process is performed even when the round billet heated at 1100 ° C. or lower is pierced and rolled at a rolling ratio of 1.2 to 4.0. It can suppress that billet outer surface temperature rises higher than 1100 degreeC by process heat_generation
- the lower limit of the finishing temperature is 800 ° C. That is, the finishing temperature is 800 to 1000 ° C.
- a preferred lower limit of the finishing temperature is 850 ° C.
- Heat supplement process The heat supplementing process is performed as necessary. If base pipe outer surface temperature after hot steel pipe (finishing temperature) is less than A 3 transformation point (Ar 3 transformation point), it can not be carried out quenching. However, if the outer surface temperature of the raw pipe after hot pipe making is 400 ° C or higher, there is no need to reheat in an off-line heating furnace, and on the conveying path connecting the constant diameter rolling mill and the quenching device (quenching device). Using the arranged auxiliary heating furnace or induction heater, the raw pipe is heated in-line to bring the outer surface temperature of the raw pipe to the Ac 3 transformation point or higher.
- the preferable upper limit of the external surface temperature of the mother pipe is heated by the auxiliary heat step is Ac 3 transformation point + 50 ° C..
- the outer surface temperature of the raw tube after being heated in the auxiliary heat process is referred to as “supplemented heat temperature”.
- the auxiliary heat temperature is measured by the following method. In the case of carrying out the heat supplementing process using the induction heater, the average value of the outer surface temperature of the main body region of the raw tube on the outlet side of the induction heater is defined as the supplementary heat temperature (° C.). In this case, the supplementary heat temperature is measured by a radiation thermometer arranged on the outlet side of the induction heater.
- the furnace temperature (° C.) of the auxiliary heating furnace coincides with the outer surface temperature of the main body region of the raw tube. Therefore, in this case, the furnace temperature of the auxiliary heating furnace is defined as the outer surface temperature (° C.) of the raw tube.
- a reheating process is implemented as needed. If the outer surface temperature of the raw tube becomes less than 400 ° C to room temperature (25 ° C) as a result of allowing the raw tube after the hot pipe to cool, use a heating furnace placed off-line in the pipe production line. Reheat the tube.
- the outer surface temperature of the raw tube heated by the reheating step is equal to or higher than the Ac 3 transformation point, and a preferable upper limit is Ac 3 transformation point + 50 ° C.
- the furnace temperature (° C.) of the heating furnace matches the outer surface temperature of the main body region of the raw tube. Therefore, when using a heating furnace in a reheating process, the furnace temperature of a heating furnace is defined as the outer surface temperature (degreeC) of a raw tube.
- the element tube is reheated as described above.
- the outer surface temperature of the raw tube heated by the reheating step is equal to or higher than the Ac 3 transformation point, and a preferable upper limit is Ac 3 transformation point + 50 ° C.
- the A 3 transformation point or higher (the outer surface temperature of the raw tube after the pipe making process is equal to or higher than the Ar 3 transformation point, or when the auxiliary heating step and the reheating step are performed, the outer surface temperature of the raw tube is Ac 3 transformation A tube having an outer surface temperature of at least a point) is quenched and quenched.
- the outer surface temperature (quenching temperature) of the raw tube at the start of quenching in the quenching process is from A 3 transformation point (Ar 3 transformation point or Ac 3 transformation point) to 1000 ° C.
- the outer surface temperature of the raw tube at the start of rapid cooling is an average value of the outer surface temperatures of the main body region.
- the average cooling rate CR from the outer surface temperature of the raw tube at the start of rapid cooling in the quenching process until the outer surface temperature of the raw tube reaches 300 ° C. is set to 15 ° C./second or more.
- the average cooling rate CR is set to 15 ° C./second or more.
- a preferable lower limit of the average cooling rate CR is 17 ° C./second, more preferably 19 ° C./second.
- a preferred rapid cooling method in the quenching step is water cooling.
- the quenching process is performed by, for example, a water-cooling device disposed on the pipe production line and downstream of the constant diameter rolling mill.
- the water cooling device includes, for example, a laminar water flow device and a jet water flow device.
- the laminar water flow device pours water from above into the raw tube. At this time, the water poured into the raw tube forms a laminar water flow.
- the jet water flow apparatus injects a jet water flow from the end of the raw pipe toward the inside of the raw pipe.
- the water cooling device may be a device other than the laminar water flow device and the jet water flow device described above.
- the water cooling device may be, for example, a water tank. In this case, the raw tube is immersed in the water tank and cooled.
- the water cooling device may also be a laminar water flow device only.
- the quenching step is performed by, for example, a water cooling device arranged outside the pipe manufacturing line.
- the water cooling device is the same as the water cooling device used in in-line quenching. Since reverse transformation can be used when performing offline quenching, the grain size number of the prior austenite crystal grains of the seamless steel pipe is higher than when only in-line quenching is performed.
- Nb solid solution difference ⁇ SR In the tube after the first quenching process and before performing the next process (for example, if the next process is a tempering process, before the tempering process, and if the next process is a reheating process, before the reheating process) Nb solid solution ratio (mass%) of Nb solid solution in steel without precipitation as Nb carbonitride and the like (Nb carbonitride and Nb nitride) with respect to the total Nb content in steel It is defined as In this case, the difference between the maximum value and the minimum value of the Nb solid solution rate in the main body region of the raw tube (hereinafter referred to as Nb solid solution rate difference ⁇ SR) is 10% or less.
- the difference in the Nb solid solution rate is realized by making the pipe at a lower temperature (1100 ° C. or less) than the conventional pipe making process and suppressing processing heat generated during piercing and rolling.
- the Nb solid solution rate of the tube after the first quenching process and before the next process is measured by the extraction residue method. Specifically, after the first quenching process and before the next process, the main body area excluding the first pipe end area and the second pipe end area is equally divided into five in the axial direction of the raw pipe, Select the center position of the segment in the axial direction of the tube as the measurement position. In a cross section perpendicular to the axial direction of the raw tube at each measurement position, test specimens are collected from four central thickness positions at 90 ° pitch around the central axis of the raw tube. At this time, the surface area of the collected test piece is 15 cm 2 .
- the test piece is electrolyzed for 0.5 g in an electrolytic solution to dissolve the matrix.
- the electrolytic solution is 10% acetylacetone + 1% tetramethylammonium chloride + remainder methanol, and the current is 200 A / m 2 .
- the precipitate is acid-decomposed and the chemical composition is analyzed by ICP (inductively coupled plasma analysis) to determine the Nb content in the precipitate. Based on the Nb content in the precipitate, the Nb solid solution rate is determined by the following formula.
- Nb solid solution rate (total Nb content in steel ⁇ Nb content in precipitate) / total Nb content in steel
- the maximum value and the minimum value are selected from the obtained Nb solid solution rates, and the difference between the maximum value and the minimum value is defined as the Nb solid solution rate difference ⁇ SR.
- the Nb solid solution rate is a raw tube after the first quenching process and is measured before the next process.
- the raw pipe quenched and quenched in the quenching process is tempered to obtain a seamless steel pipe.
- the tempering temperature is from 650 ° C. to the Ac 1 transformation point, and is adjusted based on desired mechanical properties. Specifically, the tempering temperature is adjusted so that the yield strength of the seamless steel pipe after tempering is 655 MPa to less than 862 MPa. If the yield strength is 862 MPa or more, even if the crystal grain size number is 7.0 or more and the crystal grain size number difference ⁇ GS is 1.0 or less, the SSC resistance is lowered. When the yield strength is less than 862 MPa, the SSC resistance is further increased.
- the tempering temperature for setting the yield strength of the seamless steel pipe of this embodiment having the above-described chemical composition to 655 MPa to less than 862 MPa is 650 ° C. to Ac 1 transformation point, and the preferable upper limit is 750 ° C.
- tempering temperature means the temperature inside the heat treatment furnace used in the tempering process.
- the outer surface temperature of the blank tube becomes the same as the tempering temperature (furnace temperature).
- the seamless steel pipe manufactured by the above process has excellent SSC resistance and stably has a yield strength of 655 MPa to less than 862 MPa in the circumferential direction and the axial direction.
- a plurality of seamless steel pipes having various chemical compositions were manufactured, and the SSC resistance and strength variation of the seamless steel pipes were investigated.
- a plurality of round billets were manufactured by continuous casting using molten steel.
- a seamless steel pipe was produced under the production conditions shown in Table 2 using a round billet.
- so-called in-line quenching of the case 1 and the case 2 was performed.
- the round billet of each test number was heated at the heating temperature (° C.) described in Table 2.
- a pipe making process (a drilling process, a drawing rolling process, and a constant diameter rolling process) was performed on the heated round billet to produce a raw pipe having an outer diameter of 244.5 mm and a wall thickness of 13.8 mm.
- the roll diameter of the inclined roll of the punching machine was 1390 to 1410 mm.
- the roll diameter of the inclined rolls of the drilling machines of test numbers 1 to 11 was 1390 mm
- the roll diameter of the inclined rolls of the drilling machines of test numbers 12 to 15 was 1410 mm.
- Table 2 shows the roll rotation speed (rpm), the roll peripheral speed (mm / sec), and the finishing temperature (° C.) during the drilling of the drilling machine in the pipe making process.
- the elementary tubes other than the test numbers 2 and 5 were subjected to a supplementary heating process at the supplementary heating temperature shown in Table 2. Quenching was carried out after quenching the tube after the pipe making process (test numbers 2 and 5) or after the heating step. Quench initiation temperature in the quenching step is as shown in Table 2, both were A 3 transformation point or more. The average cooling rate CR from the rapid cooling start temperature in the quenching process to the outer surface temperature of the blank tube reaching 300 ° C. was 15 ° C./second or more. Tempering was performed on the raw tube after the quenching process. The tempering temperature was as shown in Table 2, and the holding time at the tempering temperature was 30 minutes.
- the tempering temperature of any test number was also below the Ac 1 transformation point.
- the seamless steel pipe of each test number was manufactured by the above manufacturing process.
- Nb solid-solution rate difference (DELTA) SR the raw tube after a quenching process and before a tempering process was also prepared.
- the electrolytic solution was 10% acetylacetone + 1% tetramethylammonium chloride + remainder methanol, and the current was 200 A / m 2 .
- the precipitate was acid-decomposed and the chemical composition was analyzed by ICP (inductively coupled plasma analysis) to determine the Nb content in the precipitate. Based on the Nb content in the precipitate, the Nb solid solution rate was determined by the following formula.
- Nb solid solution rate (total Nb content in steel ⁇ Nb content in precipitate) / total Nb content in steel
- the maximum value and the minimum value were selected from the Nb solid solution ratios (total of 20 locations) obtained at each measurement position, and the difference between the maximum value and the minimum value was defined as Nb solid solution ratio difference ⁇ SR.
- the observation surface 100 of the test piece was observed by scraping a region from the outer surface to a depth of 1.5 mm and a region from the inner surface to a depth of 1.5 mm in the thickness direction of the seamless steel pipe.
- the length of the surface 100 in the thickness direction was the thickness T (mm) ⁇ (1.5 mm depth from the outer surface + 1.5 mm depth from the inner surface).
- the axial length of the seamless steel pipe of the observation surface 100 was 15 mm. That is, the observation surface 100 was a rectangle of (wall thickness T-3.0 mm) ⁇ 15 mm.
- the observation surface of each test piece was mechanically polished.
- the observation surface after mechanical polishing was etched using a Picral corrosive solution to reveal prior austenite grain boundaries in the observation surface.
- the average of the grain size numbers of the prior austenite crystal grains is obtained in accordance with ASTM E112 with an arbitrary 4 fields of view (500 ⁇ m ⁇ 500 ⁇ m per field) using an optical microscope with a magnification of 200 times.
- the grain size number of the prior austenite crystal grains at each measurement position was used.
- the smallest crystal grain size number was defined as the crystal grain size number based on ASTM E112 of the prior austenite crystal grains of the seamless steel pipe.
- the maximum value and the minimum value among the crystal grain size numbers (total of 40 locations) obtained at each measurement position were selected, and the difference obtained by subtracting the minimum value from the maximum value was defined as the crystal grain size difference ⁇ GS.
- a tensile test was performed at room temperature (25 ° C.) in accordance with the API standard 5CT.
- the average of the yield strength (total of 20 locations) obtained with each arc-shaped test piece was defined as the yield strength YS (MPa) of the seamless steel pipe.
- the average of the tensile strength (20 places in total) obtained with each arc-shaped test piece was defined as the tensile strength TS (MPa) of the seamless steel pipe.
- the difference between the maximum value and the minimum value of the tensile strength TS (20 locations) obtained from each arc-shaped test piece was defined as the tensile strength difference ⁇ TS (MPa).
- the SSC resistance of each round bar specimen was evaluated by a constant load test.
- the test bath was room temperature 5% sodium chloride + 0.5% acetic acid aqueous solution saturated with 1 atm hydrogen sulfide gas.
- a load stress corresponding to 90% of the actual yield strength (AYS) of each round bar test piece was applied and immersed in a test bath for 720 hours. After 720 hours from the immersion, it was confirmed whether each round bar specimen was broken. When no fracture was observed in all of the round bar test pieces (20 in total) of each test number, it was judged that the SSC resistance of the steel was high (pass). When fracture was observed in any of the round bar test pieces (20 in total) of each test number, the SSC resistance of the steel was judged to be low (failed).
- Table 2 shows the test results. “YS” in Table 2 indicates the yield strength YS (MPa), and “TS” indicates the tensile strength TS (MPa).
- SSC resistance the SSC resistance evaluation test results are described. “Accepted” means that no breakage was observed in the round bar test piece and excellent SSC resistance was exhibited. “Fail” means that a fracture was observed in the round bar specimen and the SSC resistance was low.
- the yield ratio YR is 85.0% or more
- the obtained yield strength YS is 110 ksi class (758 to 862 MPa).
- the yield ratio was 90.0% or more, and the area ratio of tempered martensite was 90% or more.
- the chemical composition was appropriate and the manufacturing conditions were also appropriate. Therefore, the yield strength YS was 655 to less than 862 MPa. Furthermore, the crystal grain size number based on ASTM E112 of the prior austenite crystal grains in the seamless steel pipe was 7.0 or more, and the crystal grain size difference ⁇ GS was 1.0 or less. Therefore, no crack was confirmed in the SSC resistance test, and excellent SSC resistance was obtained.
- the Nb solid solution difference ⁇ SR in the base tube after the quenching process and before the tempering process was all 10% or less. Therefore, the tensile strength difference ⁇ TS of the seamless steel pipe of each test number after the tempering step was 27.6 MPa or less, and a stable strength was obtained in the circumferential direction of the seamless steel pipe and the whole.
- test number 9 the number of rotations of the inclined roll exceeded 75 rpm, and the peripheral speed of the roll in the drilling process exceeded 5550 mm / sec.
- the finishing temperature of the raw tube exceeded 1000 ° C. Therefore, the crystal grain size number based on ASTM E112 of the prior austenite crystal grains in the seamless steel pipe was less than 7.0, and the crystal grain size number difference ⁇ GS exceeded 1.0.
- the SSC resistance was low.
- the Nb solid solution difference ⁇ SR exceeded 10%. Therefore, the strength difference ⁇ TS exceeded 27.6 MPa, the strength varied in the circumferential direction and the axial direction of the seamless steel pipe, and a stable strength was not obtained.
- test number 10 the heating temperature of the round billet was too high. As a result, the finishing temperature of the raw pipe in the pipe making process exceeded 1000 ° C. Therefore, the grain size number based on ASTM E112 of the prior austenite crystal grains in the seamless steel pipe was less than 7.0, and the grain size number difference ⁇ GS exceeded 1.0. As a result, the SSC resistance was low. Furthermore, the Nb solid solution difference ⁇ SR exceeded 10%. Therefore, the strength difference ⁇ TS exceeded 27.6 MPa, the strength varied in the circumferential direction and the axial direction of the seamless steel pipe, and a stable strength was not obtained.
- Test No. 11 did not contain Nb. Therefore, the grain size number based on ASTM E112 of the prior austenite crystal grains in the seamless steel pipe was less than 7.0, and the crystal grain size difference ⁇ GS exceeded 1.0. Therefore, the SSC resistance was low.
- test number 12 the Nb content was too low. Therefore, the grain size number based on ASTM E112 of the prior austenite crystal grains in the seamless steel pipe was less than 7.0. Therefore, the SSC resistance was low.
- test number 15 the tempering temperature was too low at less than 650 ° C. Therefore, the yield strength was 862 MPa or more. As a result, cracks were confirmed in the SSC resistance test, and the SSC resistance was low.
- a plurality of round billets were manufactured by continuous casting using the molten steel shown in Table 1 under the same conditions as in Example 1.
- a seamless steel pipe was produced under the production conditions shown in Table 3 using a round billet.
- the round billet of each test number was heated at the heating temperature (° C.) described in Table 3.
- a pipe making process (a drilling process, a drawing rolling process, and a constant diameter rolling process) was performed on the heated round billet to produce a raw pipe having an outer diameter of 244.5 mm and a wall thickness of 13.8 mm.
- the roll diameter of the inclined roll of the punching machine was 1390 to 1410 mm.
- the roll diameter of the inclined rolls of the drilling machines with test numbers 16 to 20 was 1390 mm
- the roll diameter of the inclined roll of the drilling machines with test numbers 21 to 23 was 1410 mm.
- Table 3 shows the roll rotation speed (rpm), the roll peripheral speed (mm / sec), and the finishing temperature (° C.) during the drilling of the punching machine in the pipe making process.
- the raw pipe of test number 16 was supplemented at the supplementary heating temperature shown in Table 3 before the quenching process.
- the base tube after the pipe making process (test numbers 17 to 23) or after the heating step (test number 16) was quenched and quenched (in-line quenching).
- Quench initiation temperature in the quenching process (quenching temperature) QT1 is as shown in Table 3, both were A 3 transformation point or more.
- the average cooling rate CR from the rapid cooling start temperature QT1 in the quenching process to the outer surface temperature of the blank tube reaching 300 ° C. was 15 ° C./second or more. Tempering was performed on the raw tube after the quenching process.
- the tempering temperature TT1 was as shown in Table 3, and the holding time at the tempering temperature TT1 was 30 minutes. The tempering temperature TT1 of any test number was also below the Ac 1 transformation point. The tempered tube was allowed to cool to room temperature (25 ° C.).
- the raw tube at room temperature was heated to the reheating quenching temperature QT2 (° C.) shown in Table 3, and the reheating temperature was rapidly cooled and quenched (off-line quenching).
- the rapid cooling start temperature in the quenching step was the same as the reheat quenching temperature QT2 shown in Table 3.
- the average cooling rate CR from the rapid cooling start temperature QT2 in the quenching process until the outer surface temperature of the raw tube reached 300 ° C. was 15 ° C./second or more.
- Tempering was performed on the blank after offline quenching.
- the tempering temperature TT2 was as shown in Table 3, and the holding time at the tempering temperature TT2 was 30 minutes.
- the tempering temperature TT2 of any test number was also below the Ac 1 transformation point.
- the seamless steel pipe of each test number was manufactured by the above manufacturing process. In each test number, in order to measure the Nb solid solution difference ⁇ SR, a blank tube after the in-line quenching process and before the first temper
- Example 1 Using the seamless steel pipe of each test number, as in Example 1, the grain size of the former austenite crystal grain according to ASTM E112, the grain size difference ⁇ GS, the yield strength YS (MPa), and the tensile strength TS ( MPa) and tensile strength difference ⁇ TS (MPa). Further, the SSC resistance test was conducted in the same manner as in Example 1. Further, in the same manner as in Example 1, Nb solid solution difference ⁇ SR was obtained for the raw tube after the in-line quenching process of each test number and before the first tempering process.
- Table 3 shows the test results.
- the yield strength YS obtained was 110 ksi class (758 to 862 MPa)
- the yield ratio YR was 90.0% or more
- the area ratio of tempered martensite was 90% or more. It was.
- the chemical composition was appropriate and the production conditions were also appropriate. Therefore, all the grain size numbers based on ASTM E112 of the prior austenite crystal grains of the seamless steel pipe were 7.0 or more, and the crystal grain size difference ⁇ GS was 1.0 or less. Therefore, excellent SSC resistance was obtained in both the axial direction and the circumferential direction of the seamless steel pipe.
- the Nb solid solution difference ⁇ SR was 10% or less. Therefore, the tensile strength difference ⁇ TS was 27.6 MPa or less, and a stable strength was obtained in the circumferential direction and the axial direction of the seamless steel pipe.
- test numbers 16 to 18, 21, and 22 offline quenching was performed after in-line quenching. Therefore, compared with test numbers 1 to 8 in which only in-line quenching was performed, the grain size number in accordance with ASTM E112 of the prior austenite crystal grains of the seamless steel pipe was further increased.
- the roll rotation speed (and the roll peripheral speed) was too high, and as a result, the finishing temperature exceeded 1000 ° C. Therefore, the Nb solid solution difference ⁇ SR in the base tube after the in-line quenching process and before the first tempering process exceeded 10%. Therefore, the tensile strength difference ⁇ TS exceeded 27.6 MPa, and a stable strength could not be obtained in the circumferential direction and the axial direction of the seamless steel pipe.
- the crystal grain size difference ⁇ GS exceeded 1.0. Therefore, cracks were confirmed in the SSC resistance test, and the SSC resistance was low.
- test number 20 the heating temperature of the round billet was too high. As a result, the finishing temperature of the raw pipe in the pipe making process exceeded 1000 ° C. Therefore, the Nb solid solution difference ⁇ SR in the base tube after the in-line quenching process and before the first tempering process exceeded 10%. Therefore, the tensile strength difference ⁇ TS exceeded 27.6 MPa, and a stable strength could not be obtained in the circumferential direction and the axial direction of the seamless steel pipe. In Test No. 20, the crystal grain size difference ⁇ GS exceeded 1.0. Therefore, cracks were confirmed in the SSC resistance test, and the SSC resistance was low.
- test number 23 the final (second) tempering temperature TT2 was too low at less than 650 ° C. Therefore, the yield strength was 862 MPa or more. As a result, cracks were confirmed in the SSC resistance test, and the SSC resistance was low.
- a plurality of round billets were manufactured by continuous casting using the molten steel shown in Table 1 under the same conditions as in Example 1.
- a seamless steel pipe was produced under the production conditions shown in Table 4 using a round billet.
- the round billet of each test number was heated at the heating temperature (° C.) described in Table 4.
- a pipe making process (a drilling process, a drawing rolling process, and a constant diameter rolling process) was performed on the heated round billet to produce a raw pipe having an outer diameter of 244.5 mm and a wall thickness of 13.8 mm.
- the roll diameter of the inclined roll of the punching machine was 1390 to 1410 mm.
- the roll diameter of the inclined rolls of the drilling machines of test numbers 24 to 28 was 1390 mm
- the roll diameter of the inclined rolls of the drilling machines of test numbers 29 to 32 was 1410 mm.
- Table 4 shows the roll rotation speed (rpm), the roll peripheral speed (mm / sec), and the finishing temperature (° C.) during the drilling of the drilling machine in the pipe making process.
- the raw tube after the pipe making process was allowed to cool to room temperature (25 ° C.).
- the raw tube at room temperature was heated to the reheating quenching temperature QT2 (° C.) shown in Table 4, and the raw tube at the reheating temperature was quenched and quenched (offline quenching).
- the rapid cooling start temperature in the quenching step was the same as the reheating quenching temperature QT2 shown in Table 4.
- the average cooling rate CR from the rapid cooling start temperature QT2 in the quenching process until the outer surface temperature of the raw tube reached 300 ° C. was 15 ° C./second or more. Tempering was performed on the blank after offline quenching.
- the tempering temperature TT2 was as shown in Table 4, and the holding time at the tempering temperature TT2 was 30 minutes.
- the tempering temperature TT2 of any test number was also below the Ac 1 transformation point.
- the seamless steel pipe of each test number was manufactured by the above manufacturing process.
- Nb solid-solution rate difference (DELTA) SR the raw tube after an offline quenching process and before a tempering process was also prepared.
- Example 1 Using the seamless steel pipe of each test number, as in Example 1, the grain size of the former austenite crystal grain according to ASTM E112, the grain size difference ⁇ GS, the yield strength YS (MPa), and the tensile strength TS ( MPa) and tensile strength difference ⁇ TS (MPa). Further, the SSC resistance test was conducted in the same manner as in Example 1.
- Nb solid solution difference ⁇ SR was determined in the same manner as in Example 1 for the raw tube after the offline quenching process of each test number and before the tempering process.
- Table 4 shows the test results.
- the yield strength YS obtained was 110 ksi class (758 to 862 MPa)
- the yield ratio YR was 90.0% or more
- the area ratio of tempered martensite was 90% or more. It was.
- the chemical composition was appropriate and the production conditions were also appropriate. Therefore, all the grain size numbers based on ASTM E112 of the prior austenite crystal grains of the seamless steel pipe were 7.0 or more, and the crystal grain size difference ⁇ GS was 1.0 or less. Therefore, excellent SSC resistance was obtained in both the axial direction and the circumferential direction of the seamless steel pipe.
- the Nb solid solution difference ⁇ SR was 10% or less. Therefore, the tensile strength difference ⁇ TS was 27.6 MPa or less, and a stable strength was obtained in the circumferential direction and the axial direction of the seamless steel pipe.
- the Nb solid solution difference ⁇ SR in the raw tube after the offline quenching process and before the tempering process exceeded 10%. Therefore, the tensile strength difference ⁇ TS exceeded 27.6 MPa, and a stable strength could not be obtained in the circumferential direction and the axial direction of the seamless steel pipe.
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Abstract
Description
95ksi(655MPa)~125ksi(862MPa)未満の高い降伏強度を有する継目無鋼管において、Nbにより結晶粒を微細化して、耐SSC性を高める。Nbは、炭窒化物、炭化物及び窒化物を形成する。Nbを含有する微細な炭窒化物、炭化物及び窒化物は、熱間製管時において、旧オーステナイト結晶粒の粗大化を抑制し、旧オーステナイト結晶粒の細粒化を促進する。Nbはさらに、再結晶温度を上昇させる。再結晶温度が上昇すれば、未再結晶温度領域が拡大し、再結晶が遅くなる。その結果、旧オーステナイト結晶粒がさらに微細化される。本実施形態の継目無鋼管は、0.010~0.050%のNbを含有することにより、Nb炭窒化物等のピンニング効果を利用して、熱間製管中のオーステナイト結晶粒の粗大化を抑制する。これにより、継目無鋼管のうち、第1管端から第2管端に向かって継目無鋼管の軸方向に500mm位置までの範囲の第1管端領域と、第2管端から第1管端に向かって継目無鋼管の軸方向に500mm位置までの範囲の第2管端領域とを除く、本体領域において、旧オーステナイト結晶粒の、ASTM E112に準拠した結晶粒度番号を7.0以上とし、かつ、結晶粒度番号の最大値と最小値との差(以下、結晶粒度差ΔGSという)を1.0以下とする。その結果、降伏強度が655MPa~862MPa未満であることを前提として、優れた耐SSC性が得られる。
継目無鋼管の周方向及び軸方向の強度ばらつきは、焼入れ工程後であって焼戻し工程前の素管においてNb炭窒化物又はNb窒化物として析出せずに鋼中に固溶しているNbの割合(以下、Nb固溶率という)に起因する。焼入れ工程後であって焼戻し工程前の素管において、周方向及び軸方向におけるNb固溶率のばらつきが小さいほど、焼戻し工程後の継目無鋼管の軸方向での強度ばらつきを低減できる。具体的には、焼入れ工程後であって焼戻し工程前における、素管の本体領域での周方向及び軸方向におけるNb固溶率の最大値と最小値の差(以下、Nb固溶率差ΔSRという)が10%以下であれば、焼戻し工程後の継目無鋼管の本体領域での周方向及び軸方向における引張強度の最大値と最小値の差(以下、引張強度差ΔTSという)が27.6MPa以下となり、強度ばらつきを十分抑制できる。
図2は、本実施形態の継目無鋼管の一例を示す図である。図2を参照して、本実施形態の継目無鋼管10は、第1管端1Eと、第2管端2Eとを備える。第2管端2Eは、継目無鋼管10の軸方向において、第1管端1Eの反対側(opposite to)に配置されている。
本実施形態の継目無鋼管の化学組成は、次の元素を含有する。
炭素(C)は、鋼の強度を高める。C含有量が低すぎれば、この効果が得られない。一方、C含有量が高すぎれば、鋼の焼割れに対する感受性が高くなる。この場合、特に鋼管の焼入れにおいては、特別な冷却手段(焼入れ方法)が必要になる。C含有量が高すぎればさらに、鋼の靱性が低下する場合がある。したがって、C含有量は0.21~0.35%である。C含有量の好ましい下限は0.23%であり、さらに好ましくは0.25%である。C含有量の好ましい上限は0.30%であり、さらに好ましくは0.27%である。
シリコン(Si)は鋼を脱酸する。Si含有量が低すぎれば、この効果が得られない。一方、Si含有量が高すぎれば、鋼の耐SSC性及び加工性が低下する。したがって、Si含有量は0.10~0.50%である。Si含有量の好ましい下限は0.15%であり、さらに好ましくは0.20%である。Si含有量の好ましい上限は0.40%であり、さらに好ましくは0.35%である。
マンガン(Mn)は鋼の焼入れ性を高め、鋼の強度を高める。Mn含有量が低すぎれば、この効果が得られない。一方、Mn含有量が高すぎれば、Mnが粒界に偏析して鋼の耐SSC性が低下する。したがって、Mn含有量は0.05~1.00%である。Mn含有量の好ましい下限は0.30%であり、さらに好ましくは0.40%である。Mn含有量の好ましい上限は0.95%であり、さらに好ましくは0.90%である。
燐(P)は不純物であり、鋼中に不可避的に含有される。Pは粒界に偏析して鋼の耐SSC性を低下する。したがって、P含有量は0.025%以下である。P含有量の好ましい上限は0.020%であり、さらに好ましくは0.015%である。P含有量はなるべく低い方が好ましい。
硫黄(S)は不純物であり、鋼中に不可避的に含有される。SはMnと結合して硫化物系介在物を形成し、鋼の耐SSC性を低下する。したがって、S含有量は0.010%以下である。S含有量の好ましい上限は0.006%であり、さらに好ましくは0.003%である。S含有量はなるべく低い方が好ましい。
アルミニウム(Al)は、鋼を脱酸する。Al含有量が低すぎれば、この効果が得られない。一方、Al含有量が高すぎれば、その効果が飽和する。Al含有量が高すぎればさらに、粗大なAl系酸化物が多数生成して鋼の耐SSC性を低下する。したがって、Al含有量は0.005~0.100%である。Al含有量の好ましい下限は0.010%であり、さらに好ましくは0.020%である。Al含有量の好ましい上限は0.070%であり、さらに好ましくは0.050%である。本明細書において、Al含有量とは、いわゆる酸可溶Al(sol.Al)の含有量を意味する。
窒素(N)は、鋼中に不可避に含有される。Nは窒化物を形成する。微細な窒化物は、結晶粒の粗大化を防止するので、Nは含有されてもよい。一方、粗大な窒化物は、鋼の耐SSC性を低下させる。したがって、N含有量は0.010%以下である。N含有量の好ましい上限は0.004%であり、さらに好ましくは0.003%である。微細な窒化物の析出によるピンニング効果を得るためのN含有量の好ましい下限は0.002%である。
クロム(Cr)は鋼の焼入れ性を高め、鋼の強度を高める。Cr含有量が低すぎれば、この効果が得られない。一方、Cr含有量が高すぎれば鋼の耐SSC性が低下する。したがって、Cr含有量は0.05~1.50%である。Cr含有量の好ましい下限は0.20%であり、さらに好ましくは0.40%である。Cr含有量の好ましい上限は1.20%であり、さらに好ましくは1.15%である。
モリブデン(Mo)は鋼の焼入れ性を高め、鋼の強度を高める。Moはさらに、鋼の焼戻し軟化抵抗性を高め、高温焼戻しによる耐SSC性を高める。Mo含有量が低すぎれば、この効果が得られない。一方、Mo含有量が高すぎれば、その効果が飽和するとともに、製造コストが嵩む。したがって、Mo含有量は0.10~1.50%である。Mo含有量の好ましい下限は0.15%であり、さらに好ましくは0.20%である。Mo含有量の好ましい上限は0.80%であり、さらに好ましくは0.60%である。
ニオブ(Nb)は、C及びNと結合して微細なNb炭窒化物、Nb炭化物、及びNb窒化物を形成する。Nbはさらに、Ti及びAlとともに複合炭化物を形成する。これらの炭窒化物等(Nb炭窒化物、Nb炭化物、及びNb窒化物及び複合炭化物)は、ピンニング効果により結晶粒を細粒化して鋼の耐SSC性を高める。これらの炭窒化物等はさらに、結晶粒度のばらつきを抑制する。Nb含有量が低すぎれば、この効果が得られない。一方、Nb含有量が高すぎれば、粗大なNb系介在物が多数生成して、鋼の耐SSC性が低下する。したがって、Nb含有量は0.010~0.050%である。Nb含有量の好ましい下限は0.013%であり、さらに好ましくは0.015%であり、さらに好ましくは0.020%である。Nb含有量の好ましい上限は0.040%であり、さらに好ましくは0.035%である。
ボロン(B)は、鋼の焼入れ性を高め、鋼の強度を高める。B含有量が低すぎれば、この効果が得られない。一方、B含有量が高すぎれば、粒界に炭窒化物が析出して、鋼の耐SSC性が低下する。したがって、B含有量は0.0003~0.0050%である。B含有量の好ましい下限は0.0005%であり、さらに好ましくは0.0008%である。B含有量の好ましい上限は0.0030%であり、さらに好ましくは0.0020%である。
チタン(Ti)はC及びNと結合して微細なTi炭窒化物を形成し、不純物であるNを固定する。Ti窒化物の生成により、結晶粒が微細化され、さらに、鋼の強度が高まる。鋼にBが含有される場合はさらに、TiはB窒化物の生成を抑制するため、Bによる焼入れ性の向上を促進する。Ti含有量が低すぎれば、これらの効果が得られない。一方、Ti含有量が高すぎれば、Nb系介在物中にTiが固溶して、Nb系介在物が粗大化する。この場合、鋼の耐SSC性が低下する。したがって、Ti含有量は0.002~0.050%である。Ti含有量の好ましい下限は0.003%であり、さらに好ましくは0.004%である。Ti含有量の好ましい上限は0.035%であり、さらに好ましくは0.030%である。
上述の継目無鋼管の化学組成はさらに、Feの一部に代えて、Vを含有してもよい。
バナジウム(V)は任意元素であり、含有されなくてもよい。含有される場合、Vは微細な炭化物を生成して焼戻し軟化抵抗を高め、高温焼戻しを可能とする。これにより、鋼の耐SSC性が高まる。しかしながら、V含有量が高すぎれば、炭化物が過剰に生成して鋼の耐SSC性がかえって低下する。したがって、V含有量は0~0.30%である。上記効果をさらに有効に得るためのV含有量の好ましい下限は0.01%であり、さらに好ましくは0.02%である。V含有量の好ましい上限は0.25%であり、さらに好ましくは0.20%である。
カルシウム(Ca)は任意元素であり、含有されなくてもよい。含有される場合、Caは鋼中の硫化物系介在物を球状化する。これにより、鋼の耐SSC性が高まる。Caを少しでも含有すれば、上記効果が得られる。しかしながら、Ca含有量が高すぎれば、介在物が過剰に多く生成し、鋼の耐SSC性が低下する。したがって、Ca含有量は0~0.0050%である。Ca含有量の好ましい下限は0.0001%であり、さらに好ましくは0.0010%であり、さらに好ましくは0.0015%である。Ca含有量の好ましい上限は0.0040%であり、さらに好ましくは0.0030%である。
希土類元素(REM)は任意元素であり、含有されなくてもよい。含有される場合、REMは鋼中の硫化物系介在物を球状化する。これにより、鋼の耐SSC性が高まる。REMを少しでも含有すれば、上記効果が得られる。しかしながら、REM含有量が高すぎれば、介在物が過剰に多く生成し、鋼の耐SSC性が低下する。したがって、REM含有量は0~0.0050%である。REM含有量の好ましい下限は0.0001%であり、さらに好ましくは0.0010%である。REM含有量の好ましい上限は0.0040%であり、さらに好ましくは0.0030%である。
本実施形態の継目無鋼管のミクロ組織は主として焼戻しマルテンサイトからなり、残部はたとえば、フェライト、ベイナイト、パーライト又はこれらの混合相等である。ここで、「主として」とは、ミクロ組織中の焼戻しマルテンサイトの総面積率が90%以上であることを意味する。
YR=YS/TS×100
本実施形態の継目無鋼管のミクロ組織ではさらに、旧オーステナイト結晶粒のASTM E112に準拠した結晶粒度番号が7.0以上である。旧オーステナイト結晶粒の粒度番号が7.0未満であれば、旧オーステナイト結晶粒が粗い。そのため、耐SSC性が低下する。旧オーステナイト結晶粒の度番号が7.0以上であれば、結晶粒が十分に微細である。そのため、優れた耐SSC性が得られる。本実施形態では、製管工程において従来よりも低温(1100℃以下)で製管し、かつ、穿孔及び圧延時に発生する加工発熱を抑制することにより、上記旧オーステナイト結晶粒度番号を実現する。
旧オーステナイト結晶粒度の測定方法は次のとおりである。継目無鋼管の第1管端領域と第2管端領域とを除く本体領域を、継目無鋼管の軸方向に5等分した各区分の軸方向中央位置を選定する。選定された各位置での継目無鋼管の軸方向に垂直な断面において、継目無鋼管の中心軸周りに45°ピッチ位置の8位置の肉厚中央位置から、継目無鋼管の軸方向と平行な表面(観察面)100を有する試験片を作製する。試験片の観察面100は、図3に示すとおり、継目無鋼管の肉厚方向において、外面から1.5mm深さまでの領域と、内面から1.5mm深さまでの領域とを削り落として、肉厚方向の長さを、肉厚T(mm)-(外面から1.5mm深さ+内面から1.5mm深さ)とする。さらに、観察面100の長さを継目無鋼管の軸方向に15mmとする。つまり、観察面100は、(肉厚T-3.0mm)×15mmの矩形とする。各試験片の観察面を機械研磨する。ピクラール(Picral)腐食液を用いて機械研磨後の観察面をエッチングして、観察面内の旧オーステナイト結晶粒界を現出させる。その後、観察面を倍率200倍の光学顕微鏡を用いて、任意の4視野(1視野あたり500μm×500μm)で、ASTM E112に準拠して、旧オーステナイト結晶粒の結晶粒度番号の平均値を求める。求めた平均値を、各測定位置での旧オーステナイト結晶粒の結晶粒度番号とする。各測定位置(合計40箇所)で得られた旧オーステナイト結晶粒の結晶粒度番号のうち、最小の結晶粒度番号を、継目無鋼管の旧オーステナイト結晶粒のASTM E112に準拠した結晶粒度番号と定義する。
本実施形態の継目無鋼管のミクロ組織ではさらに、本体領域において、継目無鋼管周方向及び軸方向のうち、任意の複数の部分で測定された結晶粒度番号の最大値と最小値の差(結晶粒度差ΔGS)が1.0以下である。結晶粒度差ΔGSが1.0を超える場合、サワー環境において、鋼材に侵入した水素が粗粒部分の脆化を引き起こし、その結果、耐SSC性が低下する。結晶粒度差ΔGSが1.0以下である場合、優れた耐SSC性が得られる。本実施形態では、製管工程中に生成するNb炭窒化物及びNb窒化物(以下、Nb炭窒化物等という)によるピンニング効果により、結晶粒が細粒化され、かつ、結晶粒度差ΔGSを1.0以下にすることができる。Nbが含有されない場合、結晶粒は絶対的に粗大化し、パイプの軸方向や周方向の温度ばらつきの影響も受けて、結晶粒度差ΔGSが1.0を超える。
結晶粒度差ΔGSは次の方法で測定される。上述の旧オーステナイト結晶粒度番号の測定方法において、結晶粒度番号を求めた40箇所の測定位置での結晶粒度番号のうち、最大値と最小値を選択する。最大値から最小値の差分を、結晶粒度差ΔGSと定義する。
本実施形態の継目無鋼管の降伏強度YSは655MPa(95ksi)~862MPa(125ksi)未満である。降伏強度YSが862MPa以上であれば、上述のミクロ組織を有していても、優れた耐SSC性が得られない。一方、降伏強度が655MPa未満であれば、高腐食性井戸用途の油井管として使用するのに必要な強度が得られない。したがって、本実施形態の継目無鋼管の降伏強度YSは655~862MPa未満である。降伏強度は、上述のとおり定義する。すなわち、降伏強度が95ksiグレード(655MPa~758MPa)である場合、0.5%全伸びの値を降伏強度(MPa)と定義する。降伏強度が110ksiグレード(758MPa~876MPa未満)である場合、0.7%全伸びの値を降伏強度(MPa)と定義する。これらの降伏強度の定義は、API規格の5CT規定に準拠している。
降伏強度YS及び引張強度TSは次の方法で測定される。継目無鋼管の第1管端領域と第2管端領域とを除く本体領域を、継目無鋼管の軸方向に5等分した各区分の軸方向中央位置を選定する。選定された各位置において、継目無鋼管の中心軸周りに90°ピッチ位置の4位置から、弧状引張試験片を採取する。弧状引張試験片の横断面(継目無鋼管の軸方向と垂直な断面)は弧状であり、弧状引張試験片の軸方向は、継目無鋼管の軸方向と平行である。弧状引張試験片を用いて、API規格の5CT規定に準拠して、常温(25℃)にて引張試験を実施する。各弧状試験片で得られた降伏強度(合計20箇所)の平均を、継目無鋼管の降伏強度YS(MPa)と定義する。さらに、各弧状試験片で得られた引張強度TS(20箇所)の最大値と最小値との差を引張強度差ΔTS(MPa)と定義する。
本実施形態の継目無鋼管の製造方法の一例を説明する。ただし、本実施形態の継目無鋼管の製造方法は後述する製造方法に限定されない。
ケース1:加熱工程-製管工程-焼入れ工程(直接焼入れ)-焼戻し工程
ケース2:加熱工程-製管工程-補熱工程-焼入れ工程-焼戻し工程
ケース3:加熱工程-製管工程-再加熱工程-焼入れ工程-焼き戻し工程
ケース4:加熱工程-製管工程-補熱工程-焼入れ工程-再加熱工程-焼入れ工程-焼戻し工程
ケース5:加熱工程-製管工程-再加熱工程-焼入れ工程-再加熱工程-焼入れ工程-焼戻し工程
ケース6:加熱工程-製管工程-(補熱工程)-焼入れ工程-焼戻し工程-再加熱工程-焼入れ工程-焼戻し工程
ケース7:加熱工程-製管工程-再加熱工程-焼入れ工程-焼戻し工程-再加熱工程-焼入れ工程-焼戻し工程
初めに、上述の化学組成を有する丸ビレットを準備する。丸ビレットの製造方法は特に限定されない。丸ビレットはたとえば、次の方法により製造される。上記化学組成を有する溶鋼を製造する。溶鋼の製造には、たとえば、転炉等を利用する。溶鋼を用いて、連続鋳造法によるブルームを製造する。溶鋼を用いて造塊法によりインゴットを製造してもよい。ブルーム及びインゴットを熱間圧延して、横断面が円形状の丸ビレットを製造する。溶鋼を用いて連続鋳造法により丸ビレットを製造してもよい。以上の方法により丸ビレットを準備する。
加熱工程により加熱された丸ビレットを穿孔及び圧延して素管を製造する。製管工程は、穿孔工程と、圧延工程とを備える。圧延工程はたとえば、延伸圧延工程と、定径圧延工程とを含む。穿孔工程では、穿孔機を用いて丸ビレットを穿孔圧延して素管にする。延伸圧延工程では、延伸圧延機を用いて、素管を延伸圧延する。延伸圧延機はたとえば、プラグミル、マンドレルミルである。定径圧延工程では、定径圧延機を用いて、素管を定径圧延する。定径圧延機はたとえば、サイザー又はストレッチレデューサである。
補熱工程は必要に応じて実施される。熱間製管後の素管の外面温度(仕上げ温度)がA3変態点(Ar3変態点)未満であれば、焼入れを実施できない。ただし、熱間製管後の素管の外面温度が400℃以上であれば、オフラインの加熱炉で再加熱する必要がなく、定径圧延機と焼入れ装置(急冷装置)とをつなぐ搬送路上に配置された補熱炉又はインダクションヒータを用いて、インラインで素管を加熱して素管の外面温度をAc3変態点以上にする。補熱工程により加熱される素管の外面温度の好ましい上限はAc3変態点+50℃である。補熱工程により加熱後の素管の外面温度を、本明細書では「補熱温度」と称する。補熱温度は次の方法で測定される。インダクションヒータを用いて補熱工程を実施する場合、インダクションヒータ出側での素管の本体領域の外面温度の平均値を、補熱温度(℃)と定義する。この場合、インダクションヒータの出側に配置された放射温度計により、補熱温度が測定される。一方、補熱炉を用いて補熱工程を実施する場合、補熱炉の炉温(℃)は、素管の本体領域の外面温度と一致する。そのためこの場合、補熱炉の炉温を素管の外面温度(℃)と定義する。
再加熱工程は必要に応じて実施される。熱間製管後の素管を放冷した結果、素管の外面温度が400℃未満~常温(25℃)になった場合、製管ラインのオフラインに配置された加熱炉を用いて、素管を再加熱する。再加熱工程により加熱される素管の外面温度はAc3変態点以上であり、好ましい上限はAc3変態点+50℃である。加熱炉の炉温(℃)は、素管の本体領域の外面温度と一致する。そのため、再加熱工程において加熱炉を用いる場合、加熱炉の炉温を素管の外面温度(℃)と定義する。
焼入れ工程では、A3変態点以上(製管工程後の素管の外面温度がAr3変態点以上、又は、補熱工程及び再加熱工程を実施した場合、素管の外面温度がAc3変態点以上)の外面温度を有する素管を、急冷して焼入れする。焼入れ工程での急冷開始時の素管の外面温度(焼入れ温度)は、A3変態点(Ar3変態点又はAc3変態点)~1000℃である。ここで、急冷開始時の素管の外面温度は、本体領域の外面温度の平均値である。さらに、焼入れ工程での急冷開始時の素管の外面温度から、素管の外面温度が300℃に至るまでの間の平均冷却速度CRを、15℃/秒以上とする。
最初の焼入れ工程後であって次の工程を実施する前(たとえば、次工程が焼戻し工程の場合は、焼戻し工程前、次工程が再加熱工程である場合、再加熱工程前)の素管において、Nb炭窒化物等(Nb炭窒化物及びNb窒化物)として析出せずに鋼中に固溶しているNbの、鋼中の全Nb含有量に対する割合をNb固溶率(質量%)と定義する。この場合、素管の本体領域でのNb固溶率の最大値と最小値との差(以下、Nb固溶率差ΔSRという)は10%以下である。Nb固溶率差ΔSRが10%を超える場合、後述の焼戻し工程を経て製造される継目無鋼管の本体領域で測定された引張強度TSの最大値と最小値との差(引張強度差ΔTS)が大きくなり、継目無鋼管中での強度ばらつきが大きくなる。Nb固溶率差ΔSRが10%以下である場合、引張強度差ΔTSが27.6MPa以下と小さくなり、継目無鋼管周方向及び軸方向での強度ばらつきが抑えられる。そのため、本実施形態の継目無鋼管は安定した高強度を有する。本実施形態では、上述の製管工程において従来よりも低温(1100℃以下)で製管し、かつ、穿孔圧延時に発生する加工発熱を抑制することにより、上記Nb固溶率差を実現する。
最初の焼入れ工程後であって次工程前の素管のNb固溶率は、抽出残渣法により測定する。具体的には、最初の焼入れ工程後であって次工程前の素管の第1管端領域と第2管端領域とを除く本体領域を、素管の軸方向に5等分し、各区分の素管軸方向の中央位置を測定位置に選定する。各測定位置での素管の軸方向に垂直な断面において、素管の中心軸周りに90°ピッチの4つ位置の肉厚中央位置から、試験片を採取する。このとき、採取された試験片の表面積は15cm2とする。試験片を電解液中で0.5g分電解してマトリクスを溶解する。電解液は、10%アセチルアセトン+1%テトラメチルアンモニウムクロライド+残部メタノールであり、電流は200A/m2とする。0.2μm穴径のフィルタで残渣(=析出物)を濾過し、析出物を抽出する。その析出物を酸分解して、ICP(誘導結合プラズマ分析)にて化学組成を分析し、析出物中のNb含有量を求める。この析出物中のNb含有量に基づいて、次の式によりNb固溶率を求める。
Nb固溶率=(鋼中の全Nb含有量-析出物中のNb含有量)/鋼中の全Nb含有量
焼入れ工程にて急冷されて焼入れされた素管を、焼戻しして継目無鋼管とする。焼戻し温度は、650℃~Ac1変態点であり、所望の力学特性に基づいて調整される。具体的には、焼戻し後の継目無鋼管の降伏強度が655MPa~862MPa未満となるように、焼戻し温度が調整される。降伏強度が862MPa以上であれば、結晶粒度番号が7.0以上であり、結晶粒度番号差ΔGSが1.0以下であっても、耐SSC性が低くなる。降伏強度が862MPa未満であれば、耐SSC性がより高まる。上述の化学組成の本実施形態の継目無鋼管の降伏強度を655MPa~862MPa未満とするための焼戻し温度は650℃~Ac1変態点であり、好ましい上限は750℃である。
表1に示す化学組成を有する溶鋼を製造した。
各試験番号の焼入れ工程後であって焼戻し工程前の素管の第1管端領域と第2管端領域とを除く本体領域を、素管の軸方向に5等分し、各区分の素管軸方向の中央位置を測定位置に選定した。各測定位置での素管の軸方向に垂直な断面において、素管の中心軸周りに90°ピッチの4つ位置の肉厚中央位置から、試験片を採取した。このとき、採取された試験片の表面積は15cm2とした。試験片を電解液中で0.5g分電解してマトリクスを溶解した。電解液は、10%アセチルアセトン+1%テトラメチルアンモニウムクロライド+残部メタノールであり、電流は200A/m2とした。0.2μm穴径のフィルタで残渣(=析出物)を濾過し、析出物を抽出した。その析出物を酸分解して、ICP(誘導結合プラズマ分析)にて化学組成を分析し、析出物中のNb含有量を求めた。この析出物中のNb含有量に基づいて、次の式によりNb固溶率を求めた。
Nb固溶率=(鋼中の全Nb含有量-析出物中のNb含有量)/鋼中の全Nb含有量
各試験番号の継目無鋼管のうち、第1管端領域及び第2管端領域を除く、本体領域を継目無鋼管の軸方向に5等分した場合の、各区分の軸方向の中央位置を測定位置に選定した。各測定位置において、継目無鋼管の中心軸周りに45°ピッチの8位置での肉厚中央位置から、継目無鋼管の軸方向に平行な表面(観察面)を有する試験片を作製した。試験片の観察面100は、図3に示すとおり、継目無鋼管の肉厚方向において、外面から1.5mm深さまでの領域と、内面から1.5mm深さまでの領域とを削り落として、観察面100の肉厚方向の長さを、肉厚T(mm)-(外面から1.5mm深さ+内面から1.5mm深さ)とした。さらに、観察面100の継目無鋼管の軸方向の長さを15mmとした。つまり、観察面100は、(肉厚T-3.0mm)×15mmの矩形であった。各試験片の観察面を機械研磨した。ピクラール(Picral)腐食液を用いて機械研磨後の観察面をエッチングして、観察面内の旧オーステナイト結晶粒界を現出させた。その後、観察面を倍率200倍の光学顕微鏡を用いて、任意の4視野(1視野あたり500μm×500μm)でで、ASTM E112に準拠して、旧オーステナイト結晶粒の結晶粒度番号の平均値を求め、各測定位置での旧オーステナイト結晶粒の結晶粒度番号とした。各測定位置(合計40箇所)で得られた旧オーステナイト結晶粒の結晶粒度番号のうち、最小の結晶粒度番号を、継目無鋼管の旧オーステナイト結晶粒のASTM E112に準拠した結晶粒度番号と定義した。さらに、各測定位置で得られた結晶粒度番号(合計40箇所)のうちの最大値と最小値を選び、最大値から最小値を差し引いた差分を結晶粒度差ΔGSと定義した。
各試験番号の継目無鋼管の第1管端領域と第2管端領域とを除く本体領域を、継目無鋼管の軸方向に5等分した各区分において、継目無鋼管の軸方向の中央位置を選定した。選定された各位置の継目無鋼管の中心軸周りに90°ピッチ位置の4位置から、弧状引張試験片を採取した。弧状引張試験片の横断面(継目無鋼管の軸方向と垂直な断面)は弧状であり、弧状引張試験片の軸方向は、継目無鋼管の軸方向と平行であった。弧状引張試験片を用いて、API規格の5CT規定に準拠して、常温(25℃)にて引張試験を実施した。各弧状試験片で得られた降伏強度(合計20箇所)の平均を、継目無鋼管の降伏強度YS(MPa)と定義した。各弧状試験片で得られた引張強度(合計20箇所)の平均を、継目無鋼管の引張強度TS(MPa)と定義した。各弧状試験片で得られた引張強度TS(20箇所)の最大値と最小地との差を引張強度差ΔTS(MPa)と定義した。
各試験番号の継目無鋼管の第1管端領域と第2管端領域とを除く本体領域を、継目無鋼管の軸方向に5等分した各区分において、継目無鋼管の軸方向の中央位置を選定した。各選定された各位置の継目無鋼管の中心軸周りに90°ピッチ位置の4位置の肉厚中央部から丸棒試験片を採取した。丸棒試験片の軸方向は、継目無鋼管の軸方向と平行であった。丸棒試験片の平行部の外径は6.35mmであり、平行部の長さは25.4mmであった。
表2に試験結果を示す。表2中の「YS」は降伏強度YS(MPa)を示し、「TS」は引張強度TS(MPa)を示す。「耐SSC性」欄には、耐SSC性評価試験結果が記載されている。「合格」は、丸棒試験片に破断が観察されず、優れた耐SSC性が示されたことを意味する。「不合格」は、丸棒試験片に破断が観察され、耐SSC性が低かったことを意味する。なお、各試験番号において、得られた降伏強度YSが95ksi級(655~758MPa未満)の場合、降伏比YRが85.0%以上であり、得られた降伏強度YSが110ksi級(758~862MPa)である場合、降伏比が90.0%以上であり、焼戻しマルテンサイトの面積率がいずれも90%以上であった。
表3に試験結果を示す。なお、各試験番号において、得られた降伏強度YSが110ksi級(758~862MPa)であり、降伏比YRが90.0%以上であり、焼戻しマルテンサイトの面積率がいずれも90%以上であった。表3を参照して、試験番号16~18、21及び22では、化学組成が適切であり、製造条件も適切であった。そのため、継目無鋼管の旧オーステナイト結晶粒のASTM E112に準拠した粒度番号がいずれも7.0以上であり、かつ、結晶粒度差ΔGSが1.0以下であった。そのため、継目無鋼管の軸方向及び周方向のいずれにおいても優れた耐SSC性が得られた。
表4に試験結果を示す。なお、各試験番号において、得られた降伏強度YSが110ksi級(758~862MPa)であり、降伏比YRが90.0%以上であり、焼戻しマルテンサイトの面積率がいずれも90%以上であった。表4を参照して、試験番号24~26、29~31では、化学組成が適切であり、製造条件も適切であった。そのため、継目無鋼管の旧オーステナイト結晶粒のASTM E112に準拠した粒度番号がいずれも7.0以上であり、かつ、結晶粒度差ΔGSが1.0以下であった。そのため、継目無鋼管の軸方向及び周方向のいずれにおいても優れた耐SSC性が得られた。
Claims (9)
- 第1管端及び第2管端を有する継目無鋼管であって、
化学組成が、質量%で、
C:0.21~0.35%、
Si:0.10~0.50%、
Mn:0.05~1.00%、
P:0.025%以下、
S:0.010%以下、
Al:0.005~0.100%、
N:0.010%以下、
Cr:0.05~1.50%、
Mo:0.10~1.50%、
Nb:0.010~0.050%、
B:0.0003~0.0050%、
Ti:0.002~0.050%、
V:0~0.30%、
Ca:0~0.0050%、及び、
希土類元素:0~0.0050%、を含有し、残部がFe及び不純物からなり、
前記継目無鋼管のうち、前記第1管端から前記第2管端に向かって前記継目無鋼管の軸方向に500mm位置までの範囲の第1管端領域と、前記第2管端から前記第1管端に向かって前記継目無鋼管の軸方向に500mm位置までの範囲の第2管端領域とを除く、本体領域において、旧オーステナイト結晶粒の、ASTM E112に準拠した結晶粒度番号が7.0以上であり、
前記本体領域において、前記結晶粒度番号の最大値と最小値との差が1.0以下であり、
前記本体領域において、降伏強度が655~862MPa未満であり、
前記本体領域において、引張強度の最大値と最小値との差が27.6MPa以下である、継目無鋼管。 - 請求項1に記載の継目無鋼管であって、
V:0.01~0.30%を含有する、継目無鋼管。 - 請求項1又は請求項2に記載の継目無鋼管であって、
Ca:0.0001~0.0050%、及び
希土類元素:0.0001~0.0050%からなる群から選択される1種以上を含有する、継目無鋼管。 - 継目無鋼管の製造方法であって、
質量%で、C:0.21~0.35%、Si:0.10~0.50%、Mn:0.05~1.00%、P:0.025%以下、S:0.010%以下、Al:0.005~0.100%、N:0.010%以下、Cr:0.05~1.50%、Mo:0.10~1.50%、Nb:0.010~0.050%、B:0.0003~0.0050%、Ti:0.002~0.050%、V:0~0.30%、Ca:0~0.0050%、及び、希土類元素:0~0.0050%を含有し、残部がFe及び不純物からなる丸ビレットを950~1100℃に加熱する工程と、
前記丸ビレットを、傾斜ロールを有する穿孔機を用いて、前記傾斜ロールの回転数を20~75rpmとして穿孔圧延し、さらに、圧延を実施して素管を製造し、最終の圧延時の素管温度を800~1000℃とする製管工程と、
前記製管工程で製造され、外面温度がA3変態点~1000℃の前記素管を急冷し、急冷開始時の前記素管の前記外面温度から、前記外面温度が300℃に至るまでの平均冷却速度を15℃/秒以上とする焼入れ工程と、
前記焼入れ工程により急冷された前記素管の外面温度を650℃~Ac1変態点で保持する焼戻しを実施して、降伏強度が655~862MPa未満の継目無鋼管を製造する焼戻し工程とを備える、継目無鋼管の製造方法。 - 継目無鋼管の製造方法であって、
質量%で、C:0.21~0.35%、Si:0.10~0.50%、Mn:0.05~1.00%、P:0.025%以下、S:0.010%以下、Al:0.005~0.100%、N:0.010%以下、Cr:0.05~1.50%、Mo:0.10~1.50%、Nb:0.010~0.050%、B:0.0003~0.0050%、Ti:0.002~0.050%、V:0~0.30%、Ca:0~0.0050%、及び、希土類元素:0~0.0050%を含有し、残部がFe及び不純物からなる丸ビレットを950~1100℃に加熱する工程と、
前記丸ビレットを、傾斜ロールを有する穿孔機を用いて、前記傾斜ロールの周速を1450~5550mm/秒として穿孔し、さらに、圧延を実施して素管を製造し、最終の圧延時の素管温度を800~1000℃とする製管工程と、
前記製管工程で製造され、外面温度がA3変態点~1000℃の前記素管を急冷し、急冷開始時の前記素管の前記外面温度から、前記外面温度が300℃に至るまでの平均冷却速度を15℃/秒以上とする焼入れ工程と、
前記焼入れ工程により急冷された前記素管の外面温度を650℃~Ac1変態点で保持する焼戻しを実施して、降伏強度が655~862MPa未満の継目無鋼管を製造する焼戻し工程とを備える、継目無鋼管の製造方法。 - 請求項4又は請求項5に記載の継目無鋼管の製造方法であってさらに、
前記焼入れ工程前であって前記製管工程後に、前記製管工程で製造され前記外面温度が400℃~Ar3変態点未満の前記素管を加熱して、前記素管の外面温度をAc3変態点~1000℃にする補熱工程を備え、
前記焼入れ工程では、前記補熱工程により加熱されて外面温度がA3変態点~1000℃となった前記素管を急冷する、継目無鋼管の製造方法。 - 請求項4又は請求項5に記載の継目無鋼管の製造方法であってさらに、
前記焼入れ工程前であって前記製管工程後に、前記製管工程で製造され前記外面温度が400℃未満の前記素管を再加熱して、前記素管の外面温度をAc3変態点~1000℃にする再加熱工程を備え、
前記焼入れ工程では、前記再加熱工程により加熱されて外面温度がA3変態点~1000℃となった前記素管を急冷する、継目無鋼管の製造方法。 - 請求項4~請求項7のいずれか1項に記載の継目無鋼管の製造方法であって、
前記丸ビレットは、
V:0.01~0.30%を含有する、継目無鋼管の製造方法。 - 請求項4~請求項8のいずれか1項に記載の継目無鋼管の製造方法であって、
前記丸ビレットは、
Ca:0.0001~0.0050%、及び
希土類元素:0.0001~0.0050%からなる群から選択される1種以上を含有する、継目無鋼管の製造方法。
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JPWO2019188740A1 (ja) * | 2018-03-26 | 2021-02-25 | 日本製鉄株式会社 | サワー環境での使用に適した鋼材 |
EP3778956A4 (en) * | 2018-03-26 | 2021-12-01 | Nippon Steel Corporation | SUITABLE STEEL MATERIAL FOR USE IN ACIDIC ENVIRONMENTS |
EP3778957A4 (en) * | 2018-03-27 | 2021-12-15 | Nippon Steel Corporation | SUITABLE STEEL MATERIAL FOR USE IN AN ACIDIC ENVIRONMENT |
JPWO2019198459A1 (ja) * | 2018-04-09 | 2021-01-14 | 日本製鉄株式会社 | 鋼管、及び、鋼管の製造方法 |
JPWO2019198468A1 (ja) * | 2018-04-09 | 2021-03-11 | 日本製鉄株式会社 | サワー環境での使用に適した鋼材 |
WO2019198468A1 (ja) * | 2018-04-09 | 2019-10-17 | 日本製鉄株式会社 | サワー環境での使用に適した鋼材 |
WO2019198459A1 (ja) * | 2018-04-09 | 2019-10-17 | 日本製鉄株式会社 | 鋼管、及び、鋼管の製造方法 |
US20210317553A1 (en) * | 2018-10-01 | 2021-10-14 | Nippon Steel Corporation | Seamless steel pipe suitable for use in sour environment |
US11905580B2 (en) * | 2018-10-01 | 2024-02-20 | Nippon Steel Corporation | Seamless steel pipe suitable for use in sour environment |
US20220098712A1 (en) * | 2019-02-15 | 2022-03-31 | Nippon Steel Corporation | Steel material suitable for use in sour environment |
EP3926058A4 (en) * | 2019-02-15 | 2024-01-31 | Nippon Steel Corp | STEEL MATERIAL SUITABLE FOR USE IN ACID ENVIRONMENT |
EP3926059A4 (en) * | 2019-02-15 | 2024-02-07 | Nippon Steel Corp | STEEL MATERIAL FOR USE IN AN ACIDIC ENVIRONMENT |
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JPWO2017200033A1 (ja) | 2019-03-14 |
JP6635194B2 (ja) | 2020-01-22 |
US11313005B2 (en) | 2022-04-26 |
MX2018014000A (es) | 2019-04-01 |
CN109154053A (zh) | 2019-01-04 |
EP3460086A1 (en) | 2019-03-27 |
US20190300979A1 (en) | 2019-10-03 |
CA3024691A1 (en) | 2017-11-23 |
EP3460086B1 (en) | 2020-11-04 |
BR112018073053A2 (pt) | 2019-02-26 |
RU2697999C1 (ru) | 2019-08-21 |
BR112018073053B1 (pt) | 2022-09-20 |
CN109154053B (zh) | 2020-08-11 |
EP3460086A4 (en) | 2019-11-27 |
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